94 research outputs found
Dark Matter in Dwarf Galaxies: High Resolution Observations
We present observations and analysis of rotation curves and dark matter halo
density profiles in the central regions of four nearby dwarf galaxies. This
observing program has been designed to overcome some of the limitations of
other rotation curve studies that rely mostly on longslit spectra. We find that
these objects exhibit the full range of central density profiles between
constant density and NFW halos. This result suggests that there is a
distribution of central density slopes rather than a unique halo density
profile.Comment: 6 pages, 4 figures. To appear in the proceedings of IAU Symposium 220
"Dark Matter in Galaxies", which took place in July 200
Giant Molecular Clouds in Local Group Galaxies
We present the first comparative study of extragalactic GMCs using complete
data sets for entire galaxies and a uniform set of reduction and analysis
techniques. We present results based on CO observations for the LMC, SMC, M33,
M31, IC10 and the nucleus of M64, and make comparisons with archival Milky Way
observations. Our sample includes large spirals and dwarf irregulars with
metallicities that vary by an order of magnitude. GMCs in HI rich galaxies are
seen to be well-correlated with HI filaments that pervade the galactic disks,
suggesting that they form from pre-existing HI structures. Virial estimates of
the ratio of CO line strength to H2 column density, XCO, suggests that a value
of 4 x 10^20 cm^-2 (K km/s)^-1 is a good value to use in most galaxies (except
the SMC) if the GMCs are virialized. However, if the clouds are only marginally
self-gravitating, as appears to be the case judging from their appearance, half
the virial value may be more appropriate. There is no clear trend of XCO with
metallicity. The clouds within a galaxy are shown to have the about the same H2
surface density and differences between galaxies seem to be no more than a
factor of ~2. We show that hydrostatic pressure appears to be the main factor
in determining what fraction of atomic gas is turned into molecules. In the
high-pressure regions often found in galactic centers, the observed properties
of GMCs appear to be different from those in the found in the Local Group. From
the association of tracers of star formation with GMCs in the LMC, we find that
about 1/4 of the GMCs exhibit no evidence of star formation and we estimate
that the lifetime of a typical GMC in these galaxies is 20--30 Myr.Comment: 17 pages, 13 figures, to appear in Protostars and Planets V. A full
color version with higher resolution figures is available at
http://astron.berkeley.edu/~blitz/PP5/ms.pd
High-Resolution Measurements of the Halos of Four Dark Matter-Dominated Galaxies: Deviations from a Universal Density Profile
We derive rotation curves for four nearby, low-mass spiral galaxies and use
them to constrain the shapes of their dark matter density profiles. This
analysis is based on high-resolution two-dimensional Halpha velocity fields of
NGC 4605, NGC 5949, NGC 5963, and NGC 6689 and CO velocity fields of NGC 4605
and NGC 5963. In combination with our previous study of NGC 2976, the full
sample of five galaxies contains density profiles that span the range from
alpha_dm = 0 to alpha_dm = 1.20, where alpha_dm is the power law index
describing the central density profile. The scatter in alpha_dm from galaxy to
galaxy is 0.44, three times as large as in Cold Dark Matter (CDM) simulations,
and the mean density profile slope is alpha_dm = 0.73, shallower than that
predicted by the simulations. These results call into question the hypothesis
that all galaxies share a universal dark matter density profile. We show that
one of the galaxies in our sample, NGC 5963, has a cuspy density profile that
closely resembles those seen in CDM simulations, demonstrating that while
galaxies with the steep central density cusps predicted by CDM do exist, they
are in the minority. In spite of these differences between observations and
simulations, the relatively cuspy density profiles we find do not suggest that
this problem represents a crisis for CDM. Improving the resolution of the
simulations and incorporating additional physics may resolve the remaining
discrepancies. We also find that four of the galaxies contain detectable radial
motions in the plane of the galaxy. We investigate the hypothesis that these
motions are caused by a triaxial dark matter halo, and place lower limits on
the ellipticity of the orbits in the plane of the disk of 0.043 - 0.175.Comment: 22 pages, 7 tables, 9 figures (7 in color; Figure 1 resolution
degraded). Accepted for publication in Ap
On the Variation of Gas Depletion Time
We present an updated status of the EDGE project, which is a survey of 125
local galaxies in the CO() and CO() lines. We combine
the molecular data of the EDGE survey with the stellar and ionized gas maps of
the CALIFA survey to give a comprehensive view of the dependence of the star
formation efficiency, or equivalently, the molecular gas depletion time, on
various local environments, such as the stellar surface density, metallicity,
and radius from the galaxy center. This study will provide insight into the
parameters that drive the star formation efficiency in galaxies at .Comment: 4 pages, 2 figures, Proceedings of IAU Symposium No. 315, 201
Dense Gas Fraction and Star Formation Efficiency Variations in the Antennae Galaxies
We use the CARMA millimeter interferometer to map the Antennae Galaxies
(NGC4038/39), tracing the bulk of the molecular gas via the 12CO(1-0) line and
denser molecular gas via the high density transitions HCN(1-0), HCO+(1-0),
CS(2-1), and HNC(1-0). We detect bright emission from all tracers in both the
two nuclei and three locales in the overlap region between the two nuclei.
These three overlap region peaks correspond to previously identified
"supergiant molecular clouds". We combine the CARMA data with Herschel infrared
(IR) data to compare observational indicators of the star formation efficiency
(SFR/H2~IR/CO), dense gas fraction (HCN/CO), and dense gas star formation
efficiency (IR/HCN). Regions within the Antennae show ratios consistent with
those seen for entire galaxies, but these ratios vary by up to a factor of 6
within the galaxy. The five detected regions vary strongly in both their
integrated intensities and these ratios. The northern nucleus is the brightest
region in mm-wave line emission, while the overlap region is the brightest part
of the system in the IR. We combine the CARMA and Herschel data with ALMA CO
data to report line ratio patterns for each bright point. CO shows a declining
spectral line energy distribution, consistent with previous studies. HCO+(1-0)
emission is stronger than HCN(1-0) emission, perhaps indicating either more gas
at moderate densities or higher optical depth than is commonly seen in more
advanced mergers.Comment: 9 pages, 4 figures, ApJ accepte
CARMA Survey toward Infrared-bright Nearby Galaxies (STING). IV. Spatially Resolved 13CO in Spiral Galaxies
We present a mapping survey of 12
nearby galaxies from the CARMA STING sample. The line intensity ratio
is derived to study the
variations in molecular gas properties. For 11 galaxies where it can be
measured with high significance, the spatially resolved on
(sub-)kiloparsec scales varies by up to a factor of 3--5 within a galaxy. Lower
values are usually found in regions with weaker . We
attribute this apparent trend to a bias against measuring large
values when is weak. Limiting our analysis to the
bright regions that are less biased, we do not find on (sub)kpc
scales correlate with galactocentric distance, velocity dispersion or the star
formation rate. The lack of correlation between SFR and indicates
that the CO optical depth is not sensitive to stellar energy input, or that any
such sensitivity is easily masked by other factors. Extending the analysis to
all regions with emission by spectral stacking, we find that 5
out of 11 galaxies show higher stacked for galactocentric radii
of kpc and , which could result from a greater contribution from diffuse
gas. Moreover, significant galaxy-to-galaxy variations are found in
, but the global does not strongly depend on dust
temperature, inclination, or metallicity of the galaxy.Comment: Accepted for publication in the Astrophysical Journal, 26 pages, 17
figure
Dark Matter in Dwarf Galaxies: Latest Density Profile Results
We present high-resolution two-dimensional velocity fields in Halpha and CO
of the nearby dwarf galaxy NGC 2976. Our observations were made at both higher
spatial resolution (~75 pc) and higher velocity resolution (13 km/s in Halpha
and 2 km/s in CO) than most previous studies. We show that NGC 2976 has a very
shallow dark matter density profile, with rho(r) lying between rho ~ r^-0.3 and
rho ~ r^0. We carefully test the effects of systematic uncertainties on our
results, and demonstrate that well-resolved, two-dimensional velocity data can
eliminate many of the systematic problems that beset longslit observations. We
also present a preliminary analysis of the velocity field of NGC 5963, which
appears to have a nearly NFW density profile.Comment: 10 pages, 4 figures (1 in color), uses newpasp.sty. To appear in the
proceedings of the conference "Satellites and Tidal Streams", La Palma,
Spain, 26-30 May 2003, eds. F. Prada, D. Martinez-Delgado, T. Mahone
The First Detection of Neutral Hydrogen in Emission in a Strong Spiral Lens
We report HI observations of eight spiral galaxies that are strongly lensing
background sources. Our targets were selected from the Sloan WFC (Wide Field
Camera) Edge-on Late-type Lens Survey (SWELLS) using the Arecibo, Karl G.
Jansky Very Large Array, and Green Bank telescopes. We securely detect
J1703+2451 at z=0.063 with a signal-to-noise of 6.7 and W50=79+/-13 km/s,
obtaining the first detection of HI emission in a strong spiral lens. We
measure a mass of M(HI)= 1.77+/-0.06(+0.35/-0.75) x 10^9 M_(sol) for this
source. We find that this lens is a normal spiral, with observable properties
that are fairly typical of spiral galaxies. For three other sources we did not
secure a detection; however, we are able to place strong constraints on the HI
masses of those galaxies. The observations for four of our sources were
rendered unusable due to strong radio frequency interference.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society (MNRAS), 10 pages, 5 figure
Resolved Properties of Extragalactic Giant Molecular Clouds
Giant molecular clouds (GMCs) are the major reservoirs of molecular gas in
galaxies, and the starting point for star formation. As such, their properties
play a key role in setting the initial conditions for the formation of stars.
We present a comprehensive combined inteferometric/single-dish study of the
resolved GMC properties in a number of extragalactic systems, including both
normal and dwarf galaxies. We find that the extragalactic GMC properties
measured across a wide range of environments, characterized by the Larson
relations, are to first order remarkably compatible with those in the Milky
Way. Using these data to investigate trends due to galaxy metallicity, we find
that: 1) these measurements do not accord with simple expectations from
photoionization-regulated star formation theory, 2) there is no trend in the
virial CO-to-H2 conversion factor on the spatial scales studied, and 3) there
are measurable departures from the Galactic Larson relations in the Small
Magellanic Cloud -- the object with the lowest metallicity in the sample --
where GMCs have velocity dispersions that are too small for their sizes. I will
discuss the stability of these clouds in the light of our recent far-infrared
analysis of this galaxy, and I will contrast the results of the virial and
far-infrared studies on the issue of the CO-to-H2 conversion factor and what
they tell us about the structure of molecular clouds in primitive galaxies.Comment: 4 pages, 3 figures, to appear in proceedings for IAU 255,
"Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies
CARMA Survey Toward Infrared-bright Nearby Galaxies (STING). III. The Dependence of Atomic and Molecular Gas Surface Densities on Galaxy Properties
We investigate the correlation between CO and HI emission in 18 nearby
galaxies from the CARMA Survey Toward IR-Bright Nearby Galaxies (STING) at
sub-kpc and kpc scales. Our sample, spanning a wide range in stellar mass and
metallicity, reveals evidence for a metallicity dependence of the HI column
density measured in regions exhibiting CO emission. Such a dependence is
predicted by the equilibrium model of McKee & Krumholz, which balances H_2
formation and dissociation. The observed HI column density is often smaller
than predicted by the model, an effect we attribute to unresolved clumping,
although values close to the model prediction are also seen. We do not observe
HI column densities much larger than predicted, as might be expected were there
a diffuse HI component that did not contribute to H_2 shielding. We also find
that the H_2 column density inferred from CO correlates strongly with the
stellar surface density, suggesting that the local supply of molecular gas is
tightly regulated by the stellar disk.Comment: 6 pages, to appear in ApJ Letter
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