A&A 374, 421-434 (2001)
DOI: 10.1051/0004-6361:20010717
D. Bettoni1 - G. Galletta2 - S. García-Burillo3 - A. Rodríguez-Franco4,5
1 - Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122
Padova, Italy
2 -
Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2,
35122 Padova, Italy
3 -
Observatorio Astronómico Nacional-OAN, Apartado 1143, 28800 Alcalá de
Henares-Madrid, Spain
4 -
Departamento de Matemática Aplicada (Biomatemática) Sección
Departamental de Optica,
Universidad Complutense de Madrid, Av. Arcos de
Jalón s/n, 28037 Madrid, Spain
5 -
Nobeyama Radio Observatory, Nobeyama, Minamimaki, Minamisaku, Nagano
384-1305, Japan
Received 23 February 2001 / Accepted 7 May 2001
Abstract
This paper studies the global ISM content in a sample of 104 accreting
galaxies, including counterrotators and polar rings, which spans the
entire Hubble sequence. The molecular, atomic and hot gas content of
accretors is compared to a newly compiled sample of normal
galaxies. We present results of a small survey of the J=1-0line of 12CO with the 15 m SEST telescope on a sample of 11
accretors (10 counterrotators and 1 polar ring). The SEST sample is
enlarged with published data from 48 galaxies, for which observational
evidence of counterrotation in the gas and/or the stars has been
found. Furthermore, the available data on a sample of 46 polar ring
galaxies has been compiled. In order to explore the existence of an
evolutionary path linking the two families of accretors, the gas
content of counterrotators and polar rings is compared.
It was found that the normalized content of cold gas (
)
in
polar rings is
1 order of magnitude higher than the reference
value derived for normal galaxies. The inferred gas masses are
sufficient to stabilize polar rings through self-gravity. In contrast,
it was found that the cold gas content of counterrotators is close to
normal for all galaxy types. Although counterrotators and polar rings
probably share a common origin, the gas masses estimated here confirm
that light gas rings accreted by future counterrotators may have
evolved faster than the self-gravitating structures of polar rings. In
this scenario, the transformation of atomic into molecular gas could
be enhanced near the transition region between the prograde and the
retrograde disks, especially in late-type accretors characterized by a
high content of primordial gas. This is tentatively confirmed in this
work: the measured H2/HI ratio seems larger in counterrotators than
in normal or polar ring galaxies for types later than S0s.
Key words: galaxies: ISM - galaxies: interactions - galaxies: evolution - galaxies: peculiar - radio lines: galaxies - submillimeter
Different scenarios have been proposed to explain the counterrotation present in elliptical and disk galaxies. Most of them invoke the capture of matter which comes from outside the acceptor galaxies. The various models examine different masses and time-scales involved in the accretion process. An external origin is also invoked to explain the existence of polar ring galaxies, where gaseous disks or rings are seen to rotate almost perpendicularly with respect to the main stellar body of the system (Whitmore et al. 1990). However, the link between polar rings and counterrotators remains unclear. Alternatively, it has been suggested that a primordial mechanism, invoking a dissipationless cosmological collapse perturbed by tidal fields, could explain the formation of counterrotating galaxies (Harsoula & Voglis 1998).
Within the accretion scenario, the morphology of the acceptor system and its dynamic evolution would depend on several factors: the nature of the accreted matter (gas and/or stars), the ratio between the accreted mass and that of the acceptor galaxy and, finally, the accretion speed. Whereas the collision between equally massive galaxies may lead to a merging like the "Antennae'', ending up as a giant elliptical galaxy with counterrotation (Hernquist & Barnes 1991), the disruption of the acceptor´s disk could be avoided by progressive infall of gas whose spin is anticorrelated with the main stellar body (Quinn & Binney 1992; Thakar & Ryden 1996; Voglis et al. 1991). However, the accretion of a gas-rich satellite may heat the stellar disk (Thakar & Ryden 1996). Observations show that lenticulars and spiral galaxies hosting counterrotation do not necessarily present disrupted stellar disks. Their stellar kinematics appear globally regular (Bettoni et al. 1991) and the stellar disks show hardly any sign of thickening (Rubin et al. 1992). The end product of the accretion process at the present epoch seems to have reached, in most of the known studied cases, an equilibrium configuration for the stellar component. Either the time-scales to reach equilibrium are short enough or, alternatively, accretion caused no traumatic changes in the kinematics of the stars.
The gas, however, is expected to reflect the consequences of the
accretion process more violently than the stars. If gas was accreted
by a disk galaxy with a non negligible amount of interstellar gas, a
strong interaction between the accreted and the primary gas is
likely. The existence of violent cloud-cloud collisions (the relative
velocity between the interacting clouds would be:
)
and the highly dissipative nature of the encounters might lead to the
onset of large-scale shocks. These might convert the atomic gas into
molecular gas (Braine & Combes 1992; Sage & Galletta 1993; Young & Knezek
1989) and eventually induce
starbursts (Wang et al. 1992; Read & Ponman 1998; García-Burillo et al.
2000). If the described scenario
holds, one would expect that the content of molecular gas would be
higher in counterrotating galaxies than in a comparison sample of
non-interacting galaxies of the same Hubble type. On the other hand,
if the origin of counterrotation is primordial, or alternatively, if
large-scale shocks are not efficient or short-lived, the H2 content
should be similar for counterrotating and normal galaxies.
It is also unclear whether polar rings and counterrotators represent different steps in the process of mass accretion. A comparison of their H2 content could reveal if there is an evolutionary link between the two families of accretors.
This paper represents a first step in answering some of the above-mentioned questions by studying the global gas content in a sample of counterrotators. In this work we estimates the content of molecular, atomic and hot gas for a sample of 58 galaxies of different morphological Hubble types and of different types of counterrotation. Molecular gas masses are derived from 12CO(J=1-0) observations made with the 15 m SEST radiotelescope on 10 galaxies with counterrotation and 1 polar ring (Sect. 2). Results from these new observations are described in Sect. 4. The published data for 48 objects where there are indications of counterrotation in the gas and/or stars have been compiled and added to this sample (see Sect. 5). The H2 content of counterrotating galaxies will be studied relative to a comparison sample of normal galaxies that was built up from the literature, as described in Sect. 6. The gas content of counterrotators has also been compared with those of polar ring galaxies, using available data from the literature (see below for detailed references). A similar comparative study has been done with the HI content (from various sources), with the warm dust mass (derived from IRAS data) and with the amount of hot gas, the dominant gas component in early type objects (values derived from X-ray data taken by ROSAT and by EINSTEIN).
The 11 galaxies in our sample observed using the SEST telescope were selected from the list of objects published by Galletta (1996). The galaxies are described individually in Sect. 4, together with the main results inferred from this CO study. The relevant parameters of the systems, such as the diameters (D25: the de-projected linear diameter corresponding to the blue isophote at 25 mag arcsec-2), absolute B magnitudes (MB), distances (d) and morphological types are shown in Cols. 3-6 of Table 1. These data have been extracted from the recently up-dated Lyon-Meudon database LEDA (Paturel et al. 1997).
The conversion factor between the integrated intensities of
12CO(J=1-0) and the H2 column densities was taken from
Strong et al. (1988), i.e.:
HI masses (
)
have been taken from various sources or
calculated from the m21 parameter of LEDA (Paturel et al. 1997), assuming:
The mass values of X-ray emitting gas ()
have been derived from
ROSAT data (Beuing et al. 1999) and from EINSTEIN data (Fabbiano et al. 1992;
Burstein et al. 1997), assuming:
Finally, warm dust masses (
)
have been calculated from IR
fluxes (S60 and S100) published by Knapp et al. (1989) and from
LEDA raw data, kindly furnished by G. Paturel, assuming:
Emission in the J=1-0 and J=2-1 transitions of 12CO among
the sample of 11 galaxies was searched for using the Swedish-ESO
Submillimeter Telescope (SEST) at La Silla, equipped with the dual
channel IRAM 115/230 GHz receivers which allow for simultaneous
observations. There were three different observation runs: November
6th-11th 1998, May 28th-31st 1999 and May 7th-11th 2000. Beam sizes
were 45
and 23
at 115 GHz and 230 GHz,
respectively. Unless explicitly stated, the temperature scale used
throughout the paper is antenna temperature, corrected from
atmospheric losses and rear spillover (
). When deriving line
ratios, it is assumed that main beam efficiencies
(115
GHz)=0.70 and
,
in order to refer
temperatures to the main beam brightness scale. Spectrometers cover
bandwidths of 995 MHz (1290 km s-1) and 543 MHz (1410 km s-1) for the J=2-1and J=1-0 lines respectively. Dual-beam switching was used, with a beam
throw of 12
to produce a flat baseline. Typical system
temperatures ranged from 190-430 K. Pointing and focus were checked
every 2-3 hours, using several SiO maser sources located near the
target galaxy. The rms accuracy of the pointing model was typically
2
,
assuring an absolute positional accuracy better than
5
.
Except for one case (NGC 3497), we made only single point maps centered on the nuclei of the galaxies. Individual scans at each position were coadded to get total integration times ranging from 1 h to 7 h. Spectra were Hanning-smoothed to a velocity resolution of 30 km s-1 (for both lines) with the exception of narrow lines for which a higher resolution was kept (see below). Linear baselines were fitted and subtracted from the smoothed spectra using the GILDAS software package.
![]() |
Figure 1: The J=1-0 and J=2-1 CO spectra for the galaxies observed by the SEST telescope. Gaussian fits on the lines are shown when available. |
Open with DEXTER |
This section presents the main results of the CO observations for the 11 galaxies of the SEST sample, preceded by a short description of the systems.
This galaxy, also denoted Anon 1029-45, has been included in a list
of dust-lane ellipticals by Hawarden et al. (1981). It has a prominent and
strongly warped dust lane going across the major axis up to
r=30
(5.2 kpc). Due to its prominent dust lane which gives the
system the appearance of an edge-on disk, it has often been classified
as S0, although it has all the properties (photometric profile, luminosity
and size) of giant ellipticals. Furthermore, long exposure plates of
the galaxy show no signature of a stellar disk.
The stellar kinematics have been studied by Bertola et al. (1988a), who
derived a maximum rotational velocity of 210 km s-1 reached at
r=20
(3.4 kpc) and a velocity dispersion of 260 km s-1. Ionized
gas in counterrotation, with spin velocities of
250 km s-1 at
r=7
(1.2 kpc), has been detected along the major axis
(Bertola et al. 1988b). There are no HI observations available for
ESO263-48; the galaxy also remains undetected in the ROSAT
survey. However, this elliptical is particularly rich in dust; from
the IRAS fluxes we estimate
.
A
continuum radio emission has been detected at 1.4 and 4.9 GHz, which
extends perpendicularly to the dust lane (Bertola et al. 1988a).
The J=2-1 and J=1-0 CO spectra of Fig. 1 show
800 km s-1-wide
emission profiles centered at v=2810 km s-1 (hereafter taken as the
CO-based systemic velocity;
). The J=1-0 spectrum is
asymmetrical with respect to
as it can be fitted by three
Gaussian-like components. Emission of the two extreme velocity
components at v=2517 km s-1 and v=3170 km s-1 can be explained by the
presence of an unresolved H2 disk with a rotation speed of
km s-1, reached within r=2 kpc (the upper limit on r is
set by the J=2-1 beam). The value of
derived from CO is
significantly larger than that which is inferred for the ionized gas
at the same radius (
250 km s-1). This discrepancy may indicate
that the H2 disk seen in projection extends farther out. The
asymmetry in the 12CO(J=1-0) spectrum is caused by the existence
of a strong component at V=2980 km s-1, redshifted by 170 km s-1
with respect to
,
and having no blue-shifted counterpart. The
latter can come from an asymmetrical distribution in the H2 disk
or, alternatively, be the signature of a warp in the molecular gas
disk (as suggested by the distorted dust lane).
The molecular gas content under the J=1-0 beam can be derived
within a radius
(4 kpc) (close to the maximum extent of
the dust lane feature). It was calculated to be
,
a high value for an elliptical galaxy.
The absolute magnitude and the intrinsic diameter of this galaxy (see Table
1) are both characteristic of a giant elliptical. IC1459 has a
massive counterrotating stellar core (
,
according to
Franx & Illingworth 1988) and its outer stellar isophotes are twisted, an indication that
the stellar body is triaxial. This galaxy shows dust absorption in the central
10
(Sparks et al. 1985) and faint pseudo-arms in the outer part
(
Malin 1985). The counterrotating core, which hosts a
compact radio source (Franx & Illingworth 1988), has a radius of
2
(200 pc)
and a projected spin velocity of
km s-1. On dynamical grounds, the
core can be described as a disk, rather than as an ellipsoid. In contrast, the
outer directly-rotating stellar body has a slower rotation figure; it reaches
km s-1at r=40
(4 kpc). The galaxy is crossed by a disk
of ionized gas, whose emission is evenly detected up to r=35
(3.5 kpc); the ionized gas rotates in the same direction than the outer stellar
body, but at a higher speed (350 km s-1) (Franx & Illingworth 1988). Therefore
counterrotation in this galaxy seems confined to the inner core and affects
only the stars. X-ray emission peaks in the galaxy nucleus (Roberts et al. 1991),
an indication of central activity.
Walsh et al. (1990) report a negative detection of HI emission,
the upper limit on
being
107
.
Our 12CO(J=1-0)
spectrum also shows a negative detection for molecular gas emission. In
contrast, the integrated 12CO(J=2-1) spectrum shows clearly a narrow
emission line of
km s-1, centered at 1782 km s-1 (see Fig. 1).
The velocity centroid of CO is redshifted by
100 km s-1 with respect
to the galaxy systemic velocity (
km s-1). This discrepancy
of velocity centroids and the narrowness of the 12CO(J=2-1) line
both indicate that emission cannot come from a rotating
molecular gas disk in equilibrium that could be associated with the ionized
disk or, alternatively, with the counterrotating core. In the discarded
scenario of a molecular gas disk, the CO line should be 3-5 times wider than
is actually observed, considering the size of our beam. Instead, the CO
profile may come from a Giant Molecular Association (GMA). Furthermore, the
derived upper limit on the mass of molecular gas,
,
is noticeably low. Additional support for the interpretation outlined
above comes from the high resolution deconvolved V-band images of the dust
distribution, obtained with the HST. The morphology of the dust lane source
in the inner 4
of the galaxy is very irregular and indicates
non equilibrium motions (Forbes et al. 1994). Finally, the estimated 3
upper
limit on the (J=2-1)/(J=1-0) ratio (>1.3) suggests that the CO emission might be
partly optically thin.
Similar molecular gas components have been found in other early-type galaxies, such as NGC404 (Wiklind & Henkel 1990), classified as a gas accreting elliptical with a minor axis dust lane (Bertola & Galletta 1978). This GMA may be a residual of one of the galaxies involved in the passed merger that is supposed to be at the origin of IC1459 (Franx & Illingworth 1988).
The size and the luminosity of IC 2006 put this galaxy among the dwarf
ellipticals. Counterrotation is present in an outer ring of atomic gas,
which is aligned with the apparent major axis of the galaxy. At the
radius of the HI ring, the galaxy luminosity decreases to mag arcsec-2 (Schweizer et al. 1989). Schweizer and collaborators describe the HI
distribution as a 2 kpc wide circular ring of radius
11 kpc,
inclined at 37
.
With the adopted parameters, the rotation speed
of the HI gas would be 200 km s-1 at this distance. HI gas remains
undetected inside the ring. In contrast, faint emission from ionized
gas is detected within 2.5 kpc of the nucleus, characterized by a
velocity gradient which is smaller and also inverted with respect to
that of the stars (from -70 km s-1 (NE) to 50 km s-1 (SW), relative to
km s-1). The counterrotating ionized gas disk is highly
turbulent, with a measured velocity dispersion of 190 km s-1 (Schweizer et al. 1989).
The upper limit set by ROSAT observations (Beuing et al. 1999) indicates
that, contrary to IC1459, IC2006 has no relevant quantity of hot
gas (see Table 1). Based on the optical photometry and the
kinematics of the outer HI ring, Schweizer et al. (1989) derive the presence
of a dark halo which contains about twice the mass of the luminous
stellar body. Our single-point 12CO(J=1-0) map sampled the galaxy
nucleus up to r=3.5 kpc. This region includes the ionized gas disk but
excludes the outer HI ring. CO emission was not detected; the latter
implies an upper limit for the central (r<3.5 kpc) molecular gas
content of
.
This galaxy belongs to the Hickson compact group H23. It is an
almost edge-on disk galaxy classified as S0-a in the LEDA database.
Counterrotation in the ionized gas is detected from optical emission
lines (Rubin et al. 1991). The gaseous disk extends up to a radius of 2 kpc, with an observed maximum rotational speed of 75 km s-1; this
velocity is significantly lower than that of the stars at the same
radius (175 km s-1). The latter may be an indication that the
gaseous disk is either warped or tilted with respect to the galaxy
plane. The optical images of this galaxy show no signature of dust
absorption. Williams & van Gorkom (1995) report a negative detection for this
galaxy in the 21 cm line, which gives an upper limit of
.
IRAS, ROSAT and, finally, our CO
observations report negative detections for NGC1216. The latter give
an upper limit of
for the molecular gas
content, within a region of radius r=7 kpc.
NGC2217 is a barred spiral (SBa) seen almost face-on. It has an
outer stellar+gaseous ring of radius 8 kpc and an inner oval
ring of radius
4 kpc, which encircles the bar.
Bettoni et al. (1990) have studied the distribution and kinematics of the ionized
gas, confined to the innermost 20
(
2 kpc) of the
galaxy. The gas distribution suggests the presence of a two-arm spiral
structure, whereas the kinematics are characterised by counterrotation
with respect to the stars, inside
(
1 kpc). However, a detailed analysis of the data by the authors shows
that the gas counterrotation is not real, and it may be better
accounted for if one assumes the gas to be in a warped disk seen in
projection. The ionized gas inside r=1 kpc would lie in a series of
polar rings almost at 90
with respect to both the bar and the
stellar disk. In the outer region (r=1-2 kpc) the plane of the gas
rings would have settled towards the disk of the stars, and changed
its inclination by nearly 90 degrees. The latter explains why the gas
and the stars rotate in the same direction for r>1 kpc. We have
classified the system as a polar ring galaxy.
The apparent counterrotation of ionized gas inside r=1 kpc produces the
largest extention of radial velocities along the bar minor axis: from v=1450 km s-1 to 1800 km s-1. The
derived from the gas and the star
kinematics agree within the margin of errors, being close to 1640 km s-1.
Bettoni et al. (1990) fit a rotation curve to their data, inferring de-projected
rotational velocities of 125 km s-1 and 150 km s-1 for the stars and the gas,
respectively, at a radius r=1 kpc.
HI observations reported by Huchtmeier (1982) show a double-horned emission
profile centered at
km s-1, close to the value found by
Bettoni et al. (1990) and with a total width at zero power of
300 km s-1. The re-scaled HI mass is
.
As there
is no high-resolution HI map of NGC2217, the location of the HI gas
is uncertain.
The spectra in Fig. 1 show the detection of molecular gas emission for
the two lines of 12CO. The two profiles differ significantly,
however. Whereas the J=1-0 line is centered at v=1720 km s-1 (i.e.,
redshifted 100 km s-1 with respect to
), the J=2-1 line peaks at
(as defined above). Although the low spatial
resolution of these observations tells us little on the precise
location of molecular clouds, the reported asymmetry of the J=1-0profile (which samples the disk up to r=2 kpc) suggests that the
distribution of H2 gas in NGC2217 is highly asymmetrical and/or
that the kinematics of molecular clouds might depart from circular
motions. Most noticeably, the integrated HI profile also shows a
pronounced asymmetry. The FWHM of the two CO lines are close to the
values found in HI and in optical emission lines (
250-300 km s-1).
We derived a molecular gas content of
up to r=2 kpc.
NGC3497 is a major-axis dust lane elliptical known by different
names (
). The ringed dusty disk shown
in the B-R color maps of Ebneter & Balick (1985) seems to have the same extent
as the stellar disk (
). As with all major-axis
dust-lane ellipticals, e.g. ESO 263-48 in this paper, the dust
signature is interpreted to be the result of an accretion episode.
The galaxy has a fainter galaxy at 2
(named NPM1G-19.0362)
and a companion with similar redshift at 5
(
).
Stellar and gas rotation curves have been derived by Bertola et al. (1988a),
who measured a radial velocity difference of
km s-1 between the western and eastern sides of the major axis (on the NE
side stars are receding). The measured systemic velocity is
km s-1. No X-ray, IR or HI data are available in
the literature.
The emission of both lines of 12CO were observed in three
positions located along the major axis of the disk: the (0,0) offset
centered on the galaxy nucleus and two off-centered positions at
.
The J=2-1 and J=1-0 spectra shown in Fig. 1 reveal the
presence of molecular gas in the central region (up to r=5 kpc) and
also the detection of the J=1-0 line of 12CO in the NE offset. In
the SW position, however, CO emission was not detected. The
12CO(J=1-0) central spectrum is fitted well by a single Gaussian
profile, centered at 3774 km s-1 and with
km s-1 (
1000 km s-1 at zero power);
therefore, it is 100 km s-1redshifted with respect to
.
In contrast, the J=2-1 profile shows three velocity components at
v=3497 km s-1, v=3693 km s-1 (close to the optically determined
)
and
v=3497 km s-1. The velocity asymmetry in the central J=1-0 spectrum may indicate
that H2 distribution is slightly asymmetrical in the disk within r=5 kpc.
A comparison between the radial velocity measured on the CO spectrum
(3570 km s-1) and the stellar velocities observed in the NE side
of the major axis (redshifted with respect to
), indicates
that molecular gas is counterrotating with respect to the stars.
The molecular gas mass within the central r=5 kpc, derived from the
12CO(J=1-0) integrated intensity, would be
.
The amount of molecular gas detected in
the NE offset is
.
Haynes et al. (2000) considers NGC4772 as a case of apparent counterrotation of
the ionized gas versus the stars. The kinematical decoupling of the
nuclear ionized gas (
kpc), observed along both the
minor and the major axes, has been interpreted as the signature of a
misaligned embedded gas bar, rather than as evidence of
counterrotation. However, this Sa galaxy shares many features with
other prototypical counterrotators. Mimicking NGC3626, the HI
content of NGC4772 is distributed in two separate rings, probably
non coplanar. As is the case for NGC 3626, the central region of
NGC4772 is HI-poor. Furthermore, the deep optical photometry
of the galaxy reveals the presence of a round, low surface brightness
disk in the outer part, reminiscent of a similar feature reported by
Buta et al. (1995) in the Sab counterrotator NGC 7217.
The J=1-0 spectrum of 12CO (Fig. 1) reveals the presence of
molecular gas (inside r=1.5 kpc). The line profile, centered at
1120 km s-1 and with
km s-1, is slightly redshifted
with respect to the optically determined
km s-1 (the same
as derived from HI).
Emission in the J=2-1 line is undetected, however. The molecular gas
mass within the central r=1.5 kpc, derived from the 12CO(J=1-0)
integrated intensity, would be
.
NGC5854 is an early spiral (Sa) characterized by a low gas content
and the absence of current star formation. Haynes et al. (2000) have studied
the stellar kinematics, using
H
and MgIb optical absorption
lines, and the kinematics of ionized gas, using the N[II] and O[III]
optical emission lines. These data reveal the existence of a
counterrotating gas disk extending up to
(0.8 kpc),
with a total velocity range of 120 km s-1. The stellar velocities
measured at
(4.5 kpc) reach
160 km s-1. Although
HI content is low, Magri (1994) detected a signal in the nucleus. The
HI profile is centered at 166 km s-1, close to the optically determined
value for
km s-1 (Fouque et al. 1992). The narrowness
of the HI spectrum (
km s-1, namely, less than the measured
stellar velocity spread) suggests a close association of the HI
component with the counterrotating ionized gas (see discussion in
Haynes et al. 2000).
Although weak, the 12CO(J=1-0) spectrum shows the existence of
H2 gas within r=2.3 kpc. There is a hint of a double-horned
profile, with two velocity components at v=1539 km s-1 (with
km s-1) and v=1930 km s-1 (with
km s-1),
equidistant from v=1735 km s-1. The CO spectrum in the J=2-1 line
confirms the detection of molecular gas. Not surprisingly, the J=2-1and J=1-0 profiles differ. The J=2-1 line shows hints of two velocity
components, although with a smaller velocity spread (v=1630 km s-1 and
1740 km s-1) and is centered at
km s-1, in reasonable agreement
with HI. However the larger velocity spread of the J=1-0 spectrum would
suggest that, compared to the counterrotating ionized gas core, the
H2 disk may extend farther out. The molecular gas mass within the
central r=2.3 kpc was derived from the 12CO(J=1-0) integrated
intensity giving;
.
NGC5898 was studied by Bettoni (1984) and Bertola & Bettoni (1988), who
discovered the first case of ionized gas counterrotation in a dust
lane elliptical in this galaxy. Their data, which extended out to
10
(1.4 kpc), have recently been completed by
Caon et al. (2000) who analysed the stellar and the gas kinematics farther
out (up to
(4.8 kpc)). The new data along the major
axis show the existence of a stellar core of radius
(0.7 kpc) which counterrotates with respect to the outer stellar
body. The ionized gas counterrotates with respect to the inner stellar
core, and it therefore corotates with the outer stars. In contrast,
gas is seen to counterrotate at all radii along the minor axis. This
might indicate that angular momentum vectors of the ionized gas and
the stars are certainly misaligned, but not antiparallel. The
observations of this galaxy at X-ray and IR wavelengths show an upper
limit for hot gas of
and a moderate quantity of
dust, of a few 104
.
The J=1-0 spectrum of 12CO (Fig. 1) shows a tentative detection of
molecular gas inside r=3.1 kpc. The line profile, centered at
2020 km s-1and with
km s-1, is slightly blueshifted
with respect to the optically determined value of
2100 km s-1, derived from
Bertola & Bettoni 1988). The observed asymmetry might arise if the H2 gas was
associated with the ionized disk, which shows a marked extension towards the
SW (where ionized gas radial velocities are blueshifted). In this scenario
molecular gas would also be counterrotating.
The 12CO emission is undetected in the J=2-1 line, however. We have
calculated the molecular gas mass within the central r=3.1 kpc, using the
12CO(J=1-0) integrated intensity, giving
.
The optical images of NGC 7007 show an elliptical-like body encircled
by an off-centered bow-shaped dust lane on the eastern
side. Dettmar et al. (1990) discovered, in this galaxy, the signature of a
counterrotating ionized gas disk by comparing the spectrograms of gas
emission (NII 6853) and stellar absorption lines
(H
). Spectra taken later (Bettoni et al. 2001a) allowed a detailed
analysis of the stellar and the gas kinematics, characterised by
maximum rotational velocities of
150 km s-1 and
175 km s-1 respectively, reached at r=10
.
The central velocity dispersion
for stars is 150 km s-1, whereas gas lines have instrumental width
(<100 km s-1). The galaxy contains a source of infrared emission
detected by IRAS, and at X-ray wavelengths the published work reports
an upper limit (see Table 1).
Our J=2-1 and J=1-0 spectra both show a weak narrow line of 30 km s-1 FWHM, centered at
2850 km s-1, close to the optical redshift
of
km s-1 reported in RC3 (de Vaucouleurs et al. 1991). These results are, however,
at odds with that quoted by Da Costa et al. (1991) (
km s-1), who
estimated
as a weighted average between gas emission and stellar absorption
data. If the Da Costa et al. (1991) value is more accurate, as indicated by the
additional spectra of Bettoni et al. (2001a), the reported difference between the CO
peak and the optical systemic velocity could be explained with an asymmetry in
the distribution of cold gas. The derived molecular gas mass,
,
could be well accounted for if the emission observed came from a few
Giant Molecular Associations (GMAs) in the center of the galaxy, as observed
in IC1459. The narrowness of lines in both transitions supports this
scenario.
NGC 7079 is a weakly barred SB0 galaxy, a member of an interacting pair.
Bettoni & Galletta (1997) detected a counterrotating disk-like structure of ionized
gas which extends up to a radius of r=2 kpc. The radial velocities for the gas
span from 2600 km s-1 to 2800 km s-1. The stellar kinematics is typical of an
undisturbed disk. The measured radial velocities (up to r=4.5 kpc) range from
km s-1 to
km s-1 and give a
km s-1 and a
central velocity dispersion of 150 km s-1. No X-ray emission has been detected
from this galaxy; the IRAS satellite detected infrared emission.
The J=2-1 and J=1-0 lines of CO are detected in this galaxy, showing the
presence of molecular gas (see Fig. 1). The line profiles, 170 km s-1
wide at zero power, are centered on the galaxy systemic velocity, derived from
optical data. The linewidths of both CO lines agree satisfactorily
with the velocity range measured for the counterrotating ionized gas. In
contrast, the CO widths are much smaller than the velocity interval measured
in the stellar lines. This may indicate that H2 gas
is confined to the inner portion of the galaxy and that it shares the same
kinematics as the counterrotating ionized gas.
The inferred molecular gas content is
up to r=2 kpc.
The newly acquired data described above would nevertheless be
insufficent to extrapolate estimates on the global gas content to all
counterrotators. In order to improve the SEST sample on statistical
grounds we added the published data from those counterrotators
(Galletta 1996; Kannappan & Fabricant 2001) with an estimate from any of the different
gas tracers:
or M
.
Masses are
derived with the same assumptions used for the SEST sample. The
properties of the enlarged sample are in first part of Table 1,
together with the list of references relevant for this
compilation. This new sample allows a complete study of the global
gas content of counterrotators, using different gas tracers in a
statistically significant sample of 58 objects.
In order to compare counterrotators and polar rings, the available data on a sample of 46 polar ring galaxies have also been compiled (see Ref. in Table 1). Data include 36 polar ring lenticulars and spirals (Schweizer et al. 1983; Whitmore et al. 1990 and this work), and 10 polar ring ellipticals, known in the literature as ellipticals with minor-axis dust-lanes, such as NGC 5128 (Bertola & Galletta 1978). Polar ring ellipticals have traditionally been classified as S0s due to the presence of a dark ring or disk in optical pictures. However, the luminosity profiles of these galaxies do not follow an exponential law, typical of stellar disks, but rather a r1/4 law, characteristic of spheroidal systems. We have therefore re-classified these galaxies as ellipticals, whenever they appear as S0s in catalogs. Not all polar ring lenticulars and spirals present in Whitmore et al. (1990)'s catalogue were finally included in our list. We only selected those systems where the perpendicularity between the ring and the stellar body is clearly visible in the images, discarding all systems appearing doubtful in our inspection of the catalogue. Altogether the sample of accreting systems includes 104 objects.
![]() |
Figure 2: (Left): Log M/D2 ratios in normal galaxies derived for the atomic and molecular gas, from the samples of Casoli et al. (1998) (filled circles) and Bettoni et al. (2001b) (open symbols). (Right): Log M/L ratios in normal galaxies for the atomic and molecular gas, from Bregman et al. (1992) (filled triangles) and Bettoni et al. (2001b) (open symbols). |
Open with DEXTER |
The main aim of this paper is to study the molecular gas content of
accreting galaxies (counterrotators and polar rings) and compare it
with the average value for normal non-interacting galaxies
as a function of the Hubble type. The first non-obvious task is the
definition of a comparison sample of normal galaxies. The sample
should contain a statistically significant number of objects. This
requirement is critical for early-type galaxies, as the majority of
accreting systems are of types earlier than Sa (morphological type
code t=1). Moreover, the sample should avoid the inclusion of abnormal
objects, suspected to be interacting and/or merging galaxies, e.g
those reported in Arp's catalogs.
In the past, two different research groups have built up comparison
samples in order to study the variation of the gas content of galaxies
along the Hubble sequence:
Bregman et al. (1992) and Casoli et al. (1998). Bregman et al. (1992) derived the content of
molecular gas, HI, X-ray emitting gas and dust, working on a sample of
467 early-type galaxies, ranging from pure ellipticals (E, t=-5) to
early spirals (Sa, t=1). In their analysis they favoured the use of
the total blue luminosity (LB) of galaxies as the necessary
normalisation factor, i.e., the inferred numbers being
and
.
They concluded, first, that the gas content of elliptical galaxies is
dominated by the hot phase(
)
and secondly,
that
and
both show a strong positive
gradient from E to Sa-type systems. However, the number of early type
galaxies detected either in H2 or in HI gas is scarce: for H2, 1 E-type detected with 11 upper limits (hereafter UL) and 6 SOs detected
with 18 UL. Poor statistics cast some doubt on their conclusions.
Casoli et al. (1998) discussed the molecular and atomic gas content for a set of 582 objects, mainly disk galaxies, normalizing the gas masses by
D252. For comparison, numbers for H2 are: 3 E detected and 7 SOs detected (with 2 UL). Results from Casoli et al. (1998) are noticeably
at odds with that of Bregman et al. (1992): the sharp increase of gas
content from t=-5 to t=1 reported by Bregman et al. (1992) (1-2 orders of
magnitude) is not confirmed using Casoli et al. (1998) data. This
discrepancy is illustrated in Fig. 2 where the mean
values are represented as a function of t for the two samples. To
reconcile these discrepant trends one must assume an unrealistic
decrease by 2 orders of magnitude in
,
going from E to
Sa systems. Results from both samples for types t<1 are however
dubious, considering the poor statistics in this range. Moreover, the
two samples include a non-negligible percentage of interacting
galaxies (estimated to be close to
20
in both
samples). Interacting galaxies should be discarded when putting
together a comparison sample of normal non-interacting objects.
These limitations were the motivation to build up a new comparison sample of normal galaxies. Paper II by Bettoni et al. (2001b) will discuss extensively the details of this compilation. The over-all numbers give a grand total of 1773 normal galaxies selected from a processed sample of 3800 objects. Bettoni et al. (2001b) purposedly excluded from the selected sample those galaxies belonging to the interacting or disturbed categories (most of them appearing in Arp 1966; Vorontsov-Velyaminov 1959; Arp & Madore 1987 catalogues). Galaxies listed in the Veron-Cetty & Veron (2000) catalogue of AGN systems have also been excluded because in some cases their peculiar activity has been attributed to gas accretion. We have taken from Bettoni et al. (2001b) the normalized values M/LB for the molecular, atomic and X-ray emitting gas, as well as for the warm dust content inferred from IRAS. The global statistics for detections (and UL) are: 247 in H2 (113 UL), 774 in HI (149 UL), 196 in X-rays (661 UL) and 861 in IR (555 UL). This sample improves the statistics for early type galaxies compared to previous works, the numbers for H2 being: 10 E-type detected (plus 18 UL) and 10 lenticulars detected (plus 17 UL). Note that the galaxies used to build the mean values for the different ISM tracers are not always the same; however, the majority of galaxies in our sample (1135) have detections or upper limits in at least two wavebands.
We have applied a survival analysis method to the different ensembles
of M/LB data. This analysis tool takes properly into account both
detections and UL in order to derive representative averages. The mean
values are derived and plotted under the label "normal galaxies'', and
are binned according to the morphological type code (with
). Most noticeably, UL lower significantly the estimated
mean M/LB values for normal galaxies of early types (see
Bettoni et al. 2001b). The derivation of mean values of molecular gas content
from Casoli et al. (1998) used survival analysis also. For HI data, all the galaxies
of their sample were detected and so no survival analysis
needs to be applied. Also, the analysis of Bregman et al. (1992) takes into account
the different detection rates of the various morphological types, but using
different non-parametric tests, based on rank.
At first sight, the comparison between the mean values derived from
these three different studies (Bregman et al. 1992; Casoli et al. 1998; Bettoni et al. 2001b) shows that
Log(
)
and Log(
)
of
Bettoni et al. (2001b) are intermediate between Casoli et al. (1998) and
Bregman et al. (1992) values. Although the cold gas content increases by a
factor
10 from E to Sa-types, this gradient is less steep than
that reported by Bregman et al. (1992) (see Fig. 2).
To identify any potential bias in the galaxy samples compared in this
work (normal galaxies, counterrotators and polar rings), we have
analysed the statistical distribution of the following intrinsic
properties:
MB, D25 and FIR flux (given by
). Kolgomorov-Smirnov tests applied to these quantities
indicate that the distributions of
MB, D25 and
are
not significantly different for the 3 samples, at a confidence level
better than 95
.
![]() |
Figure 3:
Plot of Log
![]() |
Open with DEXTER |
We discuss in this section the results obtained from the comparison of
the gas/dust content of gas-accreting and normal galaxies. Mean
values of log M/L were obtained for each morphological type, using a
code binning. We studied the deviations from the reference
values issued from the survival analysis method applied to normal
galaxies (see above). The statistical significance of any difference
found between the samples was evaluated by a Student t-test applied to
the mean of the values of Log M/L, binned according to morphological type.
Figures 3-8 illustrate this comparison, whose
results can be summarised as follows:
Log(
)
and Log(
)
show a large dispersion for
accretors of types t<0 (Figs. 3 and 4). This result holds for
counterrotators and polar rings. The values of the gas content for normal galaxies
are also highly dispersed for t<0;
![]() |
Figure 4: Variation of the molecular gas masses with the morphological type for the samples of counterrotating galaxies (open romboids), polar ring S0s and spirals (crosses) and polar ring ellipticals (asterisks). Filled triangles represent the reference values derived from the comparison sample of Bettoni et al. (2001b). |
Open with DEXTER |
![]() |
Figure 5: Change of the cold gas content (molecular and atomic)with respect to morphological type for the samples of counterrotating galaxies (open romboids), polar ring S0s and spirals (crosses) and polar ring ellipticals (asterisks). Filled triangles represent the reference values derived from the comparison sample of Bettoni et al. (2001b). |
Open with DEXTER |
![]() |
Figure 6: The ratio of molecular to atomic gas versus the morphological type. Symbols are: counterrotating galaxies (open romboids), polar ring S0s and spirals (crosses), polar ring ellipticals (asterisks) and normal galaxies (filled triangles). |
Open with DEXTER |
![]() |
Figure 7: Variation of the warm dust content with respect to morphological type for the samples of counterrotating galaxies (open romboids), polar ring S0s and spirals (crosses) and polar ring ellipticals (asterisks). Filled squares represent the reference values derived from the comparison sample of Bettoni et al. (2001b). |
Open with DEXTER |
On the other hand, the HI content of counterrotating galaxies shows no significant departure from the expected normal value (Fig. 3);
The molecular gas content of polar ring galaxies lies above normal values for all galaxy types (Fig. 4). On average, it
is calculated that
is
1 order of magnitude
higher in polar rings; this result, which agrees satisfactorily with
the conclusions of Galletta et al. (1997), is established with a 98
statistical significance. Therefore, the global content of cold gas
(
)
in S0/S-polar rings lies >1 order of magnitude above
standard values with a 99
certainty (see Fig. 5).
On the other hand, the
ratio in polar rings stays close to
normal values for all types (Fig. 6).
The molecular gas content of counterrotating galaxies is marginally
lower than normal for types t<0 (Fig. 4). This deficiency
is not firmly established, its statistical significance being low
(78). In contrast, counterrotating galaxies reach normal H2masses for t>0. Contrary to polar rings, the global content of cold
gas in counterrotators, given by Log(
), shows no
relevant departures from normal values for all types (Fig. 5).
However, the HI phase seems to dominate in early type counterrotators
(t<0). We tentatively identify an increase by 1 order of
magnitude in the
ratio from t=-6 to t=6, a factor
significantly larger than that observed in normal galaxies
(Fig. 6). The latter indicates that some mechanism
favouring the transformation of HI into H2 might be at work for
counterrotators on the right-side of the Hubble sequence (see below);
As derived from Log(
,
polar ring galaxies have a dust
content
0.5-1 order of magnitude higher than normal. This result is established
with a 99
statistical significance for spirals and lenticulars, whereas it is only
at a 90
certainty level for ellipticals. In contrast, galaxies with
counterrotation have a warm dust content not significantly different to normal galaxies
for all types (Fig. 7);
![]() |
Figure 8:
Plot of Log ![]() ![]() ![]() |
Open with DEXTER |
One fifth of counterrotators and nearly half of the polar ring
ellipticals have been detected in X-rays. On average, the estimated
masses of hot gas (see Table 1) are slightly lower than
the ones of normal galaxies. Moreover, the slope fitted to the 17
detections in the
diagram (
1.7, Fig. 8) lies
between the prototypical value for emission mainly due to hot diffuse
gas (
,
with slope
2), and the one for emission being
dominated by discrete sources (
,
with
1; see
Ciotti et al. 1991 for details). Furthermore,
values for
accretors are below the emission level predicted by cooling flows
models, assuming the recent SN I rate equal to 0.18 (i.e.,
SN I events per year per unit of 1010
)
(Cappellaro et al. 1999). We can conclude that the observed normalised X-ray
luminosity of accretors needs no huge starburst event as an explanation;
Gas and stars along polar orbits can be explained as the result of the acquisition of cold infalling gas by an accreting galaxy. The accreted gas can smear out into a ring after a few orbital periods. The fate of this ring will depend on its orientation, relative to the mass distribution of the accretor, and most importantly, on the mass/self-gravity of the gas. A polar ring may form after a high angle impact with gas which has a spin perpendicular to the equatorial plane of the accretor, remaining in an equilibrium configuration for several Hubble times. In contrast, counterrotating galaxies may form after a low angle impact with a gas disk which has a spin antiparallel to that of the accreting system.
In the most general case, however, the impact angle is intermediate between polar and planar. In this case the orbits of gas clouds will experience differential precession in the non-spherical potential of the galaxy, characterized by a quadrupole component (Steiman-Cameron & Durisen 1982). The latter applies for disk galaxies (axisymmetric) and ellipticals (triaxials). Sparke (1986) and Arnaboldi & Sparke (1994) have studied in detail the dynamics of self-gravitating annuli of matter inclined to the principal axes of axisymmetric and triaxial potentials. If the strength of gas self-gravity is negligible, the inclined ring may rapidly settle towards the equatorial plane, appearing either as a co- or as a counter-rotating disk. In contrast, if the gas ring is heavy, the self-coupling can stabilise the ring for several Hubble times. In an intermediate case, several subrings (near polar or close to the equator), characterized by different precessing rates, can coexist in a single galaxy (e.g. NGC 660).
Studying the morphology of polar rings, van Gorkom et al. (1987) and
Sage & Galletta (1993) have found that their ages may vary from 400 Myr to
4 Gyr, if the smooth rings are the oldest. Some polar rings or
inclined rings could be the result of recent acquisitions, whereas
others appear to be evolved systems. However, twisted polar rings are
uncommon and some have had time to form stars. This requires the
existence of a stabilising mechanism. The observations of atomic and
molecular gas show that the quantity of gas mass present in polar
rings is sufficient to stabilise them through self-gravity
(van Gorkom et al. 1987; Sage & Galletta 1993; Galletta et al. 1997, and this work). We derived a
global content of cold gas (
)
in polar rings which is 1-2
orders of magnitude higher than in normal galaxies.
In contrast to polar rings, the derived content of cold gas in
counterrotators is close to normal. Although counterrotators and
polar rings probably share a common origin, the estimated gas masses
confirm that light gas rings may have evolved faster. If the
mass of gas originally accreted is not sufficient to stabilise the
ring through self-gravity, the ring settles toward the equatorial
plane in less than a Hubble time. In this case, the merger relic could
be a counterrotator. Once the gas disk has settled to the plane with
an antiparallel spin it can interact with the gas of the primary
disk. Since the two components have opposite rotating directions,
there can be large-scale shocks and angular momentum annihilation when
they come into contact. Near this transition region the transformation
of atomic into molecular gas could be enhanced, especially if the
primordial gas content is high, i.e., for late-type accretors.
Confirming these expectations, the measured M(H2)/
ratio
seems larger in counterrotators than in normal galaxies for types
t>0.
In the course of this process, a starburst might be triggered in the
circumnuclear molecular gas disk (García-Burillo et al. 2000). The time-scale for
gas infall could be extremely short, being close to the free-fall
time, i.e., 107-8 Myr. The mass of gas involved in the
starburst episode however is kept low enough (10
)
for a typical counterrotating galaxy. In polar rings, although the
cold gas content is larger than the one in normal galaxies, star
formation in the dynamically stable ring proceeds calmly. Confirming
this scenario it was found that counterrotators, polar rings and
normal galaxies have a similar content of hot gas, according to their
normalized X-ray luminosities. Also, the normalized
lies
within the typical boundaries of aged (
Gy) mild starbursts,
far from the values characteristic of massive mergers (see
Read & Ponman 1998).
Acknowledgements
We would like to thank Dr. G. Paturel for kindly making available to the authors the FIR raw data of LEDA database and to Dr. F. Ochsenbein for the changes made to the Vizier's query form after our request. Thanks to the referee's comments for useful suggestions on the statistical analysis. This research made use of Vizier service (Ochsenbein et al. 2000) and of NASA's Astrophysics Data System Abstract Service, mirrored in CDS of Strasbourg. SGB and ARF thank financial support from the Spanish CICYT under grant number PB96-0104 and CICYT-PNIE under grant number 1FD1997-1442. GG has made use of funds from University of Padova (Fondi 60%-2000).
PGC | Name | t | D25 | MB | d | ![]() |
![]() |
![]() |
![]() |
References |
Kpc | Mpc | 108 M![]() |
108 ![]() |
108 ![]() |
105 ![]() |
|||||
Galaxies with counterrotation | ||||||||||
1791 | NGC 128 | -1.88 | 47 | -21.39 | 56.68 | 23.9 | <10.3 | <0.8 | 11.8 | 1 2 17 18 |
2789 | NGC 253 | 5.10 | 27 | -20.92 | 3.48 | 4.7 | 36.7 | 38.2 | 45.2 | 3a 3b 8 18 |
4992 | NGC 497 | 4.04 | 64 | -22.30 | 108.34 | 126.3 | 3a 17 | |||
9359 | NGC 936 | -1.12 | 23 | -20.35 | 17.36 | 2.7 | 4.4 | <0.1 | 1 2 3a 17 18 | |
10175 | NGC 1052 | -4.75 | 16 | -20.07 | 18.82 | <2.0 | 8.1 | <0.4 | 0.8 | 1 2 3a 3b 16 17 18 |
11693 | NGC 1216 | -0.79 | 10 | -18.76 | 65.55 | <0.5 | <3.0 | <0.1 | 6 7 13 | |
13738 | NGC 1439 | -4.71 | 15 | -19.43 | 20.56 | <0.7 | 3.3 | 1 2 16 | ||
14077 | IC 2006 | -4.17 | 8 | -18.70 | 15.36 | <0.9 | 1.7![]() |
<0.1 | 0.2 | 1 2 13 16 |
14659 | NGC 1543 | -1.94 | 19 | -19.41 | 13.18 | <11.7 | 0.6 | 1 2 3b | ||
14965 | NGC 1574 | -2.90 | 12 | -19.06 | 10.35 | <0.6 | 0.1 | 1 2 17 18 | ||
16386 | NGC 1700 | -4.57 | 45 | -21.95 | 50.86 | <21.7 | <15.8 | 1 2 | ||
25915 | NGC 2768 | -4.33 | 44 | -21.04 | 21.51 | <4.9 | 4.6 | 0.3 | 2.6 | 1 2 3a 16 |
26512 | NGC 2841 | 2.98 | 24 | -20.87 | 10.82 | 3.6 | 39.0 | 32.8 | 45.2 | 3a 3b 8 17 18 |
27840 | NGC 2983 | -0.82 | 18 | -19.56 | 24.92 | <6.0 | <0.1 | 1 2 | ||
28259 | NGC 3011 | -1.67 | 5 | -16.84 | 21.63 | 0.7 | 3a | |||
31035 | ESO 263-48 | -2.07 | 28 | -21.55 | 35.43 | 8.0 | 37.5 | 1 3b 13 | ||
33667 | NGC 3497 | -1.80 | 36 | -20.80 | 47.58 | 14.0 | 13 | |||
34257 | NGC 3593 | -0.43 | 13 | -18.16 | 8.75 | 0.1 | 2.3 | 5.1 | 5.6 | 1 2 3a 3b 4 8 15 17 18 |
34433 | NGC 3608 | -4.78 | 14 | -19.54 | 15.62 | 0.8 | 0.2 | 2 3a 18 | ||
34684 | NGC 3626 | -0.70 | 19 | -20.06 | 20.61 | 5.7 | 4.1 | 2 3a 14 | ||
35859 | UGC 6570 | -0.24 | 8 | -17.79 | 23.23 | 1.9 | 3a | |||
36914 | NGC 3900 | -0.30 | 23 | -19.91 | 25.39 | 27.9 | 10.8 | 1 2 3a 3b | ||
37235 | NGC 3941 | -2.02 | 15 | -19.80 | 14.27 | 2.6 | 2 3a | |||
38201 | NGC 4073 | -4.09 | 58 | -22.22 | 79.21 | 516.0 | <17.1 | <5.7 | 1 2 16 17 | |
38643 | NGC 4138 | -0.91 | 12 | -18.89 | 14.48 | 4.0 | 3.1 | 2 3a 8 | ||
40484 | NGC 4379 | -2.80 | 8 | -18.48 | 14.84 | <0.5 | <0.1 | 1 2 | ||
41220 | NGC 4472 | -4.72 | 34 | -21.38 | 12.02 | 33.1 | 5.2 | 0.1 | 0.2 | 1 2 3a 3b 16 18 |
41260 | NGC 4477 | -1.88 | 20 | -20.20 | 18.62 | 3.7 | <0.5 | 1.1 | 1 2 3b 17 18 | |
41939 | NGC 4546 | -2.73 | 13 | -19.60 | 13.68 | 3.3 | 1.2 | 0.7 | 1 2 3a | |
41943 | NGC 4550 | -2.06 | 21 | -19.48 | 21.90 | <1.0 | 22.4 | 0.2 | 1 2 3a 17 18 | |
42401 | NGC 4596 | -0.86 | 30 | -20.91 | 25.54 | 28.2 | <3.9 | 0.7 | 1 2 3a 8 | |
42797 | NGC 4643 | -0.60 | 16 | -19.88 | 17.75 | 1.40 | 2.2 | 2.4 | 1 2 3a 3b 17 18 | |
43149 | NGC 4684 | -1.21 | 70 | -22.77 | 87.42 | <107.8 | <30.6 | 20.7 | 1 2 3b | |
43798 | NGC 4772 | 1.07 | 14 | -19.19 | 14.02 | 7.3 | 0.5 | 2 3a 13 | ||
44114 | NGC 4816 | -3.06 | 38 | -21.32 | 94.06 | <4.2 | 1 17 | |||
44182 | NGC 4826 | 2.36 | 21 | -20.66 | 6.80 | 4.9 | 9.1 | 10.1 | 3a 3b 4 8 18 | |
45749 | NGC 5005 | 3.97 | 25 | -20.94 | 14.91 | 9.1 | 41.9 | 65.8 | 3a 3b 8 | |
47257 | NGC 5173 | -4.74 | 11 | -19.45 | 35.04 | <1.1 | 17.8 | 0.7 | 1 3a 16 | |
48815 | NGC 5297 | 4.88 | 52 | -21.55 | 34.77 | 175.3 | 63.9 | 3a 3b | ||
49044 | NGC 5322 | -4.81 | 47 | -21.42 | 26.89 | <7.5 | <4.6 | 1.5 | 1 2 16 17 18 | |
49354 | NGC 5354 | -2.13 | 20 | -20.40 | 33.33 | <6.4 | 52.5 | 9.1 | 1 3a 18 | |
52521 | NGC 5728 | 1.17 | 34 | -20.96 | 36.58 | 14.5 | 33.7 | 39.0 | 3a 3b 17 18 | |
54013 | NGC 5854 | -1.05 | 18 | -19.54 | 23.26 | 16.2 | 1.6 | <1.1 | 1 2 3a 13 | |
54625 | NGC 5898 | -4.24 | 20 | -20.40 | 28.03 | <3.6 | <2.0 | 1.0 | 0.2 | 1 2 13 16 17 |
55555 | UGC 9922A | 9 | -17.53 | 77.30 | 51.5 | 3b | ||||
62314 | NGC 6701 | 1.03 | 24 | -20.66 | 56.62 | 51.9 | 163.1 | 166.7 | 3a 3b 8 | |
62453 | NGC 6684 | -1.85 | 10 | -18.99 | 8.49 | <3.4 | <0.01 | 1 2 | ||
63620 | IC 4889 | -4.52 | 26 | -21.01 | 31.25 | <11.4 | 1.3 | 1 2 16 | ||
66069 | NGC 7007 | -2.96 | 24 | -20.24 | 37.38 | <4.1 | 0.6 | 1.7 | 1 2 13 16 | |
66934 | NGC 7079 | -1.84 | 21 | -20.60 | 34.48 | 1.2 | 0.9 | 1 2 13 | ||
67146 | NGC 7097 | -4.81 | 16 | -19.95 | 31.68 | <2.5 | 3.3 | 1 2 16 | ||
68096 | NGC 7217 | 2.50 | 16 | -20.51 | 15.14 | 7.4 | 15.1 | 31.9 | 3a 3b 8 |
69327 | NGC 7331 | 3.93 | 42 | -21.50 | 13.52 | 12.5 | 102.5 | 113.8 | 110.6 | 3a 3b 8 17 18 |
69342 | NGC 7332 | -1.97 | 19 | -19.27 | 19.27 | <0.9 | 1.6 | <9.5 | 0.2 | 1 2 3a 17 18 |
69591 | NGC 7360 | 15 | -19.09 | 63.33 | 36.9 | 3a | ||||
70090 | IC 1459 | -4.68 | 30 | -20.73 | 20.63 | 8.1 | <0.1 | <0.2 | 1.4 | 1 2 3b 13 16 17 18 19 |
72977 | UGC 12856 | 9.61 | 13 | -18.78 | 24.99 | 24.2 | 3a | |||
73126 | NGC 7796 | -3.87 | 29 | -21.05 | 41.92 | <0.8 | 1 2 | |||
Polar ring S0s and spirals | ||||||||||
2710 | IC 51 | -1.88 | 10 | -18.31 | 22.10 | 12.8 | 13.6 | 6.1 | 3a 3b 5 | |
2757 | ESO 474-26 | 2.73 | 51 | -22.13 | 216.47 | 392.2 | 177.1 | 3b 5 | ||
6101 | A 0136-08 | 18 | -17.94 | 72.91 | 46.6 | 31.2 | 3a 5 | |||
6318 | NGC 660 | 1.13 | 26 | -18.95 | 11.98 | 59.3 | 61.0 | 23.3 | 3a 3b 8 5 | |
6676 | UGC 1198 | -3.30 | 5 | -16.91 | 18.99 | 1.8 | 1.5 | 1.3 | 3a 3b 5 | |
9408 | ESO 415-26 | -2.06 | 21 | -19.35 | 58.80 | 91.3 | 42.0 | 3a 5 | ||
11505 | ESO 199-12 | 7.77 | 35 | -21.14 | 86.50 | 64.8 | 3b | |||
14740 | ESO 201-26 | 5.00 | 8 | -18.59 | 47.32 | 5.8 | 3b | |||
18883 | NGC 2217 | -0.64 | 26 | -20.42 | 18.94 | 26.8 | 0.9 | 14.7 | 1 2 3a 3b 13 | |
22945 | UGC 4261 | 10.00 | 21 | -20.87 | 86.90 | 68.6 | 14.0 | 3a 3b | ||
23355 | UGC 4332 | 3.00 | 27 | -20.21 | 73.35 | 39.7 | 33.8 | 3a 3b | ||
23393 | UGC 4323 | -4.79 | 26 | -20.12 | 56.81 | 18.9 | 3a | |||
25065 | NGC 2685 | -1.05 | 21 | -19.05 | 14.36 | <0.6 | 13.4 | 9.4 | 3.5 | 1 2 3a 3b 8 17 18 |
26018 | NGC 2748 | 4.01 | 20 | -20.48 | 23.03 | 53.0 | 12.7 | 41.3 | 3a 3b 8 5 | |
26601 | NGC 2865 | -4.12 | 24 | -20.71 | 33.27 | <4.9 | 6.7 | 1.0 | 1 2 3a 16 | |
27383 | UGC 5119 | -2.00 | 15 | -20.35 | 79.87 | 107.7 | 3a | |||
30491 | UGC 5600 | -1.68 | 13 | -18.74 | 40.40 | 45.9 | 8.4 | 11.4 | 3a 3b 5 12 | |
30708 | ESO 500-41 | 1.70 | 13 | -19.55 | 45.75 | 24.6 | 10.4 | 3b 20 | ||
32292 | NGC 3384 | -2.64 | 15 | -19.41 | 9.96 | <0.4 | <0.1 | 1.0 | 1 2 16 17 18 | |
35616 | NGC 3718 | 1.07 | 35 | -19.98 | 16.02 | <2.7 | 59.8 | <7.7 | 6.5 | 1 3a 3b 8 17 18 |
37170 | NGC 3934 | 2.70 | 14 | -18.97 | 50.75 | 48.5 | 41.2 | 14.9 | 3a 3b 5 10 | |
38906 | NGC 4174 | -0.23 | 11 | -19.46 | 52.48 | 80.6 | 3a | |||
40887 | IC 3370 | -4.72 | 36 | -21.47 | 40.12 | 5.6 | 20.2 | 1 2 3a 3b | ||
40900 | UGC 7576 | 27 | -19.22 | 95.50 | 36.4 | 3a | ||||
42951 | NGC 4650A | 0.00 | 16 | -19.49 | 36.34 | 65.4 | 16.0 | 5.0 | 3a 9 | |
43073 | NGC 4672 | 1.18 | 26 | -19.85 | 41.25 | 41.2 | 46.8 | 3a 3b | ||
46552 | NGC 5103 | 8 | -18.26 | 19.93 | <0.2 | 9.7 | 17 20 | |||
46848 | NGC 5122 | -2.48 | 12 | -18.93 | 37.57 | 13.3 | 3a | |||
47430 | ESO 576-69 | 1.30 | 19 | -19.87 | 70.70 | 205.5 | 43.1 | 3b 20 | ||
53039 | UGC 9562 | 8.43 | 6 | -17.30 | 19.24 | 10.5 | 3a | |||
54461 | UGC 9796 | 3.40 | 20 | -19.15 | 75.96 | 80.8 | 3a | |||
57173 | UGC 10205 | 1.01 | 38 | -21.05 | 90.24 | 74.6 | 64.4 | 3b 20 | ||
59352 | NGC 6286 | 3.00 | 30 | -20.59 | 78.38 | 20.2 | 275.7 | 723.3 | 3b 8 20 | |
69867 | ESO 603-21 | 4.50 | 11 | -18.70 | 45.08 | 74.4 | 21.6 | 16.1 | 3a 3b 5 | |
70332 | NGC 7468 | -4.99 | 7 | -18.38 | 29.36 | 28.1 | 3.8 | 1.6 | 1 3a 3b | |
72294 | ESO 240-16 | 3.77 | 30 | -21.06 | 179.64 | 360.9 | 3b | |||
Polar ring ellipticals | ||||||||||
4126 | NGC 404 | -4.83 | 5 | -16.78 | 3.33 | <0.01 | 1.1 | 0.3 | 0.1 | 1 2 3a 3b 8 16 |
12651 | NGC 1316 | -4.74 | 70 | -22.11 | 20.97 | 43.0 | 2.0 | 11.1 | 8.9 | 1 2 3b 11 16 17 18 |
17296 | NGC 1947 | -4.76 | 9 | -19.15 | 12.02 | <0.6 | 3.9 | 4.6 | 1 2 3b 11 17 18 | |
40455 | NGC 4374 | -4.16 | 23 | -20.71 | 12.88 | 5.1 | 11.0 | 0.4 | 1 2 3a 3b 16 17 18 | |
42139 | NGC 4589 | -4.79 | 27 | -20.88 | 29.47 | <4.3 | <11.1 | 2.1 | 1 2 16 17 18 | |
46957 | NGC 5128 | -5.09 | 16 | -19.26 | 1.94 | 0.4 | 1.3 | 0.8 | 2.5 | 1 2 3a 3b 11 17 18 |
48593 | NGC 5266 | -5.60 | 38 | -21.76 | 37.33 | 66.4 | 19.3 | 22.5 | 1 2 3b 11 | |
48655 | IC 4320 | -5.20 | 30 | -21.23 | 89.04 | <25.6 | 1 | |||
49547 | NGC 5363 | -3.75 | 23 | -20.37 | 15.78 | 2.8 | 1.0 | 4.1 | 3.9 | 1 2 3a 3b 8 11 17 18 |
66909 | NGC 7070A | -3.80 | 17 | -19.36 | 29.92 | 2.0 | 1 3 |
:
In a ring outside the galaxy.
References: 1: Knapp et al. (1989), 2: Roberts et al. (1991), 3: a) 21 cm, b) FIR from Paturel et al. (1997), 4: Sage (1993), 5: Galletta et al. (1997), 6: Williams & van Gorkom (1995), 7: Allam et al. (1996), 8: Young et al (1995), 9: Watson et al. (1994), 10: Casoli et al. (1996), 11: Sage & Galletta (1993), 12: Zhu et al. (1999), 13: this paper, 14: García-Burillo et al. (1998), 15: García-Burillo et al. (2000), 16: Beuing et al. (1999), 17: Burstein et al. (1997), 18: Fabbiano et al. (1992), 19: Walsh et al. (1990), 20: van Driel et al. (2000).