HICKSON 40:
ONE OF THE MOST EVOLVED AND HI DEFICIENT GROUPS
M. Ángeles Martínez1, Ascensión del Olmo1 , Lourdes Verdes-Montenegro1, Jaime Perea1 and Min S. Yun2
1
Instituto de Astrofísica de Andalucía (IAA), CSIC, Granada (Spain)
2
Department of Astronomy, University of Massachusetts, Amherst (USA)
Introduction
Optical and HI Data
The compact group Hickson 40 constitutes the prototype of
compact isolated HCG groups. The group consists of the five
original Hickson galaxies: two early-type galaxies (H40A and
H40B) and three spirals (H40C,H40D,H40E), plus two small
member candidates identified by Carvalho et al. (1994), at a similar
redshift, that we will note as H40F and G.
Long-slit optical spectroscopy for galaxies H40C and H40D were obtained
at the 2.5 NOT telescope at El Observatorio de el Roque de los Muchachos
(La Palma) using the ALFOSC spectrograph. The detector was a
Loral/Lesser CCD 2048x2048, with a scale in the spatial direction of
0.19’’/pixel. We use a slit width of 1.2”.
Log of the spectroscopic observations
Here we present new optical spectroscopy and VLA HI
observations of H40. This group is one of the most HI deficient
groups from Hickson´s list, with only about 10% of the expected HI
content. Detailed kinematics together with the study of the HI
distribution constitute key pieces to understand the dynamical
evolutionary history of compact groups.
Name Grism
H40C GR8
H40D GR8
R(Å/pix)
1.24
1.24
Nºexp T (s)
2
1800
2
1200
PA Spectral Range
122 5816-8340
68.3
“
fig1-espe.gif
HI observations were performed with the VLA in the C array configuration
(synthesized beam of 23’’×18’’). A velocity range between 6259.4 km/s and
6802.2 km/s was covered with a velocity resolution of 10.8 km/s. The
observations cover a total field of 30´centered in the group. The rms noise
is 0.65 mJy/beam corresponding to a column density of 1.8x1019 cm¯².
In the figure on the right we present the integrated HI emission
superposed on the J+Ks image taken with Subaru
(http://www.naoj.org, Subaru Telescope is operated by the
National Astronomical Observatory of Japan). The individual main
group members identified by Hickson are indicated and the
positions of the slits are traced.
HI emission has been only detected in two members of the group, H40C
and H40D. No HI has been detected outside from the galaxies.
Rotation curves
HI Analysis
In the Fig. to the right we present the
optical velocity curve along the major axis
of H40D. The position is expressed as
offsets with respect to the photometric
center of the galaxy, taken as the maximum
of the galaxy continuum distribution
through the slit. At the bottom, the Hα and
continuum intensity profiles along the slit
in arbitrary units are plotted. The extent of
the ionized gas emission detected in the
spectrum is ~ 20” that corresponds to less
than half of the optical disk
HI radial velocity field of H40C. The
contours go from 6520km/s, near the
galaxy center, to 6000km/s in the outer part
, with a step of 10km/s.
Channel maps of the 21 cm line.
Heliocentric velocities are indicated
in each panel. Contours correspond
to 3.8x1018 to 1.1x1019 at cm-1
The overall HI emission of the group shows a very asymmetric and distorted distribution. Only
H40C and H40D have been detected, and in both galaxies the HI is located in only a small fraction of
the disk.
Most of the HI emission is detected only in the SE side of the disk of H40C with a total mass of 9.1x108
M๏, and covering a velocity range from 6635.8 to 6399.1 km/s. That range agrees well with the
corresponding optical velocity curve, but no HI emission is detected in the opposite side.
The other region with HI emission corresponds to the NE side of the outer optical disk of H40D. We
measure a total mass of 3.3x 108 M๏ in the velocity range between 6635.8 and 6592.8 km/s. The observed
HI velocity is in agreement with the trend traced by the NE side of the optical rotation curve.
Masses and Dynamical Status
The dynamical mass of the group has been calculated using the virial
virial mass
estimator for the five galaxies and also adding H40F(v=6576km/s)
and H40G
H40F
(v=7372km/s).. In the following table ee present some dynamical parameters of the
system as a function of the number of members belonging to the group.
group.
5 Galaxies
v (km/s)
Rp (kpc)
kpc)
σv (km/s)
Mv (1012M๏)
6600
19
148
1.2
5 Galaxies+F
6596
43
136
1.3
5 Galaxies+F+G
6707
64
299
7.9
Considering only the central part of the group,
the escape velocity at the projected distance of
galaxies F and G is about 300km/s . A
comparison of the escape velocity with the
velocity difference between F and G galaxies
and the mean group velocity (24 km/s for F and
772km/s for G), suggests that galaxy F can be
considered a member of H40 whereas galaxy G
appears not to be gravitationally bounded to the
system.
DSS Image of the original
group members and the two
candidates H40F and H40G
The total luminosity of the main group amounts
to ~9x1010 L๏. This results in a total M/L for the
group of about 14 in solar units. This indicates a
low amount of dark matter besides the one
associated to the individual galaxies.
The rotation curve of H40D is highly perturbed showing a steep central gradient and
declining velocities with a sinusoidal shape in the SW side, that corresponds to the direction
of H40A, where no HI is detected. In the NE direction the velocity decreases up to a distance
of 8” from the center, later declining following a Keplerian behaviour. However this last
result should be taken with caution due to the weakness of the emission there.
The observed velocity amplitude, corrected by inclination, is of 248 km/s. This value is a bit
low for its classification as an early-type spiral. The photometric center is shifted by 0.21 kpc
to the NE with respect to the kinematical one, and has a velocity of 6745km/s, 70km/s lower
than kinematical center.
Optical velocity curve obtained along the
major axis of H40C as a function of the
distance to the photometric center. This is
defined as the maximum of the galaxy
continuum distribution through the slit,
shown as a blue line at the bottom of the
figure. The Hα intensity profile is also
represented. Emission lines are well
detected in a spatial range of 48”, which
corresponds to a 65% of the optical disk
extent.
The rotation curve shows a normal behavior, rising in the central
central part and being almost flat
outwards. Only some small velocity perturbations are present in the outer spatial sections.
The amplitude of the rotation amounts to 240 km/s, slightly high for its morphological type.
The curve is asymmetric with respect to the photometrical center,
center, as in H40D. Although it is
difficult determine the distance where the curve becomes flat, we have calculated the
kinematical center as the crosssection
leading
to
symmetrical
velocity differences between
cross
the two horizontal branches of the rotation curve and we have found that the kinematical
center is displaced 1.7 arcsecs (about 0.74 kpc)
kpc) to the SE from the photometrical one.
Conclusions
Conclusions
H40
H40 is
is composed
composed by
by six
six galaxies
galaxies and
and has
has aa dynamical
dynamical mass
mass of
of 1-2x10
1-2x101212M๏
M๏ and
and aa low
low M/L
M/L ratio
ratio ~14,
~14, not
not far
far from
from
the
the values
values found
found for
for the
the individual
individual galaxies.
galaxies. HI
HI emission
emission has
has been
been detected
detected in
in H40C
H40C and
and H40D
H40D but
but only
only in
in aa
fraction
fraction of
of the
the disks.
disks.
H40D
H40D shows
shows aa very
very distorted
distorted kinematics
kinematics with
with aa sinusoidal
sinusoidal rotation
rotation curve
curve shifted
shifted with
with respect
respect to
to the
the
photometrical
photometrical center.
center. Only
Only marginal
marginal HI
HI emission
emission is
is detected
detected in
in this
this galaxy
galaxy with
with aa deficiency
deficiency of
of 92%
92% of
of the
theexpected
expected
HI
mass,
indicating
that
it
has
been
suffering
a
continuous,
old,
and
strong
interaction
in
the
group
environment.
HI mass, indicating that it has been suffering a continuous, old, and strong interaction in the group environment.
The
The optical
optical disk
disk of
of H40C
H40C does
does not
not seem
seem to
to show
show any
any strong
strong distortions
distortions whereas
whereas itit has
has an
an asymmetric
asymmetric and
and
perturbed
perturbed HI
HI distribution
distribution with
with the
the HI
HI completely
completely depleted
depleted in
in one
one side
side of
of the
the disk
disk and
and aa 80%
80% deficiency.
deficiency. Since
Since this
this
galaxy
galaxy has
has aa velocity
velocity difference
difference of
of about
about 340km/s
340km/s with
with respect
respect to
to the
the other
other galaxies
galaxies in
in the
the group,
group, the
the most
most plausible
plausible
explanation
explanation is
is that
that this
this galaxy
galaxy is
is falling
falling into
into the
the group
group and
and itit is
is suffering
suffering striping
striping and/or
and/or heating
heating of
of its
its HI
HI
component.
component.
The
The perturbed
perturbed kinematics
kinematics of
of the
the galaxies,
galaxies, their
their low
low HI
HI content
content and
and distribution,
distribution, and
and the
the group
group dynamical
dynamical
parameters
parameters indicate
indicate that
that H40
H40 is
is aa high
high evolved
evolved group
group where
where the
the dynamics
dynamics is
is dominated
dominated by
by the
the elliptical
elliptical galaxy
galaxy
H40A
H40Aand
andthe
the spiral
spiralH40C
H40C is
is the
the latest
latest galaxy
galaxyarriving
arriving to
to the
the group.
group.