142 research outputs found
Modeling the Dust Spectral Energy Distributions of Dwarf Galaxies
Recent efforts on the modeling of the infrared spectral energy distributions
(SEDs) of dwarf galaxies are summarised here. The characterisation of the dust
properties in these low metallicity environments is just unfolding, as a result
of recently available mid-infrared to millimetre observations. From the limited
cases we know to date, it appears that the hard radiation fields that are
present in these star-bursting dwarf galaxies, as well as the rampent
energetics of supernovae shocks and winds have modified the dust properties, in
comparison with those in the Galaxy, or other gas and dust rich galaxies. The
sophistication of the SED models is limited by the availability of detailed
data in the mid infrared and particularly in the submillimetre to millimetre
regime, which will open up in the near future with space-based missions, such
as Herschel.Comment: 8 pages presented at "The Spectral Energy Distribution of Gas-Rich
Galaxies: Confronting Models with Data" Heidelberg (Germany), October 2004.
To be published in The Spectral Energy Distribution of Gas-Rich Galaxies, ed.
C. Popescu & R. Tuffs (Melville:AIP) in pres
Interstellar Gas in Low Mass Virgo Cluster Spiral Galaxies
We have measured the strengths of the [C II] 158 micron, [N II] 122 micron,
and CO (1 - 0) lines from five low blue luminosity spiral galaxies in the Virgo
Cluster, using the Infrared Space Observatory and the NRAO 12m millimeter
telescope. Two of the five galaxies have high L([C II)]/L(CO) and L(FIR)/L(CO)
ratios compared to higher mass spirals. These two galaxies, NGC 4294 and NGC
4299, have L([C II])/L(CO) ratios of >14,300 and 15,600, respectively, which
are similar to values found in dwarf irregular galaxies. This is the first time
that such enhanced L([C II])/L(CO) ratios have been found in spiral galaxies.
This result may be due to low abundances of dust and heavy elements, which can
cause the CO (1 - 0) measurements to underestimate the molecular gas content.
Another possibility is that radiation from diffuse HI clouds may dominate the
[C II] emission from these galaxies. Less than a third of the observed [C II]
emission arises from HII regions.Comment: 24 pages, Latex, 2 Figures, 6 Tables To appear in the Astronomical
Journal, July 199
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A multi-transition study of the cyclic molecule cyclopropenylidene (C2H2) in the galaxy.
We report results of multi-transition observations and modeling of the hydrocarbon ring molecule cyclopropenylidene (C\sb3H\sb2). From a survey of the 1\sb{10}-1\sb{01} (18 GHz) and 2\sb{12}-1\sb{01} (85 GHz) transitions in the Galaxy, we have found C\sb3H\sb2 present in a variety of sources including cold, dark clouds, giant molecular clouds, the envelope of a carbon star, and diffuse clouds. Up to 10 transitions of C\sb3H\sb2 ranging in wavelength from 1.3 cm to 1.3 mm were observed in the dark clouds L1498, L134N, B335 and toward several positions in TMC-1. The Large Velocity Gradient (LVG) approximation was used to model the observations. Optical depth values of C\sb3H\sb2, estimated from C\sp{13}C\sb2H\sb2 observations, are necessary to constrain the results since the range in excitation energies of the observed C\sb3H\sb2 transitions does not contrast sufficiently. The molecular hydrogen density in TMC-1 is estimated to be 3.7 10\sp4 cm\sp{-3}, while the fractional abundance of C{\sb3}H\sb2 relative to H\sb2 is 5.7 10\sp{-9}. Previous estimates assuming LTE conditions overestimate the abundance of C\sb3H\sb2. The abundance in the ridge component in Orion is estimated to be approximately 8 10\sp{-10} cm \sp{-2}. Gas phase chemical models can reproduce the high C\sb3H\sb2 abundance found in dark clouds under assumptions such as steady state conditions with (C) / (O) / 1.0, conditions of earlier evolutionary time, or \u27optimistic\u27 rate coefficients. However, large deuteration ratios (0.05 to 0.15) create difficulties for gas phase models
Discovery of PAHs in the Halo of NGC 5907
We have used sensitive archival data from the Infrared Space Observatory
(ISO) to make maps of the edge-on low SFR galaxy, NGC 5907, in 6 different MIR
bands: LW2, LW5, LW6, LW7, LW8, and LW10, covering the spectrum from 6.5 to
15.0 microns and including several narrow bands that isolate the infrared
aromatic spectral features commonly referred to as PAHs. Most of the MIR
emission is dominated by PAHs and it is likely that emission from VSGs
contribute only negligibly except in the broad IRAS-equivalent band. The flux
ratios are typical of galaxies with low SFRs or quiesent regions within
galaxies (e.g M~83) and a very high PAH/continuum ratio is observed. The PAH
emission follows the CO distribution and also shows some correlation within the
disk with the lambda 850 micron distribution. However, the PAH emission also
reaches larger galactocentric radii than the CO and other correlations suggest
that the PAHs are also more widespread. A significant new discovery is the
presence of PAHs in the halo of the galaxy. In the narrow bands that isolate
single PAH features, the emission shows structure similar to high latitude
features seen in other galaxies in other tracers. The features extend as far as
6.5 kpc from the plane but scale heights of 3.5 kpc are more typical. The
(lambda 11.3/lambda7.7) ratio also appears to increase with distance from the
major axis. To our knowledge, this is the first time PAHs have been seen in the
halo of an external galaxy. Just as significantly, they are seen in a low SFR
galaxy, suggesting that strong SNe and winds are not necessary for these large
molecules to reach high latitudes.Comment: A&A accept. 8 Sept. 05, 15 pages, 14 fig., pdf at
www.astro.queensu.ca/~irwin/pub/ngc590
The LMC+ SOFIA Legacy Program
With the goal of elucidating the effects of low metallicity on the star
formation activity, feedback and interstellar medium of low metallicity
environments, SOFIA has observed a 40' x 20' (60 pc x 30 pc) area of our
neighboring metal-poor Large Magellanic Cloud in 158 micron [CII] and 88 micron
[OIII], targeting the southern molecular ridge just south of 30Doradus. We find
extensive [CII] emission over the region, which encompasses a wide variety of
local physical conditions, from bright compact star forming regions to lower
density environments beyond, much of which does not correspond to CO
structures. Preliminary analyses indicates that most of the molecular hydrogen
is in a CO-dark gas component.Comment: Proceedings of the 7th Chile-Cologne-Bonn-Symposium "Physics and
Chemistry of Star Formation, The Dynamical ISM Across Time and Spatial
Scales", Puerto-Varas Chile, September 26-30, 2022 V. Ossenkopf-Okada, R.
Schaaf, I. Breloy (eds.
A new view on the ISM of galaxies: far-infrared and submillimetre spectroscopy with Herschel
The FIR/submm window is amongst the least explored spectral regions of the
electromagnetic spectrum. It is, however, a key to study the general properties
of the interstellar medium of galaxies, as it contains important spectral line
diagnostics from the neutral, ionized and molecular ISM. The Herschel Space
Observatory, successfully launched on 14 May 2009, is the first observatory to
cover the entire FIR/submm range between 57 and 672 mum. We discuss the main
results from the ISO era on FIR spectroscopy of galaxies and the enormous
science potential of the Herschel mission through a presentation of its
spectroscopic extragalactic key programs.Comment: 10 pages, 4 figures, accepted for publication in New Astronomy
Review
Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size-Linewidth Relationship
Stars form within molecular clouds, so characterizing the physical states of
molecular clouds is key in understanding the process of star formation. Cloud
structure and stability is frequently assessed using metrics including the
virial parameter and Larson (1981) scaling relationships between cloud radius,
velocity dispersion, and surface density. Departures from the typical Galactic
relationships between these quantities have been observed in low-metallicity
environments. The amount of H gas in cloud envelopes without corresponding
CO emission is expected to be high under these conditions; therefore, this
"CO-dark" gas could plausibly be responsible for the observed variations in
cloud properties. We derive simple corrections that can be applied to empirical
clump properties (mass, radius, velocity dispersion, surface density, and
virial parameter) to account for CO-dark gas in clumps following power-law and
Plummer mass density profiles. We find that CO-dark gas is not likely to be the
cause of departures from Larson's relationships in low-metallicity regions, but
that virial parameters may be systematically overestimated. We demonstrate that
correcting for CO-dark gas is critical for accurately comparing the dynamical
state and evolution of molecular clouds across diverse environments.Comment: 19 pages, 5 figures, accepted for publication in Ap
The Mass Function of Super Giant Molecular Complexes and Implications for Forming Young Massive Star Clusters in the Antennae (NGC 4038/39)
We have used previously published observations of the CO emission from the
Antennae (NGC 4038/39) to study the detailed properties of the super giant
molecular complexes with the goal of understanding the formation of young
massive star clusters. Over a mass range from 5E6 to 9E8 solar masses, the
molecular complexes follow a power-law mass function with a slope of -1.4 +/-
0.1, which is very similar to the slope seen at lower masses in molecular
clouds and cloud cores in the Galaxy. Compared to the spiral galaxy M51, which
has a similar surface density and total mass of molecular gas, the Antennae
contain clouds that are an order of magnitude more massive. Many of the
youngest star clusters lie in the gas-rich overlap region, where extinctions as
high as Av~100 imply that the clusters must lie in front of the gas. Combining
data on the young clusters, thermal and nonthermal radio sources, and the
molecular gas suggests that young massive clusters could have formed at a
constant rate in the Antennae over the last 160 Myr and that sufficient gas
exists to sustain this cluster formation rate well into the future. However,
this conclusion requires that a very high fraction of the massive clusters that
form initially in the Antennae do not survive as long as 100 Myr. Finally, we
compare our data with two models for massive star cluster formation and
conclude that the model where young massive star clusters form from dense cores
within the observed super giant molecular complexes is most consistent with our
current understanding of this merging system. (abbreviated)Comment: 40 pages, four figures; accepted for publication in Ap
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