P. Amram 1 - H. Plana 2 - C. Mendes de Oliveira3 - C. Balkowski 4 - J. Boulesteix1
1 - Observatoire Astronomique Marseille-Provence & Laboratoire d'Astrophysique de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 04, France
2 -
Observatorio Nacional MCT, Rua General Jose Cristino 77, São Cristóvão, 20921-400 Rio de Janeiro, RJ, Brazil
3 - Universidade de São Paulo, Instituto de Astronomia,
Geofísica e Ciências Atmosféricas, Departamento de
Astronomia, Rua do Matão 1226, Cidade Universitária,
05508-900, São Paulo, SP, Brazil
4 - Observatoire de Paris,
GEPI, CNRS and Université Paris 7, 5 place Jules Janssen, 92195
Meudon Cedex, France
Received 9 October 2002 / Accepted 19 December 2002
Abstract
We present Fabry Perot observations of galaxies in five Hickson
Compact Groups: HCG 10, HCG 19, HCG 87, HCG 91 and HCG 96. We
observed a total of 15 galaxies and we have detected ionized gas
for 13 of them. We were able to derive 2D velocity, monochromatic,
continuum maps and rotation curves for the 13 objects. Almost all
galaxies in this study are late-type systems; only HCG 19a is an
elliptical galaxy. We can see a trend of kinematic evolution
within a group and among different groups - from strongly
interacting systems to galaxies with no signs of interaction -
HCG 10 exhibits at least one galaxy strongly disturbed; HCG 19
seems to be quite evolved; HCG 87 does not show any sign of
merging but a possible gas exchange between two galaxies; HCG 91
could be accreting a new intruder; HCG 96 exhibits past and
present interactions.
Key words: galaxies: kinematics and dynamics - galaxies: evolution - galaxies: interactions - galaxies: ISM - galaxies: formation - galaxies: intergalactic medium
Galaxies in compact groups of galaxies (hereafter referred to as CGGs) are galaxies (1) which live in the most dense environments of the nearby universe (2) which experience a high rate of galaxy-galaxy interactions and (3) which are probably also progenitors of galaxies in formation (tidal dwarf galaxy candidates formed in heavily interacting systems) and in that sense, they mimic the early universe. Nevertheless, they are different from the first galaxies because they are generally large galaxies.
In 1995, we began an observation program using a Fabry-Perot instrument to obtain 2D-velocity maps of CGG with the aim of investigating the influence of the dense environments of compact groups on the kinematics and dynamics of their member galaxies. The goals of this study are: (1) to classify the groups in different evolutionary stages (merging groups, strongly interacting groups, weakly interacting groups, non-groups and single irregular galaxies); (2) to build the Tully-Fisher relation for CGGs and (3) to search for tidal dwarf galaxy candidates.
Such a study is the extension of a previous effort in the same lines, in order to determine the influence of the dense environments of galaxy clusters (e.g. Amram et al. 1996 and references therein).
Our results on several CGGs have already been published: (1) the "Cartwheel'' compact group, a quartet of galaxies containing the well-known ring galaxy called the "Cartwheel galaxy'' produced by the encounter of a small galaxy intruder through the spin axis of a large target galaxy (Amram et al. 1998); (2) HCG 16, one of the most dense compact groups known where a major merger event has taken place; this group could be a young compact group in formation through the merging of close-by objects in a dense environment (Mendes de Oliveira et al. 1998); (3) HCG 90, a dynamically evolving group where three galaxies are in interaction and will possibly merge into one single object (Plana et al. 1998); (4) HCG 18, that is not a CG but instead a large irregular galaxy with several star forming clumps (Plana et al. 2000); (5) HCG 92, the famous Stephan's Quintet where collisions among group members may have taken place and formed dwarf galaxies (Plana et al. 1999; Mendes de Oliveira et al. 2001).
We present here a new data set for five Hickson compact groups. We could derive kinematic information mainly for the late-type members. We also present a comparison of rotation curves (RCs) we derived, with previous studies using long-slit spectroscopy made by Rubin et al. (1991) (hereafter referred to as RHF1991) and Nishiura et al. (2000) (hereafter referred to as NSOMT2000).
A new set of Fabry Perot observations of three Hickson Compact
Groups: HCG 88, HCG 89 and HCG 100, for which we detect warm gas
in 15 group members, is presented in Plana et al. (2003). In a
following paper the properties of the CGGs will be compared to
those of an homogeneous sample of nearby isolated spirals with a
large range of morphological types and luminosities for which
Fabry-Perot data are also available, allowing statistical studies:
the GHASP
survey (Amram & Garrido 2002; Garrido et al. 2002, 2003).
Observations were carried out during two runs at the European Southern Observatory 3.6 m telescope (ESO 3.6 m) in August 1995 and at the Canada-France-Hawaii 3.6 m telescope (CFHT 3.6 m) in August 1996.
During the run at the ESO 3.6 m telescope, the Fabry Perot instrument CIGALE was used. It is composed of a focal reducer (bringing the original f/8 focal ratio of the Cassegrain focus to f/2), a scanning Fabry-Perot and an Image Photon Counting System (IPCS). The IPCS, with a time sampling of 1/50 s and zero readout noise, makes it possible to scan the interferometer rapidly (typically 5 s per channel), avoiding sky transparency, air-mass and seeing variation problems during the exposures.
At the CFHT 3.6 m, the multi-object spectrograph focal reducer
(MOS), attached to the f/8 Cassegrain focus, was used in the
Fabry-Perot mode. The CCD was a STIS 2 detector, 2048 2048 pixels with a read-out noise of 9.3 e- and a pixel size
on the sky of 0.86 arcsec after
binning to increase the signal
to noise ratio.
Tables 1 and 2 contain the journal of observations and observational characteristics for both runs. Reduction of the data cubes were performed using the CIGALE/ADHOC software (Boulesteix 2002). The data reduction procedure has been extensively described (Amram et al. 1998 and references therein).
Wavelength calibration was obtained by scanning the narrow Ne 6599 Å line under the same conditions as the observations.
Velocities measured relative to the systemic velocity are very
accurate, with an error of a fraction of a channel width (<
)
over the whole field.
Subtraction of bias, flat fielding of the data and cosmic-ray removal have been performed for each image of the data cube for the CFHT observations. To minimize seeing variation, each scan image was smoothed with a gaussian function of full-width at half maximum equal to the worse-seeing data of the data cube. Transparency and sky foreground fluctuations have also been corrected using field star fluxes and galaxy-free windows for the CFHT observations. Except for flat fielding, none of these operations are necessary for the IPCS data processing (ESO observations).
The signal measured along the scanning sequence was separated into
two parts: (1) an almost constant level produced by the continuum
light in a narrow passband around H
(hereafter referred
to as continuum map) and (2) a varying part produced by the
H
line (hereafter referred to as H
emission line
map or monochromatic map). The continuum level was taken
to be the mean of the three faintest channels, to avoid channel
noise effects. The H
integrated flux map was obtained by
integrating the monochromatic profile in each pixel. The
continuum as well the H
images are not flux calibrate due
to a lack of useful calibrators; hence, the contour plots are
given in arbitrary units. The velocity sampling was 11 km s-1 at CFHT and 16 km s-1 at ESO. Profiles were
spatially binned to
or
pixels in the outer
parts, in order to increase the signal-to-noise ratio. Strong OH night sky lines passing through the filters were subtracted by
determining the level of emission from extended regions away from
the galaxies (Laval et al. 1987).
In this section, we describe the main characteristics of all the
galaxies observed in each group. Table 3 summarizes the general
properties of the observed galaxies and Table 4 gives the
different galaxy position angles, inclination and maximum rotation
velocity that we derived from the continuum, monochromatic and
velocity maps. Figures 1, 3, 5, 7, 9, 11, 13, 15, 18 and 23 show
the different maps (continuum, monochromatic and velocity).
Figures 2, 4, 6, 8, 10, 12, 14, 17, 19, 21, 22, 24, 25, 26 and 27
display the different rotation curves (RCs) for each group member
and, when available, comparison with other authors' data (Figs. 14, 17 and 25). Figure 20 shows line profiles and different maps
(continuum, monochromatic and velocity for the two components of
the galaxy). A Hubble constant
is
used throughout this paper.
Table 3: Properties of HCG members.
Table 4: Kinematic properties of the HCG members.
This group is formed of four members, three are late-type objects
(SBb, Sc and Scd) and one is an elliptical galaxy (E1). A
multiphase interstellar medium has been detected in this group.
Only the two galaxies with the latest morphological types, HCG 10a and c, have been detected at 60 and 100 m (IRAS data),
suggesting the presence of dust (Allam et al. 1996). In order to
calculate the HI deficiency of the group, its HI content has been
taken as the sum of the HI content of the individual galaxies
(Verdes-Montenegro et al. 2001). HCG 10 is among the less HI deficient (
) HCGs according to
Verdes-Montenegro et al. (2001), who found an average value of
for their sample of 72 HCGs. Another
component of the cold ISM, the CO emission, has been detected only
for HCG 10a and 10c. Leon et al. (1998) computed a mean star
formation efficiency (SFE) for their sample of compact groups
(
), showing that the compact
environment has a moderate effect on triggering global star
formation. For HCG 10a and 10c the SFE is lower than the average
SFE for their sample (
and 0.14 respectively). HCG 10 has been detected as a faint X-ray emitter
by Ponman et al. (1996) using ROSAT data. HCG 10b, the elliptical
galaxy of the group, for which Pildis et al. (1995) pointed out a
shell structure surrounding the whole galaxy, was not observed in
our program, being too far (9 arcmin) from the group center (the
FOV of the instrument is 7.3 arcmin). HCG 10a has been classified
as an AGN (Shimada et al. 2000). We obtained velocity,
monochromatic and continuum maps for 10a, 10c and 10d.
The SBb galaxy HCG 10a has a bright inner region, a ring from
which two faint arms emanate and a very weak level of H emission. Shimada et al. (2000) did not detect H
line
emission in HCG 10a. Both arms are visible on the continuum
image and are traced by the HII regions detected in the galaxy.
Indeed, the emission lines surrounding the centre of the galaxy
are suspicious because the continuum emission is very bright and
the signal which could possibly be from H
emission around
the nucleus matches the intensity level of the continuum photon
noise. The values of the velocities provided by this central
emission are also highly doubtful. As a consequence of the
weakness of the H
emission, the H
velocity
determination is uncertain. A RC as uncertain as the velocity
field has been plotted, using the morphological parameters. This
RC is shown only to indicate the velocity range of the galaxy and
should not be used for further discussion.
HCG 10 c and d show clumpy emission gas distributions. For HCG 10c, the signature of the spiral-arm structure is clearly seen in
the DSS or the R-band image (not presented) as well as in the
velocity field, where wiggles on the isovelocity contours are
visible. Two bright H
knots are clearly superimposed on
the wiggles of the isovelocity contours of HCG 10c. A spiral
structure is also seen, although less obvious, on the isovelocity
contours of HCG 10d. The R-band image as well as the continuum
image of HCG 10c exhibit a strong nucleus and the brightest
central HII region presents an offset of 2.6 arcsec with respect to
the nucleus (defined as the pixel position of the maximum central
emission from the continuum map). In the continuum image, HCG 10c
also shows two very weak and diffuse extensions (in the northeast
and southwest), which could suggest the presence of a warped disk
or, alternatively, distorted spiral arms. These diffuse extensions
are oriented along the direction of the stellar major axis
(
), as deduced from the continuum images. In contrast, the
gaseous velocity field indicates that the position angle
(hereafter referred to as PA) of the major axis is oriented
northwest-southeast (
), meaning that the stellar and the
gaseous kinematics are decoupled. The velocity field is not
regular and the PA of the gaseous major axis we determined is an
average value for the whole field. The isovelocities present an
U-shape with respect to the gaseous position angle and not an
S-shape, as expected for a warp structure. Moreover, the
distortions are probably not due to a warp of the disk, since the
stellar and gaseous PAs are significantly different and a warp of
the disk should follow the stellar distribution rather than the
gaseous matter. In conclusion, the faint structures measured in
the R-image or in the continuum map are more likely extensions of
the spiral arms rather than a warp. The RC of HCG 10c shows a
linear shape and extends up to 24 arcsec (7.7 kpc), almost
reaching the optical radius (
D25/2=28 arcsec = 9 kpc) for a
velocity of
.
Since the continuum image of this
galaxy shows a bright central region, the linear shape of the RC
will probably make it difficult to reconcile with a reasonable
M/L value for the stellar disk and imposes the need for a
strong dark-matter component. The bumps around 14 arcsec (4.5 kpc)
on both sides of the RC are due to the crossing of the spiral
arms; the bump is stronger for the receding side, consistent with
the velocity field.
The velocity field of HCG 10d is rather regular and both stellar
and gaseous major axes are aligned. The RC of HCG 10d is also
quite regular and reaches almost the optical radius (
D25/2=29arcsec = 9.3 kpc) at a rotation velocity of
.
The
approaching side does not extend as far as the receding side. This
could be due to the fact that the edge of the approaching side was
placed on the edge of the receptor and of the interference filter.
Hickson (1993) originally catalogued four members in this group,
but HCG 19d was later found to be a background object, with a
velocity of over 20000
(de Carvalho et al. 1997). The
triplet contains two late-type spirals (Scd and Sdm) and one
elliptical galaxy (E2). The two spiral galaxies show signs of
morphological distortions. HCG 19 has been detected in FIR at 60
and 100
m. HCG 19b is the brightest FIR source of the group,
while for the other two members, only upper limits on the
detection are provided from IRAS data (Allam et al. 1996). In
contrast with HCG 10, the HI component detected by
Verdes-Montenegro et al. (2001) for HCG 19 shows a HI gas
deficiency (
), a value which is higher
than the mean derived for their whole sample
(
). CO emission has been detected for HCG 19b only (Leon et al. 1998). HCG 19b shows the highest SFE for
their sample of compact groups
,
the mean value being
.
This group
does not present diffuse X-ray emission (Ponman et al 1996).
HCG 19a shows a very weak H
emission but rather extended
for an elliptical galaxy. The brightest HII region is slightly
offseted from the nucleus of the galaxy by 2 arcsec (0.5 kpc). The
PAs of the major axes of the stellar and gaseous components differ
by 23
.
HCG 19a has a rather regular velocity field for an
elliptical and its RC displays an important circular rotation with
a maximum at
,
for the receding side and
,
for the approaching side. The irregularities in
the velocity field of HCG 19a are partly due to the weak S/N in
the H
profiles. Nevertheless, four main characteristics
can be noted: (1) The RC almost reaches the optical radius
(
D25/2=36 arcsec = 9.5 kpc), which is actually uncommon for
gaseous components in elliptical galaxies; (2) in the central
regions, within 8 arcsec (2.1 kpc), the isovelocities are strongly
distorted due to the offset of the brightest H
knot with
respect to the stellar nucleus; this induces a flat inner RC for
the receding side; (3) both sides of the RC are decreasing after a
certain radius (20 arcsec, which corresponds to 5.3 kpc, for the
approaching side and 28 arcsec, which corresponds to 7.4 kpc, for
the receding side); (4) whatever kinematic parameters we may
impose, we cannot make both sides of the RC match.
HCG 19b is a very faint and diffuse Scd galaxy (Hickson 1993) or
(R)SB(r)a pec (NED),
as can been seen from the DSS image and from the continuum image.
The nucleus of the galaxy is shifted by 2 arcsec (0.5 kpc) to the
east with respect to the very bright central
knot. The
velocity field is patchy but leads to a symmetric RC up to 29
arcsec (7.6 kpc). Further out, the approaching side keeps rising
up to 42 arcsec (11 kpc) while the receding side decreases up to
46 arcsec (12.1 kpc). An isolated HII region is responsible for a
sharp rise of the velocities in the outer region of the
approaching side while the decreasing trend of the receding side
is traced by diffuse
emission. The distorted shape of
the velocity field could be due to a warp, since the galaxy is
rather inclined (
). However, one cannot be sure of the
presence of such a warp because we do not observe a symmetric
S-shape in the velocity field - thus, we cannot exclude that
streaming motions could be induced by interaction of this galaxy
with the other members of the group.
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Figure 2: Generic caption for Figs. 2, 4, 6, 8, 10, 12, 14, 17, 19, 21, 22, 24, 25, 26 and 27: rotation curve of the galaxy, each side is represented by a different symbol only when one quadrant is displayed. The error bars give the dispersion in each circular annulus. The arrow along the x-axis indicates the optical radius (D25/2, from RC3 catalog), when this distance fits in the range of the plot. HCG 10a is plotted here. |
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Figure 4: HCG 10c: same as Fig. 2. |
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HCG 19c is a Sdm galaxy (Hickson 1993) or a SBm pec (NED), which
shows amorphous arms both in the continuum and in the
monochromatic maps but with a regular velocity field and RC. The
PA of the major axes of the stellar and gaseous disks differ by .
The H
distribution is non axisymmetric.
Furthermore, an outer and extended region towards HCG 19a (to the
west) is visible in the H
distribution as well as in the
velocity field. This western, extended and approaching region
induces, on one side of the RC, very low rotation velocities at
radii larger than 18 arcsec (4.7 kpc). In the central regions
(within 10 arcsec or 2.6 kpc), the agreement between both sides is
very good while at intermediate radii (between 10 and 18 arcsec
which corresponds to 2.6 and 4.7 kpc) the approaching side has
velocities typically
lower than that the ones of
receding side. On the other hand, at very low intensity levels
(
above the rms noise), a second H
component is superimposed from 10 to 22 arcsec (2.6 to 5.8 kpc),
making the link between the intermediate disk and the outer
western region. In conclusion, the external western region could
represent an extension of a distorted main disk and a signature of
an interaction with the neighbor HCG 19a.
This group is a small compact system composed of a triplet of
galaxies (a, b and c) with a background galaxy superimposed onto
it (HCG 87d). HCG 87a is a large edge-on Sbc spiral galaxy
showing a strong dust lane. HCG 87c is a late-type galaxy (Sd)
and HCG 87b is an S0 or an elliptical galaxy showing no H emission. An HST image of the group taken with the WFPC2 (English
et al. 1999) provides a striking improvement in resolution over
previous ground-based imaging. In particular, this image reveals
complex details in the dust lanes of the group's largest galaxy
member (HCG 87a), which is actually disk-shaped, but tilted
(isophotal warping); moreover the thick disk is asymmetric
radially as well as in the z-direction. In addition, a faint tidal
bridge of stars can be seen between the edge-on galaxy and the
early-type galaxy. Both HCG 87a and b have active galactic nuclei
(Shimada et al. 2000) in their centers. The third group member,
the nuclear starburst spiral galaxy, may be undergoing a burst of
active star formation (Coziol et al. 1998). Indeed, gravitational
tidal forces between interacting galaxies can provide fresh fuel
for both active nuclei and starburst phenomena by intensifying gas
flows toward the center of HCG 87a. Only a lower limit of
has been provided for the CO content of HCG 87a
(Boselli et al. 1996). No FIR emission has been reported by IRAS
(Allam et al. 1996). Ponman et al. (1996), using ROSAT data, gave
upper limits for the X-ray luminosity of the group
(
)
.
Verdes-Montenegro et al. (2001) showed that this group has a high
HI deficiency (
= 0.88). The morphological type
of the large edge-on and dusty galaxy HCG 87a is uncertain. It was
classified by Hickson (1993) as an Sbc, by Milhos et al. (1995) as
an S0 and by NED as an S0 0 pec sp. A peanut-shape bulge, boxy and
with strong X-feature, like HCG 87a, (Shaw et al. 1990; Jarvis
1986) can be reproduced in simulations from secular evolution for
an S0 galaxy (Mihos et al. 1995).
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Figure 6: HCG 10d: same as Fig. 2. |
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Figure 8: HCG 19a: same as Fig. 2. |
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We obtained H
and velocity maps for HCG 87a and HCG 87c
and for HCG 87b we could not detect any H
emission. The
emission of HCG 87a is concentrated on the northeastern half of
the galaxy. On the HST image, it can been seen that the dust lane
is more compact and opaque on the southwestern side of the galaxy
(absorbing the H
and [NII] emission line) than on the
northeastern one.
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Figure 10: HCG 19b: same as Fig. 2. |
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Figure 12: HCG 19c: same as Fig. 2. |
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The monochromatic map of HCG 87a shows four fairly intense (for an
early-type galaxy) H
knots, the brightest being close to
the center of the galaxy. HCG 87a is nearly edge-on, but probably
not exactly edge-on (
), since the disk appears
thicker on the northern side of the dust lane than on the southern
side. The linear shape of the RC leads to the conclusion that the
disk is in solid-body rotation, as already reported by Burreau &
Freeman (1999) and NSOMT2000. Nevertheless, the high inclination
of the galaxy probably strongly affects the shape of the RC by
inducing beam-smearing effects. The RC reaches 70% of the optical
radius (D25/2 = 48 arcsec or 27.4 kpc), which is fairly
extended if the galaxy is actually an S0, but probably
insufficient to reach the plateau of the RC. Furthermore, an
early-type, massive galaxy, with an extended gas disk, is not
expected to have a solid body RC, arguing in favor of a curve
strongly affected by absorption effects. The agreement between
NSOMT2000 and our data is good, except for small radii. A
comparison with the data from Burreau & Freeman (1999) also shows
good agreement. In summary, the RC of HCG 87a is truncated on one
side, most probably because of the strong dust lane in the
southwestern side of the galaxy. However, we cannot disentangle
the effects of absorption, secular evolution and environment in
the RC of HCG 87a.
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Figure 14:
HCG 87a: rotation curves using our data and
NSOMT2000. The kinematic parameters used to plot the RCs are, for
the inclination,
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Figure 16: HCG 87c: same as Fig. 2. |
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Figure 17:
HCG 87c: RCS using our data and NSOMT2000 data. The
kinematic parameters used to plot the RCs are, for the
inclination,
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The monochromatic map of HCG 87c shows bipolar HII structures,
where the brightest one corresponds almost exactly to the nucleus
of the galaxy. The velocity field and the RC clearly exhibit an
asymmetry on both sides of the galaxy for radii larger than 8
arcsec (4.6 kpc). Whatever the kinematic parameters are, both
sides of the RC cannot match at all radii. The shape of the
velocity field indicates that this asymmetry could be the result
of a warp of the disk or elliptical distortions in a planar disk.
We compare our RC with that obtained by NSOMT2000. Despite our
disagreement on the inclination (
measured by us and
by them) both curves match within 7 arcsec (4 kpc). At
larger radii, the RCs strongly diverge, the curve derived by us
being more regular and extended than that derived by NSOMT2000.
However, the RC we obtain is not symmetric, which could be due to
interactions with one or more companions in the group.
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Figure 18: HCG 91a Generic caption for Figs. 1, 3, 5, 7, 9, 11, 13, 15, 18 and 23: (Top) the upper left panel represents the continuum map (see text for details) onto which the stellar axis (SA) and the axis determined from the monochromatic map (GA) are superimposed . The SA is obtained by fitting the external isophotes of the continuum image. The lower left panel is the monochromatic map onto which the same axes (SA and GA) are plotted. The right panel shows the velocity field (in bold) superimposed onto the monochromatic map. SA and GA are also plotted here. The kinematic center is represented by a cross. |
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This quartet of galaxies (also known as VV 700) is formed by four
late-type galaxies, two of which are barred: HCG 91a and d
(Hickson 1993). A faint double tidal tail can be seen between HCG 91a and HCG 91c, from the DSS image of the group. Allam et al.
(1996) reported 60
and 100
emission for HCG 91a and HCG 91d together. No CO detection has been reported in the literature
for this group. Ebeling et al. (1994) detected point-like emission
with ROSAT in the position of the group. Verdes-Montenegro et al.
(2001) reported an HI deficiency
;
this value is a
bit lower than the mean value for their whole sample. Barnes &
Webster (2001) made a detailed study of this group in HI: the
neutral gas distribution presents two knots centered around HCG 91a and HCG 91c and at lower intensity levels a connection between
the two members of the group is present. Their line-of-sight
velocity curves suggest that there may be a connection between the
two galaxies through a gas bridge, since there is a common
velocity in the southern part of HCG 91c and the northern part of
HCG 91a. Roughly measured from their maps, we find that the
velocity amplitudes of HCG 91a and HCG 91c are
and
respectively which is roughly in agreement with
what we find. In this study, we could derive maps for HCG 91a and
HCG 91c only. We did not detect any emission for the barred galaxy
HCG 91d (SB0) and HCG 91b (Sc) was out of the field of view of the
instrument.
The monochromatic map of HCG 91a shows a clumpy and irregular
H
distribution. In particular, it shows an intense region
to the east of the galaxy and a southern arm which, at large
radii, points towards HCG 91c. A weak H
emission
superimposed onto a strong continuum is observed in the center of
the galaxy. The signature of the bar (SBc) is visible along the
minor axis of the velocity field while a twist of the
isovelocities in the outer regions indicate a rotation of the PA.
The southern spiral arm displays a roughly constant velocity,
which could be a signature of streaming motions due to the
interaction with HCG 91c. Both sides of the RC of HCG 91a match,
from the center up to 25 arcsec (11.3 kpc) and then diverge for
larger radii, with the receding side rising by about
and the approaching side decreasing by about
.
The behavior of the RC, when both sides diverge,
is probably due to streaming motions in the arms. This is more
obvious for the receding side, where the tidal arm points towards
HCG 91c. For the approaching side, one cannot observe any tidal
arm directly on the H
image (even if a diffuse pattern is
visible on the DSS and the H
images), but the perturbation
in the velocity field where the RC diverges could be the kinematic
signature of a symmetric northern tidal arm.
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Figure 19: HCG 91a: same as Fig. 2. |
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Figure 20:
HCG 91c: Upper left panel: main gas component
velocity field superimposed onto the monochromatic map.
Upper right panel: velocity field and monochromatic map
for the second gas component. Bottom left panel:
continuum map; the square in the central region represents the
region enlarged in the bottom right panel. Bottom right
panel: profiles in the center of HCG 91c. Each pixel represents
0.91 arcsec on the sky (0.4 kpc) and the x-axis range
displays the free spectral range of
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HCG 91c presents two gas components, which could be detected in
the central profiles of the galaxy, as it can be seen from the
double components in the profiles. The extension of the main gas
component is a bit larger than that of the second component. The
major-axis PAs of the components are similar. Both components are
also rotating in the same sense. Their monochromatic maps appear
to be clumpy and the velocity fields are quite regular. Their RCs
have good agreements between the receding and approaching sides
even if the RC of the first component is not as symmetric as that
for the second component. Both velocity maps show variations of
the PA with respect to the radius.
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Figure 21: HCG 91c: main component RC. Same as Fig. 2. |
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Figure 22: HCG 91c: second component RC. Same as Fig. 2. |
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Figure 24: HCG 96a: same as Fig. 2. |
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Figure 25:
HCG 96a: RCs for HCG 96a using our data, Keel et al.
(1996) and NSOMT2000 data. The kinematic parameters used to plot
the RCs are, for the inclination,
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This group, also known as Arp 182 and VV 343, shows a bright
spiral galaxy (HCG 96a) with a close companion (HCG 96c). These
two members appear to be strong IR emitters (Allam et al. 1996).
Ponman et al. (1996) listed only an upper limit for the X-ray
luminosity of this group. Leon et al. (1998) detected CO emission
for both HCG 96a and HCG 96c and showed that HCG 96a has an SFE above the mean SFE value for their whole sample. Verdes-Montenegro
et al. (2001) found a lower limit for the HI mass of the group,
leading to an HI deficiency
.
The other members of
the group are a giant E2 galaxy (HCG 96b) and HCG 96d, a galaxy
classified as an Im by Hickson et al. (1989). HCG 96a has been the
subject of numerous publications, mainly owning to its
classification as a Seyfert 2 galaxy by Mirabel & Wilson (1984).
Detailed study about the nature and the activity of this galaxy
can be found in Verdes-Montenegro et al. (1998) and Shimada et al.
(2000). Verdes-Montenegro et al. (1997) has shown that two long
low-surface brightness tails emerge from the region between H96a
and c, one to the NE and the other to the NW. They also detect a
shorter tail in the disk of H96a, N of its center.
Figures 11 and 12 show the velocity, monochromatic, continuum maps
and the RCs for the three galaxies we observed in HCG 96 (all but
HCG 96b, which has no gas). The velocity field of HCG 96a is
regular in the center but highly disturbed to the southeast. The
monochromatic map shows two bright emission regions, with the
shape of spiral arms, consistent with the spiral arms of the
galaxy. Bright emission regions can also be seen in the south and
in the northwestern part of the galaxy. The continuum image shows
a very peaked emission in the center and also hint the spiral
pattern.
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Figure 26: HCG 96c: same as Fig. 2. |
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We derived a RC for HCG 96a which looks similar (in shape) to that
of HCG 91a. We find a good agreement between the receding and
approaching sides between radii of 10 and 20 arcsec (5.7 and 11.4
kpc), where a plateau is reached. Using the continuum center we
cannot have a good agreement between receding and approaching
sides in the very central part (0 to 8 arcsec or 0 to 4.5 kpc). If
we, instead, move the position of the center by 2 pixels along the
minor axis of the galaxy, the agreement in the inner part is much
better but then, there is no agreement in the plateau region
(between 10 and 20 arcsec or 5.7 and 11.4 kpc). In any case, there
is a complete disagreement for radii larger than 20 arcsec (11.4 kpc), where the approaching side shows a steep gradient due to
peculiar isovelocities to the south and where the receding side is
much more normal, with an almost constant velocity of
.
We show the RC of HCG 96a from NSOMT2000 and
from Keel (1996). Data from Keel (1996) have been taken using a
,
close to our choice of
,
but
NSOMT2000 used a very different PA of 90
.
When we mimic
a slit of
using our data, we do reproduce the
curve derived by NSOMT2000. The curve determined by Keel (1996) is
in better agreement with ours, even if the velocity amplitude is
lower than ours in the receding part. In the approaching side the
agreement is excellent, confirming the steep decreasing curve
beyond -20 arcsec (11.4 kpc). Verdes-Montenegro et al. (1997) also
published a velocity curve for HCG 96a, but using a
,
corresponding to the direction that joins the
centers of the two galaxies HCG 96a and HCG 96c. Using our data
and simulating a long slit along the same axis, we found a curve
similar to that presented by Verdes-Montenegro et al. (1997), but
the velocity amplitudes we find are lower by
than those found in that paper.
The monochromatic map of HCG 96c does not show very strong
emission regions. The continuum map shows very peaked emission in
the center. The velocity field has a peculiar shape, where the
isovelocities are not regular. The velocity difference between the
isovelocities in the southwestern part of HCG 96c and the eastern
part of HCG 96a is less than
,
suggesting a possible
physical connection between the two objects. However, the central
velocity we found for HCG 96c is
lower than
the published systemic velocity (Verdes-Montenegro et al. 1997;
Hickson 1993). The RC of HCG 96c is very disturbed, with a
sinusoidal shape. No possible match between receding and
approaching sides of the RC could be found for this galaxy.
The monochromatic map of HCG 96d shows regular emission with no
clumpy structure, as we could expect for an Im galaxy. Images made
by Verdes-Montenegro et al. (1997) and Hickson (1993) show two
other nuclei for this galaxy. However, we could not find any
equivalent components in H.
The continuum map shows
significantly weaker continuum than for the two other members. The
velocity field shows regular isovelocities except to the northern
side, located close to the knot quoted as "A'' by Verdes-Montenegro
et al. (1997). The RC we present shows a nice match between the
two sides till 5 arcsec (2.8 kpc), after which the receding side
velocities present a large gradient (at the position of knot A)
and the velocities in the approaching side remain almost constant.
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Figure 27: HCG 96d: same as Fig. 2. |
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One of the main reasons why it is important to have the full kinematic information for each compact-group member is to have some insight about the possible interaction history of the galaxy. Mendes de Oliveira et al. (1998) described several interaction indicators and listed those present in the galaxies studied by them. Similarly, in Table 5, we list different indicators for all the galaxies studied here. Different interaction scenarios, depending on the strength of the encounter and the morphological types of the interacting systems, will leave different signatures on the velocity fields of the galaxies. Indicators like highly disturbed velocity field, double nuclei and double kinematic gas component are used to show evidences for strong interactions or merger. Other indicators such as warping, stellar and gas major axis misalignment, tidal tail, high IR luminosity and central activity are used to show indications of collisions which do not lead to merging. We have added here a new indicator: the high discrepancy between both sides of the rotation curve. It could appear somewhat redundant with the velocity field as a purely circular rotation curve is the zero momentum of the velocity field; nevertheless, exactly for that reason, it is easier to analyze. In some cases, the rotation curve can be rather symmetric and the velocity field highly disturbed, the reverse being much less common. Indeed, if the potential well is axisymmetric, the velocity field is regular and symmetric and both sides of the rotation curves match. What do we mean by highly disturbed velocity field and rotation curve? Non axisymmetric potential wells are observed in isolated as well is interacting galaxies. In isolated galaxies, it is due to "intrinsic'' reasons: e.g. the presence of a non axisymmetric structure like a bar, a triaxial bulge and/or a halo, a warp of the disk. In interacting galaxies, non axisymmetric potential wells could naturally also be a consequence of "intrinsic'' features but could also be due to external interactions with other galaxies like tidal forces and merging. Moreover, these "intrinsic'' features, when they are already present in a galaxy, they are usually amplified by the interaction and can be induced if they were not present. Nevertheless, in the "intrinsic'' case, the distortions of the velocity field are relatively weak and, even if both sides of the rotation curve do not match, the disagreement is generally relatively weak. In contrast, in the case of strong interactions and merging, the velocity field could be highly disturbed and the disagreement between both sides of the rotation curve could be large. A two-body tidal interaction like the one produced by the system Earth-Moon is supposed to add an axisymmetric component to the resulting force and the distortions in the rotation curve should be axisymmetric as well. In galactic tidal interactions, the problem is different because the time of propagation of the interaction as well as the duration of the event have to be taken into account. These parameters lead to non axisymmetric morphology and kinematics as described thirty years ago by the first simulations by Toomre & Toomre (1972). As a consequence, strong tidal interaction and a fortiori merging (interpenetration of the stellar disks which induces torques) lead to highly disturbed velocity field and rotation curves. In that case our corresponding "indicator'' will be "on''. Therefore, the degrees of disturbance of the velocity field and of the rotation curves are excellent indicators of the degree of interaction and merging. Nevertheless, one should keep in mind that an unique classification and identification pointing unambiguously to the cause of some kind of interaction from outside or from inside the galaxy is a difficult problem who is addressed here with caution.
In Table 5 we list the nine indicators for interactions and merging and we mark which galaxies show evidence of such an indicator.
We observed three of the four members of HCG 10 (HCG 10a, HCG 10c
and HCG 10d). HCG 10a should be taken with caution due to the
weakness of the
emission for this galaxy. HCG 10c shows
six indicators for interactions. Only one is in favour of a merger
process. Also the misalignment between gas and stellar major axis
indicates that this member had an interaction in the past. On the
other hand, HCG 10d does not show any indicators for a past or
present interaction. HCG 10 a and c show extended spiral arms that
could be in fact tidal tails. HCG 10c is the most disturbed of the
observed galaxies, suggesting it is the most evolved of the group
in contrast with HCG 10d, which seems to be untouched and which
could be a new object entering the group.
We observed the three accordant members of HCG 19 (HCC 19a, HCC 19b and HCC 19c). HCG 19a and HCG 19b show respectively four and five indicators in favour for past/ongoing interactions and HCG 19c only three. The common property of HCG 19a and HCG 19b is the very weak emission they show. Also both show disturbed velocity fields and HCG 19c, even if its velocity field is regular, has a disturbed extension to the west. None of the group members show evidence that it may be the result of a merger. HCG 19c may play the role of a gas tank fuelling HCG 19a. Despite the relative low number of positive interaction indicators, HCG 19 seems to be an evolved group: HCG 19b may have gone through a starburst phase consuming the HI around it and HCG 19c could be in the process of fuelling HCG 19a with gas.
We observed two of the four members of HCG 87 (HCC 87a and HCC 87c), HCG 87b does not show gas in emission and HCG 87d is a background object. This group shows evidences for an interaction scenario but not for a merging one. Nevertheless, HCG 87c has five positive indicators for interaction and the central activity for the three group members could be the result of gas exchange between the galaxies. HCG 87a is difficult to analyze because of the absorption due to the dust and the projection effects.
We observed three of the four members of HCG 91 (HCC 91a, HCG 91c and HCC 91d). HCG 91a exhibits a large tail in direction of HCG 91c and this galaxy presents six positive indicators for past/ongoing interactions. The velocity field is strongly disturbed due to a huge tail having an almost constant velocity. On the other hand HCG 91c shows a double gaseous component strongly suggesting a past interaction with four positive indicators. We could not find emission in HCG 91d neither a connection of this galaxy with HCG 91a. A probable scenario for this group is the passage of HCG 91c through the group forming the tail of HCG 91a. The HI distribution published by Barnes & Webster (2001) seems to support this idea, because it shows a bridge between the two galaxies.
We observed three of the four members of HCG 96 (HCC 96a, HCG 96c and HCC 96d). HCG 96a shows seven positive indicators for past or present interaction. There is a connection between HCG 96a and HCG 96c. HCG 96c and HCG 96d show four positive indicators. These three galaxies present highly disturbed velocity fields and HCG 96d shows a central double nuclei. This argues in favor of this group being highly evolved.
In a following paper, the data presented here will be interpreted, along with those shown in other previous publications which analyzed other compact group galaxies (Amram et al. 1998; Mendes de Oliveira et al. 1998, 2001; Plana et al. 1998, 1999, 2000, 2003) in order to determine an evolutionary sequence of groups in different dynamical stages, in an attempt to date compact groups, from the least evolved ones to those close to total merging.
Table 5:
Interaction Indicators. Description of the columns:
(1) Highly disturbed velocity field.
(2) Disagreement between both sides of the rotation curve.
(3) Central double nuclei.
(4) Double kinematic gas component.
(5) Changing position angle (PA) along major axis (MA). Rotation of external isophotes.
(6) Gaseous vs stellar MA misalignment. The indicator is "on'' if the ranges of values do not intersect.
(7) Tidal Tail, shell, filaments.
(8) High
.
(9) Central activity.
The symbol "+'' means that the indicator is "on'' otherwise the
symbol is "-''. The symbol
means that the data do not
allow measurements.
Acknowledgements
The authors thank J. L. Gach for helping during the observations. The authors also thank the referee for his/her comments. H.P. acknolwedges the financial support of the Brazilian CNPQ, under contract 150089/98-8. H.P. thanks Brazillian PRONEX program, for financial help to attend the conference "Galaxies: the Third Dimension'' in Cozumel (Mexico), Dec. 3-7 2001. CMdO acknowledges funding from FAPESP, Cnpq and PRONEX. This research has made use of the NASA/IPAC Extragalactic Data Base (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Table 1: Journal of Perot-Fabry observations.
Table 2: Observational characteristics.
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Figure 1: Generic caption for Figs. 1, 3, 5, 7, 9, 11, 13, 15, 18 and 23: (Top) the upper left panel represents the continuum map (see text for details) onto which the stellar axis (SA) and the axis determined from the monochromatic map (GA) are superimposed. The SA is obtained by fitting the external isophotes of the continuum image. The lower left panel is the monochromatic map onto which the same axes (SA and GA) are plotted. The right panel shows the velocity field (in bold) superimposed onto the monochromatic map. SA and GA are also plotted here. The kinematic center is represented by a cross. |
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Figure 3: HCG 10c: same as Fig. 1. |
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Figure 5: HCG 10d: same as Fig. 1. The galaxy is not centered in the window, since it was at the edge of the field of view of the receptor. Furthermore, the southern regions of the monochromatic image and of the velocity field as well as the approaching side of the rotation curve could be slightly truncated as it can been suspected from the continuum image which does not fit in the window. |
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Figure 7: HCG 19a: same as Fig. 1. |
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Figure 9: HCG 19b: same as Fig. 1. |
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Figure 11: HCG 19c: same as Fig. 1. |
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Figure 13: HCG 87a: same as Fig. 1. |
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Figure 15: HCG 87c: same as Fig. 1. |
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Figure 23: HCG 96a: same as Fig. 1. |
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