1,318 research outputs found
Observations of Stripped Edge-on Virgo Cluster Galaxies
We present observations of highly inclined, HI deficient, Virgo cluster
spiral galaxies. Our high-resolution VLA HI observations of edge-on galaxies
allow us to distinguish extraplanar gas from disk gas. All of our galaxies have
truncated H-alpha disks, with little or no disk gas beyond a truncation radius.
While all the gas disks are truncated, the observations show evidence for a
continuum of stripping states: symmetric, undisturbed truncated gas disks
indicate galaxies that were stripped long ago, while more asymmetric disks
suggest ongoing or more recent stripping. We compare these timescale estimates
with results obtained from two-dimensional stellar spectroscopy of the outer
disks of galaxies in our sample. One of the galaxies in our sample, NGC 4522 is
a clear example of active ram-pressure stripping, with 40% of its detected HI
being extraplanar. As expected, the outer disk stellar populations of this
galaxy show clear signs of recent (and, in fact, ongoing) stripping. Somewhat
less expected, however, is the fact that the spectrum of the outer disk of this
galaxy, with very strong Balmer absorption and no observable emission, would be
classified as ``k+a'' if observed at higher redshift. Our observations of NGC
4522 and other galaxies at a range of cluster radii allow us to better
understand the role that clusters play in the structure and evolution of disk
galaxies.Comment: 4 pages, 2 figures, to appear in the proceedings of the Island
Universes conference held in Terschelling, Netherlands, July 2005, ed. R. de
Jong, version with high resolution figures can be downloaded from
ftp://ftp.astro.yale.edu/pub/hugh/papers/iu_crowl_h.ps.g
Arp 302: Non-starburst Luminous Infrared Galaxies
Arp 302, a luminous infrared source (L_{IR} = 4.2x10^{11} Lsun), consisting
of two spiral galaxies (VV340A and VV340B) with nuclear separation of 40'', has
the highest CO luminosity known. Observations with the BIMA array at 5'' X 7''
resolution reveal that the CO emission is extended over 23.0 kpc in the edge-on
spiral galaxy, VV340A, corresponding to 6.7x10^{10} Msun of H_2. In the
companion face-on galaxy, VV340B, the CO emission is extended over ~10.0 kpc,
with 1.1x10^{10} Msun of H_2. The large CO extent is in strong contrast to
starburst systems, such as Arp 220, in which the CO extent is typically 1
kpc. Furthermore, (H_2) is found to be 6.0 Lsun/Msun throughout
both galaxies. Thus the high IR luminosity of Arp 302 is apparently not due to
starbursts in the nuclear regions, but is due to its unusually large amount of
molecular gas forming stars at a rate similar to giant molecular clouds in the
Milky Way disk. Arp 302 consists of a pair of very gas-rich spiral galaxies
that may be interacting and in a phase before a likely onset of starbursts.Comment: AAS Latex plus two postscript figures. ApJ Letters (accepted
Radiative Transfer Effects in He I Emission Lines
We consider the effect of optical depth of the 2 ^{3}S level on the nebular
recombination spectrum of He I for a spherically symmetric nebula with no
systematic velocity gradients. These calculations, using many improvements in
atomic data, can be used in place of the earlier calculations of Robbins. We
give representative Case B line fluxes for UV, optical, and IR emission lines
over a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3},
and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for all
lines arising from quantum levels with n < 11 is also available from the
authors.
We present a special set of fitting formulae for the physical conditions
relevant to low metallicity extragalactic H II regions: T=12,000-20,000 K,
n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physical
conditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A,
and 6678 A, are changed less than 1%, in agreement with previous studies.
However, the 7065 A corrections are much smaller than those calculated by
Izotov & Thuan based on the earlier calculations by Robbins. This means that
the 7065 A line is a better density diagnostic than previously thought. Two
corrections to the fitting functions calculated in our previous work are also
given.Comment: To be published in 10 April 2002 ApJ; relevant code available at
ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu
Weak Lensing Effects on the Galaxy Three-Point Correlation Function
We study the corrections to the galaxy three-point correlation function
(3PCF) induced by weak lensing magnification due to the matter distribution
along the line of sight. We consistently derive all the correction terms
arising up to second order in perturbation theory and provide analytic
expressions as well as order of magnitude estimates for their relative
importance. The magnification contributions depend on the geometry of the
projected triangle on the sky plane, and scale with different powers of the
number count slope and redshift of the galaxy sample considered. We evaluate
all terms numerically and show that, depending on the triangle configuration as
well as the galaxy sample considered, weak lensing can in general significantly
contribute to and alter the three-point correlation function observed through
galaxy and quasar catalogs.Comment: 24 pages, 11 figures; version accepted for publication in Phys. Rev.
D; v2: typos corrected, figure caption clarifie
SBS 0335-052W - an Extremely Low Metallicity Dwarf Galaxy
We present Multiple Mirror Telescope (MMT) and Keck II telescope
spectrophotometry and 3.5m Calar Alto telescope R, I photometry of the western
component of the extremely low-metallicity blue compact galaxy SBS 0335-052.
The components, separated by 22 kpc, appear to be members of a unique,
physically connected system. It is shown that SBS 0335-052W consists of at
least three stellar clusters and has the same redshift as SBS 0335-052. The
oxygen abundance in its two brightest knots is extremely low, 12+log(O/H)=
7.22+/-0.03 and 7.13+/-0.08, respectively. These values are lower than in SBS
0335-052 and are nearly the same as those in I Zw 18. The (R-I) color profiles
are very blue in both galaxies due to the combined effects of ionized gas and a
young stellar population emission. We argue that SBS 0335-052W is likely to be
a nearby, young dwarf galaxy.Comment: 18 pages, 4 EPS figures, to appear in ApJ, 1 July 199
Discovery of X-ray emission rom the distant lensing cluster of galaxies CL2236-04 at z = 0.552
X-ray emission from the distant lensing cluster CL2236-04 at = 0.552 was
discovered by ASCA and ROSAT/HRI observations. If the spherical symmetric mass
distribution model of the cluster is assumed, the lensing estimate of the
cluster mass is a factor of two higher than that obtained from X-ray
observations as reported for many distant clusters. However, the elliptical and
clumpy lens model proposed by Kneib et al.(1993) is surprisingly consistent
with the X-ray observations assuming that the X-ray emitting hot gas is
isothermal and in a hydrostatic equilibrium state. The existence of the cooling
flow in the central region of the cluster is indicated by the short central
cooling time and the excess flux detected by ROSAT/HRI compared to the ASCA
flux. However, it is shown that even if the AXJ2239-0429 has a cooling flow in
the central region, the temperature measured by ASCA which is the mean
emission-weighted cluster temperature in this case, should not be cooler than
and different from the virial temperature of the cluster. Therefore, we
conclude that the effect of the clumpiness and non-zero ellipticity in the mass
distribution of the cluster are essential to explain the observed feature of
the giant luminous arc, and there is no discrepancy between strong lensing and
X-ray estimation of the mass of the cluster in this cluster.Comment: 18 pages, including 4 postscripts figs, LaTex. To appear in Part 1 of
The Astrophysical Journa
WR bubbles and HeII emission
We present the very first high quality images of the HeII 4686 emission in
three high excitation nebulae of the Magellanic Clouds. A fourth high
excitation nebula, situated around the WR star BAT99-2, was analysed in a
previous letter. Using VLT FORS data, we investigate the morphology of the ring
nebulae around the early-type WN stars BAT99-49 & AB7. We derive the total HeII
fluxes for each object and compare them with the most recent theoretical WR
models. Using Halpha, [OIII] and HeI 5876 images along with long-slit
spectroscopy, we investigate the physical properties of these ring nebulae and
find only moderate chemical enrichment. We also surveyed seven other LMC WR
stars but we failed to detect any HeII emission but note that the nebula around
BAT99-11 shows a N/O ratio and an oxygen abundance slightly lower than the LMC
values, while the nebula around BAT99-134 presents moderate chemical enrichment
similar to the one seen near BAT99-2, 49 and AB7. The third high excitation
nebula presented in this paper, N44C, does not harbor stars hotter than mid-O
main sequence stars. It was suggested to be a fossil X-ray nebula ionized but
our observations of N44C reveal no substantial changes in the excitation
compared to previous results reported in the literature.Comment: 13 pages, 8 figures (7 in jpg), accepted by A&A, also available from
http://vela.astro.ulg.ac.be/Preprints/P81/index.htm
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