Issue |
A&A
Volume 512, March-April 2010
|
|
---|---|---|
Article Number | A36 | |
Number of page(s) | 15 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200913591 | |
Published online | 25 March 2010 |
The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals
B. Vollmer1 - M. Soida2 - A. Chung3 - R. Beck4 - M. Urbanik2 - K. T. Chyzy2 - K. Otmianowska-Mazur2 - J. H. van Gorkom5
1 - CDS, Observatoire astronomique de Strasbourg, UMR 7550, 11 rue de
l'université, 67000 Strasbourg, France
2 - Astronomical Observatory, Jagiellonian University, Kraków, Poland
3 - Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge,
MA 02138, USA
4 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
5 - Department of Astronomy, Columbia University, 538 West 120th
Street, New York, NY 10027, USA
Received 3 November 2009 / Accepted 11 January 2010
Abstract
The influence of the environment on the polarized and total power
radio continuum emission of cluster spiral galaxies is investigated.
We present deep scaled array VLA 20 and 6 cm observations
including polarization
of 8 Virgo spiral galaxies. These data are combined with existing
optical, H I, and H
data. Ram pressure compression leads to sharp edges of the total power
distribution
at one side of the galactic disk. These edges coincide with H I
edges.
In edge-on galaxies the extraplanar radio emission can extend further
than the H I
emission. In the same galaxies asymmetric gradients in the degree of
polarization give additional
information on the ram pressure wind direction. The local total power
emission is not sensitive to the effects of ram
pressure. The radio continuum spectrum might flatten in the compressed
region only
for very strong ram pressure.
This implies that neither the local star formation rate
nor the turbulent small-scale magnetic field are significantly
affected by ram pressure. Ram pressure compression
occurs mainly on large scales (
1 kpc)
and is primarily detectable in
polarized radio continuum emission.
Key words: galaxies: interactions - galaxies: ISM - galaxies: magnetic fields - radio continuum: galaxies
1 Introduction
Radio continuum emission is due to relativistic electrons of density
gyrating
around the interstellar magnetic field B:
,
where S is the intensity of synchrotron emission
and
is the component of the total magnetic field in the sky plane. The
galactic magnetic field can be divided
into a small-scale and large-scale component compared to the
resolution of the radio continuum observations, which is typically
about 1 kpc in nearby galaxies. The small-scale magnetic field
is
due to turbulent gas motions and is therefore tangled. The
large-scale magnetic field is due to a galactic dynamo. Polarized
emission is due to the regularly
oriented, large-scale magnetic field, but it can also be caused by an
alignment of anisotropic small-scale magnetic fields, produced by
stretching and compression of
small-scale magnetic structures. While the large-scale unidirectional
fields yield a non-zero Faraday rotation, this is not
the case for aligned anisotropic small-scale fields.
The small-scale magnetic field is typically a factor of
2-5 larger
than the regular large-scale magnetic field in spiral arms and
1-2 times larger in the interarm regions (Beck 2001). Whenever
there is
enhanced turbulence due to enhanced star formation, the small-scale
magnetic field is increased and the large-scale magnetic field is
diminished (see, e.g., Beck 2007).
Table 1: Integration times and rms.
Deep VLA observations at 6 cm have shown that
the
distribution of polarized radio continuum emission of Virgo cluster
spiral galaxies is strongly asymmetric, with elongated ridges
located in the outer galactic disk (Vollmer et al. 2004a,b; Chyzy
et al. 2006,
2007; Vollmer
et al. 2007).
These features
are not found in similar observations of field galaxies, where the
distribution of
6 cm
polarized emission is generally
relatively symmetric and strongest in the interarm regions (Beck
2005). The
polarized radio continuum emission is sensitive to
compression and shear motions within the galactic disks occurring
during the interaction between the galaxy and its cluster
environment. These interactions can be of tidal nature (with the
cluster
potential: Byrd & Valtonen 1990; Valluri 1993, rapid
flybys of
massive galaxies, galaxy ``harassment'': Moore et al. 1998) or
hydrodynamic nature (ram pressure stripping: Gunn & Gott 1972).
On the other hand, total radio continuum emission is sensitive
to
star formation which gives rise to the radio-FIR correlation
(see, e.g. Murphy et al. 2008).
Based on 1.4 GHz radio continuum observations Gavazzi
et al. (1991)
showed that this correlation that is
shared by spiral galaxies in a huge luminosity interval, is different
for cluster and isolated galaxies: the cluster galaxies have an
increased
radio/FIR ratio. A similar increase has been observed at
4.85 GHz and 10.55 GHz (Niklas et al. 1995) for 6 out
of 45 observed Virgo spirals including NGC 4388 and
NGC 4438.
Gavazzi & Boselli (1999)
studied the
distribution of the radio/NIR luminosity (RLF) of late-type galaxies
in 5 nearby galaxy clusters. They found that the RLF of Cancer,
ACO 262 and Virgo are consistent with that of isolated
galaxies.
However, galaxies in ACO 1367 and Coma have their radio
emissivity
enhanced by a factor 5
with respect to isolated objects.
Multiple systems in the Coma cluster bridge also show an enhanced
radio/NIR emission. Gavazzi & Boselli (1999) argue
that the latter
effect is due to increased star formation caused by tidal
interactions, whereas the enhanced radio/NIR ratio in ACO 1367
and Coma is due to ram pressure compression of the magnetic field.
Murphy et al. (2009)
investigated the radio-FIR relation of Virgo
cluster galaxies. They showed that ram pressure affected galaxies
have global radio flux densities that are enhanced by a factor of 2-3
compared to isolated galaxies.
The typical spectral slope of synchrotron emission from spiral
galaxies
between 1.4 GHz and 4.85 GHz is (Gioia
et al. 1982;
Klein 1990).
Völk & Xu (1994)
proposed that a shock-induced reacceleration of relativistic electrons
during ram pressure compression might lead to an enhanced total power
emission and a flattening of the radio continuum spectrum.
However, based on integrated galaxy properties,
Niklas (1995)
and Vollmer et al. (2004a,b)
do not find any significant difference
between the mean spectral index of Virgo cluster and field spiral
galaxies.
It is still an open question if environmental interactions in a galaxy cluster can locally enhance the total power radio continuum emission of a galaxy and if they can alter the spectral index as suggested by Vollmer et al. (2004a,b) for NGC 4522 in the Virgo cluster.
In this article we present the total power radio continuum observations of 8 Virgo spiral galaxies observed by Vollmer et al. (2007) in polarization. These galaxies were carefully selected based on the following criteria: (i) they show signs of interaction with the cluster environment such as tidal interactions and/or ram pressure stripping; (ii) VIVA H I data (Chung et al. 2009) are available; and (iii) their 6 cm total power emission is strong. In previous work we elaborated interaction scenarios for most of the galaxies in our sample.
Deep VLA 20 and 6 cm observations including
polarization are presented in Sect. 2. In
Sect. 3
we present for each galaxy the
(i) 6 cm total power emission distribution superposed
on DSS B band image together with the
apparent vectors;
(ii) 20 cm total power emission distribution on a DSS
B band image together with the apparent
vectors;
(iii) 6 cm polarized emission distribution on the H I distribution;
(iv) H
emission
distribution (from Goldmine et al. 2003) on the spectral index
map; and (v) 6 cm polarized emission distribution on
the degree of polarization.
The galaxy properties at different wavelengths are compared to each
other in Sect. 4.
We discuss our results in
Sect. 5
and give our conclusions in Sect. 6.
2 Observations
The 8 Virgo spiral galaxies were observed at 4.85 GHz between
November 8, 2005
and January 10, 2006 with the Very Large Array (VLA) of the National
Radio Astronomy Observatory (NRAO)
in the D array configuration. The
band passes were
MHz.
We used 3C286 as the flux
calibrator and 1254+116 as the phase calibrator, the latter of which
was observed every 40 min. Maps were made for both wavelengths
using
the AIPS task IMAGR with ROBUST = 3. The final
cleaned maps were
convolved to a beam size of
.
The bright radio
source M 87 caused sidelobe effects enhancing the rms noise
level of
NGC 4438. In addition, we observed the 8 galaxies at
1.4 GHz on August 15, 2005
in the C array configuration.
The band passes were
MHz.
We used the same calibrators as
for the 4.85 GHz observations. The final cleaned maps were
convolved to a beam size of
.
The rms levels of the 20 and 6 cm total power and
polarized intensity data are shown in Table 1.
We obtain apparent
vectors
by rotating the observed
vector
by
,
uncorrected for Faraday rotation.
For NGC 4388, NGC 4402, NGC 4438, and NGC 4501 our 20 cm total power data have artifacts most likely due to the tenuous UV coverage. We therefore preferred the VIVA H I 20 cm continuum images (Chung et al. 2009; for the more sophisticated data reduction of NGC 4438 see Vollmer et al. 2009).
3 Results
3.1 Galaxy properties
3.1.1 NGC 4321
![]() |
Figure 1:
NGC 4321: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
NGC 4321 is the only galaxy in our sample which shows
relatively symmetric emission
distributions at all wavelengths as it is observed in field spiral
galaxies (Fig. 1).
The 6 cm and 20 cm total power distributions follow
the stellar and gas distribution.
The radio continuum spectrum is flat (
with
)
in the regions of the main spiral arms and steep (
)
elsewhere.
The polarized emission and the degree of polarization are highest in
the
interarm regions.
In addition, the distribution of the degree of polarization shows an
azimuthally symmetric radial gradient with a higher degree of
polarization towards the outer disk. All characteristics cited above
are typical for an unperturbed spiral galaxy (Beck 2005,
and references therein).
The regions of highest degrees of polarization (up to 40%) are found
at the western edge of the polarized emission distribution.
![]() |
Figure 2:
NGC 4388: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
3.1.2 NGC 4388
NGC 4388 harbors a Seyfert 2 nucleus with an
associated bright radio source and a 15''-long jet to the north (Hummel
& Saikia 1991).
The disk and jet emission perpendicular to the galactic plane has
already been detected at 6 cm by Hummel et al. (1983).
In Fig. 2
we observe emission from the nuclear outflow in polarized 6 cm
emission with vertical magnetic field vectors north of the nucleus.
The radio continuum disk emission at 20 cm and 6 cm
and the H I emission
distribution are truncated at about half the optical radius. These
emission distributions are more extended to the east
than to the west. The 6 cm total power extension perpendicular
to the
major axis east of the nucleus needs confirmation. It is present in
observations made on two separate days.
We do not detect a disk-wide radio halo.
The eastern 6 cm polarized emission maximum is located 20''south of
the disk plane, on the edge of the H I
emission.
The western 6 cm polarized emission maximum is slightly
elongated
parallel to the minor axis.
The radio continuum spectrum is flat (
)
everywhere in the disk. We observe an asymmetric distribution of the
degree
of polarization at 6 cm with 10% of polarization at
the southeastern edge, whereas the degree of polarization is
much less at the northern edge of the galactic disk.
3.1.3 NGC 4396
The 20 cm and 6 cm total power emission of NGC 4396 is truncated within the optical disk and shows a tail structure to the northwest (Fig. 3). As the H I tail, the radio continuum tail is bend to the north, but it is less extended than the H I tail. We do not detect a disk-wide radio halo. The 6 cm polarized emission is mainly found in the southeast of the galactic disk extending into the tail. Faint polarized emission perpendicular to the galactic disk is found in the eastern region. The radio continuum spectrum is flat over the whole starforming disk of NGC 4396. The distribution of the degree of polarization at 6 cm increases towards the tail with a maximum value of 30% at the northwestern tip. It also increases to the north in the eastern polarized emission feature.
3.1.4 NGC 4402
Table 2: Integrated flux densities.
![]() |
Figure 3:
NGC 4396: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
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![]() |
Figure 4:
NGC 4402: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
As the H I emission distribution, the 20 cm and 6 cm total power emission distribution are truncated at about half of the optical radius (Chung et al. 2009, Fig. 4). In addition, we observe a disk-wide extension to the north, whereas the southern edge of the emission distributions is sharp (see also Crowl et al. 2005). Although the total power disk emission is symmetric along the major axis, the 6 cm polarized emission is more extended to the west than to the east. Moreover, we find faint extraplanar polarized emission northeast of the galactic disk. The extended extraplanar radio continuum emission north of the disk shows a steepening of the radio continuum spectrum. The distribution of the degree of polarization has a north-south and an east-west gradient, the latter being dominant. At the southern edge the degree of polarization is 10%, whereas it increases up to 40% at the western edge of the galactic disk.
3.1.5 NGC 4438
![]() |
Figure 5:
NGC 4438: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
NGC 4438 harbors a Seyfert 2 nucleus with an associated strong
nuclear radio source
(Hummel & Saikia 1991).
In addition, we observe prominent extraplanar total power emission at
20 cm and 6 cm extending further than emission at
other wavelengths (Fig. 5).
The 6 cm polarized extraplanar emission has a shell-like
distribution with a
pronounced maximum in the south. We also observe polarized emission
from the galactic disk south from the nucleus.
The spectral index of the western extraplanar emission region of
NGC 4438 is constant. Its value is uncertain because of
M 87's
strong sidelobes at 20 cm which makes the
20 cm flux density uncertain. In Vollmer et al. (2009) we argue
that we miss a substantial part of the flux
density of NGC 4438 at 20 cm due to these sidelobes
and that
the spectral index is most likely
,
i.e. the typical
value for synchrotron emission. There is a positive gradient of the
degree of
polarization towards the border of the extraplanar synchrotron
emission where the degree of polarization increases to
30%. The
southern half of the extraplanar
radio emission shows a degree of polarization of 20%.
3.1.6 NGC 4501
![]() |
Figure 6:
NGC 4501: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
As the H I distribution, the
20 cm and 6 cm total power emission distributions are
truncated at about the optical radius (Fig. 6).
Whereas the southwestern edge is sharp, there is extended emission to
the northeast. The 6 cm polarized emission shows a long ridge
extending over
almost the entire south western edge of the galactic disk.
There is a secondary maximum north of the nucleus.
Whereas the radio continuum spectrum is flat (
)
in the northeastern spiral arm,
it is steep (
)
in the southwestern spiral arms.
We observe a general steepening of the spectrum to the southeast
of the galactic disk.
The degree of polarization shows an asymmetric distribution. It
increases towards the
southwestern and northeastern edges of the galactic disk. Whereas it
rises to 20% in the northeast, it increases to 30% in the southwest.
3.1.7 NGC 4535
![]() |
Figure 7:
NGC 4535: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
The 20 cm and 6 cm total power emission extends to 0.7 times
the optical radius, well inside the H I distribution
(Fig. 7).
The total power emission associated with the western optical arm is
stronger than that of the rest of the disk.
Polarized 6 cm continuum emission is only detected in the
region of the western optical arm. The radio continuum spectrum is flat
(
)
in the regions of the starforming
spiral arms and steep (
)
elsewhere.
A high degree of polarization (
40%) is found in the southern
part of the polarized emission arm.
3.1.8 NGC 4654
![]() |
Figure 8:
NGC 4654: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER |
The 20 cm and 6 cm total power emission roughly
follows the stellar
distribution of the galactic disk (Fig. 8). Along the
major axis, the emission
is stronger to the northwest than to the southeast.
We observe a sharp edge of the emission distribution to the
northwest and some faint extended emission in the direction of the
prominent
southeastern H I tail. The 6 cm
polarized emission distribution is
asymmetric with a maximum south of the galaxy center well within the
H I distribution. The radio
continuum spectrum is flat (
)
in the regions of the starforming spiral arms and steep (
)
elsewhere.
The degree of 6 cm polarization increase to
20% towards
the southern edge of the polarized emission distribution.
The highest degrees of polarization (
30%) are found at the south
eastern tip.
4 Comparison between galaxies
In this section we compare the total power distributions at 20/6 cm to the gas and stellar disks followed by the polarized radio continuum emission and the spectral index. The degree of polarization at 6 cm and the rotation measure between 20 and 6 cm are presented at the end of this section.
4.1 Total power emission
The total power emission is a mixture of synchrotron and thermal emission. The intensity of the synchrotron emission depends on the density of cosmic-ray electrons and the square of the total magnetic field in the sky plane. The density of thermal and of cosmic ray electrons depends on the star formation activity of the galaxy. In an isolated spiral galaxy the total magnetic field is dominated by the turbulent small-scale component. As the result of ram pressure compression the total power emission can be locally enhanced by (i) an increased local star formation rate and/or (ii) compression of the magnetic field.
Only NGC 4321 shows a symmetric total power distribution as it is observed in unperturbed field spiral galaxies. The 20 cm and 6 cm total power distributions of two (NGC 4501, NGC 4654) out of three mildly inclined spiral galaxies show sharp edges with steep gradients at one side of the disk. These edges coincide with H I edges and are due to ram pressure (Soida et al. 2006; Vollmer et al. 2008). Murphy et al. (2009) combined Spitzer FIR and VLA 20 cm radio continuum imaging to study the FIR-radio correlation in Virgo spiral galaxies. For 6 out of 10 sample galaxies, they found regions along their outer edges that are highly deficient in the radio compared with models relying on the FIR-radio correlation of field galaxies. The observed sharp edges in the total power distribution might be linked to this phenomenon of radio deficient edges.
We observe asymmetric extraplanar emission in 3 out
of 4 edge-on
galaxies (NGC 4396, NGC 4402, NGC 4438; see
Sect. 5.1).
Whereas the extraplanar
radio continuum emission follows the H I
emission in the northwest of
NGC 4396, it is more extended than the H I
emission in NGC 4402
and NGC 4438. At our limiting flux density level there is no
total power emission associated with the
ionized nuclear outflow in NGC 4388 (Veilleux et al. 1999). In all
moderately inclined galaxies (NGC 4321, NGC 4501,
NGC 4535, NGC 4654)
the 20 cm and 6 cm emission follow the recent massive
star formation as
observed in the H
line. The
southeastern H I tail of NGC 4654
(Phookun & Mundy 1995)
has
no radio continuum counterpart. This is most probably due to the
lack of star formation in this tail.
4.2 Polarized continuum and H I emission
Polarized continuum emission is caused by the regular large-scale magnetic field. Polarized emission can be enhanced by large-scale shear or compression motions.
Only NGC 4321 has a relatively symmetric polarized emission distribution which is highest in the interarm regions. All other galaxies have asymmetric elongated ridges of polarized emission located in the outer parts of the galactic disks (Vollmer et al. 2007). These ridges are located within the H I distribution, except for NGC 4438, where the western local maximum of polarized emission is located within the molecular gas disk (Vollmer et al. 2005). Most of the ridges are close to the outer edge of the H I distribution. Especially in NGC 4501 the H I and polarized emission distributions show a sharp edge to the southeast where ram pressure acts on the galaxy (see Vollmer et al. 2008). Such a sharp edge is also observed in the northeast of NGC 4654 (see Soida et al. 2006). Only the southern polarized ridge of NGC 4654 and that of NGC 4535 are well inside the H I distribution. Given the asymmetric velocity field of NGC 4535 in the region of the maximum of polarized radio continuum emission (Chung et al. 2009), the enhancement of the polarized emission is most probably due to shear motions. Moreover, all edge-on galaxies show extraplanar polarized emission which extends further than the H I emission. In NGC 4388 this extraplanar polarized emission is probably due to the AGN outflow (Veilleux et al. 1999). In NGC 4396 it is located on the southeast and extends to the north. In NGC 4402 it is located in the western side of the disk where the action of ram pressure is maximum (Crowl et al. 2005). Whereas the extraplanar polarized emission is faint in NGC 4396 and NGC 4402, it is prominent in NGC 4438, extending further than emission at any other wavelength.
The polarized emission at 20 cm suffers severe Faraday rotation and depolarization, especially in edge-on galaxies. We will use this emission only to calculate the rotation measure in Sect. 4.5.
4.3 Spectral index
As a general trend, we find flat radio continuum spectra
(
)
associated with regions of recent massive star
formation, indicating an enhanced fraction of thermal electrons. This
corresponds to the
classical behavior of unperturbed spiral galaxies (e.g., Sukumar
et al. 1987;
Berkhuijsen et al. 2003;
Tabatabaei et al. 2007a,b;
Heesen et al. 2009).
For example, in NGC 4396 the radio continuum spectrum
of the northwestern extraplanar emission region is flat (
), because of its active star
formation.
On the other hand, the extraplanar radio continuum emission north of
NGC 4402 shows a steepening of the spectrum due to the aging
of the relativistic electrons. This is consistent with the scenario
of Crowl et al. (2005)
where it is assumed that the radio halo is compressed on the southern
side and pushed out of the galactic disk on the
northern side. Surprisingly, the same steepening of the spectrum
is seen on the southwestern side of NGC 4501 where ram
pressure compresses the gas (Vollmer et al. 2008). This is
contrary
to what we found in NGC 4522 (Vollmer et al. 2004a,b) where
we observed
a spectral flattening in the compressed region. Thus, in our sample we
do not observe a flattening of the spectrum
in regions of enhanced polarized emission. A shock-induced
reacceleration of
relativistic electrons as proposed by Völk & Xu (1994) is still a
probable explanation for the flat spectrum associated to the
ridge of polarized emission in NGC 4522.
4.4 Degree of polarization
The degree of polarization is defined as the ratio between polarized and total power (mostly synchrotron) emission. The degree of polarization is a measure for the fraction of the regularly oriented, large-scale magnetic field with respect to the total magnetic field.
In our galaxy sample the degree of polarization varies
between 10 and 40%. In the face-on galaxy
NGC 4321 the degree of polarization is highest in the
interarm regions as it is expected for an unperturbed galaxy.
Moreover, we observe an azimuthally symmetric radial gradient
as it is observed in field spirals (e.g., M 83: Neininger
et al. 1993;
NGC 6946: Ehle & Beck 1993). As an
example for an edge-on spiral galaxy,
Dumke & Krause (1998)
combined 6 cm Effelsberg and VLA data of NGC 891
(optical radius R25=13.5').
They found a symmetrically increasing degree of polarization towards
the
edges of the emission distribution to 10%.
In our sample, NGC 4388, NGC 4396, and NGC 4402 show vertically asymmetric gradients of the degree of polarization. In addition, NGC 4396, NGC 4402, NGC 4501, and NGC 4654 show azimuthally asymmetric distributions of the degree of polarization. The rising degree of polarization towards the tails in NGC 4396 and NGC 4654 might be due to magnetic field ordering or shear motions in the gas which is pulled away from or re-accreting onto the galactic disk. Thus 5 out of 8 sample galaxies show these asymmetries. This is different from the behavior of field spirals (Beck 2005 and references therein; e.g. Neininger 1992; Sukumar & Allen 1991) and is thus most probably due to the interaction between the galaxy and the cluster environment.
4.5 Rotation measure
Table 3: Polarized intensity weighted mean values of the rotation measure distribution in rad/m2.
The orientation of polarization vectors is changed in a magnetic plasma by Faraday rotation that is proportional to the line-of-sight integral over the density of thermal electrons multiplied by the strength of the regular field component along the line of sight. Aligned anisotropic fields do not lead to Faraday rotation.The rotation measure was calculated with polarization angle
maps
which were clipped at
in polarized emission.
This leads to a maximum uncertainty of the rotation measure of
16 rad/m2.
Since the 20 cm data suffer from
severe Faraday depolarization, we detect significant polarized
emission at 20 cm only in NGC 4321,
NGC 4501, NGC 4396, NGC 4535,
and NGC 4654.
The intensity-weighted mean values of the rotation measure for our
galaxies are presented in Table 3.
Since the rotation
measure is caused by the regular magnetic field component along the
line-of-sight, we expect the rotation measure from an axisymmetric
field
to change sign from
one side of the major axis to the other. This is the reason why we
give two values for the NGC 4654 which is the only galaxy
where we
can significantly determine rotation measures on both sides of the
minor axis.
In general, all face-on galaxies show detectable rotation measures
and we find rotation measures between -7
and 10 rad/m2. However, since
the difference in frequency is large
and Faraday depolarization can be strong at 20 cm, one has to
take these measurements with caution. The actual rotation measures
are most probably much larger. For comparison,
Wezgowiec et al. (2007)
report rotation measures between 4.85
and 10.45 GHz obtained from Effelsberg observations
of +14-+46 rad/m2 for NGC 4501
and -57/+60 rad/m2 for
NGC 4654.
Ultimately, we would like to observe these galaxies at 3 cm to
produce more reliable rotation measure maps. These will help us to
discriminate between the 2 creation scenarios of the polarized
ridges: in the case of shear or compression of a random field, the
resulting ordered field is anisotropic and cannot produce Faraday
rotation.
A detectable rotation measure would show regular large-scale fields
and thus help to distinguish them from anisotropic ones. A high
rotation measure in regions of the ridges of enhanced polarized
emission might indicate that a pre-existing large-scale field has
been amplified by compression or shear.
5 Discussion
5.1 Disk-wide radio halos
Only NGC 4402 out of three edge-on galaxies shows a disk-wide
radio halo
that is compressed on one side and extends to 3.7 kpc
above the disk plane on the other side (Crowl et al. 2005).
With increasing distance from the galactic disk the total
power surface brightness decreases and the radio continuum spectrum
steepens
due to the aging of relativistic electrons generated in the galactic
disk. This halo shows extraplanar polarized emission at the eastern
and western extremities and vertical magnetic field lines in the east,
unlike classical X-structures (Beck 2005 and
references therein; e.g. Tüllmann et al. 2000),
supporting the scenario of a ram pressure pushed/stripped synchrotron
halo.
Dahlem et al. (2006) showed that disk-wide radio halos exist in galaxies with a low mean mass surface density and a high mean energy input per unit surface area (Fig. 7 of Dahlem et al. 2006). To investigate if our edge-on spirals should host a radio halo, we calculated the mean stellar mass surface density using the H band magnitudes and optical diameters from Goldmine. For the mean energy input per unit surface area we use the 20 cm flux densities and radial extents. For NGC 4388 we subtracted the emission from the active nucleus. Figure 9 shows the border between galaxies with and without radio halos together with the data points of our 3 edge-on galaxies. Based on this plot NGC 4388 and NGC 4402 are expected to show a disk-wide radio halo, but only NGC 4402 does. A radio halo probably exists in NGC 4388, but may have been missed due to the higher noise in our maps compared to NGC 4402. If this is not the case, the pressure of the intracluster medium, that confines the radio halo might be higher for NGC 4402 than for NGC 4388.
![]() |
Figure 9: Mean stellar mass density as a function of the mean energy input per unit surface area. The solid line roughly indicates the transition zone between galaxies with and without radio halos (Dahlem et al. 2006). The error bars represent uncertainties of a factor of 2. |
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5.2 Extraplanar radio continuum emission can extend further than the H I emission
In our sample of 8 Virgo spiral galaxies we have two examples of asymmetric extraplanar total power emission extending further than the H I emission: NGC 4402, and NGC 4438. For comparison, in NGC 891 the distribution of neutral hydrogen has a vertical extent comparable to that of the radio halo (Oosterloo et al. 2007). This is not the case for NGC 4402 (Fig. 10) where the radio halo is more extended to the north than the H I distribution.
![]() |
Figure 10: NGC 4402: vertical profiles to the north of the 20 cm continuum (solid line) and H I (dashed line) emission distribution at a resolution of 20''. |
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The extraplanar radio continuum emission of NGC 4438, located up to 5 kpc from the galactic disk, is unique in the sense that it has an almost constant surface brightness and spectral index (Vollmer et al. 2009).
NGC 4396 and NGC 4402 show extraplanar
6 cm polarized emission in the southwestern
(NGC 4396) and northeastern (NGC 4402) part of the
disk extending 2.5 kpc
above the galactic plane. For NGC 4396 its association to the
extraplanar total
power emission is uncertain, because a relatively bright point
source occupies the same region. In NGC 4402 this extraplanar
polarized emission
is associated with total power emission from the radio halo.
NGC 4396 thus resembles NGC 4402, but it lacks a
detectable radio halo, because of its low mean star formation rate
(Fig. 9).
The radio continuum emission of the galaxies in our sample and
in the
literature (i) is more extended than the extraplanar H
and H I emission in
NGC 4402, NGC 4438, and NGC 4569 (Chyzy
et al. 2006);
(ii) follows the H
and H I emission in
NGC 4501, and NGC 4522 (Vollmer et al. 2004a,b);
and (iii) is less extended than the H I
emission in NGC 4535 and NGC 4654.
The asymmetric extraplanar radio continuum emission extending further
than the H I emission
may be due to (i) the ionization of ram pressure
stripped H I;
(ii) more efficient stripping of the cosmic ray gas with its
associated magnetic field, or (iii) nuclear outflows.
5.3 Ram pressure compression does not change the local total power emission nor the spectral index
In all galaxies with asymmetric polarized emission ridges we do not
detect significant enhanced total power emission associated with
these ridges (Sect. 4.1).
This means that the local
star formation and the small-scale turbulent magnetic field, which
is sensitive to star formation in unperturbed galaxies, are not
affected by the external ram pressure compression. Murphy
et al. (2009)
came to the same conclusion investigating the radio-FIR relation
of perturbed Virgo spiral galaxies based on Spitzer 70 m and
VIVA 20 cm data. They found no local but a slight global radio
excess (by a factor of
2)
in NGC 4330, NGC 4388, and
NGC 4522. We suggest that this excess is due to a slight
compression of the
small-scale turbulent magnetic field.
On the other hand, the large-scale magnetic field and thus the
polarized emission is influenced by the external ram pressure
compression.
This compression leads to the observed asymmetric ridges of polarized
emission.
Our result, that the small-scale magnetic field is not significantly
compressed whereas the large-scale field is, can be understood in terms
of relevant timescales.
Whereas the timescale for ram pressure stripping is several
10 Myr
(see, e.g., Vollmer et al. 2001),
the free fall time of molecular clouds leading to star formation is
several Myr (see, e.g., Krumholz & Tan 2007). Thus
the dynamics of the mainly
molecular gas in the star forming spiral arms are decoupled from the
overall large-scale (1 kpc) motions.
In the same line, the spectral index between 20 and 6 cm in our sample galaxies only depends on the local star formation rate, as it is the case for unperturbed galaxies (Sukumar et al. 1987; Berkhuijsen et al. 2003; Tabatabaei et al. 2007a,b; Heesen et al. 2009), Sect. 4.3. This is contrary to what we found in NGC 4522 (Vollmer et al. 2004a,b) where the radio continuum spectrum flattens toward the edge of the disk where the gas is compressed. Since NGC 4522 is close to peak ram pressure (Vollmer et al. 2006), a flattening of the spectrum in the compressed region might only occur in galaxies undergoing very strong ram pressure stripping. Unfortunately, the quality of the 20 cm data of the second galaxy in our sample which is close to peak ram pressure, NGC 4438, does not permit to determine the spectral index within the galactic disk.
We thus conclude
that ram pressure by the intracluster medium leads to gas compression
on
large scales (1 kpc;
see also Vollmer et al. 2008 for a
detailed discussion of NGC 4501) without a significant
enhancement of
the star formation and the associated small-scale turbulent magnetic
field.
This is consistent with the results of Koopmann & Kenney (2004)
who found that only 2% of their 52 Virgo cluster spiral galaxies have
an enhanced star formation distribution. Only one galaxy shows a
truncated and
enhanced star formation distribution. NGC 4321,
NGC 4501, and NGC 4522 are classified
as having truncated normal star formation distributions,
NGC 4535 and NGC 4654
as having normal star formation distributions.
5.4 The degree of polarization gives additional information on the nature of the interaction
We suggest that the vertically and azimuthally asymmetric increase of the degree of polarization (Sect. 4.4) is due to ram pressure stripping and thus indicates the ram pressure wind direction or the direction of the galaxy's motion in a static intracluster medium. This is consistent with Murphy et al. (2009) who found radio deficits with respect to the FIR surface brightness in the regions where we observe an enhanced degree of polarization in NGC 4402 and NGC 4522 (Vollmer et al. 2004a,b). The absence of a local radio-deficit region at the southern edge of NGC 4388's disk is probably due to the radio continuum emission from the nuclear outflow (Veilleux et al. 1999).
In NGC 4388 (Vollmer & Huchtmeier 2003), NGC 4402 (Crowl et al. 2005), and NGC 4501 (Vollmer et al. 2008) the degree of polarization is highest in the direction of the ram pressure wind. In unperturbed spiral galaxies galactic rotation leads to an azimuthally symmetric large-scale magnetic field. Due to beam depolarization the degree of polarization decreases towards the edge of the galactic disk in highly inclined galaxies. We interpret the increase of the degree of polarization along the major axis in NGC 4396 and NGC 4402 as ionized ISM that deviates significantly from galactic rotation. In NGC 4402 the western maximum of polarized emission thus traces ram pressure stripped ionized gas with its associated magnetic field. This is consistent with the ram pressure scenario of Crowl et al. (2005) where the ram pressure wind comes from the southeast. By extrapolating this interpretation to NGC 4396, we suggest that the ram pressure wind direction is south.
6 Conclusions
Deep VLA 20 and 6 cm radio continuum data including
polarization of
a sample of 8 Virgo spiral galaxies are presented and combined with
optical DSS, VIVA H I (Chung
et al. 2009),
and Goldmine
H
data. We study the spatial distributions of the spectral
index, the degree of polarization, and the rotation measure and
derive the following conclusions:
- 1.
- Ram pressure leads to sharp edges of the total power distribution on one side of the galactic disk (NGC 4501, NGC 4654). The radio continuum edge coincides with the H I edge.
- 2.
- In edge-on galaxies, the extraplanar radio continuum emission (total power and polarized intensity) can extend further than the H I emission. The most prominent example is NGC 4438.
- 3.
- In the case of edge-on galaxies, we find azimuthally asymmetric distributions of the degree of polarization (NGC 4388, NGC 4396, NGC 4402). This asymmetry gives important information on the ram pressure wind direction.
- 4.
- Ram pressure does not alter the local total power emission
nor the
spectral index. Only very strong ram pressure might lead to a
flattening of
the radio continuum spectrum. This means that the local star formation
and the
small-scale turbulent magnetic field, which is sensitive to star
formation in unperturbed galaxies, are not influenced by external ram
pressure compression. We thus conclude that ram pressure by the
intracluster medium leads to compression on large-scales (
1 kpc). In addition, the absence of enhanced total power emission also implies that star formation is not significantly enhanced in the compressed regions.
This research has made use of the GOLD Mine Database. This work was supported by the Polish-French (ASTRO-LEA-PF) cooperation program and by the Polish Ministry of Sciences and Higher Education grant 3033/B/H03/2008/35. The authors would like to thank E.M. Berkhuijsen for careful reading of the manuscript.
References
- Beck, R. 2001, Space Sci. Rev., 99, 243 [NASA ADS] [CrossRef] [Google Scholar]
- Beck, R. 2005, Cosmic Magnetic Fields, ed. R. Wielebinski, & R. Beck. (Berlin: Springer), Lect. Notes Phys., 664, 41 [Google Scholar]
- Beck, R. 2007, A&A, 470, 539 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Berkhuijsen, E. M., Beck, R., & Hoernes, P. 2003, A&A, 398, 937 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Byrd, G., & Valtonen, M. 1990, ApJ, 350, 89 [NASA ADS] [CrossRef] [Google Scholar]
- Cayatte, V., van Gorkom, J. H., Balkowski, C., & Kotanyi, C. 1990, AJ, 100, 604 [NASA ADS] [CrossRef] [Google Scholar]
- Chung, A., van Gorkom, J. H., Kenney, J. D. P., Crowl, H. H., & Vollmer, B. 2009, AJ, 138, 1741 [NASA ADS] [CrossRef] [Google Scholar]
- Chyzy, K. T. 2008, A&A, 482, 755 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Chyzy, K. T., Soida, M., Bomans, D. J., et al. 2006, A&A, 447, 465 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Chyzy, K. T., Ehle, M., & Beck, R. 2007, 474, 415 [Google Scholar]
- Crowl, H. H., Kenney, J. D. P., van Gorkom, J. H., & Vollmer, B. 2005, AJ, 130, 65 [NASA ADS] [CrossRef] [Google Scholar]
- Dahlem, M., Lisenfeld, U., & Rossa, J. 2006, A&A, 457, 121 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Dumke, M., & Krause, M. 1998, ed. D. Breitschwerdt, M. J. Freyberg, & J. Truemper, Proc. IAU Colloq., 166, 555 [Google Scholar]
- Ehle, M., & Beck, R. 1993, 273, 45 [Google Scholar]
- Gavazzi, G., & Boselli, A. 1999, A&A, 343, 93 [NASA ADS] [Google Scholar]
- Gavazzi, G., Boselli, A., & Kennicutt, R. 1991, AJ, 101, 1207 [NASA ADS] [CrossRef] [Google Scholar]
- Gavazzi, G., Boselli, A., Donati, A., Franzetti, P., & Scodeggio, M. 2003, A&A, 400, 451 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Gioia, I. M., Gregorini, L., & Klein, U. 1982, A&A, 116, 164 [NASA ADS] [Google Scholar]
- Gunn, J. E., & Gott, J. R. III 1972, ApJ, 176, 1 [NASA ADS] [CrossRef] [Google Scholar]
- Heesen, V., Beck, R., Krause, M., & Dettmar, R.-J. 2009, A&A, 494, 563 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Hummel, E., & Saikia, D. J. 1991, A&A, 249, 43 [NASA ADS] [Google Scholar]
- Hummel, E., van Gorkom, J. H., & Kotanyi, C. G. 1983, ApJ, 267, L5 [NASA ADS] [CrossRef] [Google Scholar]
- Kenney, J. D. P., Rubin, V. C., Planesas, P., & Young, J. S. 1995, ApJ, 438, 135 [NASA ADS] [CrossRef] [Google Scholar]
- Klein, U. 1990, in Windows on Galaxies, ed. G. Fabbiano, J. S. Gallagher, & A. Renzini (Dordrecht: Kluwer), Ap&SS Library, 160., 157 [Google Scholar]
- Koopmann, R. A., & Kenney, J. D. P. 2004, ApJ, 613, 866 [NASA ADS] [CrossRef] [Google Scholar]
- Krumholz, M. R., & Tan, J. C. 2007, ApJ, 654, 304 [NASA ADS] [CrossRef] [Google Scholar]
- Moore, B., Lake, G., & Katz, N. 1998, ApJ, 495, 139 [NASA ADS] [CrossRef] [Google Scholar]
- Murphy, E. J., Helou, G., Kenney, J. D. P., Armus, L., & Braun, R. 2008, ApJ, 678, 828 [NASA ADS] [CrossRef] [Google Scholar]
- Murphy, E. J., Kenney, J. D. P., Helou, G., Chung, A., & Howell, J. H. 2009, ApJ, 694, 1435 [NASA ADS] [CrossRef] [Google Scholar]
- Neininger, N. 1992, A&A, 263, 30 [NASA ADS] [Google Scholar]
- Neininger, N., Beck, R., Sukumar, S., & Allen, R. J. 1993, A&A, 274, 687 [NASA ADS] [Google Scholar]
- Niklas, S., Klein, U., & Wielebinski, R. 1995, A&A, 293, 56 [NASA ADS] [Google Scholar]
- Oosterloo, T., Fraternali, F., & Sancisi, R. 2007, AJ, 134, 1019 [NASA ADS] [CrossRef] [Google Scholar]
- Phookun, B., & Mundy, L. G. 1995, ApJ, 453, 154 [NASA ADS] [CrossRef] [Google Scholar]
- Soida, M., Otmianowska-Mazur, K., Chyzy, K., & Vollmer, B. 2006, A&A, 458, 727 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Sukumar, S., & Allen, R. J. 1991, ApJ, 382, 100 [NASA ADS] [CrossRef] [Google Scholar]
- Sukumar, S., Klein, U., & Gräve, R. 1987, A&A, 184, 71 [NASA ADS] [Google Scholar]
- Tabatabaei, F. S., Krause, M., & Beck, R. 2007a, A&A, 472, 785 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Tabatabaei, F. S., Beck, R., Krgel, E., et al. 2007b, A&A, 475, 133 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Tüllmann, R., Dettmar, R.-J., Soida, M., Urbanik, M., & Rosa, J. 2000, A&A, 364, L36 [NASA ADS] [Google Scholar]
- Valluri, M. 1993, ApJ, 408, 57 [NASA ADS] [CrossRef] [Google Scholar]
- Veilleux, S., Bland-Hawthorn, J., Cecil, G., Tully, R. B., & Miller, S. T. 1999, ApJ, 520, 111 [NASA ADS] [CrossRef] [Google Scholar]
- Völk, H. J., & Xu, C. 1994, IR Phys. Tech., 35, 527 [Google Scholar]
- Vollmer, B., & Huchtmeier, W. 2003, A&A, 406, 427 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Vollmer, B., Cayatte, V., Balkowski, C., & Duschl, W. J. 2001, ApJ, 561, 708 [NASA ADS] [CrossRef] [Google Scholar]
- Vollmer, B., Thierbach, M., & Wielebinski, R. 2004a, A&A, 418, 1 [Google Scholar]
- Vollmer, B., Beck, R., Kenney, J. P. D., & van Gorkom, J. H. 2004b, AJ, 127, 3375 [NASA ADS] [CrossRef] [Google Scholar]
- Vollmer, B., Braine, J., Combes, F., & Sofue, Y. 2005, A&A, 441, 473 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Vollmer, B., Soida, M., Otmianowska-Mazur, K., et al. 2006, A&A, 453, 883 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Vollmer, B., Soida, M., Beck, R., et al. 2007, A&A, 464, L37 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Vollmer, B., Soida, M., Chung, A., et al. 2008, A&A, 483, 89 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Vollmer, B., Soida, M., Chung, A., et al. 2009, A&A, 496, 669 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Wezgowiec, M., Urbanik, M., Vollmer, B., et al. 2007, A&A, 471, 93 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
Footnotes
- ... (NRAO)
- NRAO is a facility of National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
All Tables
Table 1: Integration times and rms.
Table 2: Integrated flux densities.
Table 3: Polarized intensity weighted mean values of the rotation measure distribution in rad/m2.
All Figures
![]() |
Figure 1:
NGC 4321: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
NGC 4388: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
NGC 4396: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
NGC 4402: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
NGC 4438: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
NGC 4501: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
NGC 4535: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
NGC 4654: From top left to bottom right:
6 cm total power emission distribution on DSS B band
image together with the apparent |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Mean stellar mass density as a function of the mean energy input per unit surface area. The solid line roughly indicates the transition zone between galaxies with and without radio halos (Dahlem et al. 2006). The error bars represent uncertainties of a factor of 2. |
Open with DEXTER | |
In the text |
![]() |
Figure 10: NGC 4402: vertical profiles to the north of the 20 cm continuum (solid line) and H I (dashed line) emission distribution at a resolution of 20''. |
Open with DEXTER | |
In the text |
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