68 research outputs found
Direct Detections of Young Stars in Nearby Elliptical Galaxies
Small amounts of star formation in elliptical galaxies are suggested by
several results: surprisingly young ages from optical line indices, cooling
X-ray gas, and mid-IR dust emission. Such star formation has previously been
difficult to directly detect, but using UV Hubble Space Telescope (HST) Wide
Field Camera 3 (WFC3) imaging, we have identified individual young stars and
star clusters in four nearby ellipticals. This technique is orders of magnitude
more sensitive than other methods, allowing detections of star formation to
10^(-5) Msun/yr. Ongoing star formation is detected in all galaxies, including
three ellipticals that have previously exhibited potential signposts of star
forming conditions (NGC 4636, NGC 4697, and NGC 4374), as well as the typical
"red and dead" NGC 3379. The current star formation in our closest targets,
where we are most complete, is between 1-8x10^(-5) Msun/yr. The star formation
history was roughly constant from 0.5-1.5 Gyr (at 3-5x10^(-4) Msun/yr), but
decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a
mass between 10^2 - 10^4 Msun. The specific star formation rates of ~10^(-16)
yr^(-1) (at the present day) or ~10^(-14) yr^(-1) (when averaging over the past
Gyr) imply that a fraction 10^(-8) of the stellar mass is younger than 100 Myr
and 10^(-5) is younger than 1 Gyr, quantifying the level of frosting of recent
star formation over the otherwise passive stellar population. There is no
obvious correlation between either the presence or spatial distribution of
postulated star formation indicators and the star formation we detect.Comment: Accepted for publication in ApJ. 14 pages, 11 figure
An Interaction of a Magellanic Leading Arm High Velocity Cloud with the Milky Way Disk
The Leading Arm of the Magellanic System is a tidally formed HI feature
extending \sim 60\arcdeg from the Magellanic Clouds ahead of their direction
of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey
(GASS), supplemented with data from the Australia Telescope Compact Array, we
have found evidence for an interaction between a cloud in the Leading Arm and
the Galactic disk where the Leading Arm crosses the Galactic plane. The
interaction occurs at velocities permitted by Galactic rotation, which allows
us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the
Leading Arm crosses the Galactic Plane at a Galactic radius of
kpc.Comment: 14 pages, 5 figures, accepted to Astrophysical Journal Letters. Full
resolution version available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/LeadingArm_apjl.pd
GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc
We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS
data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick
disk, and Star clusters) fields. Its detected individual stars populate an
approximately one magnitude range of its luminosity function (LF). Using
synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we
find that the colors and magnitudes of GHOSTS I's individual stars are most
consistent with being young helium-burning and asymptotic giant branch stars at
a distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be actively
forming stars, so we tentatively classify it as a dwarf irregular (dIrr)
galaxy, although future HST observations deep enough to resolve a larger
magnitude range in its LF are required to make a more secure classification.
GHOSTS I's absolute magnitude is , making it one
of the least luminous dIrr galaxies known, and its metallicity is lower than
[Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and its
ellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites at
comparable luminosity. There are no luminous massive galaxies or galaxy
clusters within ~ 4 Mpc from GHOSTS I that could be considered as its host,
making it a very isolated dwarf galaxy in the Local Universe.Comment: 8 pages, 7 figures. Accepted for publication in Ap
Milky Way Disk-Halo Transition in HI: Properties of the Cloud Population
Using 21cm HI observations from the Parkes Radio Telescope's Galactic All-Sky
Survey, we measure 255 HI clouds in the lower Galactic halo that are located
near the tangent points at 16.9 < l < 35.3 degrees and |b| < 20 degrees. The
clouds have a median mass of 700 Msun and a median distance from the Galactic
plane of 660 pc. This first Galactic quadrant (QI) region is symmetric to a
region of the fourth quadrant (QIV) studied previously using the same data set
and measurement criteria. The properties of the individual clouds in the two
quadrants are quite similar suggesting that they belong to the same population,
and both populations have a line of sight cloud-cloud velocity dispersion of
sigma_cc ~ 16 km/s. However, there are three times as many disk-halo clouds at
the QI tangent points and their scale height, at h=800 pc, is twice as large as
in QIV. Thus the observed line of sight random cloud motions are not connected
to the cloud scale height or its variation around the Galaxy. The surface
density of clouds is nearly constant over the QI tangent point region but is
peaked near R~4 kpc in QIV. We ascribe all of these differences to the
coincidental location of the QI region at the tip of the Milky Way's bar, where
it merges with a major spiral arm. The QIV tangent point region, in contrast,
covers only a segment of a minor spiral arm. The disk-halo HI cloud population
is thus likely tied to and driven by large-scale star formation processes,
possibly through the mechanism of supershells and feedback.Comment: 14 pages, 20 figures, to be published in ApJ (accepted August 3 2010
The Spitzer Space Telescope First-Look Survey: KPNO MOSAIC-1 R-band Images and Source Catalogs
(Abridged) We present R-band images covering more than 11 square degrees of
sky obtained with the KPNO 4-m telescope in preparation for the Spitzer Space
Telescope First Look Survey. The FLS was designed to characterize the
mid-infrared sky at depths 2 orders of magnitude deeper than previous surveys.
The extragalactic component is the first cosmological survey done with Spitzer.
Two relatively large regions of the sky were observed: the main FLS extra
galactic field (17h18m+59d30m) and ELAIS-N1 field (16h10m+54d30m).
The overall quality of the images is high. The relative astrometric accuracy
is better than 0.1'' and the typical seeing is 1.1''. Images are relatively
deep since they reach a median 5-sigma depth limiting magnitude of R=25.5
(Vega). Catalogs have been extracted using SExtractor using thresholds in area
and flux for which the number of false detections is below 1% at R=25. Only
sources with S/N greater than 3 have been retained in the final catalogs.
Comparing the galaxy number counts from our images with those of deeper R-band
surveys, we estimate that our observations are 50% complete at R=24.5. These
limits in depth are sufficient to identify a substantial fraction of the
infrared sources which will be detected by Spitzer.Comment: 21 pages, 15 Postscript figures, uses aastex, AJ accepted. Images and
catalogs at http://ssc.spitzer.caltech.edu/fls/extragal/noaor_data.htm
Discovery of a Gas-Rich Companion to the Extremely Metal-Poor Galaxy DDO 68
We present HI spectral-line imaging of the extremely metal-poor galaxy DDO
68. This system has a nebular oxygen abundance of only 3% Z, making
it one of the most metal-deficient galaxies known in the local volume.
Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal
content, making it a significant outlier in the mass-metallicity and
luminosity-metallicity relationships. The origin of such a low oxygen abundance
in DDO 68 presents a challenge for models of the chemical evolution of
galaxies. One possible solution to this problem is the infall of pristine
neutral gas, potentially initiated during a gravitational interaction. Using
archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large
Array, we have discovered a previously unknown companion of DDO 68. This
low-mass (M 2.810 M), recently
star-forming (SFR 1.410 M yr,
SFR 710 M yr) companion has
the same systemic velocity as DDO 68 (V 506 km s; D
12.740.27 Mpc) and is located at a projected distance of 42 kpc. New HI
maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide
evidence that DDO 68 and this companion are gravitationally interacting at the
present time. Low surface brightness HI gas forms a bridge between these
objects.Comment: Accepted for publication in the Astrophysical Journal Letter
HI Clouds in the Lower Halo: I. The Galactic All-Sky Survey Pilot Region
We have detected over 400 HI clouds in the lower halo of the Galaxy within
the pilot region of the Galactic All-Sky Survey (GASS), a region of the fourth
quadrant that spans 18 degrees in longitude, 40 degrees in latitude and is
centered on the Galactic equator. These clouds have a median peak brightness
temperature of 0.6 K, a median velocity width of 12.8 km/s, and angular sizes
<1 degree. The motion of these clouds is dominated by Galactic rotation with a
random cloud-to-cloud velocity dispersion of 18 km/s. A sample of clouds likely
to be near tangent points was analyzed in detail. These clouds have radii on
the order of 30 pc and a median HI mass of 630 Msun. The population has a
vertical scale height of 400 pc and is concentrated in Galactocentric radius,
peaking at R=3.8 kpc. This confined structure suggests that the clouds are
linked to spiral features, while morphological evidence that many clouds are
aligned with loops and filaments is suggestive of a relationship with star
formation. The clouds might result from supernovae and stellar winds in the
form of fragmenting shells and gas that has been pushed into the halo rather
than from a galactic fountain.Comment: 16 pages. Accepted for publication in Ap
GASKAP -- The Galactic ASKAP Survey
A survey of the Milky Way disk and the Magellanic System at the wavelengths
of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule
will be carried out with the Australian Square Kilometre Array Pathfinder
telescope. The survey will study the distribution of HI emission and absorption
with unprecedented angular and velocity resolution, as well as molecular line
thermal emission, absorption, and maser lines. The area to be covered includes
the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg,
spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire
area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The
brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K
at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific
goals, from studies of galaxy evolution to star formation, with particular
contributions to understanding stellar wind kinematics, the thermal phases of
the interstellar medium, the interaction between gas in the disk and halo, and
the dynamical and thermal states of gas at various positions along the
Magellanic Stream.Comment: 45 pages, 8 figures, Pub. Astron. Soc. Australia (in press
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