1,022 research outputs found
Neutral Gas Distributions and Kinematics of Five Blue Compact Dwarf Galaxies
We present the results of high spatial resolution HI observations of five
intrinsically compact dwarf galaxies which are currently experiencing a strong
burst of star formation. The HI maps indicate that these systems have a complex
and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these
Blue Compact Dwarf (BCD) galaxies exhibit strong central concentrations in
their neutral gas distributions which may provide a clue to the origin of their
strong star-burst activity. Furthermore, while all of the systems do appear to
be rotating, based on observed velocity gradients, the kinematics are complex.
All systems have non-ordered kinematic structure at some level; some of the
extended gas is not necessarily kinematically connected to the main system.
The observed gas distributions and kinematics place constraints on
evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf
irregulars, and dwarf ellipticals have been postulated to explain their high
star formation rates and low luminosity, low metallicity nature. The BCDs
appear to have higher central mass concentrations in both gas and stellar
content than the dwarf irregulars, indicating that evolutionary scenarios
connecting these two classes will require mass redistribution. In addition, the
fact that BCDs are rotationally supported systems indicates that BCDs are
unlikely to evolve into dwarf ellipticals without substantial loss of angular
momentum. Thus, while such evolutionary scenarios may still be possible with
the aid of mergers or tidal interactions, the isolated nature of BCDs suggests
that the majority of BCDs will not fade to become objects similar to the
present day dwarf ellipticals.Comment: 19 pages, 15 figures. To appear in A
A Population of Metal-Poor Galaxies with ~L* Luminosities at Intermediate Redshifts
We present new spectroscopy and metallicity estimates for a sample of 15
star-forming galaxies with redshifts in the range 0.29 - 0.42. These objects
were selected in the KPNO International Spectroscopic Survey via their strong
emission lines seen in red objective-prism spectra. Originally thought to be
intermediate-redshift Seyfert 2 galaxies, our new spectroscopy in the far red
has revealed these objects to be metal-poor star-forming galaxies. These
galaxies follow a luminosity-metallicity (L-Z) relation that parallels the one
defined by low-redshift galaxies, but is offset by a factor of more than ten to
lower abundances. The amount of chemical and/or luminosity evolution required
to place these galaxies on the local L-Z relation is extreme, suggesting that
these galaxies are in a very special stage of their evolution. They may be
late-forming massive systems, which would challenge the current paradigm of
galaxy formation. Alternatively, they may represent intense starbursts in
dwarf-dwarf mergers or a major infall episode of pristine gas into a
pre-existing galaxy. In any case, these objects represent an extreme stage of
galaxy evolution taking place at relatively low redshift.Comment: 6 pages, 3 figures; to appear in 10 April 2009 ApJ
Comparing Infrared Star-Formation Rate Indicators with Optically-Derived Quantities
We examine the UV reprocessing efficiencies of warm dust and polycyclic
aromatic hydrocarbons (PAHs) through an analysis of the mid- and far-infrared
surface luminosity densities of 85 nearby H-selected star-forming
galaxies detected by the volume-limited KPNO International Spectroscopic Survey
(KISS). Because H selection is not biased toward continuum-bright
objects, the KISS sample spans a wide range in stellar masses
(-), as well as H luminosity
(-), mid-infrared 8.0m luminosity
(-), and [Bw-R] color (-.1-2.2). We find that
mid-infrared polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer IRAC
8.0m band correlates with star formation, and that the efficiency with
which galaxies reprocess UV energy into PAH emission depends on metallicity. We
also find that the relationship between far-infrared luminosity in the Spitzer
MIPS 24m band pass and H-measured star-formation rate varies from
galaxy to galaxy within our sample; we do not observe a metallicity dependence
in this relationship. We use optical colors and established mass-to-light
relationships to determine stellar masses for the KISS galaxies; we compare
these masses to those of nearby galaxies as a confirmation that the
volume-limited nature of KISS avoids strong biases. We also examine the
relationship between IRAC 3.6m luminosity and galaxy stellar mass, and
find a color-dependent correlation between the two.Comment: 15 pages, 10 figure
Radio line and continuum observations of quasar-galaxy pairs and the origin of low reshift quasar absorption line systems
There are a number of known quasars for which our line of sight to the high redshift quasar passes within a few Holmberg radii of a low redshift galaxy. In a few of these cases, spectra of the quasar reveal absorption by gas associated with the low redshift galaxy. A number of these pairs imply absorption by gas which lies well outside the optical disk of the associated galaxy, leading to models of galaxies with 'halos' or 'disks' of gas extending to large radii. The authors present observations of 4 such pairs. In three of the four cases, they find that the associated galaxy is highly disturbed, typically due to a gravitational interaction with a companion galaxy, while in the fourth case the absorption can be explained by clouds in the optical disk of the associated galaxy. They are led to an alternative hypothesis concerning the origin of the low redshift absorption line systems: the absorption is by gas clouds which have been gravitationally stripped from the associated galaxy. These galaxies are rapidly evolving, and should not be used as examples of absorption by clouds in halos of field spirals. The authors conclude by considering the role extended gas in interacting systems plays in the origin of higher redshift quasar absorption line systems
Infrared Properties of a Complete Sample of Star-Forming Dwarf Galaxies
We present a study of a large, statistically complete sample of star-forming
dwarf galaxies using mid-infrared observations from the {\it Spitzer Space
Telescope}. The relationships between metallicity, star formation rate (SFR)
and mid-infrared color in these systems show that the galaxies span a wide
range of properties. However, the galaxies do show a deficit of 8.0 \um\
polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0
\um\ luminosity which is only 0.004 \lstarf\ while the median -band
luminosity is 0.05 \lstarb. Despite many of the galaxies being 8.0 \um\
deficient, there is about a factor of 4 more extremely red galaxies in the
[3.6] [8.0] color than for a sample of normal galaxies with similar optical
colors. We show correlations between the [3.6] [8.0] color and luminosity,
metallicity, and to a lesser extent SFRs that were not evident in the original,
smaller sample studied previously. The luminosity--metallicity relation has a
flatter slope for dwarf galaxies as has been indicated by previous work. We
also show a relationship between the 8.0 \um\ luminosity and the metallicity of
the galaxy which is not expected given the competing effects (stellar mass,
stellar population age, and the hardness of the radiation field) that influence
the 8.0 \um\ emission. This larger sample plus a well-defined selection
function also allows us to compute the 8.0 \um\ luminosity function and compare
it with the one for the local galaxy population. Our results show that below
10 \solar, nearly all the 8.0 \um\ luminosity density of the local
universe arises from dwarf galaxies that exhibit strong \ha\ emission -- i.e.,
8.0 \um\ and \ha\ selection identify similar galaxy populations despite the
deficit of 8.0 \um\ emission observed in these dwarfs.Comment: 13 pages, 11 figures, Published in Ap
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