2,447 research outputs found
The Nature of the Peculiar Virgo Cluster Galaxies NGC 4064 and NGC 4424
We present a detailed study of the peculiar HI-deficient Virgo cluster spiral
galaxies NGC 4064 and NGC 4424, using CO 1-0 interferometry, optical
imaging and integral-field spectroscopic observations, in order to learn what
type of environmental interactions have afected these galaxies. Optical imaging
reveals that NGC 4424 has a strongly disturbed stellar disk, with banana-shaped
isophotes and shells. NGC 4064, which lies in the cluster outskirts, possesses
a relatively undisturbed outer stellar disk and a central bar. In both galaxies
H-alpha emission is confined to the central kiloparsec. CO observations reveal
bilobal molecular gas morphologies, with H-alpha emission peaking inside the CO
lobes, implying a time sequence in the star formation process.Gas kinematics
reveals strong bar-like non-circular motions in the molecular gas in both
galaxies, suggesting that the material is radially infalling. In NGC 4064 the
stellar kinematics reveal strong bar-like non-circular motions in the central 1
kpc. On the other hand, NGC 4424 has extremely modest stellar rotation
velocities (Vmax ~ 30 km s-1), and stars are supported by random motions as far
out as we can measure it. The observations suggest that the peculiarities of
NGC 4424 are the result of an intermediate-mass merger plus ram pressure
stripping. In the case of NGC 4064, the evidence suggests an already stripped
"truncated/normal" galaxy that recently suffered a minor merger or tidal
interaction with another galaxy. We propose that galaxies with
"truncated/compact" H-alpha morphologies such as these are the result of the
independent effects of ram pressure stripping, which removes gas from the outer
disk, and gravitational interactions such as mergers, which heat stellar disks,
drive gas to the central kpc and increase the central mass concentrations.Comment: 42 pages, 21 figure
NGC 4314. III. Inflowing Molecular Gas Feeding a Nuclear Ring of Star Formation
NGC 4314 is an early-type barred galaxy containing a nuclear ring of recent
star formation. We present CO(1-0) interferometer data of the bar and
circumnuclear region with 2.3 x 2.2 arcsec spatial resolution and 13 km/s
velocity resolution acquired at the Owens Valley Radio Observatory . These data
reveal a clumpy circumnuclear ring of molecular gas. We also find a peak of CO
inside the ring within 2 arcsec of the optical center that is not associated
with massive star formation. We construct a rotation curve from these CO
kinematic data and the mass model of Combes et al. (1992). Using this rotation
curve, we have identified the location of orbital resonances in the galaxy.
Assuming that the bar ends at corotation, the circumnuclear ring of star
formation lies between two Inner Lindblad Resonances, while the nuclear stellar
bar ends near the IILR. Deviations from circular motion are detected just
beyond the CO and H-alpha ring, where the dust lanes along the leading edge of
the bar intersect the nuclear ring. These non-circular motions along the minor
axis correspond to radially inward streaming motions at speeds of 20 - 90 km/s
and clearly show inflowing gas feeding an ILR ring. There are bright HII
regions near the ends of this inflow region, perhaps indicating triggering of
star formation by the inflow.Comment: 25 pages, uses aasms.sty. 7 Postscript figures, 12 JPEG figures.
Figures may be retrieved from
ftp://clyde.as.utexas.edu/pub/N4314COfigs.tar.g
Search for cold and hot gas in the ram pressure stripped Virgo dwarf galaxy IC3418
We present IRAM 30m sensitive upper limits on CO emission in the ram pressure
stripped dwarf Virgo galaxy IC3418 and in a few positions covering HII regions
in its prominent 17 kpc UV/Ha gas-stripped tail. In the central few arcseconds
of the galaxy, we report a possible marginal detection of about 1x10^6 M_sun of
molecular gas (assuming a Galactic CO-to-H_2 conversion factor) that could
correspond to a surviving nuclear gas reservoir. We estimate that there is less
molecular gas in the main body of IC3418, by at least a factor of 20, than
would be expected from the pre-quenching UV-based star formation rate assuming
the typical gas depletion timescale of 2 Gyr. Given the lack of star formation
in the main body, we think the H_2-deficiency is real, although some of it may
also arise from a higher CO-to-H_2 factor typical in low-metallicity, low-mass
galaxies. The presence of HII regions in the tail of IC3418 suggests that there
must be some dense gas; however, only upper limits of < 1x10^6 M_sun were found
in the three observed points in the outer tail. This yields an upper limit on
the molecular gas content of the whole tail < 1x10^7 M_sun, which is an amount
similar to the estimates from the observed star formation rate over the tail.
We also present strong upper limits on the X-ray emission of the stripped gas
in IC3418 from a new Chandra observation. The measured X-ray luminosity of the
IC3418 tail is about 280 times lower than that of ESO 137-001, a spiral galaxy
in a more distant cluster with a prominent ram pressure stripped tail.
Non-detection of any diffuse X-ray emission in the IC3418 tail may be due to a
low gas content in the tail associated with its advanced evolutionary state
and/or due to a rather low thermal pressure of the surrounding intra-cluster
medium.Comment: 15 pages, 11 figures, A&A accepte
The influence of the cluster environment on the star formation efficiency of 12 Virgo spiral galaxies
The influence of the environment on gas surface density and star formation
efficiency of cluster spiral galaxies is investigated. We extend previous work
on radial profiles by a pixel-to pixel analysis looking for asymmetries due to
environmental interactions. The star formation rate is derived from GALEX UV
and Spitzer total infrared data. As in field galaxies, the star formation rate
for most Virgo galaxies is approximately proportional to the molecular gas
mass. Except for NGC 4438, the cluster environment does not affect the star
formation efficiency with respect to the molecular gas. Gas truncation is not
associated with major changes in the total gas surface density distribution of
the inner disk of Virgo spiral galaxies. In three galaxies, possible increases
in the molecular fraction and the star formation efficiency with respect to the
total gas, of factors of 1.5 to 2, are observed on the windward side of the
galactic disk. A significant increase of the star formation efficiency with
respect to the molecular gas content on the windward side of ram
pressure-stripped galaxies is not observed. The ram-pressure stripped
extraplanar gas of 3 highly inclined spiral galaxies shows a depressed star
formation efficiency with respect to the total gas, and one of them (NGC 4438)
shows a depressed rate even with respect to the molecular gas. The
interpretation is that stripped gas loses the gravitational confinement and
associated pressure of the galactic disk, and the gas flow is diverging, so the
gas density decreases and the star formation rate drops. However, the stripped
extraplanar gas in one highly inclined galaxy (NGC 4569) shows a normal star
formation efficiency with respect to the total gas. We propose this galaxy is
different because it is observed long after peak pressure, and its extraplanar
gas is now in a converging flow as it resettles back into the disk.Comment: 34 pages, 24 figures, accepted for publication by A&
Virgo Galaxies with Long One-Sided HI Tails
In a new HI imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo
galaxies in Atomic gas), we find 7 spiral galaxies with long HI tails. The
morphology varies but all the tails are extended well beyond the optical radii
on one side. These galaxies are found in intermediate-low density regions
(0.6-1 Mpc in projection from M87). The tails are all pointing roughly away
from M87, suggesting that these tails may have been created by a global cluster
mechanism. While the tidal effects of the cluster potential are too small, a
rough estimate suggests that simple ram-pressure stripping indeed could have
formed the tails in all but two cases. At least three systems show HI
truncation to within the stellar disk, providing evidence for a gas-gas
interaction. Although most of these galaxies do not appear disturbed optically,
some have close neighbors, suggesting that tidal interactions may have moved
gas outwards making it more susceptible to the ICM ram-pressure or viscosity.
Indeed, a simulation study of one of the tail galaxies, NGC 4654, suggests that
the galaxy is most likely affected by the combined effect of a gravitational
interaction and ram-pressure stripping. We conclude that these one-sided HI
tail galaxies have recently arrived in the cluster, falling in on highly radial
orbits. It appears that galaxies begin to lose their gas already at
intermediate distances from the cluster center through ram-pressure or
turbulent viscous stripping and tidal interactions with neighbours, or a
combination of both.Comment: 4 pages, 3 figures (including 1 plate), accepted for accepted for
publication in ApJ Letter (vol. 659, L115), a version with full resolution
Plate 1 is available from
http://www.astro.umass.edu/~achung/astro-ph/viva_tail.pd
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