116 research outputs found
Night Matters—Why the Interdisciplinary Field of “Night Studies” Is Needed
The night has historically been neglected in both disciplinary and interdisciplinary research. To some extent, this is not surprising, given the diurnal bias of human researchers and the difficulty of performing work at night. The night is, however, a critical element of biological, chemical, physical, and social systems on Earth. Moreover, research into social issues such as inequality, demographic changes, and the transition to a sustainable economy will be compromised if the night is not considered. Recent years, however, have seen a surge in research into the night. We argue that “night studies” is on the cusp of coming into its own as an interdisciplinary field, and that when it does, the field will consider questions that disciplinary researchers have not yet thought to ask
Distances to Galactic high-velocity clouds. Complex C
We report the first determination of a distance bracket for the high-velocity
cloud (HVC) complex C. Combined with previous measurements showing that this
cloud has a metallicity of 0.15 times solar, these results provide ample
evidence that complex C traces the continuing accretion of intergalactic gas
falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as
well as He, the distance bracket implies a mass of 3-14x10^6 M_sun, and the
complex represents a mass inflow of 0.1-0.25 M_sun/yr. We base our distance
bracket on the detection of CaII absorption in the spectrum of the blue
horizontal branch star SDSS J120404.78+623345.6, in combination with a
significant non-detection toward the BHB star BS 16034-0114. These results set
a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more
weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the
BHB star BS 16079-0017.Comment: Accepted for publication in ApJ Letter
Dwarf Galaxy Dark Matter Density Profiles Inferred from Stellar and Gas Kinematics
We present new constraints on the density profiles of dark matter (DM) halos
in seven nearby dwarf galaxies from measurements of their integrated stellar
light and gas kinematics. The gas kinematics of low mass galaxies frequently
suggest that they contain constant density DM cores, while N-body simulations
instead predict a cuspy profile. We present a data set of high resolution
integral field spectroscopy on seven galaxies and measure the stellar and gas
kinematics simultaneously. Using Jeans modeling on our full sample, we examine
whether gas kinematics in general produce shallower density profiles than are
derived from the stars. Although 2/7 galaxies show some localized differences
in their rotation curves between the two tracers, estimates of the central
logarithmic slope of the DM density profile, gamma, are generally robust. The
mean and standard deviation of the logarithmic slope for the population are
gamma=0.67+/-0.10 when measured in the stars and gamma=0.58+/-0.24 when
measured in the gas. We also find that the halos are not under concentrated at
the radii of half their maximum velocities. Finally, we search for correlations
of the DM density profile with stellar velocity anisotropy and other baryonic
properties. Two popular mechanisms to explain cored DM halos are an exotic DM
component or feedback models that strongly couple the energy of supernovae into
repeatedly driving out gas and dynamically heating the DM halos. We investigate
correlations that may eventually be used to test models. We do not find a
secondary parameter that strongly correlates with the central DM density slope,
but we do find some weak correlations. Determining the importance of these
correlations will require further model developments and larger observational
samples. (Abridged)Comment: 29 pages, 18 figures, 10 tables, accepted for publication in Ap
A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfs from
the Sloan Digital Sky Survey Data Release 4. We have selected the stars through
photometric cuts and spectroscopic modeling, backed up by a set of visual
inspections. Roughly 6000 of the stars are new discoveries, roughly doubling
the number of spectroscopically confirmed white dwarfs. We analyze the stars by
performing temperature and surface gravity fits to grids of pure hydrogen and
helium atmospheres. Among the rare outliers are a set of presumed helium-core
DA white dwarfs with estimated masses below 0.3 Msun, including two candidates
that may be the lowest masses yet found. We also present a list of 928 hot
subdwarfs.Comment: Accepted by the Astrophysical Journal Supplements, 25 pages, 24
figures, LaTeX. The electronic catalog, as well as diagnostic figures and
links to the spectra, is available at http://das.sdss.org/wdcat/dr4
Chandra Observations of Type Ia Supernovae: Upper Limits to the X-ray Flux of SN 2002bo, SN 2002ic, SN 2005gj, and SN 2005ke
We set sensitive upper limits to the X-ray emission of four Type Ia
supernovae (SNe Ia) using the Chandra X-ray Observatory. SN 2002bo, a normal,
although reddened, nearby SN Ia, was observed 9.3 days after explosion. For an
absorbed, high temperature bremsstrahlung model the flux limits are 3.2E-16
ergs/cm^2/s (0.5-2 keV band) and 4.1E-15 ergs/cm^2/s (2-10 keV band). Using
conservative model assumptions and a 10 km/s wind speed, we derive a mass loss
rate of \dot{M} ~ 2E-5 M_\odot/yr, which is comparable to limits set by the
non-detection of Halpha lines from other SNe Ia. Two other objects, SN 2002ic
and SN 2005gj, observed 260 and 80 days after explosion, respectively, are the
only SNe Ia showing evidence for circumstellar interaction. The SN 2002ic X-ray
flux upper limits are ~4 times below predictions of the interaction model
currently favored to explain the bright optical emission. To resolve this
discrepancy we invoke the mixing of cool dense ejecta fragments into the
forward shock region, which produces increased X-ray absorption. A modest
amount of mixing allows us to accommodate the Chandra upper limit. SN 2005gj is
less well studied at this time. Assuming the same circumstellar environment as
for SN 2002i, the X-ray flux upper limits for SN 2005gj are ~4 times below the
predictions, suggesting that mixing of cool ejecta into the forward shock has
also occurred here. Our reanalysis of Swift and Chandra data on SN 2005ke does
not confirm a previously reported X-ray detection. The host galaxies NGC 3190
(SN 2002bo) and NGC 1371 (SN 2005ke) each harbor a low luminosity (L_X ~ 3-4E40
ergs/s) active nucleus in addition to wide-spread diffuse soft X-ray emission.Comment: 16 pages, to appear in ApJ (20 Nov 2007
Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1
The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are
tracers of energetic processes in and around the Milky Way. Clouds with
near-solar metallicity about one kpc above the disk trace the circulation of
material between disk and halo (the Galactic Fountain). The Magellanic Stream
consists of gas tidally extracted from the SMC, tracing the dark matter
potential of the Milky Way. Several other HVCs have low-metallicity and appear
to trace the continuing accretion of infalling intergalactic gas. These
assertions are supported by the metallicities (0.1 to 1 solar) measured for
about ten clouds in the past decade. Direct measurements of distances to HVCs
have remained elusive, however. In this paper we present four new distance
brackets, using VLT observations of interstellar \CaII H and K absorption
toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7
kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8
to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and
with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated
with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for
cloud g1, which appears to be in the outflow phase of the Fountain. Our
measurements further demonstrate that the Milky Way is accreting substantial
amounts of gaseous material, which influences the Galaxy's current and future
dynamical and chemical evolution.Comment: Accepted by Ap
The Hercules-Aquila Cloud
We present evidence for a substantial overdensity of stars in the direction
of the constellations of Hercules and Aquila. The Cloud is centered at a
Galactic longitude of about 40 degrees and extends above and below the Galactic
plane by at least 50 degrees. Given its off-centeredness and height, it is
unlikely that the Hercules-Aquila Cloud is related to the bulge or thick disk.
More likely, this is a new structural component of the Galaxy that passes
through the disk. The Cloud stretches about 80 degrees in longitude. Its
heliocentric distance lies between 10 and 20 kpc so that the extent of the
Cloud in projection is roughly 20 kpc by 15 kpc. It has an absolute magnitude
of -13 and its stellar population appears to be comparable to, but somewhat
more metal-rich than, M92.Comment: ApJ (Letters), in pres
Candidate spectroscopic binaries in the Sloan Digital Sky Survey
We have examined the radial velocity data for stars spectroscopically
observed by the Sloan Digital Sky Survey (SDSS) more than once to investigate
the incidence of spectroscopic binaries, and to evaluate the accuracy of the
SDSS stellar radial velocities. We find agreement between the fraction of stars
with significant velocity variations and the expected fraction of binary stars
in the halo and thick disk populations. The observations produce a list of 675
possible new spectroscopic binary stars and orbits for eight of them.Comment: 7 pages, 17 figures. Accepted for publication in Astronomy and
Astrophysics. Table 4 is available at
http://sb9.astro.ulb.ac.be/~pourbaix/Papers/data/SDSS/table4.da
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