54 research outputs found
Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402
We present optical, HI and radio continuum observations of the highly
inclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for ram-pressure
stripping and dense cloud ablation. VLA HI and radio continuum maps show a
truncated gas disk and emission to the northwest of the main disk emission. In
particular, the radio continuum emission is asymmetrically extended to the
north and skewed to the west. The Halpha image shows numerous HII complexes
along the southern edge of the gas disk, possibly indicating star formation
triggered by the ICM pressure. BVR images at 0.5" resolution obtained with the
WIYN Tip-Tilt Imager show a remarkable dust lane morphology: at half the
optical radius, the dust lane of the galaxy curves up and out of the disk,
matching the HI morphology. Large dust plumes extend upward for ~1.5 kpc from
luminous young star clusters at the SE edge of the truncated gas disk. These
star clusters are very blue, indicating very little dust reddening, which
suggests dust blown away by an ICM wind at the leading edge of the interaction.
To the south of the main ridge of interstellar material, where the galaxy is
relatively clean of gas and dust, we have discovered 1 kpc long linear dust
filaments with a position angle that matches the extraplanar radio continuum
tail; we interpret this angle as the projected ICM wind direction. One of the
observed dust filaments has an HII region at its head. We interpret these dust
filaments as large, dense clouds which were initially left behind as the
low-density ISM is stripped, but are then ablated by the ICM wind. These
results provide striking new evidence on the fate of molecular clouds in
stripped cluster galaxies.Comment: 17 pages, 4 figures, accepted for publication in AJ. See
ftp://ftp.astro.yale.edu/pub/hugh/papers/crowl_n4402.ps.gz for a version with
high-resolution figure
Ongoing Gas Stripping in the Virgo Cluster Spiral NGC 4522
The Virgo cluster galaxy NGC 4522 is one of the best spiral candidates for
ICM-ISM stripping in action. Optical broadband and H-alpha images from the WIYN
telescope of the highly inclined galaxy reveal a relatively undisturbed stellar
disk and a peculiar distribution of H-alpha emission. Ten percent of the
H-alpha emission arises from extraplanar HII regions which appear to lie within
filamentary structures >3 kpc long above one side of the disk. The filaments
emerge from the outer edge of a disk of bright H-alpha emission which is
abruptly truncated beyond 0.35R(25). Together the truncated H-alpha disk and
extraplanar H-alpha filaments are reminiscent of a bow shock morphology, which
strongly suggests that the interstellar medium (ISM) of NGC 4522 is being
stripped by the gas pressure of the intracluster medium (ICM). The galaxy has a
line-of-sight velocity of 1300 km/sec with respect to the mean Virgo cluster
velocity, and thus is expected to experience a strong interaction with the
intracluster gas. The existence of HII regions apparently located above the
disk plane suggests that star formation is occuring in the stripped gas, and
that newly formed stars will enter the galaxy halo and/or intracluster space.
The absence of HII regions in the disk beyond 0.35R(25), and the existence of
HII regions in the stripped gas suggest that even molecular gas has been
effectively removed from the disk of the galaxy.Comment: to appear in The Astronomical Journal, 16 pages, 5 figures, 1 tabl
Star Formation Thresholds in Galactic Disks
We report the first results of a detailed study of the star formation law in
a sample of 32 nearby spiral galaxies with well-measured rotation curves, HI
and H (as traced by CO) surface density profiles, and new \Ha CCD
photometry. Our results strongly support the view that the formation of
gravitationally bound interstellar clouds regulates the onset of widespread
star formation -- at least in the outer regions of galactic disks.Comment: Will appear in July 1 ApJ. Abbreviated abstract. Postscript version
available at http://www.astro.caltech.edu/~clm
The Evolution of the ISM in the Mildly Disturbed Spiral Galaxy NGC 4647
We present matched-resolution maps of HI and CO emission in the Virgo Cluster
spiral NGC 4647. The galaxy shows a mild kinematic disturbance in which one
side of the rotation curve flattens but the other side continues to rise. This
kinematic asymmetry is coupled with a dramatic asymmetry in the molecular gas
distribution but not in the atomic gas. An analysis of the gas column densities
and the interstellar pressure suggests that the H2/HI surface density ratio on
the east side of the galaxy is three times higher than expected from the
hydrostatic pressure contributed by the mass of the stellar disk. We discuss
the probable effects of ram pressure, gravitational interactions, and
asymmetric potentials on the interstellar medium and suggest it is likely that
a m=1 perturbation in the gravitational potential could be responsible for all
of the galaxy's features. Kinematic disturbances of the type seen here are
common, but the curious thing about NGC 4647 is that the molecular distribution
appears more disturbed than the HI distribution. Thus it is the combination of
the two gas phases that provides such interesting insight into the galaxy's
history and into models of the interstellar medium.Comment: ApJ, accepte
Testing Modified Newtonian Dynamics with Rotation Curves of Dwarf and Low Surface Brightness Galaxies
Dwarf and low surface brightness galaxies are ideal objects to test modified
Newtonian dynamics (MOND), because in most of these galaxies the accelerations
fall below the threshold below where MOND supposedly applies. We have selected
from the literature a sample of 27 dwarf and low surface brightness galaxies.
MOND is successful in explaining the general shape of the observed rotation
curves for roughly three quarters of the galaxies in the sample presented here.
However, for the remaining quarter, MOND does not adequately explain the
observed rotation curves. Considering the uncertainties in distances and
inclinations for the galaxies in our sample, a small fraction of poor MOND
predictions is expected and is not necessarily a problem for MOND. We have also
made fits taking the MOND acceleration constant, a_0, as a free parameter in
order to identify any systematic trends. We find that there appears to be a
correlation between central surface brightness and the best-fit value of a_0,
in the sense that lower surface brightness galaxies tend to have lower a_0.
However, this correlation depends strongly on a small number of galaxies whose
rotation curves might be uncertain due to either bars or warps. Without these
galaxies, there is less evidence of a trend, but the average value we find for
a_0 ~ 0.7*10^-8 cm s^-2 is somewhat lower than derived from previous studies.
Such lower fitted values of a_0 could occur if external gravitational fields
are important.Comment: 12 pages, accepted for publication in Ap
Ram pressure stripping and galaxy orbits: The case of the Virgo cluster
We investigate the role of ram pressure stripping in the Virgo cluster using
N-body simulations. Radial orbits within the Virgo cluster's gravitational
potential are modeled and analyzed with respect to ram pressure stripping. The
N-body model consists of 10000 gas cloud complexes which can have inelastic
collisions. Ram pressure is modeled as an additional acceleration on the clouds
located at the surface of the gas distribution in the direction of the galaxy's
motion within the cluster. We made several simulations changing the orbital
parameters in order to recover different stripping scenarios using realistic
temporal ram pressure profiles. We investigate systematically the influence of
the inclination angle between the disk and the orbital plane of the galaxy on
the gas dynamics. We show that ram pressure can lead to a temporary increase of
the central gas surface density. In some cases a considerable part of the total
atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk
after the stripping event. A quantitative relation between the orbit parameters
and the resulting HI deficiency is derived containing explicitly the
inclination angle between the disk and the orbital plane. The comparison
between existing HI observations and the results of our simulations shows that
the HI deficiency depends strongly on galaxy orbits. It is concluded that the
scenario where ram pressure stripping is responsible for the observed HI
deficiency is consistent with all HI 21cm observations in the Virgo cluster.Comment: 29 pages with 21 figures. Accepted for publication in Ap
Diffuse HI Disks in Isolated Galaxies
In order to investigate the contribution of diffuse components to their total
HI emission, we have obtained high precision HI line flux densities with the
100m Green Bank Telescope for a sample of 100 isolated spiral and irregular
galaxies which we have previously observed with the 43m telescope. A comparison
of the observed HI line fluxes obtained with the two different telescopes,
characterized by half-power beam widths of 9 arcmin and 21 arcmin respectively,
exploits a ``beam matching'' technique to yield a statistical determination of
the occurrence of diffuse HI components in their disks. A simple model of the
HI distribution within a galaxy well describes ~75 % of the sample and accounts
for all of the HI line flux density. The remaining galaxies are approximately
evenly divided into two categories: ones which appear to possess a
significantly more extensive HI distribution than the model predicts, and ones
for which the HI distribution is more centrally concentrated than predicted.
Examples of both extremes can be found in the literature but little attention
has been paid to the centrally concentrated HI systems. Our sample has
demonstrated that galaxies do not commonly possess extended regions of low
surface brightness HI gas which is not accounted for by our current
understanding of the structure of HI disks. Eight HI-rich companions to the
target objects are identified, and a set of extragalactic HI line flux density
calibrators is presented.Comment: 26 page
Towards the Secondary Bar: Gas Morphology and Dynamics in NGC 4303
The bulk of the molecular line emission in the double barred galaxy NGC4303
as observed in its CO(1-0) line with the OVRO mm-interferometer comes from two
straight gas lanes which run north-south along the leading sides of the
large-scale primary bar. Inside a radius of ~ 400 pc the molecular gas forms a
spiral pattern which, for the northern arm, can be traced to the nucleus.
Comparison of the OVRO and archival HST data with dynamical models of gas flow
in the inner kiloparsec of single- and double-barred galaxies shows that the
observed global properties of the molecular gas are in agreement with models
for the gas flow in a strong, large-scale bar, and the two-arm spiral structure
seen in CO in the inner kiloparsec can already be explained by a density wave
initiated by the potential of that bar. Only a weak correlation between the
molecular gas distribution and the extinction seen in the HST V-H map is found
in the inner 400 pc of NGC4303: The innermost part of one arm of the nuclear CO
spiral correlates with a weak dust filament in the color map, while the overall
dust distribution follows a ring or single-arm spiral pattern well correlated
with the UV continuum. This complicated nuclear geometry of the stellar and
gaseous components allows for two scenarios: (A) A self-gravitating m=1 mode is
present forming the spiral structure seen in the UV continuum. In this case the
gas kinematics would be unaffected by the small (~ 4'') inner bar. (B) The UV
continuum traces a complete ring which is heavily extincted north of the
nucleus. Such a ring forms in hydrodynamic models of double bars, but the
models cannot account for the UV emission observed on the leading side of the
inner bar. (abridged)Comment: 47 pages, 14 figures, accepted for publication in Ap
The Disk and Extraplanar Environment of NGC 247
The stellar content of the spiral galaxy NGC 247 is investigated. The main
sequence turn-off (MSTO) in the inner 12 kpc of the disk corresponds to an age
of 6 Myr. A mean star formation rate (SFR) of 0.1 solar masses per year during
the past 16 Myr is computed from star counts. The color of the red supergiant
plume does not change with radius, suggesting that the mean metallicity of
young stars does not vary by more than 0.1 dex. The number of bright main
sequence stars per local stellar mass density climbs towards larger radii out
to a distance of 12 kpc; the scale lengths that characterize the radial
distributions of young and old stars in the disk thus differ. The density of
bright main sequence stars with respect to projected HI mass gradually drops
with increasing radius. The population of very young stars disappears in the
outer disk; the MSTO at galactocentric radii between 12 and 15 kpc corresponds
to 16 Myr, while between 15 and 18 kpc the age is > 40 Myr. Red giant branch
(RGB) stars are resolved at a projected minor axis galactocentric distance of
12 kpc. There is a broad spread in metallicity among the RGB stars, with a mean
[M/H] = -1.2. The RGB-tip occurs at i' = 24.5 +/- 0.1, indicating that the
distance modulus is 27.9 +/- 0.1. Luminous AGB stars with an age 3 Gyr are also
seen in this field.Comment: Includes 16 eps figures; to appear in the Astrophysical Journa
A tale of two galaxies: light and mass in NGC891 and NGC7814
The two edge-on galaxies NGC891 and NGC7814 are representative of two extreme
morphologies: the former is disk-dominated while the latter is almost entirely
bulge-dominated. It has been argued (van der Kruit 1983) that since the two
galaxies, which are optically so different, have similar rotation curves their
total mass distributions cannot be related in any way to the light
distributions. This would lead to the conclusion that dark matter is the
dominating component of the mass. We have derived new rotation curves from
recent, high-sensitivity HI observations and have found that the shapes of the
rotation curves are significantly different for the two galaxies. They indicate
that in NGC7814 the mass is more concentrated to the centre as compared to
NGC891. This reflects the distribution of light which is more centrally
concentrated in NGC7814 than in NGC891. Mass and light do seem to be closely
related. This is confirmed by the analysis of the rotation curves in mass
components: solutions close to the maximum light (bulge + disk) do provide
excellent fits. In NGC891 bulge and disk can explain the rotation curve without
any need for dark matter out to ~15 kpc. In NGC7814 the bulge dominates in the
inner parts; further out the rotation curve is well reproduced by a maximum
disk but its M/L ratio is excessively high. A substantial dark matter
contribution, closely coupled to the luminous component, seems, therefore,
necessary.Comment: 10 pages, 9 figures, accepted for publication in A&
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