152 research outputs found
Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies
A new installment of neutral hydrogen mappings of Blue Compact Dwarf
galaxies, as defined by optical morphology, in and near the Virgo cluster is
presented. The primary motivation was to search for outlying clouds of HI as
potential interactive triggers of the enhanced star formation, and therefore
the mapped galaxies were selected for large HI} mass, large optical diameter,
and large velocity profile width. Approximately half the sample proved to have
one or more small, low column density star-free companion clouds, either
detached or appearing as an appendage in our maps, at resolution of order 4
kpc. Comparison is made to a sample of similarly mapped field BCD galaxies
drawn from the literature; however, the Virgo cluster sample of mapped BCDs is
still too small for conclusive comparisons to be made.
We found, on the one hand, little or no evidence for ram pressure stripping
nor, on the other, for extremely extended low column density HI envelopes. The
HI rotation curves in most cases rise approximately linearly, and slowly, as
far out as we can trace the gas.Comment: To appear in AJ, Dec. 200
The GALFA-HI Compact Cloud Catalog
We present a catalog of 1964 isolated, compact neutral hydrogen clouds from
the Galactic Arecibo L-Band Feed Array Survey Data Release One (GALFA-HI DR1).
The clouds were identified by a custom machine-vision algorithm utilizing
Difference of Gaussian kernels to search for clouds smaller than 20'. The
clouds have velocities typically between |VLSR| = 20-400 km/s, linewidths of
2.5-35 km/s, and column densities ranging from 1 - 35 x 10^18 cm^-2. The
distances to the clouds in this catalog may cover several orders of magnitude,
so the masses may range from less than a Solar mass for clouds within the
Galactic disc, to greater than 10^4 Solar Masses for HVCs at the tip of the
Magellanic Stream. To search for trends, we separate the catalog into five
populations based on position, velocity, and linewidth: high velocity clouds
(HVCs); galaxy candidates; cold low velocity clouds (LVCs); warm, low
positive-velocity clouds in the third Galactic Quadrant; and the remaining warm
LVCs. The observed HVCs are found to be associated with previously-identified
HVC complexes. We do not observe a large population of isolated clouds at high
velocities as some models predict. We see evidence for distinct histories at
low velocities in detecting populations of clouds corotating with the Galactic
disc and a set of clouds that is not corotating.Comment: 34 Pages, 9 Figures, published in ApJ (2012, ApJ, 758, 44), this
version has the corrected fluxes and corresponding flux histogram and masse
Star formation history of mass-selected galaxies in the ELAIS-N1 Field
We measure the specific star formation rates of \textit{K}-band selected
galaxies from the ELAIS-N1 by stacking GMRT data at 610 MHz. We identify a
sample of SFGs, spanning and
, using a
combination of multi-wavelength diagnostics obtained from the deep LoTSS
multi-wavelength catalogue. We measure the flux densities in the radio map and
estimate the radio SFR in order to probe the nature of the galaxies below the
noise and confusion limits. The massive galaxies in our sample have the lowest
sSFRs which is in agreement with previous studies. For the different
populations, we show that the sSFR-mass relation steepens with redshift, with
an average slope of for
the whole sample, and for
the SFGs. Our results indicate that galaxy populations undergo 'downsizing',
whereby most massive galaxies form their stars earlier and more rapidly than
low mass galaxies. Both populations show a strong decrease in their sSFR toward
the present epoch. The sSFR evolution with redshift is best described by a
power law , where for all galaxies, and for SFGs. Comparing our measured
sSFRs to results from literature, we find a general agreement in the
\textit{sSFR-M} plane.Comment: 17 pages, 13 figures, accepted for publication in MNRA
The HII region G35.673-00.847: another case of triggered star formation?
As part of a systematic study that we are performing with the aim to increase
the observational evidence of triggered star formation in the surroundings of
HII regions, we analyze the ISM around the HII region G35.673-00.847, a poorly
studied source. Using data from large-scale surveys: Two Micron All Sky Survey,
Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), MIPSGAL,
Galactic Ring Survey (GRS), VLA Galactic Plane Survey (VGPS), and NRAO VLA Sky
Survey (NVSS) we performed a multiwavelength study of G35.673-00.847 and its
surroundings. The mid IR emission, shows that G35.673-00.847 has an almost
semi-ring like shape with a cut towards the galactic west. The radius of this
semi-ring is about 1.5' (~1.6 pc, at the distance of ~3.7 kpc). The distance
was estimated from an HI absorption study and from the analysis of the
molecular gas. Indeed, we find a molecular shell composed by several clumps
distributed around the HII region, suggesting that its expansion is collecting
the surrounding material. We find several YSO candidates over the molecular
shell. Finally, comparing the HII region dynamical age and the fragmentation
time of the molecular shell, we discard the collect and collapse as the
mechanism responsible for the YSOs formation, suggesting other processes such
as radiative driven implosion and/or small-scale Jeans gravitational
instabilities.Comment: Accepted for publication in A&A, 18 October 2010. Some figures were
degraded to reduce file siz
Compact HI clouds from the GALFA-HI survey
The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the
entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR),
at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin.
The unprecedented resolution and sensitivity of the GALFA-HI survey have
resulted in the detection of numerous isolated, very compact HI clouds at low
Galactic velocities, which are distinctly separated from the HI disk emission.
In the limited area of ~4600 deg surveyed so far, we have detected 96 of
such compact clouds. The detected clouds are cold with a median T
(the kinetic temperature in the case in which there is no non-thermal
broadening) of 300 K. Moreover, these clouds are quite compact and faint, with
median values of 5 arcmin in angular size, 0.75 K in peak brightness
temperature, and cm in HI column density. Most of the
clouds deviate from Galactic rotation at the 20-30 km/s level, and a
significant fraction show evidence for a multiphase medium and velocity
gradients. No counterparts for these clouds were found in other wavebands. From
the modeling of spatial and velocity distributions of the whole compact cloud
population, we find that the bulk of the compact clouds are related to the
Galactic disk, and their distances are likely to be in the range of 0.1 to a
few kpc. We discuss various possible scenarios for the formation and
maintenance of this cloud population and its significance for Galactic ISM
studies.Comment: Accepted for publication in the Astrophysical Journa
Physical Properties of Complex C Halo Clouds
Observations from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey
of the tail of Complex C are presented and the halo clouds associated with this
complex cataloged. The properties of the Complex C clouds are compared to
clouds cataloged at the tail of the Magellanic Stream to provide insight into
the origin and destruction mechanism of Complex C. Magellanic Stream and
Complex C clouds show similarities in their mass distributions (slope = -0.7
and -0.6, respectively) and have a common linewidth of 20 - 30 km/s (indicative
of a warm component), which may indicate a common origin and/or physical
process breaking down the clouds. The clouds cataloged at the tail of Complex C
extend over a mass range of 10^1.1 to 10^4.8 solar masses, sizes of 10^1.2 to
10^2.6 pc, and have a median volume density of 0.065 cm^(-3) and median
pressure of (P/k) = 580 K cm^{-3}. We do not see a prominent two-phase
structure in Complex C, possibly due to its low metallicity and inefficient
cooling compared to other halo clouds. From assuming the Complex C clouds are
in pressure equilibrium with a hot halo medium, we find a median halo density
of 5.8 x 10^(-4) cm^(-3), which given a constant distance of 10 kpc, is at a
z-height of ~3 kpc. Using the same argument for the Stream results in a median
halo density of 8.4 x 10^(-5) x (60kpc/d) cm^(-3). These densities are
consistent with previous observational constraints and cosmological
simulations. We also assess the derived cloud and halo properties with three
dimensional grid simulations of halo HI clouds and find the temperature is
generally consistent within a factor of 1.5 and the volume densities, pressures
and halo densities are consistent within a factor of 3.Comment: Accepted for publication in AJ. 54 pages, including 6 tables and 16
figure
The Star Clusters in the Starburst Irregular Galaxy NGC 1569
We examine star clusters in the irregular, starburst galaxy NGC 1569 from HST
images. In addition to the two known super star clusters, we identify 45 other
clusters that are compact but resolved. Integrated UVI colors of the clusters
span a large range, and suggest that ages range from 3 Myrs to 1 Gyr. However,
most of the clusters were formed at the tail end of the recent starburst.
Numerous clusters in addition to the know super star clusters are similar in
luminosity to a small globular cluster. We examined the radial surface
brightness of four of the clusters. Their half-light radii and core radii are
in the range observed in present-day globular clusters. Therefore, conditions
that produced the recent starburst have also been those necessary for producing
compact, bright star clusters.
We examine resolved stars in the outer parts of the two super star clusters.
Cluster A is dominated by bright blue stars with a small population of red
supergiants. Sub-components A1 and A2 have similar colors and a two-dimensional
color map does not offer evidence that one component is dominated by red
supergiants and the other not. The contradiction of the presence of red super-
giants with Wolf-Rayet stars may instead not be a contradiction at all since
there coexistence in a coeval population is not inconsistent with the evolution
of massive stars. Cluster B is dominated by red supergiants, and this is
confirmed by the presence of the stellar CO absorption feature in an integrated
spectrum. The various age indicators are consistent with a picture in which
cluster B is of order 10--20 Myrs old, and cluster A is >4-5 Myrs old.Comment: To be published in AJ, November 200
A multiwavelength study of the star forming region IRAS 18544+0112
This work aims at investigating the molecular and infrared components in the
massive young stellar object (MYSO) candidate IRAS 18544+0112. The purpose is
to determine the nature and the origin of this infrared source. To analyze the
molecular gas towards IRAS 18544+0112, we have carried out observations in a
90" x 90" region around l = 34.69, b = -0.65, using the Atacama Submillimeter
Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO J=3-2, HCO+ J=4-3 and CS
J=7-6 lines with an angular resolution of 22". The infrared emission in the
area has been analyzed using 2MASS and Spitzer public data. From the molecular
analysis, we find self-absorbed 12CO J=3-2 profiles, which are typical in star
forming regions, but we do not find any evidence of outflow activity. Moreover,
we do not detect either HCO+ J=4-3 or CS J=7-6 in the region, which are species
normally enhanced in molecular outflows and high density envelopes. The 12CO
J=3-2 emission profile suggests the presence of expanding gas in the region.
The Spitzer images reveal that the infrared source has a conspicuous extended
emission bright at 8 um with an evident shell-like morphology of ~ 1.5 arcmin
in size (~ 1.4 pc at the proposed distance of 3 kpc) that encircles the 24 um
emission. The non-detection of ionized gas related to IRAS 18544+0112, together
with the fact that it is still embedded in a molecular clump suggest that IRAS
18544+0112, has not reached the UCHII region stage yet. Based on near infrared
photometry we search for YSO candidates in the region and propos that 2MASS
18565878+0116233 is the infrared point source associated with IRAS 18544+0112.
Finally, we suggest that the expansion of a larger nearby HII region,
G034.8-0.7, might be related to the formation of IRAS 18544+0112.Comment: 14 pages, accepted for publication in A&A. Figures degraded to reduce
file siz
HI Distribution and Kinematics of UGCA 86
We present 21-cm HI line and 408 MHz and 1.4 GHz continuum observations of
the dwarf galaxy UGCA 86 with the DRAO synthesis telescope. UGCA 86 is detected
in the continuum at 408 MHz (S_{408} = 120 +/- 30 mJy) and 1.4 GHz (S_{1400} =
79 +/- 3 mJy). The HI structure of UGCA 86 is complex, with two separate
components: a rotating disk and a highly elongated spur that is kinematically
disjunct from the disk. The HI disk is centered on the optical galaxy with
similar axial ratio and orientation of the major axis. An area of the disk with
a peculiar velocity of 25 km/s relative to the regular rotation of the disk is
found on the southern side, where most of the star formation is concentrated.
The spur is seen along the minor axis of UGCA 86 and overlaps in part with the
disk. Towards the optical center of UGCA 86, the velocity difference between
the spur and the disk is 40 km/s, about one third of the rotation velocity of
the HI disk at 6 kpc from the center. This implies a large radial component of
the orbital velocity of the spur, and therefore a significantly non-circular
orbit. The velocity dispersion of the disk is 8.8 km/s, whereas the velocity
dispersion of the spur varies from 10 km/s to 30 km/s. A possible tidal origin
of the spur is considered in view of the proximity of the large Scd galaxy IC
342. However, evidence that the spur is located far outside the plane of the HI
disk, and the absence of evidence for a warp in the outer HI disk, poses a
problem for the interpretation of the spur as a tidal tail induced by IC 342.
Detailed modelling of the IC 342/UGCA 86 system will be required before a tidal
origin of the spur can be dismissed conclusively. The possibility that the spur
is part of the nascent cloud of UGCA 86 or the remains of a small dwarf galaxy
is presented as an alternative interpretation.Comment: 27 pages, 11 figures, external as gif. A PDF file with figures
included is temporarily available from
http://www.ras.ucalgary.ca/~stil/ugca86.html Accepted for publication in the
Astrophysical Journa
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