Here are some images and supporting information for the January AAS press release on galaxy transformation in Abell 2125. The embargo time for media release is 0920 EST on Jan. 6, 2004. Most images are displayed at low resolution; click for the larger ones. The cluster is at redshift z=0.25 (920 Mpc or about 3 billion light-years distant); extensive redshift data indicate that it is dominated by mergers among substructures around a rich and X-ray bright core, all lying within a long filament seen near our line of sight. Multiwavelength data show us the fate of a lage spiral galaxy which has raced through the core's intergalactic gas and is having its own gas stripped away as we watch. The press release with images and video may be seen at the STScI press site. Other partners in this project are Frazer Owen (National Radio Astronomy Observatory), Michael Ledlow (Gemini Observatory), and Daniel Wang (University of Massachusetts. Neal Miller (NASA GSFC) and Glenn Morrison (IPAC) have also been involved in studying the dynamics of the cluster Abell 2125.
Supporting information for media:
One-page PDF version of the Atlanta AAS display presentation.
PowerPoint briefing presentation from
AAS media session.
Greg Bothun's image of the Coma cluster
for comparison with Alan Dressler's HST data
on Abell 851 at redshift z=0.4 graphically shows how spirals
have vanished from clusters over cosmic time (part of the Butcher-Oemler
effect).
There is a
nice
animated simulation of ram-pressure stripping removing
gas from a spiral presented by C. Balkowski, B. Vollmer, and V. Cayatte. Sequence of observations and conclusions, going back to about 1994: Above: the inner
part of Abell 2125 from two-color WFPC2 imaging. The triple central
galaxy is just off the left edge. C153 is the galaxy near the left side
showing plumes of emission-line material (red). Here are two displays of the same data now zoomed around C153 itself.
Linear (left) and logarithmic stretches are both helpful in showing
the inner chaotic dust structure and the plumes of star-forming
regions extending up (more or less north) and to the left from
it. These, in turn, are regions where other data suggest gas has
recently been swept from the galaxy. If there isn't already a
galaxy called the Flying Fish, this one will do. Color composite images of
Abell 2125 from the KPNO 4m Mosaic Imager.
Available as JPEGS covering 3x4 arcminutes
(a2125mos2.jpg) and, scrunched down,
6x8 arcminutes (a2125mos2wf.jpg);
even that last one is only about 1/16 of the full field by area.
Mark Hanna at NOAO has sent a trial TIFF
of the cluster core after cleaning up the star-saturation artifacts.
For your browsing pleasure, I've set up color composites of the
entire 8000x8000 Mosaic field, at full resolution
(12-Mbyte jpeg) or
half-size (2.8-Mbyte jpeg). Getting these
made required using both my workstation and laptop to cover for
one another's shortcomings... This image shows the major clumps in
Abell 2125 (there's another big group of galaxies in the 4 o-clock
direction to the SW), plus interesting background clusters, foreground
interacting galaxies, and the ubiquitous background of faint
blue galaxies. And looking north during the summer, we were graced by
the trails of not just one, but two, tumbling Russian SL-16 booster stages,
that aren't quite completely eliminated in image stacking.
Now for the multiwavelength evidence as to what kind of bad things
happened to this galaxy. The contours below are of [O II] emission from
ionized gas, observed with the KPNO 4m and CCD Mosaic, overlaid on
the HST V-band image (approximately emitted B band). This shows gas
in a trail from the galaxy, which makes sense if it has passed
through the center of the cluster's hot gas recently. The galaxy
is part of a subsystem with redshift 2000 km/s higher than the main
cluster (which likely has most of the X-ray gas seen by Chandra). This
sure looks like gas stripping in action. And for more excitement, the Chandra data by D. Wang et al. show not
only the usual intracluster gas, but when windowed for soft X-rays
only, show a trail from C153 matching the one seen in [O II]
emission. These data are shown contoured in an overlay with the
KPNO Mosaic blue-light image, since they extend well beyond the WFPC2
image borders. (This image already is full size, cribbed from a recent
proposal): Here is the cluster core comparing optical galaxy light and the
adaptivey smoothed X-ray structure. The low- and high-energy
X-rays (split at 2 keV) are shown as blue and violet, superimposed
on the visual image from the Kitt Peak Mosaic camera.
This contrasts the hotter intracluster gas with the cooler
gas in the tail of C153. Cooler by comparison with the 30 million K
of the cluster core, anyway.
This just in: Frazer has straightened out some alignment issues,
so here is an overlay of the highest-resolution VLA data on a color-enhanced
version of the HST WFPC2 image. It now suggests that we may see
radio emission along the minor axis, as one might expect from a
starburst- or AGN-driven wind, as well as from star formation in the galaxy and
the bright star-forming regions to the left (west) in the fishtail.
The central radio source is best explained by an active nucleus. Dense environments clearly foster the transformation of galaxies, but it has
proven difficult to untangle the roles of various processes in cluster
environments. We have found a uniquely strong case for ongoing stripping of
gas from the galaxy C153 in Abell 2125. The cluster, at z=0.25, includes
merging subsystems with a relative line-of-sight velocity near 2000 km/s.
C153, identified using the VLA as a strong radio source powered by star
formation, is the brightest cluster member with activity of this kind, and
part of the less populous blueshifted grouping. Several lines of evidence
indicate that it is being swept by a stripping event. (1) A tail of ionized
gas is seen in [O II] emission, which extends at least 70 kpc toward the cluster
core, coinciding with a soft X-ray feature seen in the Chandra observations
reported by Wang et al. (2) HST WFPC2 images reveal disturbed and clumpy
morphology, including luminous star-forming complexes and chaotic dust features. (3) The
spectral energy distribution and Gemini GMOS absorption-line spectrum indicate
a massive burst of star formation ~108 years ago superimposed on an older and much fainter population. (4) The stellar and gas kinematics are
decoupled, with multiple gas velocity systems including counter-rotating
components. The large velocity difference between the galaxy and (most of the)
intracluster medium may contribute to the signatures being more prominent
than hitherto seen. The starburst age is consistent with estimates of the time
since the closest encounter of the major subsystems during the cluster-level
merger. We continue to explore whether a starburst outflow or tidal damage
has added to the role of stripping by the ICM, and how star formation has
proceeded in the gas after leaving the galaxy disk. This work was supported by NASA through HST grant GO-07279.01-96A, and by the
NSF through facilities at NRAO, Kitt Peak, and Gemini-North. And here's the Chandra abstract: The structure of the universe is believed to have formed by clustering
hierarchically from small to large scales. Much of this evolution
occurs very slowly but at a few special times more, rapid, violent activity
may occur as major subunits collide at high velocities. Abell 2125
(z=0.247) appears to be undergoing such an event as shown by modeling of the
optical velocity field and by the detection with the VLA of an unusually large
number of associated radio active galaxies. The core of the
cluster contains four strong radio galaxies, one of which (C153) as imaged
by HST shows a very peculiar, disrupted morphology and a radial velocity
of 2000 km/s relative to the cluster mean. We present a spectacular
80 ksec Chandra imaging of Abell 2125. This X-ray imaging, together with
extensive complementary multi-wavelength data (see the accompanied
presentation), reveals for the first time the direct evidence
for galaxy transformation and destruction during the cluster formation.
The Chandra data unambiguously
separate the X-ray contributions from discrete sources and large-scale
diffuse gas in the Abell 2125 complex, which consists of various
merging clusters/groups of galaxies and low-surface brightness emission.
This enables us to study processes
affecting galaxy evolution during this special time from scales of Mpc down
to a few Kpc. On large scales, a patch of relatively prominent low-surface
brightness X-ray emission has a significantly lower temperature
metal abundance than in the Abell 2125 core cluster, characteristic
of a group environment. At higher resolution, soft X-ray enhancements are found
to be associated with bright radio galaxies in the core cluster. A distinct
X-ray trail, in particular, appears on one side of C153 suggesting that
the ISM is being stripped from this galaxy. The overall level of activity
plus the special time for the cluster-cluster merger suggests that an
important phase of galaxy evolution can take place during such events. This work was supported by NASA through Chandra grant GO1-2126. Here is a crude sketch
showing the cluster merger, constituent galaxies, and trail
of gas from C153. I had a little trouble getting the galaxies to
fade to the background color; ignore the black moats around some
of them. Here, deep blue is supposed to represent the hot cluster
gas seen in X-rays, whose effect accounts for the loss of gas from C153.
The Chandra data show that there is some large-scale lumpiness in
this gas, attributable to the cluster interaction.
[Ignore the cross effect on the galaxies too...]
C153 likely passed very close to the center of
the bigger cluster.
From our point of view, the the foreground cluster's large
central galaxy looks very close to the two big ones in the background.
There is additional excitement in the form of stars pulled out of some of these
galaxies by tidal forces.
This is a half-size image; click for the full-size one. Some additional relevant links:
AAS abstracts:
Last changes: 2 January 2004