The volume densities of giant molecular clouds in M 83

Heiner, J. S.; Allen, R. J.; Wong, O. I.; Kruit, van der

doi:10.1051/0004-6361:200810430

Cited by 14 publications

(18 citation statements)

References 28 publications

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“…We are assuming that the maximum values are real and not a selection effect, whereas a sensitivity limit has been reached at larger galactocentric radii. The fluxes show the same spread as we reported for M83 (Heiner et al 2008b) – namely 2 dex or less at any given galactocentric radius. Since associations of O and B stars are measured, the number of those stars per cluster could decrease with the radius, causing the general decline.…”

Section: Resultssupporting

confidence: 83%

“…The M81 and M83 results were presented in Heiner et al (2008a,b). Both galaxies show no clear drop in total hydrogen volume densities out to larger galactocentric radii.…”

Section: Discussionmentioning

confidence: 99%

“…In Heiner et al (2008a), we used 600 pc, although almost all measured values were below 400 pc. In Heiner et al (2008b), we used 480 pc. Erroneously including H i patches that are not produced under the influence of the assumed central UV source results in low computed total hydrogen volume densities.…”

Section: Methodsmentioning

confidence: 99%

“…Observations of nearby galaxies by the GALEX satellite provide the FUV data, the Very Large Array (VLA) radio telescope provides H i column densities from 21‐cm emission and optical emission lines in the ionized gas near the OB association provide estimates of the local dust‐to‐gas ratio. Allen, Atherton & Tilanus (1986) presented the first evidence that the dissociation of H 2 into H i occurs on kiloparsec scales in the nearby galaxy M83, and in the meantime several nearby galaxies have been studied in more detail including M81 (Allen et al 1997; Heiner et al 2008a), M101 (Smith et al 2000) and (again) M83 Heiner et al (2008b). We will refer to this approach as the ‘PDR method’, which we describe and apply here to M33.…”

Section: Introductionmentioning

confidence: 99%

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Atomic hydrogen produced in M33 photodissociation regions

Heiner

Allen

Kruit

2011

Monthly Notices of the Royal Astronomical Society

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We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far‐ultraviolet (FUV) photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess FUV emission that reaches the GMC from the OB association and of the excess 21‐cm radio H i emission produced after these FUV photons convert H2 into H i on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs and is especially sensitive to a range of densities well below the critical density for CO(1–0) emission. We describe our ‘PDR method’ in more detail and apply it using GALEX FUV and Very Large Array 21‐cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80‐pc resolution. The cloud densities we derive range from 1 to 500 cm−3, with no clear dependence on the galactocentric radius; these results are generally similar to those obtained earlier in the cases of M81, M83 and M101 using the same method.

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Section: Resultssupporting

confidence: 83%

“…The M81 and M83 results were presented in Heiner et al (2008a,b). Both galaxies show no clear drop in total hydrogen volume densities out to larger galactocentric radii.…”

Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Atomic hydrogen produced in M33 photodissociation regions

Heiner

Allen

Kruit

2011

Monthly Notices of the Royal Astronomical Society

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“…The alternative method we present here is based on the physics of photodissociation of molecular hydrogen and the discovery by Allen et al (1986) that this process was responsible for the production of atomic hydrogen in the prominent spiral arms of M83. The method was first featured in Allen et al (1997), then in Smith et al (2000), Heiner et al (2008a), andHeiner et al (2008b). The method provides estimates of the total hydrogen volume densities (atomic + molecular) in gas clouds located in close proximity to regions of recent star formation.…”

Section: The Pdr Methodsmentioning

confidence: 99%

A New Method for Obtaining the Star Formation Law in Galaxies

Heiner

Allen

Kruit

2010

ApJ

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We present a new observational method to evaluate the exponent of the star formation law as initially formulated by Schmidt, i.e., the power-law expression assumed to relate the rate of star formation in a volume of space to the local total gas volume density present there. Total volume densities in the gas clouds surrounding an OB association are determined with a simple model which considers the atomic hydrogen as a photodissociation product on the cloud surfaces. The photodissociating photon flux incident on the cloud is computed from the far-UV luminosity of the OB association and the geometry. As an example, we have applied this "PDR Method" to a sample of star-forming regions in M33 using Very Large Array (VLA) 21 cm data for the H i and Galaxy Evolution Explorer (GALEX) imagery in the far-UV. With these two observables, our approach provides an estimate of the total volume density of hydrogen (atomic + molecular) in the gas clouds surrounding the young star cluster. A graph in logarithmic coordinates of the cluster UV luminosity versus the total density in the surrounding gas provides a direct measure of the exponent of the star formation law. However, we show that this plot is severely affected by observational selection, which renders large areas of the diagram inaccessible to the data. An ordinary least-squares regression fit to a straight line, therefore, gives a strongly biased result. In the present case, the slope of such a fit primarily reflects the boundary defined when the 21 cm line becomes optically thick and is no longer a reliable measure of the H i column density. We use a maximum likelihood statistical approach which can deal with truncated and skewed data, and also takes account of the large uncertainties in the total gas densities which we derive. The exponent we obtain for the Schmidt law in M33 is 1.4 ± 0.2.

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The opacity of spiral galaxy disks: IX. Dust and gas surface densities

Holwerda¹,

Allen²,

Blok³

et al. 2013

Astron Nachr

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Our aim is to explore the relation between gas, atomic and molecular, and dust in spiral galaxies. Gas surface densities are from atomic hydrogen and CO line emission maps. To estimate the dust content, we use the disk opacity as inferred from the number of distant galaxies identified in twelve HST/WFPC2 fields of ten nearby spiral galaxies. The observed number of distant galaxies is calibrated for source confusion and crowding with artificial galaxy counts and here we verify our results with sub-mm surface brightnesses from archival Herschel-SPIRE data. We find that the opacity of the spiral disk does not correlate well with the surface density of atomic (H I) or molecular hydrogen (H2) alone implying that dust is not only associated with the molecular clouds but also the diffuse atomic disk in these galaxies. Our result is a typical dust-to-gas ratio of 0.04, with some evidence that this ratio declines with galactocentric radius, consistent with recent Herschel results. We discuss the possible causes of this high dust-to-gas ratio; an over-estimate of the dust surface-density, an under-estimate of the molecular hydrogen density from CO maps or a combination of both. We note that while our value of the mean dust-to-gas ratio is high, it is consistent with the metallicity at the measured radii if one assumes the Pilyugin & Thuan (2005) calibration of gas metallicity.

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The volume densities of giant molecular clouds in M 83

Cited by 14 publications

References 28 publications

Atomic hydrogen produced in M33 photodissociation regions

Atomic hydrogen produced in M33 photodissociation regions

A New Method for Obtaining the Star Formation Law in Galaxies

The opacity of spiral galaxy disks: IX. Dust and gas surface densities

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