2,756 research outputs found
High-resolution [C II] imaging of HDF850.1 reveals a merging galaxy at z=5.185
New high-resolution maps with the IRAM Interferometer of the redshifted [C
II] 158 micron line and the 0.98mm dust continuum of HDF850.1 at z = 5.185 show
the source to have a blueshifted northern component and a redshifted southern
component, with a projected separation of 0.3 arcsec, or 2 kpc. We interpret
these components as primordial galaxies that are merging to form a larger
galaxy. We think it is the resulting merger-driven starburst that makes
HDF850.1 an ultraluminous infrared galaxy, with an L(IR) of 1E13 Lsun. The
observed line and continuum brightness temperatures and the constant
line-to-continuum ratio across the source imply (1) high [C II] line optical
depth, (2) a [C II] excitation temperature of the same order as the dust
temperature, and (3) dust continuum emission that is nearly optically thick at
158 microns. These conclusions for HDF850.1 probably also apply to other
high-redshift submillimeter galaxies and quasar hosts in which the [C II] 158
micron line has been detected, as indicated by their roughly constant [C
II]-to-158 micron continuum ratios, in sharp contrast to the large dispersion
in their [C II]-to-FIR luminosity ratios. In brightness temperature units, the
[C II] line luminosity is about the same as the predicted CO(1-0) luminosity,
implying that the [C II] line can also be used to estimate the molecular gas
mass, with the same assumptions as for CO.Comment: Accepted by Astronomy and Astrophysic
Massive molecular outflows at high spatial resolution
We present high-spatial resolution Plateau de Bure Interferometer CO(2-1) and
SiO(2-1) observations of one intermediate-mass and one high-mass star-forming
region. The intermediate-mass region IRAS20293+3952 exhibits four molecular
outflows, one being as collimated as the highly collimated jet-like outflows
observed in low-mass star formation sources. Furthermore, comparing the data
with additional infrared H2 and cm observations we see indications that the
nearby ultracompact HII region triggers a shock wave interacting with the
outflow. The high-mass region IRAS19217+1651 exhibits a bipolar outflow as well
and the region is dominated by the central driving source. Adding two more
sources from the literature, we compare position-velocity diagrams of the
intermediate- to high-mass sources with previous studies in the low-mass
regime. We find similar kinematic signatures, some sources can be explained by
jet-driven outflows whereas other are better constrained by wind-driven models.
The data also allow to estimate accretion rates varying from a few times
10^{-5}Msun/yr for the intermediate-mass sources to a few times 10^{-4}Msun/yr
for the high-mass source, consistent with models explaining star formation of
all masses via accretion processes.Comment: 14 pages text, 4 tables, 8 figures, accepted for Ap
The Stellar Populations and Evolution of Lyman Break Galaxies
Using deep near-IR and optical observations of the HDF-N from the HST NICMOS
and WFPC2 and from the ground, we examine the spectral energy distributions
(SEDs) of Lyman break galaxies (LBGs) at 2.0 < z < 3.5. The UV-to-optical
rest-frame SEDs of the galaxies are much bluer than those of present-day spiral
and elliptical galaxies, and are generally similar to those of local starburst
galaxies with modest amounts of reddening. We use stellar population synthesis
models to study the properties of the stars that dominate the light from LBGs.
Under the assumption that the star-formation rate is continuous or decreasing
with time, the best-fitting models provide a lower bound on the LBG mass
estimates. LBGs with ``L*'' UV luminosities are estimated to have minimum
stellar masses ~ 10^10 solar masses, or roughly 1/10th that of a present-day L*
galaxy. By considering the effects of a second component of maximally-old
stars, we set an upper bound on the stellar masses that is ~ 3-8 times the
minimum estimate. We find only loose constraints on the individual galaxy ages,
extinction, metallicities, initial mass functions, and prior star-formation
histories. We find no galaxies whose SEDs are consistent with young (< 10^8
yr), dust-free objects, which suggests that LBGs are not dominated by ``first
generation'' stars, and that such objects are rare at these redshifts. We also
find that the typical ages for the observed star-formation events are
significantly younger than the time interval covered by this redshift range (~
1.5 Gyr). From this, and from the relative absence of candidates for quiescent,
non-star-forming galaxies at these redshifts in the NICMOS data, we suggest
that star formation in LBGs may be recurrent, with short duty cycles and a
timescale between star-formation events of < 1 Gyr. [Abridged]Comment: LaTeX, 37 pages, 21 figures. Accepted for publication in the
Astrophysical Journa
Observations on the Formation of Massive Stars by Accretion
Observations of the H66a recombination line from the ionized gas in the
cluster of newly formed massive stars, G10.6-0.4, show that most of the
continuum emission derives from the dense gas in an ionized accretion flow that
forms an ionized disk or torus around a group of stars in the center of the
cluster. The inward motion observed in the accretion flow suggests that despite
the equivalent luminosity and ionizing radiation of several O stars, neither
radiation pressure nor thermal pressure has reversed the accretion flow. The
observations indicate why the radiation pressure of the stars and the thermal
pressure of the HII region are not effective in reversing the accretion flow.
The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient
to form massive stars within the time scale imposed by their short main
sequence lifetimes. A simple model of disk accretion relates quenched HII
regions, trapped hypercompact HII regions, and photo-evaporating disks in an
evolutionary sequence
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The High-Density Ionized Gas in the Central Parsec of the Galaxy
We report a study of the line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2'' over a field of 60'' (2 pc) and a velocity range of . This field encompasses most of the Galactic center's "minispiral." With an isothermal homogeneous H II model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the line data along with Very Large Array data from the line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are and 5000-13,000 K, respectively. The and line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a three-dimensional model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the "Bar" region, which is located 0.1-0.2 pc south of and behind Sgr A*. Finally, a total Lyman continuum flux of photons is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to ~250 O9-type zero-age-main-sequence stars.Astronom
Faint Radio Sources and Star Formation History
Faint extragalactic radio sources provide important information about the
global history of star formation. Sensitive radio observations of the Hubble
Deep Field and other fields have found that sub-mJy radio sources are
predominantly associated with star formation activity rather than AGN. Radio
observations of star forming galaxies have the advantage of being independent
of extinction by dust. We use the FIR-radio correlation to compare the radio
and FIR backgrounds, and make several conclusions about the star forming
galaxies producing the FIR background. We then use the redshift distribution of
faint radio sources to determine the evolution of the radio luminosity
function, and thus estimate the star formation density as a function of
redshift.Comment: 12 pages, 9 figures, latex using texas.sty, to appear in the CD-ROM
Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and
Cosmology, held in Paris, France, Dec. 14-18, 1998. Eds.: J. Paul, T.
Montmerle, and E. Aubourg (CEA Saclay). No changes to paper, just updated
publication info in this commen
Mass Flows in Cometary UCHII Regions
High spectral and spatial resolution, mid-infrared fine structure line
observations toward two ultracompact HII (UCHII) regions (G29.96 -0.02 and Mon
R2) allow us to study the structure and kinematics of cometary UCHII regions.
In our earlier study of Mon R2, we showed that highly organized mass motions
accounted for most of the velocity structure in that UCHII region. In this
work, we show that the kinematics in both Mon R2 and G29.96 are consistent with
motion along an approximately paraboloidal shell. We model the velocity
structure seen in our mapping data and test the stellar wind bow shock model
for such paraboloidal like flows. The observations and the simulation indicate
that the ram pressures of the stellar wind and ambient interstellar medium
cause the accumulated mass in the bow shock to flow along the surface of the
shock. A relaxation code reproduces the mass flow's velocity structure as
derived by the analytical solution. It further predicts that the pressure
gradient along the flow can accelerate ionized gas to a speed higher than that
of the moving star. In the original bow shock model, the star speed relative to
the ambient medium was considered to be the exit speed of ionized gas in the
shell.Comment: 34 pages, including 14 figures and 1 table, to be published in ApJ,
September 200
Spherical Infall in G10.6-0.4: Accretion Through an Ultracompact HII Region
We present high resolution (0.''12 x 0.''079) observations of the
ultracompact HII region G10.6-0.4 in 23 GHz radio continuum and the NH3(3,3)
line. Our data show that the infall in the molecular material is largely
spherical, and does not flatten into a molecular disk at radii as small as 0.03
pc. The spherical infall in the molecular gas matches in location and velocity
the infall seen in the ionized gas. We use a non-detection to place a stringent
upper limit on the mass of an expanding molecular shell associated with
pressure driven expansion of the HII region. These data support a scenario in
which the molecular accretion flow passes through an ionization front and
becomes an ionized accretion flow onto one or more main sequence stars, not the
classical pressure-driven expansion scenario. In the continuum emission we see
evidence for externally ionized clumps of molecular gas, and cavities evacuated
by an outflow from the central source.Comment: Accepted for publication in Astrophysical Journal Letter
The Chandra Deep Field-North Survey. XIV. X-ray detected Obscured AGNs and Starburst Galaxies in the Bright Submm Source Population
We provide X-ray constraints and perform the first X-ray spectral analyses
for bright (f_850>=5mJy; S/N>=4) SCUBA sources in an 8.4'x8.4' area of the 2 Ms
Chandra Deep Field-North survey containing the Hubble Deep Field-North. X-ray
emission is detected from 7 of the 10 bright submm sources in this region,
corresponding to an X-ray detected submm source density of ~360 deg^-2 (>~36%
of the bright submm source population). Two of the X-ray detected sources have
nearby (within 3") X-ray companions, suggesting merging/interacting sources or
gravitational lensing effects, and 3 lie within the approximate extent of a
proto-cluster candidate. Five of the X-ray detected sources have flat X-ray
spectral slopes, suggesting obscured AGN activity. X-ray spectral analyses
suggest that one of these AGNs may be a Compton-thick source; of the other 4
AGNs, 3 appear to be Compton-thin sources and one has poor constraints. The
rest-frame unabsorbed X-ray luminosities of these AGNs are more consistent with
those of Seyfert galaxies than QSOs. Thus, the low X-ray detection rate of
bright submm sources by moderately deep X-ray surveys appears to be due to the
relatively low luminosities of the AGNs rather than Compton-thick absorption. A
comparison of these sources to the well-studied heavily obscured AGN NGC6240
shows that the average AGN contribution is negligible at submm wavelengths. The
X-ray properties of the other 2 X-ray detected sources are consistent with
those expected from luminous star formation; however, we cannot rule out the
possibility that low-luminosity AGNs are present. The 3 X-ray undetected
sources appear to lie at high redshift (z>4) and could be either AGNs or
starbust galaxies.Comment: AJ in press (February 2003), 16 pages, includes emulateapj5.st
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