371 research outputs found
Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
Recent studies of methanol and ground-state OH masers at very high spectral
resolution have shed new light on small-scale maser processes. The nearby
source W3(OH), which contains numerous bright masers in several different
transitions, provides an excellent laboratory for high spectral resolution
techniques. We present a model of W3(OH) based on EVN observations of the
rotationally-excited 6030 and 6035 MHz OH masers taken at 0.024 km/s spectral
resolution. The 6.0 GHz masers are becoming brighter with time and show
evidence for tangential proper motions. We confirm the existence of a region of
magnetic field oriented toward the observer to the southeast and find another
such region to the northeast in W3(OH), near the champagne flow. The 6.0 GHz
masers trace the inner edge of a counterclockwise rotating torus feature.
Masers at 6030 MHz are usually a factor of a few weaker than at 6035 MHz but
trace the same material. Velocity gradients of nearby Zeeman components are
much more closely correlated than in the ground state, likely due to the
smaller spatial separation between Zeeman components. Hydroxyl maser peaks at
very long baseline interferometric resolution appear to have structure on
scales both smaller than that resolvable as well as on larger scales.Comment: 21 pages using emulateapj.cls including 16 figures and 2 tables,
accepted to Ap
Spatially Resolved Millimeter Interferometry of SMMJ02399-0136: a Very Massive Galaxy at z=2.8
We report high-resolution millimeter mapping with the IRAM Plateau de Bure
interferometer of rest-frame 335 micron continuum and CO(3-2) line emission
from the z=2.8 submillimeter galaxy SMMJ02399-0136. The continuum emission
comes from a ~3" diameter structure whose elongation is approximately east-west
and whose centroid is coincident within the astrometric errors with the
brightest X-ray and rest-UV peak (L1). The line data show that this structure
is most likely a rapidly rotating disk. Its rotation velocity of >420 km/s
implies a total dynamical mass of >3x10^11 solar masses within an intrinsic
radius of 8 kpc, most of which is plausibly in the form of stars and gas.
SMMJ02399-0136 is thus a very massive system, whose formation at z~3 is not
easy to understand in current CDM hierarchical merger cosmogonies.Comment: 19 pages, 6 figures (5 PS + 1 GIF), accepted by ApJ; added color
versions of Figures 1, 5, &
Molecular Gas in the Lensed Lyman Break Galaxy cB58
We have used the IRAM Plateau de Bure Interferometer to map CO(3-2) emission
from the gravitationally lensed Lyman break galaxy MS1512-cB58. This is the
first detection of a molecular emission line in any Lyman break system; its
integrated intensity implies a total molecular gas mass of 6.6e9 Msun, while
its width implies a dynamical mass of 1.0e10 csc^2i Msun (for a flat Lambda=0.7
cosmology). These estimates are in excellent concordance with nearly all
parameters of the system measured at other wavelengths, and yield a consistent
picture of past and future star formation with no obvious discrepancies
requiring explanation by differential lensing. In particular, we find that the
age and remaining lifetime of the current episode of star formation are likely
to be similar; the surface densities of star formation and molecular gas mass
are related by a Schmidt law; and the fraction of baryonic mass already
converted into stars is sufficient to account for the observed enrichment of
the interstellar medium to 0.4 Zsun. Barring substantial gas inflow or a major
merger, the stars forming in the current episode will have mass and coevality
at z=0 similar to those of a spiral bulge. Assuming cB58 is a typical Lyman
break galaxy apart from its magnification, its global parameters suggest that
the prescriptions for star formation used in some semi-analytic models of
galaxy evolution require moderate revision, although the general prediction
that gas mass fraction should increase with redshift is validated. [abridged]Comment: 41 pages, 6 figures, accepted by Ap
Assessment of Damage to Nucleic Acids and Repair Machinery in Salmonella typhimurium Exposed to Chlorine
Water disinfection is usually evaluated using mandatory methods based on cell culturability. However, such methods do not consider the potential of cells to recover, which should also be kept as low as possible. In this paper, we hypothesized that a successful disinfection is achieved only when the applied chlorine leads to both intracellular nucleic acid damage and strong alterations of the DNA repair machinery. Monitoring the SOS system responsiveness with a umuCâ-âlacZ reporter fusion, we found that the expression of this important cellular machinery was altered after the beginning of membrane permeabilization but prior to the total decline of both the cell culturability and the nucleic acid integrity as revealed by Sybr-II staining. Rapid measurement of such nucleic acid alterations by fluorochrome-based staining could be used as an alternative method for assessing the effectiveness of disinfection with chlorine
Sensitive Radio Observations of High Redshift Dusty QSOs
We present sensitive radio continuum imaging at 1.4 GHz and 4.9 GHz of seven
high redshift QSOs selected for having a 240 GHz continuum detection, which is
thought to be thermal dust emission. We detect radio continuum emission from
four of the sources: BRI 0952-0115, BR 1202-0725, LBQS 1230+1627B, and BRI
1335-0417. The radio source in BR 1202-0725 is resolved into two components,
coincident with the double mm and CO sources. We compare the results at 1.4 GHz
and 240 GHz to empirical and semi-analytic spectral models based on star
forming galaxies at low redshift. The radio-to-submm spectral energy
distribution for BR 1202-0725, LBQS 1230+1627B, and BRI 1335-0417 are
consistent with that expected for a massive starburst galaxy, with implied
massive star formation rates of order 1000 solar masses per year (without
correcting for possible amplification by gravitational lensing). The
radio-to-submm spectral energy distribution for BRI 0952-0115 suggests a
low-luminosity radio jet source driven by the AGN.Comment: 12 pages, Latex emulateapj format, including 1 table and 3 figures.
The Astrophysical Journal, to appear in the January 2000 issu
Dynamical Masses of Low Mass Stars in the Taurus and Ophiuchus Star Forming Regions
We report new dynamical masses for 5 pre-main sequence (PMS) stars in the
L1495 region of the Taurus star-forming region (SFR) and 6 in the L1688 region
of the Ophiuchus SFR. Since these regions have VLBA parallaxes these are
absolute measurements of the stars' masses and are independent of their
effective temperatures and luminosities. Seven of the stars have masses
solar masses, thus providing data in a mass range with little data, and of
these, 6 are measured to precision . We find 8 stars with masses in the
range 0.09 to 1.1 solar mass that agree well with the current generation of PMS
evolutionary models. The ages of the stars we measured in the Taurus SFR are in
the range 1-3 MY, and MY for those in L1688. We also measured the
dynamical masses of 14 stars in the ALMA archival data for Akeson~\&~Jensen's
Cycle 0 project on binaries in the Taurus SFR. We find that the masses of 7 of
the targets are so large that they cannot be reconciled with reported values of
their luminosity and effective temperature. We suggest that these targets are
themselves binaries or triples.Comment: 20 page
A Census of the Young Cluster IC 348
We present a new census of the stellar and substellar members of the young
cluster IC 348. We have obtained images at I and Z for a 42'x28' field
encompassing the cluster and have combined these measurements with previous
optical and near-infrared photometry. From spectroscopy of candidate cluster
members appearing in these data, we have identified 122 new members, 15 of
which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015
M_sun by recent evolutionary models. The latest census for IC 348 now contains
a total of 288 members, 23 of which are later than M6 and thus are likely to be
brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a
16'x14' field centered on the cluster, we construct an IMF that is unbiased in
mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where
the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses
down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and
then declines into the substellar regime. In comparison, the similarly-derived
IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak
near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of
the IMFs in IC 348 and Taurus are reflected in the distributions of spectral
types, which peak at M5 and K7, respectively. These data provide compelling,
model-independent evidence for a significant variation of the IMF with
star-forming conditions.Comment: 47 pages, 14 figures, 3rd para of 4.5.3 has been added, this is final
version in press at ApJ, also found at
http://cfa-www.harvard.edu/sfgroup/preprints.htm
The Radio-to-Submm Spectral Index as a Redshift Indicator
We present models of the 1.4 GHz to 350 GHz spectral index, alpha(350/1.4),
for starburst galaxies as a function of redshift. The models include a
semi-analytic formulation, based on the well quantified radio-to-far infrared
correlation for low redshift star forming galaxies, and an empirical
formulation, based on the observed spectrum of the starburst galaxies M82 and
Arp 220. We compare the models to the observed values of alpha(350/1.4) for
starburst galaxies at low and high redshift. We find reasonable agreement
between the models and the observations, and in particular, that an observed
spectral index of alpha(350/1.4) > +0.5 indicates that the target source is
likely to be at high redshift, z > 1. The evolution of alpha(350/1.4) with
redshift is mainly due to the very steep rise in the Raleigh-Jeans portion of
the thermal dust spectrum shifting into the 350 GHz band with increasing
redshift. We also discuss situations where this relationship could be violated.
We then apply our models to examine the putative identifications of submm
sources in the Hubble Deep Field, and conclude that the submm sources reported
by Hughes et al. are likely to be at high redshifts, z > 1.5.Comment: standard LATEX file plus 1 postscript figure. Added references and
revised figure. second figure revision. Final Proof version. to appear in
Astrophysical Journal Letter
Looking for Pure Rotational H_2 Emission from Protoplanetary Disks
We report on a limited search for pure-rotational molecular hydrogen emission
associated with young, pre-main-sequence stars. We looked for H_2 v=0 J = 3->1
and J = 4->2 emission in the mid-infrared using the Texas Echelon-Cross-Echelle
Spectrograph (TEXES) at NASA's 3m Infrared Telescope Facility. The high
spectral and spatial resolution of our observations lead to more stringent
limits on narrow line emission close to the source than previously achieved.
One star, AB Aur, shows a possible (2sigma) H_2 detection, but further
observations are required to make a confident statement. Our non-detections
suggest that a significant fraction, perhaps all, of previously reported H_2
emission towards these objects could be extended on scales of 5" or more.Comment: 14 pages including 2 figures. Accepted by ApJ Letter
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift
Black-hole masses of the highest redshift quasars (4 <~ z <~ 6) are estimated
using a previously presented scaling relationship, derived from reverberation
mapping of nearby quasars, and compared to quasars at lower redshift. It is
shown that the central black holes in luminous z >~ 4 quasars are very massive
(>~ 10^9 solar masses). It is argued that the mass estimates of the high-z
quasars are not subject to larger uncertainties than those for nearby quasars.
Specifically, the large masses are not overestimates and the lack of similarly
large black-hole masses in the nearby Universe does not rule out their
existence at high-z. However, AGN host galaxies do not typically appear fully
formed or evolved at these early epochs. This supports scenarios in which black
holes build up mass very fast in a radiatively inefficient (or obscured) phase
relative to the stars in their galaxies. Additionally, upper envelopes of
black-hole mass of approximately 10^{10} solar masses and bolometric luminosity
of ~ 10^{48} erg/s are observed at all redshifts.Comment: 17 pages including 7 figures (5 in color) and 1 table. To appear in
ApJ, v600, January 1, 200
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