4,291 research outputs found
Ram-pressure stripped molecular gas in the Virgo spiral galaxy NGC 4522
IRAM 30m 12CO(1-0) and 12CO(2-1) HERA observations are presented for the
ram-pressure stripped Virgo spiral galaxy NGC 4522. The CO emission is detected
in the galactic disk and the extraplanar gas. The extraplanar CO emission
follows the morphology of the atomic gas closely but is less extended. The CO
maxima do not appear to correspond to regions where there is peak massive star
formation as probed by Halpha emission. The presence of molecular gas is a
necessary but not sufficient condition for star formation. Compared to the disk
gas, the molecular fraction of the extraplanar gas is 30% lower and the star
formation efficiency of the extraplanar gas is about 3 times lower. The
comparison with an existing dynamical model extended by a recipe for
distinguishing between atomic and molecular gas shows that a significant part
of the gas is stripped in the form of overdense arm-like structures. It is
argued that the molecular fraction depends on the square root of the total
large-scale density. Based on the combination of the CO/Halpha and an
analytical model, the total gas density is estimated to be about 4 times lower
than that of the galactic disk. Molecules and stars form within this dense gas
according to the same laws as in the galactic disk, i.e. they mainly depend on
the total large-scale gas density. Star formation proceeds where the local
large-scale gas density is highest. Given the complex 3D morphology this does
not correspond to the peaks in the surface density. In the absence of a
confining gravitational potential, the stripped gas arms will most probably
disperse; i.e. the density of the gas will decrease and star formation will
cease.Comment: 11 pages, 15 figures, A&A accepted for publicatio
Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501
VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The
HI disk is sharply truncated to the southwest, well within the stellar disk. A
region of low surface-density gas, which is more extended than the main HI
disk, is discovered northeast of the galaxy center. These data are compared to
existing 6cm polarized radio continuum emission, Halpha, and optical broad band
images. We observe a coincidence between the western HI and polarized emission
edges, on the one hand, and a faint Halpha emission ridge, on the other. The
polarized emission maxima are located within the gaps between the spiral arms
and the faint Halpha ridge. Based on the comparison of these observations with
a sample of dynamical simulations with different values for maximum ram
pressure and different inclination angles between the disk and the orbital
plane,we conclude that ram pressure stripping can account for the main observed
characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and
is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The
southwestern ridge of enhanced gas surface density and enhanced polarized
radio-continuum emission is due to ram pressure compression. It is argued that
the faint western Halpha emission ridge is induced by nearly edge-on ram
pressure stripping. NGC 4501 represents an especially clear example of early
stage ram pressure stripping of a large cluster-spiral galaxy.Comment: 22 pages, 25 figures, accepted for publication in A&
Phase Separation in Binary Fluid Mixtures with Continuously Ramped Temperature
We consider the demixing of a binary fluid mixture, under gravity, which is
steadily driven into a two phase region by slowly ramping the temperature. We
assume, as a first approximation, that the system remains spatially isothermal,
and examine the interplay of two competing nonlinearities. One of these arises
because the supersaturation is greatest far from the meniscus, creating
inversion of the density which can lead to fluid motion; although isothermal,
this is somewhat like the Benard problem (a single-phase fluid heated from
below). The other is the intrinsic diffusive instability which results either
in nucleation or in spinodal decomposition at large supersaturations.
Experimental results on a simple binary mixture show interesting oscillations
in heat capacity and optical properties for a wide range of ramp parameters. We
argue that these oscillations arise under conditions where both nonlinearities
are important
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&
VIVA, VLA Imaging of Virgo spirals in Atomic gas: I. The Atlas & The HI Properties
We present the result of a new VLA HI Imaging survey of Virgo galaxies, VIVA
(the VLA Imaging survey of Virgo galaxies in Atomic gas). The survey includes
high resolution HI data of 53 carefully selected late type galaxies (48 spirals
and 5 irregular systems). The goal is to study environmental effects on HI gas
properties of cluster galaxies to understand which physical mechanisms affect
galaxy evolution in different density regions, and to establish how far out the
impact of the cluster reaches. As a dynamically young cluster, Virgo contains
examples of galaxies experiencing a variety of environmental effects. Its
nearness allows us to study each galaxy in great detail. We have selected Virgo
galaxies with a range of star formation properties in low to high density
regions (at the projected distance from M87, d_87=0.3-3.3 Mpc). Contrary to pr
evious studies, more than half of the galaxies in the sample (~60%) are fainter
than 12 mag in B_T. Overall, the selected galaxies represent the late type
Virgo galaxies (S0/a to Sd/Irr) down to m_p<~14.6 fairly well in morphological
type, systemic velocity, subcluster membership, HI mass and deficiency. In this
paper (VIVA I: the atlas and the HI properties), we present HI maps and
properties, and describe the HI morphology and kinematics of individual
galaxies in detail (abbreviated).Comment: K band magnitudes for 6 galaxies in Table 3 have been corrected. One
of the labels in Figure 8 is corrected and an omission in the acknowledgments
has been added. The latter two were correct in the previous astro-ph version
but are wrong in the journal version. A full resolution with the complete HI
atlas can be downloaded at http://www.astro.yale.edu/viva/pub.htm
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