463 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
Recommended from our members
PACS and SPIRE photometer maps of M 33: First results of the HERschel M 33 Extended Survey (HERM33ES)
Context. Within the framework of the HERM33ES key program, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M 33, exploiting the high resolution and sensitivity of Herschel. Aims. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 μm wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Methods. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 μm and 500 μm using also MIPS/Spitzer  data, to derive first estimates of the dust physical conditions. Results. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M 33. An underlying diffuse disk is seen in all SPIRE maps (250–500 μm). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with β fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60 K ± 10 K. The temperature of the cold component drops significantly from ~24 K in the inner 2 kpc radius to 13 K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for β = 1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M 33
Recommended from our members
Cool gas and dust in M 33: Results from the HERschel M 33 Extended Survey (HERM33ES)
We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M 33 is a first step towards young low-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H2 content. In this Letter, we measure the dust emission cross-section σ using SPIRE and recent CO and H i observations; a variation in σ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the H i column, yields a morphology similar to that observed in CO. The H2/H i mass ratio decreases from about unity to well below 10% and is about 15% averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33
Properties of compact 250 μm emission and H II regions in M 33 (HERM33ES)
Aims. Within the framework of the HERM33ES key program, using the high resolution and sensitivity of the Herschel photometric
data, we study the compact emission in the Local Group spiral galaxy M33 to investigate the nature of the compact SPIRE emission
sources. We extracted a catalogue of sources at 250 μm in order to investigate the nature of this compact emission. Taking advantage
of the unprecedented Herschel resolution at these wavelengths, we also focus on a more precise study of some striking Hα shells in
the northern part of the galaxy.
Methods. We present a catalogue of 159 compact emission sources in M33 identified by SExtractor in the 250 μm SPIRE band that
is the one that provides the best spatial resolution. We also measured fluxes at 24 μm and Hα for those 159 extracted sources. The
morphological study of the shells also benefits from a multiwavelength approach including Hα, far-ultraviolet from GALEX, and
infrared from both Spitzer IRAC 8 μm and MIPS 24 μm in order to make comparisons.
Results. For the 159 compact sources selected at 250 μm, we find a very strong Pearson correlation coefficient with the MIPS 24 μm
emission (r_(24) = 0.94) and a rather strong correlation with the Hα emission, although with more scatter (r_(Hα) = 0.83). The morphological
study of the Hα shells shows a displacement between far-ultraviolet, Hα, and the SPIRE bands. The cool dust emission from
SPIRE clearly delineates the Hα shell structures.
Conclusions. The very strong link between the 250 μm compact emission and the 24 μm and Hα emissions, by recovering the star formation
rate from standard recipes for H II regions, allows us to provide star formation rate calibrations based on the 250 μm compact
emission alone. The different locations of the Hα and far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to
a dynamical age of a few Myr for the Hα shells and the associated cool dust
Cool gas and dust in M33: Results from the Herschel M33 extended survey (HERM33ES)
We present an analysis of the first space-based far-IR-submm observations of
M 33, which measure the emission from the cool dust and resolve the giant
molecular cloud complexes. With roughly half-solar abundances, M33 is a first
step towards young low-metallicity galaxies where the submm may be able to
provide an alternative to CO mapping to measure their H content. In this
Letter, we measure the dust emission cross-section using SPIRE and
recent CO and \HI\ observations; a variation in is present from a
near-solar neighborhood cross-section to about half-solar with the maximum
being south of the nucleus. Calculating the total H column density from the
measured dust temperature and cross-section, and then subtracting the \HI\
column, yields a morphology similar to that observed in CO. The H/\HI\ mass
ratio decreases from about unity to well below 10% and is about 15% averaged
over the optical disk. The single most important observation to reduce the
potentially large systematic errors is to complete the CO mapping of M 33.Comment: 5 pages, 5 figures Accepted for publication in Astronomy and
Astrophysic
Properties of compact 250 \mu m emission and HII regions in M33 (HERM33ES)
Within the framework of the HERM33ES Key Project, using the high resolution
and sensitivity of the Herschel photometric data, we study the compact emission
in the Local Group spiral galaxy M33 to investigate the nature of the compact
SPIRE emission sources. We extracted a catalogue of sources at 250um in order
to investigate the nature of this compact emission. Taking advantage of the
unprecedented Herschel resolution at these wavelengths, we also focus on a more
precise study of some striking Halpha shells in the northern part of the
galaxy. We present a catalogue of 159 compact emission sources in M33
identified by SExtractor in the 250um SPIRE band that is the one that provides
the best spatial resolution. We also measured fluxes at 24um and Halpha for
those 159 extracted sources. The morphological study of the shells also
benefits from a multiwavelength approach including Halpha, far-UV from GALEX,
and infrared from both Spitzer IRAC 8um and MIPS 24um in order to make
comparisons. For the 159 compact sources selected at 250um, we find a very
strong Pearson correlation coefficient with the MIPS 24um emission (r24 = 0.94)
and a rather strong correlation with the Halpha emission, although with more
scatter (rHa = 0.83). The morphological study of the Halpha shells shows a
displacement between far-ultraviolet, Halpha, and the SPIRE bands. The cool
dust emission from SPIRE clearly delineates the Halpha shell structures. The
very strong link between the 250um compact emission and the 24um and Halpha
emissions, by recovering the star formation rate from standard recipes for HII
regions, allows us to provide star formation rate calibrations based on the
250um compact emission alone. The different locations of the Halpha and
far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to
a dynamical age of a few Myr for the Halpha shells and the associated cool
dust.Comment: 4 pages, 3 figures, Accpeted for publication in the A&A Herschel
Special Issu
Molecular Gas in Candidate Double Barred Galaxies III. A Lack of Molecular Gas?
Most models of double-barred galaxies suggest that a molecular gas component
is crucial for maintaining long-lived nuclear bars. We have undertaken a CO
survey in an attempt to determine the gas content of these systems and to
locate double barred galaxies with strong CO emission that could be candidates
for high resolution mapping. We observed 10 galaxies in CO J=2-1 and J=3-2 and
did not detect any galaxies that had not already been detected in previous CO
surveys. We preferentially detect emission from galaxies containing some form
of nuclear activity. Simulations of these galaxies require that they contain 2%
to 10% gas by mass in order to maintain long-lived nuclear bars. The fluxes for
the galaxies for which we have detections suggest that the gas mass fraction is
in agreement with these models requirements. The lack of emission in the other
galaxies suggests that they contain as little as 7 x 10^6 solar masses of
molecular material which corresponds to < 0.1% gas by mass. This result
combined with the wide variety of CO distributions observed in double barred
galaxies suggests the need for models of double-barred galaxies that do not
require a large, well ordered molecular gas component.Comment: 17 pages (3 figures embedded on pg 17). To appear in the March 10
issue of the Astrophysical Journa
Spitzer Observations of Low Luminosity Isolated and Low Surface Brightness Galaxies
We examine the infrared properties of five low surface brightness galaxies
(LSBGs) and compare them with related but higher surface brightness galaxies,
using Spitzer Space Telescope images and spectra. All the LSBGs are detected in
the 3.6 and 4.5um bands, representing the stellar population. All but one are
detected at 5.8 and 8.0um, revealing emission from hot dust and aromatic
molecules, though many are faint or point-like at these wavelengths. Detections
of LSBGs at the far-infrared wavelengths, 24, 70, and 160um, are varied in
morphology and brightness, with only two detections at 160um, resulting in
highly varied spectral energy distributions. Consistent with previous
expectations for these galaxies, we find that detectable dust components exist
for only some LSBGs, with the strength of dust emission dependent on the
existence of bright star forming regions. However, the far-infrared emission
may be relatively weak compared with normal star-forming galaxies.Comment: 20 pages, 8 figures, accepted to Ap
Low, Milky-Way like, Molecular Gas Excitation of Massive Disk Galaxies at z~1.5
We present evidence for Milky-Way-like, low-excitation molecular gas
reservoirs in near-IR selected massive galaxies at z~1.5, based on IRAM Plateau
de Bure Interferometer CO[3-2] and NRAO Very Large Array CO[1-0] line
observations for two galaxies that had been previously detected in CO[2-1]
emission. The CO[3-2] flux of BzK-21000 at z=1.522 is comparable within the
errors to its CO[2-1] flux, implying that the CO[3-2] transition is
significantly sub-thermally excited. The combined CO[1-0] observations of the
two sources result in a detection at the 3 sigma level that is consistent with
a higher CO[1-0] luminosity than that of CO[2-1]. Contrary to what is observed
in submillimeter galaxies and QSOs, in which the CO transitions are thermally
excited up to J>=3, these galaxies have low-excitation molecular gas, similar
to that in the Milky Way and local spirals. This is the first time that such
conditions have been observed at high redshift. A Large Velocity Gradient
analysis suggests that molecular clouds with density and kinetic temperature
comparable to local spirals can reproduce our observations. The similarity in
the CO excitation properties suggests that a high, Milky-Way-like, CO to H_2
conversion factor could be appropriate for these systems. If such
low-excitation properties are representative of ordinary galaxies at high
redshift, centimeter telescopes such as the Expanded Very Large Array and the
longest wavelength Atacama Large Millimeter Array bands will be the best tools
for studying the molecular gas content in these systems through the
observations of CO emission lines.Comment: 5 pages, 4 figures. ApJ Letters in pres
- …