A&A 396, 815-832 (2002)
DOI: 10.1051/0004-6361:20021420
L. Verdes-Montenegro - A. Del Olmo - J. I. Iglesias-Páramo - J. Perea - J. M. Vílchez - M. S. Yun - W. K. Huchtmeier
1 - Instituto de Astrofísica de
Andalucía, CSIC, Apdo. Correos 3004, 18080 Granada, Spain
2 - Instituto de Astrofísica de
Andalucía, CSIC, Apdo. Correos 3004, 18080 Granada, Spain
3 - Laboratoire d'Astrophysique de Marseille,
Traverse du Siphon - Les Trois Lucs,
13376 Marseille,
France
4 - Instituto de Astrofísica de
Andalucía, CSIC, Apdo. Correos 3004, 18080 Granada, Spain
5 - Instituto de Astrofísica de
Andalucía, CSIC, Apdo. Correos 3004, 18080 Granada, Spain
6 - Astronomy Department, University of Massachusetts, Amherst, MA 01003,
USA
7 - Max-Planck-Institut für
Radioastronomie, Auf dem Hügel 69, 53121 Bonn,
Germany
Received 16 May 2002 / Accepted 25 September 2002
Abstract
HCG 54 has been classified as a compact galaxy group by Hickson,
but its nature is uncertain because of its unusual properties.
We present here a study of HCG 54 based on deep optical images and
spectroscopy as well as high spatial and spectral
resolution HI observations obtained at the VLA. Both optical and radio
data indicate clearly that HCG 54 is a product of a recent
merger involving at least two galaxies.
Our optical images have revealed a blue elongated structure associated
with HCG 54a embedded in a rounder and redder stellar envelope.
Several ripples or shells whose center is located
near HCG 54a and b are also detected.
These observed features are similar to those found in the
numerical simulations of tidal interactions involving
two equal mass disk galaxies.
This system is embedded in an HI cloud 12 kpc in
diameter, and a 20 kpc HI tidal tail emerges from its western edge.
Both the HI distribution and kinematics are consistent with
a recent history of a deeply penetrating interaction or a merger.
Based on the analysis of our new data
we propose that HCG 54 is the remnant of
a merger in an advanced stage, whose main body is what has been called
HCG 54a, while HCG 54b marks the location of a strong starburst
induced by the interaction, as evidenced by the Wolf Rayet
stellar population that we detect.
Optical spectra of HCG 54c and d show HII region features and
are interpreted as tidal debris of the collision undergoing active
star formation.
Key words: galaxies: interactions - galaxies: kinematics and dynamics - galaxies: evolution - galaxies: structure - radio lines: galaxies - galaxies: individual: HCG 54
HCG 54 was proposed by Hickson (1989)
to be a small group composed of four dwarf galaxies, named a through d.
The brightest galaxy has a "Sdm'' morphological type
while the rest are classified as "Im''. Some
authors have later claimed (e.g., Arkhipova et al. 1981)
that HCG 54 is in fact a single galaxy composed of several bright
HII regions.
Vílchez & Iglesias-Páramo (1998) found multiple
emission knots in H.
HCG 54 occupies the lower end
of the total luminosity, velocity dispersion and optical diameter
distributions of the HCGs sample,
with
,
and
kpc.
We were motivated to study HGC 54 in the context of our
ongoing studies of HCGs (e.g. Verdes-Montenegro et al. 2001;
Vílchez & Iglesias-Páramo 1998) in order to confirm
or discard its compact group nature based on optical images and spectra,
together with HI VLA data.
A large HI tidal tail,
clear signature of a tidal interaction, as well as
the presence of multiple optical ripples or shells,
added new motivations to this study.
In particular, the presence of ripples/shells and the apparent Sdm morphology
pose an interesting theoretical problem, since the formation of these
features has been generally attributed to a
bright elliptical accreting a small companion (see
e.g. Hernquist & Quinn 1987).
The observations are presented in Sect. 2, the results discussed
in Sect. 3 and we present possible scenarios in Sect. 4.
A distance of 19.6 Mpc (
)
is assumed throughout this paper for HCG 54.
We have imaged HCG 54 in the Johnson R band at the
2.5 m NOT telescope at el Roque de los Muchachos (La Palma)
using the ALFOSC spectrograph during the nights of March 21st and
27th and May 16th 2001.
The main characteristics of the observations are given
in Table 1. The detector was a Loral/Lesser (CCD#7)
,
with a spatial scale of 0.189
/pixel, which gives a field of
about
.
In order to cover a larger field (16
6
16
7)
tracing the direction of the HI tail (see Sect. 3.3), a total of
10 shifted exposures were taken during the 1st run for 600 s each.
The seeing was varying between
1
1 and 1
4 during the observations.
Ten bias exposures were taken through this run and were used to construct a
median bias which was subtracted from each image. Pixel-to-pixel variations
were evaluated with
a median normalized twilight flat-field.
Then a bidimensional fourth order Legendre polynomial
was fitted using IMSURFIT
task in IRAF. Images were divided by this flat field.
After correction of instrumental effects, the atmospheric extinction was
determined and corrected
from observations of selected fields from Landolt list of standard
stars (PG0918 and PG1047). The rms errors of the standard stars in the final
calibration are smaller than 0.07 mag.
Finally all individual images were combined in a mosaic
by using the IRAF SQIID package created by K. M. Merrit at NOAO, and
using SQMOS, XYGET and NIRCOMBINE routines, together with a mask built for each
frame that prevented the mean of the frame edges, as well as bad CCD lines.
The resulting image is shown in Fig. 1, where
three previously unclassified galaxies A1126+2051, A1127+2054 and
A1127+2057 are also identified (see Sects. 3.1 and 3.2).
![]() |
Figure 1:
Full field of the R-Johnson band optical image of HCG 54
obtained as explained
in the text (Sect. 2.1). The dotted lines delineate the HI integrated emission
at a level of 0.5 ![]() |
Open with DEXTER |
![]() |
Figure 2: Combined frame of B, R and r' bands in order to better recover the faintest emission. Contours are shown for the central part that appears saturated in our greyscale. The objects identified by Hickson et al. (1989) as the members of HCG 54 are indicated with letters a to d. We also mark the tidal optical features described in Sect. 3.1 t1 to t4, and an example of the unresolved knots (k) that we detect close to HCG 54. |
Open with DEXTER |
![]() |
Figure 3: B-R map of HCG 54. Averaged colors of different regions are indicated by arrows, and the displayed range goes from 0 (white) to 0.8 (black). We have superimposed two R band isophotes as a reference. |
Open with DEXTER |
![]() |
Figure 4: R band image of HCG 54 showing the direction of the slits used for the spectroscopical observations in the main body of HCG 54 (see Sect. 2.2), and the sections indicated in Tables 3 and 4. |
Open with DEXTER |
![]() |
Figure 5: a) Up: spectrum in the direction joining HCG 54a and b (sp3), where the position is given in offsets with respect to the peak of emission of HCG 54b. The indicated regions are described in Sect. 3.2. Bottom: R band profile along the slit. |
Open with DEXTER |
![]() |
Figure 5: b) Up: spectrum in the direction joining HCG 54c and d (sp4), where the position is given in offsets with respect to the peak of emission of HCG 54c. Bottom: R band profile along the slit. |
Open with DEXTER |
![]() |
Figure 5: c) Spectrum in the direction joining HCG 54d and A1126+2051 (sp1, Fig. 4), for the sections where emission was detected. Positions are given in offsets with respect to the continuum peak of HCG 54d along the slit. |
Open with DEXTER |
![]() |
Figure 6: Full spectrum of HCG 54a. |
Open with DEXTER |
![]() |
Figure 7: a) Full spectrum of HCG 54b. |
Open with DEXTER |
![]() |
Figure 7: b) Selected range of the same spectrum shown in a). |
Open with DEXTER |
In the second run,
a deeper red band exposure was obtained for the central part
in order to better define the faintest optical structures.
The red image is a composition of six 1200 s images with the
Sloan-Gunn r' filter.
A blue image was also obtained in order to be able to derive colour indices
of the different components of the group.
The Johnson B filter image taken in the 3rd run
is a composition of four 1200 s frames.
Slight dithering was applied between successive frames
for both filters, in order to avoid cosmic rays and bad pixels.
All frames (R, r' and B band images) were finally combined
in a search for the best definition of the weakest structures
(Fig. 2). None of the nights of these
two last runs were photometric, so no calibration was obtained for
the r' and B band images. Instead we adopted the one proposed
by Hickson et al. (1989) for the (B - R) color map (Fig. 3),
using
the integrated B-R color of HCG 54b as a reference.
Image | Date |
![]() |
Filter |
Mosaic | 21/03/2001 | 6000 | R-Johnson |
Center | 27/03/2001 | 7200 | r' Gunn |
Center | 16/05/2001 | 4800 | B-Johnson |
![]() |
Figure 8:
a) Left: HI column density contours 0.5, 0.7, 1.2, 2.0,
2.7, 3.4, 4.4, 6.7, 8.4, 10.1, 13.5, 16.9, 20.2 to 22.6
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 9:
Channel maps of the 21 cm line radiation
superimposed on the R image of the group.
Heliocentric velocities are
indicated in each panel. Contours correspond to
2.4, 4.0, 5.6, 7.3, 8.9, 10.5, 12.1, 13.7, 15.3 K, and the
rms noise of the maps is 1.2 K. The synthesized beam
(15
![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 9: continued. |
Open with DEXTER |
The spectra were also obtained with ALFOSC at the NOT 2.5m telescope, with the
same detector used for the images and using Grism#4, 7 and 8,
during April 2000 and March 2001. Table 2 summarizes the long-slit spectra
taken for this study. The format is as follows: Col. 1 spectrum
identification, 2 direction through which the spectrum was taken,
3 date of observation, 4 Grism used, 5 spectral
dispersion, 6 number of exposures, 7 duration of each exposure in
seconds, 8 spectral range, 9 position angle of the slit in
degrees measured from N to E, and 10 width of the slit.
The slit widths were chosen to match approximately the seeing of
the nights. The direction of the slits listed in Table 2
as well as the different zones from which the spectra have been extracted
along the slits are marked in Fig. 4.
Observations with Grism#4 were performed in order to derive general
spectral characteristics and kinematics of the galaxies,
and to determine redshifts for possible satellite galaxies in the field.
Grisms#7 and #8 were used to characterize the kinematics with
a higher spectral resolution than for Grism#4.
Observations with Grism#7 covered with good spectral resolution
the range including [OII]3727 and [SII]6717/6731.
In the observations performed with Grism#8 the
[NII], H
and [SII] lines were detected and used
to characterize the kinematics.
The spectra were reduced according to the usual methods, including subtraction of a mean bias calculated for each night, division by a median flat-field obtained for each configuration, as well as wavelength calibration based on HeNe calibration lamp spectra obtained after and before each exposure. The rms of the bidimensional calibration was 0.4 Å for Grism#4 and 0.1 Å for Grism#7 and Grism#8. A mean sky was obtained from object-free sections of each spectrum. In all cases several exposures were taken in order to increase the signal to noise ratio and to remove cosmic rays. The derived physical parameters described in Sect. 3.2 are given in Tables 3 and 4.
In order to obtain
redshifts and velocity dispersions we used the cross-correlation
technique derived by Tonry & Davis (1979). The templates used
for the spectra showing
emission lines were: the brightest spatial section
of HCG 54a and HCG 54c for the Grism#8 and Grism#7 spectra
respectively, together with a synthetic spectrum built from the
rest frame wavelengths of the emission lines.
In the case of the absorption line
spectra, where our goal was to determine the redshift of A1126+2051
and A1127+205, we used as template the spectrum of the radial velocity
standard giant star HD186176 obtained with the same configuration.
In order to improve the signal to noise ratio of the derived curves
a binning of 2 pixels (0
38)
has been applied to the spectra sp5 and sp6
in the spatial direction
and of 3 pixels (0
57) to sp1 (Fig. 4).
The radial velocity data in the direction joining HCG 54a and b are shown in
Fig. 5a, in the HCG 54c - d direction in Fig. 5b
together with the section of sp1 corresponding to HCG 54d, and in the
HCG 54d - A1126+2051 in Fig. 5c.
The spectra for HCG 54a and b taken with Grism#4 are shown in Fig. 6 and
7 respectively.
Spectrophotometric standard stars were observed in order to get a proper
calibration of the line fluxes in sp3 and sp4 (Fig. 4).
However, since the nights of April 27 and 28 were non-photometric, only the
relative fluxes of the emission lines are reliable.
The rest of spectra were taken under photometric conditions.
Name | Direction | Date | Grism | R(Å/px) | N |
![]() |
Wav.-range (Å) | PA (deg) | Slit width (
![]() |
sp1 | HCG 54d-A1126+2051 | 21/03/01 | GR4 | 3.0 | 3 | 1800 | 3018-9006 | 86 | 1.2 |
sp2 | A1127+2057 | 21/03/01 | GR4 | 3.0 | 2 | 1200 | 3018-9006 | 115 | 1.2 |
sp3 | HCG 54a-b | 27/03/01 | GR4 | 3.0 | 3 | 1200 | 3018-9006 | 65 | 1.0 |
sp4 | HCG 54c-d | 28/03/01 | GR4 | 3.0 | 3 | 1200 | 3018-9006 | 20 | 1.0 |
sp5 | HCG 54a-b | 27/04/00 | GR8 | 1.2 | 3 | 1800 | 5816-8326 | 65 | 1.2 |
sp6 | HCG 54c-d | 21/03/01 | GR7 | 1.5 | 2 | 1200 | 3815-6815 | 15 | 1.2 |
We have mapped HCG 54 with the VLA in the C array in August 1997.
The synthesized beam is 20
16
(1.9
1.5 kpc at a distance of 19.6 Mpc) after natural weighting of the data.
A velocity range between 1085 km s-1 and
1730 km s-1 was covered with a velocity resolution of 10.4 km s-1 .
The rms noise is 0.33 mJy/beam corresponding to a column
density of 1.2
1019 at cm-2.
Assuming an HI linewidth of 30 km s-1, the achieved HI
mass detection limit is about 106
.
We have detected emission above 3
in the velocity range
1334.2 to 1490.2 km s-1 . The integrated emission is presented
in Fig. 8a superposed on the R-band image,
and the velocity field is presented in Fig. 8b. The velocity
channel maps are shown in Fig. 9.
The total spectrum has been obtained by integrating the emission in the
individual channel maps (Fig. 10, solid line).
The total HI line flux
detected, 6.23 Jy km s-1, is in good agreement
with the single dish measurement by Huchtmeier (1997;
Fig. 10, dotted line). The total
HI mass derived is 5.6
108
.
ION | ![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
[OII] | 3727 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
[NeIII] | 3869 |
![]() |
- | - | - | - | - |
H8+HeI | 3889 |
![]() |
- | - | - | - | - |
H![]() |
3970 |
![]() |
- | - | - | - | - |
H![]() |
4100 |
![]() |
- | - | - | - | - |
H![]() |
4340 |
![]() |
![]() |
- | - | - | 0.43![]() |
[OIII] | 4363 |
![]() |
![]() |
- | - | - | - |
HeI | 4472 |
![]() |
- | - | - | - | - |
H![]() |
4861 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
[OIII] | 4959 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
[OIII] | 5007 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
HeI | 5876 |
![]() |
- |
![]() |
- | - | - |
[OI] | 6300 |
![]() |
- | - | - | - | - |
H![]() |
6563 |
![]() |
![]() |
![]() |
![]() |
![]() |
2.81 0.15 |
[NII] | 6584 | - | - |
![]() |
- | - | - |
HeI | 6678 |
![]() |
![]() |
- | - | - | - |
[SII] | 6717,31 |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
HeI | 7065 |
![]() |
- | - | - | - | - |
[AIII] | 7136 |
![]() |
- | - | - | - | - |
1.3*I(6584)/I(3727) | - | 0.07 | 0.10 | - | - | - | |
log
![]() |
0.90 | 0.87 | 0.85 | 0.98 | 0.76 | 0.89 | |
P = [(1.3*I(5007)/I(H![]() ![]() |
0.79 | 0.36 | 0.35 | 0.52 | 0.33 | 0.34 | |
log[1.3*I(5007)/I(3727)] | 0.58 | -0.24 | -0.26 | 0.04 | -0.31 | -0.28 | |
t[OIII] (104 K) | 1.05 | - | - | - | - | - | |
t[OII] (104 K) | 1.14 | - | - | - | - | - | |
104 O2+/H+ | 1.46 | - | - | - | - | - | |
104 O+/H+ | 0.36 | - | - | - | - | - | |
12 + Log O/H | 8.26 | 8.30 | 8.30 | 8.20 | 8.10:, 8.60: | 8.25: | |
LogNe2+/O2+ | -0.67 | - | - | - | - | - |
![]() |
![]() |
![]() |
![]() |
|
[OII] | 3727 |
![]() |
![]() |
![]() |
H![]() |
4340 |
![]() |
- | - |
H![]() |
4861 |
![]() |
![]() |
![]() |
[OIII] | 4959 |
![]() |
![]() |
![]() |
[OIII] | 5007 |
![]() |
![]() |
![]() |
H![]() |
6563 |
![]() |
![]() |
![]() |
[SII] | 6717,31 |
![]() |
![]() |
![]() |
log
![]() |
0.82 | 0.99 | 0.77 | |
P = [(1.3*I(5007)/I(H![]() ![]() |
0.51 | 0.54 | 0.43 | |
log[1.3*I(5007)/I(3727)] | 0.01 | 0.07 | -0.12 | |
12 + Log O/H | 8.10:, 8.40: | 8.20 | 8.10:, 8.45: |
![]() |
Figure 10: Integrated HI emission as a function of the heliocentric velocity for the range where we detect emission. The solid line corresponds to our VLA data, while the dashed ones are single dish data from Huchtmeier (1997). |
Open with DEXTER |
![]() |
Figure 11:
a) Upper-left: R band emission of A1127+2054 in greyscale with
overlapped isophotes 22.6, 22.9, 23.3, 24.1 mag arcsec-2.
b) Upper right: light profile of this galaxy in the R band
as a function of the equivalent radius.
c) Lower left: integrated HI emission of this galaxy where the contours correspond to
3.3, 6.6, 10.0 and 11.6 ![]() |
Open with DEXTER |
The brightest central part of HCG 54 (Fig. 2) shows a dominant knot (HCG 54a)
embedded in an elongated feature visible in the blue and red bands
at a PA of 65.
It is more extended
to the SW, where a very compact bright knot is located (HCG 54b) which is even
more prominent in H
(see Vílchez & Iglesias-Páramo 1998).
The elongated area defined by HCG 54a and b is connected to the NE with
the weaker but still prominent knots HCG 54c and d.
Large variations in the B-R color index are apparent throughout the whole system (from 0.15 in HCG 54d to 0.70 around HCG 54a, Fig. 3). The elongated structure centered in HCG 54a has a blue color (0.5), likely a signature of a recent star forming episode, and is embedded in a rounder region that shows the reddest colours in HCG 54 (B-R = 0.7).
The B-R color index for HCG 54b is
surprisingly red for a strong star formation burst like the one hosted by this
galaxy. However, we argue that this is due to the contamination by the H,
[O III] and H
emission lines. After computing the equivalent
widths of these lines
Å,
Å,
Å,
Å),
we estimated that the corrected B-R color should be
0.46 mag bluer than the one measured directly from the color map. Thus,
we would obtain
B - R = 0.20, which is more consistent with a very young stellar
population.
Three shell-like structures are found (noted as t1 to t3 in Fig. 2),
centered roughly on HCG 54a and b.
A fourth optical feature is located to the SW and marked as t4.
We detect all of these features in a R-band surface brightness range of
24 to 27 mag/(
)2.
The outer shells (t2 and t3) although redder than the internal knots,
have blue colours (
0.5) typical of irregular galaxies
(see Fukugita et al. 1995).
The inner shell (t1) is slightly redder (0.60), probably due to contamination
from the central parts of HCG 54. Numerous unresolved knots are detected
everywhere in the area, such as the one marked in Fig. 2 with a "k''.
No stellar counterpart to the HI long tail is detected down to
27 mag/(
)2 in R.
We found three faint galaxies in our large R-band frame (Fig. 1),
that are named A1126+2051, A1127+2057 and A1127+2054. The first
two are background galaxies (Sect. 3.2)
while the last one is at the velocity of the system according
to the HI data (Sect. 3.3). The optical image of this galaxy
shown in Fig. 11 reveals that the inner and the outer
isophotes are off-centered. When measured at an R
isophote of 26 mag/(
)2 the optical size is 2.6 kpc
and the total magnitude in R is
MR = -14.2 mag.
The surface brightness derived as a function of radius is exponential,
consistent with that of a disk system (see Fig. 11b).
From the Grism#4 spectra sp1 (Fig. 4) and sp2 (Table 2) we have determined that A1126+2051 and A1127+2057 are not at the group redshift. A1126+2051 shows absorption lines of CaII, G band, Mg I and Na lines and has a redshift of z = 0.15 (see also Sect. 3.3). A1127+2057 also shows an absorption spectrum typical of early type galaxies and gas at a redshift of z = 0.052.
We have studied the kinematics of the central parts of the group with the
spectra obtained with Grism#7 and #8. The spectrum along the HCG 54a-b direction (sp5, Fig. 4)
shows emission lines along the 249 spatial sections (47
), and the
obtained velocity curve is plotted in Fig. 5a, where the zero
position corresponds to the center of HCG 54b.
We mark in the figure the continuum extent of HCG 54a and mark with an arrow
the position of the emission line maxima.
The spatial sections closer to HCG 54a (5
diameter)
traces a distorted rotation curve with an amplitude of
45 km s-1. At both sides of the center of HCG 54a we find discontinuities
in the velocity that correspond in the R band to distorted isophotes.
The sections in the direction of HCG 54c (R1 in Fig. 5a) have
three well differentiated knots with emission lines, whose left end
is the external part of HCG 54c. The strongest knot presents a
velocity gradient of 50 km s-1 in 4
that corresponds to the
contact area between HCG 54a and
HCG 54c (R3). The velocities in the direction of HCG 54b
show irregular changes of up to 70 km s-1, as is the case of
R2 (Fig. 5a) that appears to be an HII region detached from the general trend
of the velocity curve. The area of HCG 54b has a peculiar
kinematics (Fig. 5a) that could be consistent
with infalling gas, in the case that the background emission is hidden
by extinction so that we are only observing the motions of the
foreground component.
In Fig. 5b we show the velocities in the slit through
HCG 54c-d (sp6, Fig. 4), and the regions corresponding to
HCG 54c and d in the R image are also marked.
These regions have a weak continuum, and
none of them show a rotation curve. HCG 54d has irregular motions,
while HCG 54c shows a nearly constant velocity following a U-shape with
an amplitude 30 km s-1.
We do not detect continuum emission toward
the third emitting region in the direction of HCG 54a (Fig. 5b), while
the velocities increase continuously until it reaches the values
characteristic of HCG 54a.
Finally we find a region with a decoupled velocity (R4)
that is located between HCG 54c and d
In the lower resolution spectrum crossing HCG 54d (sp1, Fig. 4)
we detect a velocity gradient of only 70 km s-1 within 7
.
In Fig. 5c we show the sections where we have detected emission,
where the center corresponds to the continuum peak of HCG 54d along
the slit.
The presence of several knots in our R band image along the
slit direction suggests that the continuity of the observed
velocity gradient might be due to smearing of individual components.
We have extracted individual spectra for 9 zones from the data taken
with Grism#4 (sp3 and sp4, Fig. 4). Six of them are along the slit
position joining HCG 54a-b and three along the HCG 54c-d direction
(Fig. 4). The spectrum of HCG 54a (sp3, Fig. 4) shows strong Balmer
absorption lines as well as lines of CaII, Gband, and possibly MgI,
while Mg
is barely detected. The spectrum shows a blue continuum,
characteristic of a post-starburst
population (Fig. 6).
The spectrum corresponding to knot b is remarkable (Fig. 7), with a
high excitation and the presence of WR features at
4650 Å over a flat continuum, indicative of a young and strong burst of star
formation. Figure 7a shows the full spectrum of HCG 54b, while
Fig. 7b shows a detail of the spectrum around the Wolf-Rayet (WR)
feature. The temperature sensitive [OIII] line at
4363 Å
was measured in this spectrum, giving a t([OIII]) temperature of
10 485 K, implying an oxygen abundance of
(see Table 3). According to Schaerer & Vacca (1998), the measured equivalent
width of the WR
4650 Å feature in knot b,
Å, implies an age of the burst between 3 and 4 Myr.
For
the zones showing emission line spectra (sp3, sp4, Fig. 4) we present in
Tables 3 and 4 their fluxes relative to H
as well as the derived
physical conditions of the ionized gas.
Reddening corrected line fluxes relative to H
are presented
for the 9 individual spectra extracted along slit positions sp3 and sp4 (Table 2, Fig. 4).
For each zone given in Tables 3 and 4 the following ionization
structure parameters have been derived in order to perform the
abundance analysis (as detailed below):
,
denotes the
abundance parameter after Pagel et al. (1980) which is defined as
[I(3727) + I(5007) + I(4959) ]/I(H
), P denotes the abundance
parameter defined by Pilyugin (2000), as quoted in Tables 3 and 4,
.
The excitation, defined as
,
and
the nitrogen to oxygen abundance indicator,
,
are also quoted in Table 2. Note the small range of variation of
in contrast with the large variations shown by the
excitation along the slit. The electron temperatures t[OIII]
and t[OII], corresponding to the ionization zones of [OIII] and
[OII] respectively, have been derived for region
1 and listed
in Table 3.
For this region the ionic and total abundances of oxygen,
O2+/H+, O+/H+ and O/H respectively, as well as the
abundance ratio of neon to oxygen, Ne2+/O2+, have been
calculated and are presented in the table. For the rest of the studied
regions, only the total abundance of oxygen has been estimated (see
Tables 3 and 4).
Given that the temperature sensitive line
[OIII]lambda 4363A was measured only for knot b, we have to rely on the
empirical calibration in order to derive their abundances. We have used the
calibration of the oxygen abundance versus
(cf. Pagel et al. 1980) as parameterized by McGaugh in Kobulnicky et al. (1999), and
following the P-method (Pilyugin 2000), in order to provide an
estimation of 12+Log O/H.
The [NII]/[OII] line ratio, when observed,
was used to discriminate between the lower and upper branch, though often
the 6584 line was severely blended with H
and was not
measured. For those zones with
we have assumed an
average abundance of 12+logO/H = 8.2 as indicated by the calibration.
All the zones selected are consistent with 12+Log O/H = 8.2, within the
errors of the empirical calibration. This abundance is typical of galaxies
like the Large Magellanic Cloud and the outer disks of late type spirals.
The integrated emission of neutral hydrogen (Fig. 8a) shows a NE-SW distribution with extensions to the SE and SW, and a long tail with a projected size of 20 kpc to the NE. The velocity field is quite perturbed, but still shows a velocity gradient similar to a rotating disk with a twisted major axis (Fig. 8b). This reflects itself also in the asymmetry of the HI line integrated profile (Fig. 10).
The situation is more complex when the channel maps (Fig. 9) are examined. The sizes of the optical knots are small compared with the VLA synthesized beam, and tracing the HI kinematics with respect to the optical features is difficult. Nevertheless these channel maps reveal the details that are lost in the integrated emission image shown in Fig. 8a. Except for the large HI tail to the northeast, most of the HI emission arises within the faint optical extent of HCG 54. Bright HI features directly associated with the bright optical ridge of emission are seen in the channel maps with velocity range between 1365-1490 km s-1. The overall velocity field is along the length of the bright optical ridge in a manner consistent with that of rotation, but clear evidence for a velocity gradient in the perpendicular direction is also present, increasing in velocity from NW to SE. Since stars and gas inside the tidal radius are generally unaffected by a tidal interaction, the observed kinematic disturbance suggests an involvement of a deeply penetrating interaction or a merger.
Most of the high surface brightness HI features associated
with the fainter outer optical envelope occur on the west side of
the optical galaxy, closely associated with the optical tidal
features t1, t2, and t4 (see Fig. 2).
Both the Y-shaped HI morphology
in Fig. 8a and the shifting of the focus of the iso-velocity
contours to the west of the optical peaks in Fig. 8b are direct
results of the large amount of HI associated with these tidal
features. This one-sided appearance may indicate that only
one of the progenitor systems was HI-rich if HCG 54 is mainly
a product of a merger involving two late type galaxies - i.e.
the progenitor responsible for the tidal feature t3 had
relatively little HI associated with its stellar disk.
The HI extension to the southwest, closely associated with
the optical feature t4, occurs mainly in the velocity ranges
of 1334 to 1428 km s-1 and contains
of HI. The long HI tail associated with either t1 and/or t2occurs in the velocity range of 1376 to 1459 km s-1 and
contains about
of HI. Combined together,
these two outer HI features account for more than 40% of the
total HI detected in this system.
Several HI clumps that are detached from the main body of HI are
also seen at
levels in these channel maps, and they
indicate a more extensive debris field associated with this
system. A velocity coherent string of HI clumps forming a nearly
complete loop or a ring is seen in the channel maps within the velocity
range of 1438 to 1469 km s-1, largely to the northeast. Some of these
features make up part of the 20 kpc long HI tail to the north,
but their appearance in the channel maps, particularly at 1438 km s-1 and 1448 km s-1, suggest a more ring-like morphology, similar to
those seen in collisional ring galaxies (e.g. Higdon 1996).
The stream of HI clumps extending to the west of
the main body seen in Fig. 8a form a second distinct, velocity coherent
structure appearing at velocities of 1386-1407 km s-1, and these
channel maps suggest its origin being more to the south of the
main body rather than a linear east-west structure. The total HI
mass associated with this feature is about
.
The overall velocity field of the bulk of HI
delineates an elongated structure with an axis ratio that
would correspond, if intrinsically circular, with an approximate
inclination of 50
and a position angle of 70
,
similar to
the one traced by the direction of HCG 54a-b (
).
The amplitude derived from the velocity field (140 km s-1 ) and
deprojected according to this inclination gives an overall velocity
gradient of
183 km s-1. If this is interpreted as a Keplerian
rotation, we estimate a dynamical mass for the
system of
.
A dwarf companion galaxy A1127+2054 is found at a projected distance
of 27 kpc northeast of HCG 54a (see
Sect. 3.1, Figs. 1 and 11). Its location also corresponds to
a distance of 8 kpc from the tip of the 20 kpc long HI tail,
in the same projected direction. The HI channel maps show associated
emission in 3 channels covering velocities of 1386-1407 km s-1.
A close examination suggests a
central depression in the integrated emission (see Fig. 11c).
The velocity field is consistent with a slowly rotating disk with
a velocity amplitude of 30 km s-1 (Fig. 11d).
The HI extent at 3
1019 at cm-2 (3
level)
is 45
25
(4.3 kpc
2.4 kpc) and contains
of HI. The atomic component seems
to be perturbed since it is not well centered on the optical component.
In the superposition of the integrated HI emission over the deep optical
image shown in Fig. 8a, the HI isolated peak located
about 2
east of HCG 54a appears to have an optical counterpart
in a bright, compact galaxy. From the optical spectrum we obtain,
however, that this is a chance superposition as this galaxy A1126+2051
is found to be a background object with z=0.15
(see Sect. 3.2).
At the position of this galaxy we find
HI emission in two channel maps, with no signature of systematic motions.
Since the gas is located close to the center of the group, it is
possibly part of the tidal debris. This is supported when a
sharpening of the optical image
of A1126+2051 is performed, suggesting the existence of two objects,
a symmetric disk structure whose peak is located in the slit,
and then corresponds probably to the background galaxy, plus a
blob that might be a stellar counterpart of the HI emission.
Using the data from the line-free channels, a 1.4 GHz continuum
image was constructed. No significant continuum emission
associated with the optical galaxy is detected at the limit of about 0.5 mJy beam-1. The New VLA Sky Survey
image also gives a comparable upper limit of about 1.5 mJy
for about a 3 times larger beam. HCG 54 was detected by IRAS
in the 12
m, 60
m, and 100
m bands with flux densities of
0.24, 0.50, and 0.84 Jy, respectively, with the derived FIR luminosity
of
(Verdes-Montenegro et al. 1998).
The upper limit of 1.5 mJy for 1.4 GHz radio continuum makes
this object slightly underluminous in the radio with a lower limit on the
q-value of about 2.7 (see Yun et al. 2001).
The enhanced IR luminosity resulting from the relatively high
dust temperature, inferred from the FIR
(
)
and mid-IR (
)
color,
offers a natural explanation for the relatively weak radio continuum.
These warm infrared colors are also consistent with the evidence for
significant WR activity detected in the optical spectra
(see Sect. 2.2).
We have identified clear signs of tidal interactions and possibly a merger toward HCG 54. The morphology and kinematics of the atomic gas, a sensible tracer of interactions, is strongly perturbed, including a long (20 kpc) HI tidal tail, plus two more HI extensions that emerge from the main body. The stellar component also shows features that are characteristic of interactions or mergers, in particular several shells around the bright central area. We propose here that the formation of the HI tail and the optical ripples involved interactions and a merger of at least two galaxies.
The proximity of HCG 54 to our Local Universe (v = 1470 km s-1)
makes the study of its large scale environment difficult.
We find in the NASA Extragalactic Database (NED)
that all galaxies with measured
redshifts in a square of 1 Mpc radius centered in HCG 54
have velocities larger than 4000 km s-1.
Within a distance of 500 kpc none of
the galaxies without redshift determination have a similar size or
magnitude as the HCG 54 members. In fact,
the closest and brightest galaxy to HCG 54, A1127+2057,
is a background galaxy (see Sect. 3). Similarly, another
optically selected potential neighbor A1126+2051 is also
shown to be a background galaxy.
The VLA primary beam (30)
covers a diameter of 170 kpc
at the distance of HCG 54, and we found only one object
(A1127+2054, see Sect. 3) in this area at the group redshift
in HI emission, within the observed range of 645 km s-1.
The dwarf disk galaxy A1127+2054 is the only identified companion in
the environment of HCG 54 and is located at 20 kpc from its center.
Shells are usually thought to form in unequal mass interactions
involving a bright elliptical accreting a small companion
(see e.g. Quinn 1984; Hernquist & Quinn 1987).
The work by Schweizer & Seitzer (1988) showed that
a smooth triaxial potential, or even a disk,
can also give rise to the formation of
shells and ripples in the stellar component of
an interacting smaller companion.
It does not seem to be the case in HCG 54,
where we do not detect a large bulge component nor any other massive system.
The two galaxies should have been most probably late-type systems, judging
from the post-starburst nature of the spectrum shown by HCG 54a
(without clearly detected features typical of early type systems) which
could be invoked as the possible nucleus of the system.
Furthermore, the low chemical abundance
derived from the ionized gas, 12+log(O/H)
8.3, is more typical
of irregular galaxies.
Few examples of shells around
low mass late type galaxies are found in the literature, as NGC 7673
(Homeier & Gallagher 1999). If we relax the low mass
criteria there is also NGC 3310 (see Mulder & van Driel,
1996) which is a peculiar Sbc. In both cases a tidal interaction origin
has been attributed to the shells.
In general, little observational evidence exists for
ripples in disk-disk interactions, (see e.g. Homeier & Gallagher
1999; Charmandaris & Appleton 1996; Kemp & Meaburn 1993 or
Schweizer & Seitzer 1988).
Probably for this reason few models are found focused on reproducing
HCG 54-like systems. In this sense, the simulations presented by
Hernquist & Spergel (1992) seem to provide with a more suitable model
to describe the system. The model represents
a merger of two equal disks galaxies in a close collision
from a parabolic orbit. At the final stages the remnant has
a central bar-like structure embedded in a disk like envelope,
together with several shells at different radii.
This resembles quite well the
optical structure of HCG 54, centered on HCG 54a,
with an associated bar-like bluer structure, whose colours are suggestive
of star formation,
in the middle of a red disk-like component and surrounded by 3 shells.
The shells are centered somewhere close to HCG 54a and b,
but the data do not allow a determination of the precise location of
their focus (Sect. 3.1). The outer shells (t1 & t3, see Fig. 2)
have bluer colors, consistent with an origin in the late type
progenitor disk. The inner one (t2) is slightly redder,
and this might be due to contamination by the
population of the disk-like component associated with HCG 54a.
Therefore the morphology of HCG 54 can be well explained
by the collision of two similar mass disk systems.
The model does not allow further test or comparison,
since no prediction is given neither for the kinematics nor for the
stellar populations/colors of the colliding systems.
Here we will try to shed more light in the
identification of the involved systems based on the
rest of the information provided by our optical and HI data.
We have tried to determine the number of present galaxies using the
observed HI mass
).
We have calculated the expected mass as a function of
optical luminosity and morphological type
via the relationships obtained by Haynes & Giovanelli (1984).
Assuming that HCG 54 is composed of
4 irregular galaxies as classified in Hickson catalog, we predict a mass
of
,
but for a single irregular galaxy
the expected mass would be
.
Unfortunately these cases are indistinguishable since they are within one sigma, and all
we can conclude is that the HI content of
HCG 54 does not deviate significantly from normal.
HCG 54a is the best defined object of the system, with a defined
nucleus, embedded in a very elongated blue component and surrounded
by a rounder and redder stellar envelope.
This ensemble seems to be aligned with a perturbed but still visible
HI velocity gradient. This might indicate that HCG 54a is one
of the interacting galaxies, whose disk is a red round
component that still keeps memory of the original rotation according to
the HI kinematics (Fig. 7b). We find
however several arguments against this hypothesis. The atomic gas is
decoupled from the stellar component both in morphology and
kinematics.
The optical emission is displaced to the north by 1 kpc with
respect to the atomic gas, and this shift is too large to be explained
only by stripping of the northern HI into the long tail.
Furthermore, the HI velocity gradient does not have a counterpart
in the optical velocities. The signature of
a rotation curve is not found along any of the observed directions,
except for a small velocity gradient
of 50 km s-1 in the inner 5
of HCG 54a (see Sect. 3.2).
The HI radial velocities along the direction of the optical spectra
differ from the optical ones by
30 km s-1, also consistent
with a decoupling between the stellar and gaseous components.
Finally, since the HI velocity gradient is the only trace of a
possible disk system in the central part of HCG 54, then the second
disk would be a minor body when compared with HCG 54a, in
contradiction with the equal masses involved in the simulations.
Furthermore, if the observed HI velocity gradient of
180 km s-1 had
to be attributed to a single galaxy, it would be more characteristic of
an Sb type system, which is not observed here.
A more plausible scenario is that we are observing a mix of the two components of the original systems, that have almost coalesced and now present a similar distribution as found in the simulations. The irregular shape of the optical velocity curve supports an advanced merger stage. HCG 54b shows a clear WR feature in the spectrum, indicative of a recent and strong burst of star formation that was possibly triggered by the interaction, while the underlying population, if present, is negligible, as suggested by the very high equivalent widths of the brightest emission lines (see Sect. 3.1.). In fact we found a possible signature of gas infall in our spectra (see Sect. 3.2) that would be feeding the recent violent star formation observed. HCG 54c and d do not have a well defined photometric structure, but consist of an aggregate of knots, and have HII regions features in their spectra. The very blue color of HCG 54d suggests a pure star formation episode younger than several million years old. They plausibly constitute the debris of the interacting systems. The HI component of the kinematics we observe might be the relic of the orbital motions of the interacting systems. However projected radial motions cannot be negelected and detailed numerical simulations including a kinematical study are needed to interpret the data.
In summary, no clear individual galaxies could be identified toward HCG 54 while strong signs of interactions and a merger are found. The overall morphology is in good agreement with the model by Hernquist & Spergel (1992), and we propose that HCG 54 is in the final stages of a merger of two similar mass disk systems.
A1127+2054 is not likely to be a tidal dwarf since it is not
located within the tail and has its own rotating HI disk or cloud.
The fact that the associated HI feature is not well centered on
the stellar body is consistent with the tidal disruption scenario.
No stellar counterpart to the HI tail is detected down to a
surface brightness level of 27 mag/(
)2 in R along its length.
This tidal disruption scenario has some potential difficulties however.
Assuming an inclination of ,
the estimated dynamical
mass for A1127+2054 inside 2.1 kpc radius is only about
,
which is about 4% of the dynamical mass of HCG 54.
On the other hand, the estimated HI mass alone for the 20 kpc
long tidal tail is about
(see Sect. 3.3),
and raising such a massive tidal tail from the gravitational
potential of HCG 54 may require a far more massive perturber.
An alternative scenario for the formation of the large HI tail is the merger of two equal mass progenitors
as we have already proposed in the previous section in order to
account for the disrupted stellar features around HCG 54.
In numerical simulations of galaxy mergers including
gas particles such as by Mihos & Hernquist (1996), formation of
one or more massive gaseous tails is commonly seen. For example,
the projected appearance of the HI tail with respect to the
stellar remnant in HCG 54 is similar to the "''
stages shown in Figs. 1 and 2 of Mihos & Hernquist (when
mirrored about the vertical axis). This
corresponds to a late stage in this particular simulation
where the stellar cores finally merge, similar to what we
infer in HCG 54. If we are allowed to press this comparison
a little further, the extended HI structures seen to the south and
southeast of the stellar body may be interpreted as the tidal
tail emerging from the second progenitor, as seen in the
simulation. Of course, these inferences are qualitative
at best since the initial conditions of the Mihos & Hernquist
simulation are likely to be different from those of the
merger involved in HCG 54.
In this scenario, an active role of A1127+2054 in shaping the appearance of the large HI tail is still allowed. For example, a dwarf companion UGC 957 is found within one of the large HI tails resulting from the merger of two massive disk galaxies in NGC 520 (see Hibbard & van Gorkom 1996) even though UGC 957 may be only a third party to the ongoing or recent merger.
We propose that HCG 54 is a system that consists of at least two late type galaxies plus a dwarf companion involved in recent tidal interactions and a merger. Two low mass disk systems merged and induced the formation of a central blue, bar-like structure embedded in a red, round stellar envelope, outside which lies several stellar shells. A luminous starburst is associated with HCG 54b while HCG 54c and d are the remnant debris of the recent merger undergoing modest star formation activity. This merger is probably in a very advanced state. The 20 kpc long gaseous tidal tail discovered in HI may be a result of the same merger or driven by the dwarf galaxy A1127+2054 found at a projected distance of 8 kpc from the tip. In either scenario, it is likely playing a role in shaping the appearance of this tidal tail. Unfortunately current theoretical or numerical models do not provide information on the kinematics or the distribution of induced star formation in low mass disk mergers. Few similar systems have been discussed in the literature, and the present data should provide valuable inputs for future modeling of mergers involving low mass galaxies.
Acknowledgements
We wish to thank Dr. Masegosa and Dr. Márquez for the help with the R observations at the NOT and their valuable comments. LV-M, AO and JP are partially supported by DGI (Spain) Grant AYA2000-1564 and Junta de Andalucía (Spain). JMV is partially supported by DGI (Spain) Grant AYA2001-3939-C03-01 and Junta de Andalucía (Spain).