1,846 research outputs found
Monte Carlo Predictions of Far-Infrared Emission from Spiral Galaxies
We present simulations of Far Infrared (FIR) emission by dust in spiral
galaxies, based on the Monte Carlo radiative transfer code of Bianchi, Ferrara
& Giovanardi (1996). The radiative transfer is carried out at several
wavelength in the Ultraviolet, optical and Near Infrared, to cover the range of
the stellar Spectral Energy Distribution (SED). Together with the images of the
galactic model, a map of the energy absorbed by dust is produced. Using
Galactic dust properties, the spatial distribution of dust temperature is
derived under the assumption of thermal equilibrium. A correction is applied
for non-equilibrium emission in the Mid Infrared. Images of dust emission can
then be produced at any wavelength in the FIR.
We show the application of the model to the spiral galaxy NGC 6946. The
observed stellar SED is used as input and models are produced for different
star-dust geometries. It is found that only optically thick dust disks can
reproduce the observed amount of FIR radiation. However, it is not possible to
reproduce the large FIR scalelength suggested by recent observation of spirals
at 200 um, even when the scalelength of the dust disk is larger than that for
stars. Optically thin models have ratios of optical/FIR scalelengths closer to
the 200um observations, but with smaller absolute scalelengths than optically
thick cases. The modelled temperature distributions are compatible with
observations of the Galaxy and other spirals. We finally discuss the
approximations of the model and the impact of a clumpy stellar and dust
structure on the FIR simulations.Comment: 19 pages, 6 figures, accepted by A&
SCUBA imaging of NGC 7331 dust ring
We present observations of the spiral galaxy NGC 7331 using the
Sub-millimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell
Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at
wavelengths of 450 and 850-micron. The dust ring is in good correspondence with
other observations of the ring in the mid-infrared (MIR), CO and
radio-continuum, suggesting that the observed dust is associated with the
molecular gas and star formation. A B-K colour map shows an analogous ring
structure with an asymmetry about the major axis, consistent with the
extinction being produced by a dust ring. The derived temperature of the dust
lies between 16 and 31 K and the gas-to-dust ratio between 150 and 570,
depending on the assumed dust emission efficiency index (beta=1.5 or 2.).Comment: 5 pages, 6 figures, to be published in MNRA
Abundant dust found in intergalactic space
Galactic dust constitutes approximately half of the elements more massive
than helium produced in stellar nucleosynthesis. Notwithstanding the formation
of dust grains in the dense, cool atmospheres of late-type stars, there still
remain huge uncertainties concerning the origin and fate of galactic stardust.
In this paper, we identify the intergalactic medium (i.e. the region between
gravitationally-bound galaxies) as a major sink for galactic dust. We discover
a systematic shift in the colour of background galaxies viewed through the
intergalactic medium of the nearby M81 group. This reddening coincides with
atomic, neutral gas previously detected between the group members. The
dust-to-HI mass ratio is high (1/20) compared to that of the solar neighborhood
(1/120) suggesting that the dust originates from the centre of one or more of
the galaxies in the group. Indeed, M82, which is known to be ejecting dust and
gas in a starburst-driven superwind, is cited as the probable main source.Comment: 5 pages, 3 figures, 1 table. ApJ Letters in pres
Dust properties of external galaxies; NGC 891 revisited
We compare 850um SCUBA images of NGC 891 with the corresponding V-band
optical depth predicted from radiation transfer simulations. These two tracers
of dust show a very similar distribution along the minor axis and a reasonable
agreement along the major axis. Assuming that the grains responsible for
optical extinction are also the source of 850um emission we derive a
submillimeter emissivity (emission efficiency) for dust in the NGC 891 disk.
This quantity is found to be a factor of 2-3 higher than the generally-accepted
(but highly uncertain) values adopted for the Milky Way. It should be stated,
however, that if a substantial fraction of dust in NGC 891 is clumped, the
emissivity in the two galaxies may be quite similar. We use our newly-acquired
emissivity to convert our 850um images into detailed maps of dust mass and,
utilizing 21cm and CO-emission data for NGC 891, derive the gas-to-dust ratio
along the disk. We compute an average ratio of 260 -- a value consistent with
the Milky Way and external spirals within the uncertainties in deriving both
the dust mass and the quantity of molecular gas. The bulk of dust in NGC 891
appears to be closely associated with the molecular gas phase although it may
start to follow the distribution of atomic hydrogen at radii >9 kpc (i.e. >0.5
R_25). Using the optical depth of the NGC 891 disk, we quantify how light
emitted at high redshift is attenuated by dust residing in foreground spirals.
For B-band observations of galaxies typically found in the Hubble Deep Field,
the amount of light lost is expected to be small (~ 5%). This value depends
critically on the maximum radial extent of cold dust in spiral disks (which is
poorly known). It may also represent a lower limit if galaxies expel dust over
time into the intergalactic medium.Comment: 22 pages, 7 figures, A&A accepte
Radiation Pressure Supported Starburst Disks and AGN Fueling
We consider the structure of marginally Toomre-stable starburst disks under
the assumption that radiation pressure on dust grains provides the dominant
vertical support against gravity. This is particularly appropriate when the
disk is optically thick to its own IR radiation, as in the central regions of
ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for
star formation changes in the optically-thick limit, with the star formation
rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface
density and kappa is the mean opacity. We show that optically thick starburst
disks have a characteristic flux and dust effective temperature of F ~ 10^{13}
L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with
observations and find good agreement. We extend our model from many-hundred
parsec scales to sub-parsec scales and address the problem of fueling AGN. We
assume that angular momentum transport proceeds via global torques rather than
a local viscosity. We account for the radial depletion of gas due to star
formation and find a strong bifurcation between two classes of disk models: (1)
solutions with a starburst on large scales that consumes all of the gas with
little fueling of a central AGN and (2) models with an outer large-scale
starburst accompanied by a more compact starburst on 1-10 pc scales and a
bright central AGN. The luminosity of the latter models is in many cases
dominated by the AGN. We show that the vertical thickness of the starburst disk
on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration
in some Type 2 AGN. We also argue that the disk of young stars in the Galactic
Center may be the remnant of such a compact nuclear starburst.Comment: 26 pages, 9 figures, emulateapj, accepted to ApJ, minor changes,
discussion tightened, references adde
Star Formation Rate from Dust Infrared Emission
We examine what types of galaxies the conversion formula from dust infrared
(IR) luminosity into the star formation rate (SFR) derived by
Kennicutt (1998) is applicable to. The ratio of the observed IR luminosity,
, to the intrinsic bolometric luminosity of the newly (\la 10
Myr) formed stars,
, of a galaxy can be determined by a mean dust opacity in the
interstellar medium and the activity of the current star formation. We find
that these parameters area being is very large, and many nearby normal and active star-forming
galaxies really fall in this area. It results from offsetting two effects of a
small dust opacity and a large cirrus contribution of normal galaxies relative
to starburst galaxies on the conversion of the stellar emission into the dust
IR emission. In conclusion, the SFR determined from the IR luminosity under the
assumption of like Kennicutt (1998) is reliable within
a factor of 2 for all galaxies except for dust rich but quiescent galaxies and
extremely dust poor galaxies.Comment: Accepted by ApJL: 6 pages (emulateapj5), 2 figures (one is an extra
figure not appeared in ApJL
A Mini-survey of X-ray Point Sources in Starburst and Non-Starburst Galaxies
We present a comparison of X-ray point source luminosity functions of 3
starburst galaxies (the Antennae, M82, and NGC 253) and 4 non-starburst spiral
galaxies (NGC 3184, NGC 1291, M83, and IC 5332). We find that the luminosity
functions of the starbursts are flatter than those of the spiral galaxies; the
starbursts have relatively more sources at high luminosities. This trend
extends to early-type galaxies which have steeper luminosity functions than
spirals. We show that the luminosity function slope is correlated with 60
micron luminosity, a measure of star formation. We suggest that the difference
in luminosity functions is related to the age of the X-ray binary populations
and present a simple model which highlights how the shape of the luminosity
distribution is affected by the age of the underlying X-ray binary population.Comment: 8 pages, 4 figures. accepted for publication in Ap
Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402
We present optical, HI and radio continuum observations of the highly
inclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for ram-pressure
stripping and dense cloud ablation. VLA HI and radio continuum maps show a
truncated gas disk and emission to the northwest of the main disk emission. In
particular, the radio continuum emission is asymmetrically extended to the
north and skewed to the west. The Halpha image shows numerous HII complexes
along the southern edge of the gas disk, possibly indicating star formation
triggered by the ICM pressure. BVR images at 0.5" resolution obtained with the
WIYN Tip-Tilt Imager show a remarkable dust lane morphology: at half the
optical radius, the dust lane of the galaxy curves up and out of the disk,
matching the HI morphology. Large dust plumes extend upward for ~1.5 kpc from
luminous young star clusters at the SE edge of the truncated gas disk. These
star clusters are very blue, indicating very little dust reddening, which
suggests dust blown away by an ICM wind at the leading edge of the interaction.
To the south of the main ridge of interstellar material, where the galaxy is
relatively clean of gas and dust, we have discovered 1 kpc long linear dust
filaments with a position angle that matches the extraplanar radio continuum
tail; we interpret this angle as the projected ICM wind direction. One of the
observed dust filaments has an HII region at its head. We interpret these dust
filaments as large, dense clouds which were initially left behind as the
low-density ISM is stripped, but are then ablated by the ICM wind. These
results provide striking new evidence on the fate of molecular clouds in
stripped cluster galaxies.Comment: 17 pages, 4 figures, accepted for publication in AJ. See
ftp://ftp.astro.yale.edu/pub/hugh/papers/crowl_n4402.ps.gz for a version with
high-resolution figure
The Rest-Frame Optical Properties of z~3 Galaxies
We present the results of a near-infrared imaging survey of z~3 Lyman Break
Galaxies (LBGs). The survey covers a total of 30 arcmin^2 and includes 118
photometrically selected LBGs with K_s band measurements, 63 of which also have
J band measurements, and 81 of which have spectroscopic redshifts. Using the
distribution of optical {\cal R} magnitudes from previous work and {\cal R}-K_s
colors for this sub-sample, we compute the rest-frame optical luminosity
function of LBGs. At the brightest magnitudes, where it is fairly well
constrained, this luminosity function strikingly exceeds locally determined
optical luminosity functions. The V-band luminosity density of only the
observed bright end of the z~3 LBG luminosity function already approaches that
of all stars in the local universe. For the 81 galaxies with measured
redshifts, we investigate the range of LBG stellar populations implied by the
photometry which generally spans the range 900--5500 AA in the rest-frame.
While there are only weak constraints on the parameters for most of the
individual galaxies, there are strong trends in the sample as a whole. A
unified scenario which accounts for the observed trends in bright LBGs is one
in which a relatively short period of very rapid star-formation (hundreds of
M_sun/yr) lasts for roughly 50--100 Myr, after which both the extinction and
star-formation rate are considerably reduced and stars are formed at a more
quiescent, but still rapid, rate for at least a few hundred Myr. In our sample,
a considerable fraction (~20%) of the LBGs have best-fit star-formation ages ~>
1 Gyr, implied stellar masses of ~> 10^10 M_sun, and are still forming stars at
\~30 M_sun/yr.Comment: 61 pages including 19 figures. Accepted for publication in Ap
Mapping the submillimeter spiral wave in NGC 6946
We have analysed SCUBA 850\mum images of the (near) face-on spiral galaxy NGC
6946, and found a tight correlation between dust thermal emission and molecular
gas. The map of visual optical depth relates well to the distribution of
neutral gas (HI+H2) and implies a global gas-to-dust ratio of 90. There is no
significant radial variation of this ratio: this can be understood, since the
gas content is dominated by far by the molecular gas. The latter is estimated
through the CO emission tracer, which is itself dependent on metallicity,
similarly to dust emission. By comparing the radial profile of our visual
optical depth map with that of the SCUBA image, we infer an emissivity (dust
absorption coefficient) at 850\mum that is 3 times lower than the value
measured by COBE in the Milky Way, and 9 times lower than in NGC 891. A
decomposition of the spiral structure half way out along the disk of NGC 6946
suggests an interarm optical depth of between 1 and 2. These surprisingly high
values represent 40-80% of the visual opacity that we measure for the arm
region (abridged).Comment: 12 pages, 9 figures, accepted in A&
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