375 research outputs found
A Search for Near-Infrared Emission From the Halo of NGC 5907 at Radii of 10 kpc to 30 kpc
We present a search for near-infrared (3.5-5 micron) emission from baryonic
dark matter in the form of low-mass stars and/or brown dwarfs in the halo of
the nearby edge-on spiral galaxy NGC 5907. The observations were made using a
256 by 256 InSb array with a pixel scale of 17" at the focus of a
liquid-helium-cooled telescope carried above the Earth's atmosphere by a
sounding rocket. In contrast to previous experiments which have detected a halo
around NGC 5907 in the V, R, I, J and K bands at galactic radii 6kpc < r <
10kpc, our search finds no evidence for emission from a halo at 10kpc < r <
30kpc. Assuming a halo mass density scaling as r^(-2), which is consistent with
the flat rotation curves that are observed out to radii of 32kpc, the lower
limit of the mass-to-light ratio at 3.5-5 microns for the halo of NGC 5907 is
250 (2 sigma) in solar units. This is comparable to the lower limit we have
found previously for NGC 4565 (Uemizu et al. 1998). Based on recent models, our
non-detection implies that hydrogen- burning stars contribute < 15% of the mass
of the dark halo of NGC 5907. Our results are consistent with the previous
detection of extended emission at r < 10kpc if the latter is caused by a
stellar population that has been ejected from the disk because of tidal
interactions. We conclude that the dark halo of NGC 5907, which is evident from
rotation curves that extend far beyond 10kpc, is not comprised of hydrogen
burning stars.Comment: 12 pages, LateX, plus 6 ps figures. Accepted by ApJ. minor changes,
added references, corrected typo
Chandra and HST Confirmation of the Luminous and Variable X-ray Source IC 10 X-1 as a Possible Wolf-Rayet, Black-Hole Binary
We present a Chandra and HST study of IC 10 X-1, the most luminous X-ray
binary in the closest starburst galaxy to the Milky Way. Our new hard X-ray
observation of X-1 confirms that it has an average 0.5-10 keV luminosity of
1.5e38 erg/s, is strongly variable (a factor of ~2 in >3 ks), and is spatially
coincident (within 0.'23 +/-0.'30) with the Wolf-Rayet (WR) star [MAC92] 17A in
IC 10. The spectrum of X-1 is best fit by a power law with photon index of ~1.8
and a thermal plasma with kT~1.5 keV, although systematic residuals hint at
further complexity. Taken together, these facts suggest that X-1 may be a black
hole belonging to the rare class of WR binaries; it is comparable in many ways
to Cyg X-3. The Chandra observation also finds evidence for extended X-ray
emission co-spatial with the large non-thermal radio superbubble surrounding
X-1.Comment: ApJL in press (Oct 2003), 4 pages, 4 figures (w/ fig1 at severely
reduced quality), latest emulateapj.cls use
The Evolution of the ISM in the Mildly Disturbed Spiral Galaxy NGC 4647
We present matched-resolution maps of HI and CO emission in the Virgo Cluster
spiral NGC 4647. The galaxy shows a mild kinematic disturbance in which one
side of the rotation curve flattens but the other side continues to rise. This
kinematic asymmetry is coupled with a dramatic asymmetry in the molecular gas
distribution but not in the atomic gas. An analysis of the gas column densities
and the interstellar pressure suggests that the H2/HI surface density ratio on
the east side of the galaxy is three times higher than expected from the
hydrostatic pressure contributed by the mass of the stellar disk. We discuss
the probable effects of ram pressure, gravitational interactions, and
asymmetric potentials on the interstellar medium and suggest it is likely that
a m=1 perturbation in the gravitational potential could be responsible for all
of the galaxy's features. Kinematic disturbances of the type seen here are
common, but the curious thing about NGC 4647 is that the molecular distribution
appears more disturbed than the HI distribution. Thus it is the combination of
the two gas phases that provides such interesting insight into the galaxy's
history and into models of the interstellar medium.Comment: ApJ, accepte
X-ray/Optical/Radio Observations of a Resolved Supernova Remnant in NGC 6822
The supernova remnant (SNR), Ho 12, in the center of the dwarf irregular
galaxy NGC 6822 was previously observed at X-ray, optical, and radio
wavelengths. By using archival Chandra and ground-based optical data, we found
that the SNR is spatially resolved in X-rays and optical. In addition, we
obtained a ~5" resolution radio image of the SNR. These observations provide
the highest spatial resolution imaging of an X-ray/optical/radio SNR in that
galaxy to date. The multi-wavelength morphology, X-ray spectrum and
variability, and narrow-band optical imagings are consistent with a SNR. The
SNR is a shell-shaped object with a diameter of about 10" (24 pc). The
morphology of the SNR is consistent across the wavelengths while the Chandra
spectrum can be well fitted with a nonequilibrium ionization model with an
electron temperature of 2.8 keV and a 0.3-7 keV luminosity of 1.6e37 erg/s. The
age of the SNR is estimated to be 1700-5800 years.Comment: 6 pages, 3 figures, accepted for publication in the Astronomical
Journa
The N/O Plateau of Blue Compact Galaxies: Monte Carlo Simulations of the Observed Scatter
Chemical evolution models and Monte Carlo simulation techniques have been
combined for the first time to study the distribution of blue compact galaxies
on the N/O plateau. Each simulation comprises 70 individual chemical evolution
models. For each model, input parameters relating to a galaxy's star formation
history (bursting or continuous star formation, star formation efficiency),
galaxy age, and outflow rate are chosen randomly from ranges predetermined to
be relevant. Predicted abundance ratios from each simulation are collectively
overplotted onto the data to test its viability. We present our results both
with and without observational scatter applied to the model points. Our study
shows that most trial combinations of input parameters, including a simulation
comprising only simple models with instantaneous recycling, are successful in
reproducing the observed morphology of the N/O plateau once observational
scatter is added. Therefore simulations which include delay of nitrogen
injection are no longer favored over those which propose that most nitrogen is
produced by massive stars, if only the plateau morphology is used as the
principal constraint. The one scenario which clearly cannot explain plateau
morphology is one in which galaxy ages are allowed to range below 250 Myr. We
conclude that the present data for the N/O plateau are insufficient by
themselves for identifying the portion of the stellar mass spectrum most
responsible for cosmic nitrogen production.Comment: 41 pages, 15 figures; accepted by ApJ, to appear Aug. 20, 200
A Quantitative Comparison of SMC, LMC, and Milky Way UV to NIR Extinction Curves
We present an exhaustive, quantitative comparison of all of the known
extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with
our understanding of the general behavior of Milky Way extinction curves. The
R_V dependent CCM relationship and the sample of extinction curves used to
derive this relationship is used to describe the general behavior of Milky Way
extinction curves. The ultraviolet portion of the SMC and LMC extinction curves
are derived from archival IUE data, except for one new SMC extinction curve
which was measured using HST/STIS observations. The optical extinction curves
are derived from new (for the SMC) and literature UBVRI photometry (for the
LMC). The near-infrared extinction curves are calculated mainly from 2MASS
photometry supplemented with DENIS and new JHK photometry. For each extinction
curve, we give R_V = A(V)/E(B-V) and N(HI) values which probe the same dust
column as the extinction curve. We compare the properties of the SMC and LMC
extinction curves with the CCM relationship three different ways: each curve by
itself, the behavior of extinction at different wavelengths with R_V, and
behavior of the extinction curve FM fit parameters with R_V. As has been found
previously, we find that a small number of LMC extinction curves are consistent
with the CCM relationship, but majority of the LMC and all of the SMC curves do
not follow the CCM relationship. For the first time, we find that the CCM
relationship seems to form a bound on the properties of all of the LMC and SMC
extinction curves. This result strengthens the picture of dust extinction
curves exhibit a continuum of properties between those found in the Milky Way
and the SMC Bar. (abridged)Comment: 18 pages, 10 figures, ApJ in pres
Electron cotunneling transport in gold nanocrystal arrays
We describe current-voltage characteristics I(V) of alkyl-ligated gold
nanocrystals arrays in long screening length limit. Arrays with
different alkyl ligand lengths have been prepared to tune the electronic tunnel
coupling between the nanocrystals. For long ligands, electronic diffusion
occurs through sequential tunneling and follows activated laws, as function of
temperature and as function of electric field . For better conducting arrays, i.e.
with small ligands, the transport properties crossover to the cotunneling
regime and follows Efros-Shklovskii laws as function of temperature and as function of electric field . The data shows that electronic
transport in nanocrystal arrays can be tuned from the sequential tunneling to
the cotunneling regime by increasing the tunnel barrier transparency
Neutral Hydrogen 21cm Absorption at Redshift 0.673 towards 1504+377
We detect the 21 cm line of neutral hydrogen in absorption at a redshift of
0.673 towards the 1 Jy radio source 1504+377. The 1504+377 radio source is
located toward the center of what appears to be an inclined disk galaxy at z =
0.674. The 21 cm absorption line shows multiple velocity components over a
velocity range of about 100 km sec, with a total HI column density:
N(HI) = cm. The
velocity-integrated optical depth of this system is the largest yet seen for
redshifted HI 21 cm absorption line systems (Carilli 1995). The 21 cm
absorption line is coincident in redshift with a previously detected broad
molecular absorption line system (Wiklind and Combes 1996). We do not detect HI
21 cm absorption associated with the narrow molecular absorption line system at
z = 0.67150, nor do we detect absorption at these redshifts by the 18 cm lines
of OH, nor by the 2 cm transition of HCO. There is no evidence for a bright
optical AGN in 1504+377, suggesting significant obscuration through the disk --
a hypothesis supported by the strong absorption observed. The 1504+377 system
resembles the ``red quasar'' PKS 1413+135, which has been modeled as a
optically obscured AGN with a very young radio jet in the center of a gas rich
disk galaxy (Perlman et al. 1996). The presence of very bright radio jets at
the centers of these two disk galaxies presents a challenge to unification
schemes for extragalactic radio sources and to models for the formation of
radio loud AGN.Comment: 17 pages, postscrip
Halo White Dwarfs and the Hot Intergalactic Medium
We present a schematic model for the formation of baryonic galactic halos and
hot gas in the Local Group and the intergalactic medium. We follow the
dynamics, chemical evolution, heat flow and gas flows of a hierarchy of scales,
including: protogalactic clouds, galactic halos, and the Local Group itself.
Within this hierarchy, the Galaxy is built via mergers of protogalactic
fragments. We find that early bursts of star formation lead to a large
population of remnants (mostly white dwarfs), which would reside presently in
the halo and contribute to the dark component observed in the microlensing
experiments. The hot, metal-rich gas from early starbursts and merging
evaporates from the clouds and is eventually incorporated into the
intergalactic medium. The model thus suggests that most microlensing objects
could be white dwarfs (m \sim 0.5 \msol), which comprise a significant
fraction of the halo mass. Furthermore, the Local Group could have a component
of metal-rich hot gas similar to, although less than, that observed in larger
clusters. We discuss the known constraints on such a scenario and show that all
local observations can be satisfied with present data in this model. The
best-fit model has a halo that is 40% baryonic, with an upper limit of 77%.Comment: 15 pages, LaTex, uses aas2pp4.sty, 7 postscript figures.
Substantially revised and enlarged to a full-length article. Somewhat
different quantitative results, but qualitative conclusions unchange
Far Infrared and Submillimeter Emission from Galactic and Extragalactic Photo-Dissociation Regions
Photodissociation Region (PDR) models are computed over a wide range of
physical conditions, from those appropriate to giant molecular clouds
illuminated by the interstellar radiation field to the conditions experienced
by circumstellar disks very close to hot massive stars. These models use the
most up-to-date values of atomic and molecular data, the most current chemical
rate coefficients, and the newest grain photoelectric heating rates which
include treatments of small grains and large molecules. In addition, we examine
the effects of metallicity and cloud extinction on the predicted line
intensities. Results are presented for PDR models with densities over the range
n=10^1-10^7 cm^-3 and for incident far-ultraviolet radiation fields over the
range G_0=10^-0.5-10^6.5, for metallicities Z=1 and 0.1 times the local
Galactic value, and for a range of PDR cloud sizes. We present line strength
and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158
micron, [O I] 63 and 145 micron, [C I] 370 and 609 micron, CO J=1-0, J=2-1,
J=3-2, J=6-5 and J=15-14, as well as the strength of the far-infrared
continuum. These plots will be useful for the interpretation of Galactic and
extragalactic far infrared and submillimeter spectra observable with ISO,
SOFIA, SWAS, FIRST and other orbital and suborbital platforms. As examples, we
apply our results to ISO and ground based observations of M82, NGC 278, and the
Large Magellenic Cloud.Comment: 54 pages, 20 figures, accepted for publication in The Astrophysical
Journa
- …