2,226 research outputs found
A Stellar Mass Threshold for Quenching of Field Galaxies
We demonstrate that dwarf galaxies (10^7 < M_stellar < 10^9 Msun) with no
active star formation are extremely rare (<0.06%) in the field. Our sample is
based on the NASA-Sloan Atlas which is a re-analysis of the Sloan Digital Sky
Survey Data Release 8. We examine the relative number of quenched versus star
forming dwarf galaxies, defining quenched galaxies as having no Halpha emission
(EW_Halpha < 2 AA) and a strong 4000AA-break. The fraction of quenched dwarf
galaxies decreases rapidly with increasing distance from a massive host,
leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of
a massive host galaxy to be in the field. We demonstrate that there is a
stellar mass threshold of M_stellar < 1.0x10^9 Msun below which quenched
galaxies do not exist in the field. Below this threshold, we find that none of
the 2951 field dwarf galaxies are quenched; all field dwarf galaxies show
evidence for recent star formation. Correcting for volume effects, this
corresponds to a 1-sigma upper limit on the quenched fraction of 0.06%. In more
dense environments, quenched galaxies account for 23% of the dwarf population
over the same stellar mass range. The majority of quenched dwarf galaxies
(often classified as dwarf elliptical galaxies) are within 2 virial radii of a
massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond
4 virial radii. Thus, for galaxies with stellar mass less than 1.0x10^9 Msun,
ending star-formation requires the presence of a more massive neighbor,
providing a stringent constraint on models of star formation feedback.Comment: 9 pages, 6 figures, accepted to Ap
Ultraviolet images of the gravitationally lensed quadruple quasar Q2237+0305 with the HST
We analyze observations of the quadruple lensed quasar Q2237+0305, taken with
the WFPC2 camera in the F336W and F300W bands. 25 exposures were performed
within 15 hours real time on 3 November 1995. On a timescale of 3--4 hours, we
observe no variation in component A of greater than 0.02 mag. The other
components are constant over a period of 10 hours to within about 0.05 mag. In
the final 5 hours there is some evidence (not conclusive) for variation of
component D by about 0.1 mag. Component A was brighter than component B by
about 0.3 mag. Components C and D were fainter than component A by about 1.3
and 1.4 mag. Any fifth (central) component was at least 6.5 mag fainter than
component A. Using the PC chip, we measure the relative distances of the four
components with high accuracy. Our values are systematically larger than other
investigators' (by 0.1% to 2.0%). The F336W filter had been chosen for the
observations because it could have allowed us to see extended Ly-alpha emission
from the Broad-Line Region (BLR). However, the quasar components are consistent
with a point source. We conclude that there cannot be a Ly-alpha feature in the
image plane brighter than about 23.5 mag in F336W and further from the quasar
core than 100 mas. According to a lensing model by Rix, Scheider & Bahcall
(1992), this would preclude any such features in the source plane further than
20 mas (~ 100 h^{-1} pc, assuming q_0 = 0.5) from the quasar core and brighter
than 25 mag before magnification.Comment: 16 pages, Latex, 9 figures, accepted by MNRAS, 3 tables adde
Efficacy and Methodology of Urban Pigeon Control with DRC-1339
During 1990 and 1991, feral pigeon (Columbia livia) baiting programs using compound DRC-1339 were conducted in 6 cities in Kentucky and Tennessee. U. S. Department of Agriculture, Animal and Plant Health Inspection Service, Animal Damage Control (ADC) personnel evaluated each problem site, selected bait sites, supervised prebaiting, mixed, and applied toxic bait In most cases, local personnel conducted the prebaiting and disposed of dead birds. Seven pigeon flocks containing 95-735 birds were reduced by up to 100% with 1 or 2 baitings with 0.37% DRC-1339-treated bait Most birds died at their roosting areas or on rooftops where they were not visible to the public. Use of the bait sites by non target birds was minimal, and no non target kills were found during carcass pickup. DRC-1339 appears to be a very effective, selective, and safe means of urban pigeon population reduction
The Baryon Content of Extremely Low Mass Dwarf Galaxies
We investigate the gas content and baryonic Tully-Fisher relationship for
extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 >
Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists
of 101 galaxies for which we have obtained follow-up HI observations using the
Arecibo Observatory and Green Bank Telescope. This represents the largest
homogeneous sample of dwarfs at low luminosities with well-measured HI and
optical properties. The sample spans a range of environments, from dense groups
to truly isolated galaxies. The average neutral gas fraction is f_gas=0.6,
significantly exceeding that of typical gas-rich galaxies at higher
luminosities. Dwarf galaxies are therefore less efficient at turning gas into
stars over their lifetimes. The strong environmental dependence of the gas
fraction distribution demonstrates that while internal processes can reduce the
gas fractions to roughly f_gas=0.4, external processes are required to fully
remove gas from a dwarf galaxy. The average rotational velocity of our sample
is vrot=50 km/s. Including more massive galaxies from the literature, we fit a
baryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slope
compares well with CDM models that assume an equal baryon to dark matter ratio
at all masses. While gas stripping or other processes may modify the baryon to
dark matter ratio for dwarfs in the densest environments, the majority of dwarf
galaxies in our sample have not preferentially lost significant baryonic mass
relative to more massive galaxies.Comment: 33 pages, 8 figures. Accepted to ApJ. Data available at
http://www.ociw.edu/~mgeha/researc
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