548 research outputs found
HI in Virgo's "Red and Dead" Dwarf Ellipticals - A Tidal Tail and Central Star Formation
We investigate a sample of 3 dwarf elliptical galaxies in the Virgo Cluster
which have significant reservoirs of HI. We present deep optical imaging (from
CFHT and KPNO), HI spectra (Arecibo) and resolved HI imaging (VLA) of this
sample. These observations confirm their HI content and optical morphologies,
and indicate that the gas is unlikely to be recently accreted. The sample has
more in common with dwarf transitionals, although dwarf transitionals are
generally lower in stellar mass and gas fraction. VCC 190 has an HI tidal tail
from a recent encounter with the massive spiral galaxy NGC 4224. In VCC 611,
blue star-forming features are observed which were unseen by shallower SDSS
imaging.Comment: 11 pages, 6 figures, accepted in A
HIghMass - High HI Mass, HI-Rich Galaxies at : Combined HI and H Observations
We present resolved HI and CO observations of three galaxies from the
HIghMass sample, a sample of HI-massive (), gas-rich
( in top for their ) galaxies identified in the ALFALFA
survey. Despite their high gas fractions, these are not low surface brightness
galaxies, and have typical specific star formation rates (SFR) for their
stellar masses. The three galaxies have normal star formation rates for their
HI masses, but unusually short star formation efficiency scale lengths,
indicating that the star formation bottleneck in these galaxies is in the
conversion of HI to H, not in converting H to stars. In addition, their
dark matter spin parameters () are above average, but not
exceptionally high, suggesting that their star formation has been suppressed
over cosmic time but are now becoming active, in agreement with prior H
observations.Comment: 20 pages, 13 figure
The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems
We present VLA HI spectral line imaging of 5 sources discovered by ALFALFA.
These targets are drawn from a larger sample of systems that were not uniquely
identified with optical counterparts during ALFALFA processing, and as such
have unusually high HI mass to light ratios. These candidate "Almost Dark"
objects fall into 4 categories: 1) objects with nearby HI neighbors that are
likely of tidal origin; 2) objects that appear to be part of a system of
multiple HI sources, but which may not be tidal in origin; 3) objects isolated
from nearby ALFALFA HI detections, but located near a gas-poor early-type
galaxy; 4) apparently isolated sources, with no object of coincident redshift
within ~400 kpc. Roughly 75% of the 200 objects without identified counterparts
in the .40 database (Haynes et al. 2011) fall into category 1. This
pilot sample contains the first five sources observed as part of a larger
effort to characterize HI sources with no readily identifiable optical
counterpart at single dish resolution. These objects span a range of HI mass
[7.41 < log(M) < 9.51] and HI mass to B-band luminosity ratios (3 <
M/L < 9). We compare the HI total intensity and velocity
fields to SDSS optical imaging and to archival GALEX UV imaging. Four of the
sources with uncertain or no optical counterpart in the ALFALFA data are
identified with low surface brightness optical counterparts in SDSS imaging
when compared with VLA HI intensity maps, and appear to be galaxies with clear
signs of ordered rotation. One source (AGC 208602) is likely tidal in nature.
We find no "dark galaxies" in this limited sample. The present observations
reveal complex sources with suppressed star formation, highlighting both the
observational difficulties and the necessity of synthesis follow-up
observations to understand these extreme objects. (abridged)Comment: Astronomical Journal, in pres
Gas-Bearing Early-Type Dwarf Galaxies in Virgo: Evidence for Recent Accretion
We investigate the dwarf (M_B> -16) galaxies in the Virgo cluster in the
radio, optical, and ultraviolet regimes. Of the 365 galaxies in this sample, 80
have been detected in HI by the Arecibo Legacy Fast ALFA survey. These
detections include 12 early-type dwarfs which have HI and stellar masses
similar to the cluster dwarf irregulars and BCDs. In this sample of 12, half
have star-formation properties similar to late type dwarfs, while the other
half are quiescent like typical early-type dwarfs. We also discuss three
possible mechanisms for their evolution: that they are infalling field galaxies
that have been or are currently being evolved by the cluster, that they are
stripped objects whose gas is recycled, and that the observed HI has been
recently reaccreted. Evolution by the cluster adequately explains the
star-forming half of the sample, but the quiescent class of early-type dwarfs
is most consistent with having recently reaccreted their gas.Comment: 18 pages, 9 figure
Hot debris dust around HD 106797
Photometry of the A0 V main-sequence star HD 106797 with AKARI and
Gemini/T-ReCS is used to detect excess emission over the expected stellar
photospheric emission between 10 and 20 micron, which is best attributed to hot
circumstellar debris dust surrounding the star. The temperature of the debris
dust is derived as Td ~ 190 K by assuming that the excess emission is
approximated by a single temperature blackbody. The derived temperature
suggests that the inner radius of the debris disk is ~ 14 AU. The fractional
luminosity of the debris disk is 1000 times brighter than that of our own
zodiacal cloud. The existence of such a large amount of hot dust around HD
106797 cannot be accounted for by a simple model of the steady state evolution
of a debris disk due to collisions, and it is likely that transient events play
a significant role. Our data also show a narrow spectral feature between 11 and
12 micron attributable to crystalline silicates, suggesting that dust heating
has occurred during the formation and evolution of the debris disk of HD
106797.Comment: Accepted to ApJ Letters, 8 pages, 2 figure
A direct measurement of the baryonic mass function of galaxies & implications for the galactic baryon fraction
We use both an HI-selected and an optically-selected galaxy sample to
directly measure the abundance of galaxies as a function of their "baryonic"
mass (stars + atomic gas). Stellar masses are calculated based on optical data
from the Sloan Digital Sky Survey (SDSS) and atomic gas masses are calculated
using atomic hydrogen (HI) emission line data from the Arecibo Legacy Fast ALFA
(ALFALFA) survey. By using the technique of abundance matching, we combine the
measured baryonic function (BMF) of galaxies with the dark matter halo mass
function in a LCDM universe, in order to determine the galactic baryon fraction
as a function of host halo mass. We find that the baryon fraction of low-mass
halos is much smaller than the cosmic value, even when atomic gas is taken into
account. We find that the galactic baryon deficit increases monotonically with
decreasing halo mass, in contrast with previous studies which suggested an
approximately constant baryon fraction at the low-mass end. We argue that the
observed baryon fractions of low mass halos cannot be explained by reionization
heating alone, and that additional feedback mechanisms (e.g. supernova blowout)
must be invoked. However, the outflow rates needed to reproduce our result are
not easily accommodated in the standard picture of galaxy formation in a LCDM
universe.Comment: Accepted to ApJ. 45 pages (aastex), 19 figures; added references and
updated fig.18 for version
The Arecibo Legacy Fast ALFA Survey: The Galaxy Population Detected by ALFALFA
Making use of HI 21 cm line measurements from the ALFALFA survey (alpha.40)
and photometry from the Sloan Digital Sky Survey (SDSS) and GALEX, we
investigate the global scaling relations and fundamental planes linking stars
and gas for a sample of 9417 common galaxies: the alpha.40-SDSS-GALEX sample.
In addition to their HI properties derived from the ALFALFA dataset, stellar
masses (M_*) and star formation rates (SFRs) are derived from fitting the
UV-optical spectral energy distributions. 96% of the alpha.40-SDSS-GALEX
galaxies belong to the blue cloud, with the average gas fraction f_HI =
M_HI/M_* ~ 1.5. A transition in SF properties is found whereby below M_* ~
10^9.5 M_sun, the slope of the star forming sequence changes, the dispersion in
the specific star formation rate (SSFR) distribution increases and the star
formation efficiency (SFE) mildly increases with M_*. The evolutionary track in
the SSFR-M_* diagram, as well as that in the color magnitude diagram are linked
to the HI content; below this transition mass, the star formation is regulated
strongly by the HI. Comparison of HI- and optically-selected samples over the
same restricted volume shows that the HI-selected population is less evolved
and has overall higher SFR and SSFR at a given stellar mass, but lower SFE and
extinction, suggesting either that a bottleneck exists in the HI to H_2
conversion, or that the process of SF in the very HI-dominated galaxies obeys
an unusual, low efficiency star formation law. A trend is found that, for a
given stellar mass, high gas fraction galaxies reside preferentially in dark
matter halos with high spin parameters. Because it represents a full census of
HI-bearing galaxies at z~0, the scaling relations and fundamental planes
derived for the ALFALFA population can be used to assess the HI detection rate
by future blind HI surveys and intensity mapping experiments at higher
redshift.Comment: 21 pages (2 columns), 14 figures. Accepted for publication in ApJ.
Version with full-resolution figures is available at
http://egg.astro.cornell.edu/alfalfa/pubs/Huang2012b_120702.pd
Grain growth in the inner regions of Herbig Ae/Be star disks
We present new mid-infrared spectroscopy of
the emission from warm circumstellar dust grains
in Herbig Ae/Be stars. Our survey significantly
extends the sample that was studied by Bouwman et
al. (2001). We find a correlation between the
strength of the silicate feature and its shape.
We interpret this as evidence for the removal of
small (0.1 mu m) grains from the disk surface
while large (1-2 mu m) grains persist. If the
evolution of the grain size distribution is
dominated by gravitational settling, large grains
are expected to disappear first, on a timescale
which is much shorter than the typical age of our
programme stars. Our observations thus suggest a
continuous replenishment of micron sized grains
at the disk surface. If the grain replenishment
is due to the dredge-up of dust from the disk
interior, the mineralogy we observe is
representative of the bulk composition of dust in
these stars. Based on observations obtained at
the European Southern Observatory (ESO), La
Silla, and on observations with ISO, an ESA
project with instruments funded by ESA Member
States (especially the PI countries: France,
Germany, The Netherlands and the UK) and with the
participation of ISAS and NASA
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