993 research outputs found
Galaxy Evolution and Star Formation Efficiency in the Last Half of the Universe
We present the results of a CO(1-0) emission survey with the IRAM 30m of 30
galaxies at moderate redshift (z ~ 0.2-0.6) to explore galaxy evolution and in
particular the star formation efficiency, in the redshift range filling the gap
between local and very high-z objects. Our detection rate is about 50%. One of
the bright objects was mapped at high resolution with the IRAM interferometer,
and about 50% of the total emission found in the 27 arcsec (97 kpc) single dish
beam is recovered by the interferometer, suggesting the presence of extended
emission. The FIR-to-CO luminosity ratio is enhanced, following the increasing
trend observed between local and high-z ultra-luminous starbursts.Comment: 6 pages, 5 figures, To appear in the proceedings of "SF2A-2007:
Semaine de l'Astrophysique Francaise", (J. Bouvier, A. Chalabaev, and C.
Charbonnel eds
Dense molecular globulettes and the dust arc towards the runaway O star AE Aur (HD 34078)
Some runaway stars are known to display IR arc-like structures around them,
resulting from their interaction with surrounding interstellar material. The
properties of these features as well as the processes involved in their
formation are still poorly understood. We aim at understanding the physical
mechanisms that shapes the dust arc observed near the runaway O star AEAur
(HD34078). We obtained and analyzed a high spatial resolution map of the
CO(1-0) emission that is centered on HD34078, and that combines data from both
the IRAM interferometer and 30m single-dish antenna. The line of sight towards
HD34078 intersects the outer part of one of the detected globulettes, which
accounts for both the properties of diffuse UV light observed in the field and
the numerous molecular absorption lines detected in HD34078's spectra,
including those from highly excited H2 . Their modeled distance from the star
is compatible with the fact that they lie on the 3D paraboloid which fits the
arc detected in the 24 {\mu}m Spitzer image. Four other compact CO globulettes
are detected in the mapped area. These globulettes have a high density and
linewidth, and are strongly pressure-confined or transient. The good spatial
correlation between the CO globulettes and the IR arc suggests that they result
from the interaction of the radiation and wind emitted by HD 34078 with the
ambient gas. However, the details of this interaction remain unclear. A wind
mass loss rate significantly larger than the value inferred from UV lines is
favored by the large IR arc size, but does not easily explain the low velocity
of the CO globulettes. The effect of radiation pressure on dust grains also
meets several issues in explaining the observations. Further observational and
theoretical work is needed to fully elucidate the processes shaping the gas and
dust in bow shocks around runaway O stars. (Abridged)Comment: Accepted for publication in Astronomy & Astrophysic
Molecular Gas in Candidate Double-Barred Galaxies II. Cooler, Less Dense Gas Associated with Stronger Central Concentrations
We have performed a multi-transition CO study of the centers of seven
double-barred galaxies that exhibit a variety of molecular gas morphologies to
determine if the molecular gas properties are correlated with the nuclear
morphology and star forming activity. Near infrared galaxy surveys have
revealed the existence of nuclear stellar bars in a large number of barred or
lenticular galaxies. High resolution CO maps of these galaxies exhibit a wide
range of morphologies. Recent simulations of double-barred galaxies suggest
that variations in the gas properties may allow it to respond differently to
similar gravitational potentials. We find that the 12CO J=3-2/J=2-1 line ratio
is lower in galaxies with centrally concentrated gas distributions and higher
in galaxies with CO emission dispersed around the galactic center in rings and
peaks. The 13CO/12CO J=2-1 line ratios are similar for all galaxies, which
indicates that the J=3-2/J=2-1 line ratio is tracing variations in gas
temperature and density, rather than variations in optical depth. There is
evidence that the galaxies which contain more centralized CO distributions are
comprised of molecular gas that is cooler and less dense. Observations suggest
that the star formation rates are higher in the galaxies containing the warmer,
denser, less centrally concentrated gas. It is possible that either the bar
dynamics are responsible for the variety of gas distributions and densities
(and hence the star formation rates) or that the star formation alone is
responsible for modifying the gas properties.Comment: 27 pages + 6 figures; to appear in the April 20, 2003 issue of Ap
A Constant Bar Fraction out to Redshift z~1 in the Advanced Camera for Surveys Field of the Tadpole Galaxy
Bar-like structures were investigated in a sample of 186 disk galaxies larger
than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with
the HST ACS. We found 22 clear cases of barred galaxies, 21 galaxies with small
bars that appear primarily as isophotal twists in a contour plot, and 11 cases
of peculiar bars in clump-cluster galaxies, which are face-on versions of chain
galaxies. The latter bars are probably young, as the galaxies contain only weak
interclump emission. Four of the clearly barred galaxies at z~0.8-1.2 have
grand design spirals. The bar fraction was determined as a function of galaxy
inclination and compared with the analogous distribution in the local Universe.
The bar fraction was also determined as a function of galaxy angular size.
These distributions suggest that inclination and resolution effects obscure
nearly half of the bars in our sample. The bar fraction was also determined as
a function of redshift. We found a nearly constant bar fraction of 0.23+-0.03
from z~0 to z=1.1. When corrected for inclination and size effects, this
fraction is comparable to the bar fraction in the local Universe, ~0.4, as
tabulated for all bar and Hubble types in the Third Reference Catalogue of
Galaxies. The average major axis of a barred galaxy in our sample is ~10 kpc
after correcting for redshift with a LambdaCDM cosmology. Galaxy bars were
present in normal abundance at least ~8 Gy ago (z~1); bar dissolution cannot be
common during a Hubble time unless the bar formation rate is comparable to the
dissolution rate.Comment: to appear in ApJ, Sept 1, 2004, Vol 612, 18 pg, 12 figure
Tentative Detection of the Nitrosylium Ion in Space
We report the tentative detection in space of the nitrosylium ion, NO.
The observations were performed towards the cold dense core Barnard 1-b. The
identification of the NO =2--1 line is supported by new laboratory
measurements of NO rotational lines up to the =8--7 transition
(953207.189\,MHz), which leads to an improved set of molecular constants: \,MHz, \,kHz, and \,MHz. The profile of the feature assigned to NO exhibits two
velocity components at 6.5 and 7.5 km s, with column densities of and cm, respectively. New
observations of NO and HNO, also reported here, allow to estimate the following
abundance ratios: (NO)/(NO), and
(HNO)/(NO). This latter value provides important constraints
on the formation and destruction processes of HNO. The chemistry of NO and
other related nitrogen-bearing species is investigated by the means of a
time-dependent gas phase model which includes an updated chemical network
according to recent experimental studies. The predicted abundance for NO
and NO is found to be consistent with the observations. However, that of HNO
relative to NO is too high. No satisfactory chemical paths have been found to
explain the observed low abundance of HNO. HSCN and HNCS are also reported here
with an abundance ratio of . Finally, we have searched for NNO,
NO, HNNO, and NNOH, but only upper limits have been obtained for
their column density, except for the latter for which we report a tentative
3- detection.Comment: To appear in the Astrophysical Journal October 20, 201
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
Atomic Carbon in Galaxies
We present new measurements of the ground state fine-structure line of atomic
carbon at 492 GHz in a variety of nearby external galaxies, ranging from spiral
to irregular, interacting and merging types. In comparison with CO(1-0), the
CI(1-0) intensity stays fairly comparable in the different environments, with
an average value of the ratio of the line integrated areas in Kkm/s of
CI(1-0)/CO(1-0) = 0.2 +/- 0.2. However, some variations can be found within
galaxies, or between galaxies. Relative to CO lines, CI(1-0) is weaker in
galactic nuclei, but stronger in disks, particularly outside star forming
regions. Also, in NGC 891, the CI(1-0) emission follows the dust continuum at
1.3mm extremely well along the full length of the major axis where molecular
gas is more abundant than atomic gas. Atomic carbon therefore appears to be a
good tracer of molecular gas in external galaxies, possibly more reliable than
CO. Atomic carbon can contribute significantly to the thermal budget of
interstellar gas. Cooling due to C and CO amounts typically to 2 x 10^{-5} of
the FIR continuum or 5% of the CII line. However, C and CO cooling reaches 30%
of the gas total, in Ultra Luminous InfraRed Galaxies, where CII is abnormally
faint. Together with CII/FIR, the emissivity ratio CI(1-0)/FIR can be used as a
measure of the non-ionizing UV radiation field in galaxies.Comment: 26 pages, 8 figure
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