126 research outputs found
Dust emission in star-forming dwarf galaxies: General properties and the nature of the sub-mm excess
We studied the global characteristics of dust emission in a large sample of
emission-line star-forming galaxies. The sample consists of two subsamples. One
subsample (SDSS sample) includes ~4000 compact star-forming galaxies from the
SDSS, which were also detected in all four bands at 3.4, 4.6, 12, and 22 mum of
the WISE all-sky survey. The second subsample (Herschel sample) is a sample of
28 compact star-forming galaxies observed with Herschel in the FIR range. Data
of the Herschel sample were supplemented by the photometric data from the
Spitzer observations, GALEX, SDSS, WISE, 2MASS, NVSS, and FIRST surveys, as
well as optical and Spitzer spectra and data in sub-mm and radio ranges. It is
found that warm dust luminosities of galaxies from the SDSS sample and cold and
warm dust luminosities of galaxies from the Herschel sample are strongly
correlated with Hbeta luminosities, which implies that one of the main sources
of dust heating in star-forming galaxies is ionising UV radiation of young
stars. Using the relation between warm and cold dust masses for estimating the
total dust mass in star-forming galaxies with an accuracy better than ~0.5 dex
is proposed. On the other hand, it is shown for both samples that dust
temperatures do not depend on the metallicities. The dust-to-neutral gas mass
ratio strongly declines with decreasing metallicity, similar to that found in
other studies of local emission-line galaxies, high-redshift GRB hosts, and
DLAs. On the other hand, the dust-to-ionised gas mass ratio is about one
hundred times as high implying that most of dust is located in the neutral gas.
It is found that thermal free-free emission of ionised gas in compact
star-forming galaxies might be responsible for the sub-mm emission excess. This
effect is stronger in galaxies with lower metallicities and is also positively
affected by an increased star-formation rate.Comment: 22 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
The Flux Ratio Method for Determining the Dust Attenuation of Starburst Galaxies
The presence of dust in starburst galaxies complicates the study of their
stellar populations as the dust's effects are similar to those associated with
changes in the galaxies' stellar age and metallicity. This degeneracy can be
overcome for starburst galaxies if UV/optical/near-infrared observations are
combined with far-infrared observations. We present the calibration of the flux
ratio method for calculating the dust attenuation at a particular wavelength,
Att(\lambda), based on the measurement of F(IR)/F(\lambda) flux ratio. Our
calibration is based on spectral energy distributions (SEDs) from the PEGASE
stellar evolutionary synthesis model and the effects of dust (absorption and
scattering) as calculated from our Monte Carlo radiative transfer model. We
tested the attenuations predicted from this method for the Balmer emission
lines of a sample starburst galaxies against those calculated using radio
observations and found good agreement. The UV attenuation curves for a handful
of starburst galaxies were calculated using the flux ratio method, and they
compare favorably with past work. The relationship between Att(\lambda) and
F(IR)/F(\lambda) is almost completely independent of the assumed dust
properties (grain type, distribution, and clumpiness). For the UV, the
relationship is also independent of the assumed stellar properties (age,
metallicity, etc) accept for the case of very old burst populations. However at
longer wavelengths, the relationship is dependent on the assumed stellar
properties.Comment: accepted by the ApJ, 18 pages, color figures, b/w version at
http://mips.as.arizona.edu/~kgordon/papers/fr_method.htm
On the Submillimeter Opacity of Protoplanetary Disks
Solid particles with the composition of interstellar dust and power-law size
distribution dn/da propto a^{-p} for a 3 lambda and 3 <
p < 4 will have submm opacity spectral index beta(lambda) = dln(kappa)/dln(nu)
approx (p-3) beta_{ism}, where beta_{ism} approx 1.7 is the opacity spectral
index of interstellar dust material in the Rayleigh limit. For the power-law
index p approx 3.5 that characterizes interstellar dust, and that appears
likely for particles growing by agglomeration in protoplanetary disks, grain
growth to sizes a > 3 mm will result in beta(1 mm) < ~1. Grain growth can
naturally account for beta approx 1 observed for protoplanetary disks, provided
that a_{max} > ~ 3 lambda.Comment: Submitted to ApJ. 17 pages, 6 figure
Warm-Dense Molecular Gas in the ISM of Starbursts, LIRGs and ULIRGs
The role of star formation in luminous and ultraluminous infrared galaxies is
a hotly debated issue: while it is clear that starbursts play a large role in
powering the IR luminosity in these galaxies, the relative importance of
possible enshrouded AGNs is unknown. It is therefore important to better
understand the role of star forming gas in contributing to the infrared
luminosity in IR-bright galaxies. The J=3 level of 12CO lies 33K above ground
and has a critical density of ~1.5 X 10^4 cm^-3. The 12CO(J=3-2) line serves as
an effective tracer for warm-dense molecular gas heated by active star
formation. Here we report on 12CO (J=3-2) observations of 17 starburst spirals,
LIRGs and ULIRGs which we obtained with the Heinrich Hertz Submillimeter
Telescope on Mt. Graham, Arizona.
Our main results are the following: 1. We find a nearly linear relation
between the infrared luminosity and warm-dense molecular gas such that the
infrared luminosity increases as the warm-dense molecular gas to the power
0.92; We interpret this to be roughly consistent with the recent results of Gao
& Solomon (2004a,b). 2. We find L_IR/M_H2 ratios ranging from ~10 to ~128
L_sun/M_sun using a standard CO-H2 conversion factor of 3 X 10^20 cm^-2 (K km
s^-1)^-1. If this conversion factor is ~an order of magnitude less, as
suggested in a recent statistical survey (Yao et al. 2003), then 2-3 of our
objects may have significant contributions to the L_IR by dust-enshrouded AGNs.Comment: 15 Pages, 2 figures, Accepted for Publication in Ap
Molecular Line Observations of the Small Protostellar Group L1251B
We present molecular line observations of L1251B, a small group of pre- and
protostellar objects, and its immediate environment in the dense C18O core
L1251E. These data are complementary to near-infrared, submillimeter and
millimeter continuum observations reported by Lee et al. (2006, ApJ, 648, 491;
Paper I). The single-dish data of L1251B described here show very complex
kinematics including infall, rotation and outflow motions, and the
interferometer data reveal these in greater detail. Interferometer data of N2H+
1-0 suggest a very rapidly rotating flattened envelope between two young
stellar objects, IRS1 and IRS2. Also, interferometer data of CO 2-1 resolve the
outflow associated with L1251B seen in single-dish maps into a few narrow and
compact components. Furthermore, the high resolution data support recent
theoretical studies of molecular depletions and enhancements that accompany the
formation of protostars within dense cores. Beyond L1251B, single-dish data are
also presented of a dense core located ~150" to the east that, in Paper I, was
detected at 850 micron but has no associated point sources at near- and
mid-infrared wavelengths. The relative brightness between molecules, which have
different chemical timescales, suggests it is less chemically evolved than
L1251B. This core may be a site for future star formation, however, since line
profiles of HCO+, CS, and HCN show asymmetry with a stronger blue peak, which
is interpreted as an infall signature.Comment: 46 pages, 18 figures. Accepted for publication in Ap
Dark cloud cores and gravitational decoupling from turbulent flows
We test the hypothesis that the starless cores may be gravitationally bound
clouds supported largely by thermal pressure by comparing observed molecular
line spectra to theoretical spectra produced by a simulation that includes
hydrodynamics, radiative cooling, variable molecular abundance, and radiative
transfer in a simple one-dimensional model. The results suggest that the
starless cores can be divided into two categories: stable starless cores that
are in approximate equilibrium and will not evolve to form protostars, and
unstable pre-stellar cores that are proceeding toward gravitational collapse
and the formation of protostars. The starless cores might be formed from the
interstellar medium as objects at the lower end of the inertial cascade of
interstellar turbulence. Additionally, we identify a thermal instability in the
starless cores. Under par ticular conditions of density and mass, a core may be
unstable to expansion if the density is just above the critical density for the
collisional coupling of the gas and dust so that as the core expands the
gas-dust coupling that cools the gas is reduced and the gas warms, further
driving the expansion.Comment: Submitted to Ap
NGC 2264 IRS1: The central engine and its cavity
We present a high-resolution study of NGC 2264 IRS1 in CS(2-1) and in the
3-mm continuum using the IRAM Plateau de Bure Interferometer. We complement
these radio data with images taken at 2.2, 4.6, and 11.9 micron. The combined
information allow a new interpretation of the closest environment of NGC 2264
IRS1. No disk around the B-type star IRS1 was found. IRS1 and its low-mass
companions are located in a low-density cavity which is surrounded by the
remaining dense cloud core which has a clumpy shell-like structure. Strong
evidence for induced on-going star formation was found in the surroundings of
IRS1. A deeply embedded very young stellar object 20 arcsec to the north of
IRS1 is powering a highly collimated bipolar outflow. The object 8 in the
closer environment of IRS1 is a binary surrounded by dusty circumbinary
material and powering two bipolar outflows.Comment: 17 pages, 6 figures, The paper is accepted and will appear in the
Astrophysical Journal, Vol 599, No 1 (issue December 10). A high-resolution
postscript version of this paper is available here (
http://www.astro.uni-jena.de/Users/martin/publi.html). Furthermore, you can
find a high resolution PDF file here (
http://www.tls-tautenburg.de/research/tls-research/pub2003.html
Candidate Rotating Toroids around High-Mass (Proto)Stars
Using the OVRO, Nobeyama, and IRAM mm-arrays, we searched for
``disk''-outflow systems in three high-mass (proto)star forming regions:
G16.59-0.05, G23.01-0.41, and G28.87+0.07. These were selected from a sample of
NH3 cores associated with OH and H2O maser emission and with no or very faint
continuum emission. Our imaging of molecular line (including rotational
transitions of CH3CN and 3mm dust continuum emission revealed that these are
compact, massive, and hot molecular cores (HMCs), that is likely sites of
high-mass star formation prior to the appearance of UCHII regions. All three
sources turn out to be associated with molecular outflows from CO and/or HCO+
J=1--0 line imaging. In addition, velocity gradients of 10 -- 100 km/s per pc
in the innermost densest regions of the G23.01 and G28.87 HMCs are identified
along directions roughly perpendicular to the axes of the corresponding
outflows. All the results suggest that these cores might be rotating about the
outflow axis, although the contribution of rotation to gravitational
equilibrium of the HMCs appears to be negligible. Our analysis indicates that
the 3 HMCs are close to virial equilibrium due to turbulent pressure support.
Comparison with other similar objects where rotating toroids have been
identified so far shows that in our case rotation appears to be much less
prominent; this can be explained by the combined effect of unfavorable
projection, large distance, and limited angular resolution with the current
interferometers.Comment: Accepted by ApJ main journal, the paper with the original quality
figures are available from
http://subarutelescope.org/staff/rsf/publication.htm
Detection of the 13CO(J=6-5) Transition in the Starburst Galaxy NGC 253
We report the detection of 13CO(J=6-5) emission from the nucleus of the
starburst galaxy NGC 253 with the redshift (z) and Early Universe Spectrometer
(ZEUS), a new submillimeter grating spectrometer. This is the first
extragalactic detection of the 13CO(J=6-5) transition, which traces warm, dense
molecular gas. We employ a multi-line LVG analysis and find ~ 35% - 60% of the
molecular ISM is both warm (T ~ 110 K) and dense (n(H2) ~ 10^4 cm^-3). We
analyze the potential heat sources, and conclude that UV and X-ray photons are
unlikely to be energetically important. Instead, the molecular gas is most
likely heated by an elevated density of cosmic rays or by the decay of
supersonic turbulence through shocks. If the cosmic rays and turbulence are
created by stellar feedback within the starburst, then our analysis suggests
the starburst may be self-limiting.Comment: 4 pages, 2 figures, accepted by ApJ Letter
Molecular ions in L1544. II. The ionization degree
The maps presented in Paper I are here used to infer the variation of the
column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from
the dust peak. These results are interpreted with the aid of a crude chemical
model which predicts the abundances of these species as a function of radius in
a spherically symmetric model with radial density distribution inferred from
the observations of dust emission at millimeter wavelengths and dust absorption
in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+)
column density ratio is of order 0.2 towards the L1544 dust peak as compared to
N(DCO+)/N(HCO+) = 0.04. We conclude that this result as well as the general
finding that N2H+ and N2D+ correlate well with the dust is caused by CO being
depleted to a much higher degree than molecular nitrogen in the high density
core of L1544. Depletion also favors deuterium enhancement and thus N2D+, which
traces the dense and highly CO-depleted core nucleus, is much more enhanced
than DCO+. Our models do not uniquely define the chemistry in the high density
depleted nucleus of L1544 but they do suggest that the ionization degree is a
few times 10^{-9} and that the ambipolar diffusion time scale is locally
similar to the free fall time. It seems likely that the lower limit which one
obtains to ionization degree by summing all observable molecular ions is not a
great underestimate of the true ionization degree. We predict that atomic
oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in
the central highly depleted region of the core.Comment: 31 pages, 8 figures, to be published in Ap
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