THE CO-TO-H<sub>2</sub>CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP

Leroy, Adam K.; Bolatto, Alberto D.; Gordon, K. D.; Sandström, Karin; Gratier, P.; Rosolowsky, Erik; Engelbracht, C. W.; Mizuno, N.; Corbelli, E.; Fukui, Y.; Kawamura, Akiko

doi:10.1088/0004-637x/737/1/12

Cited by 584 publications

(981 citation statements)

References 79 publications

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“…Within the Milky Way and Local Group (aside from the Small Magallenic Cloud [SMC]), the conversion factor appears to display a remarkably narrow range of X CO ≈ 2 − 4 × 10 20 cm −2 /K km s −1 (or α CO ≈ 3 − 6 M pc −2 (K km s −1 ) −1 Bloemen et al, 1986;Solomon et al, 1987;Blitz et al, 2007;Delahaye et al, 2011;Leroy et al, 2011;Donovan Meyer et al, 2012b,a). The independent H 2 mass measurements in these observations come from virial mass measurements, dust to gas ratio assumptions (or measurements), and γ-ray observations.…”

Section: Deriving H 2 Gas Masses From High-redshift Galaxiesmentioning

confidence: 99%

Dusty star-forming galaxies at high redshift

2014

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Far-infrared and submillimeter wavelength surveys have now established the important role of dusty, star-forming galaxies (DSFGs) in the assembly of stellar mass and the evolution of massive galaxies in the Universe. The brightest of these galaxies have infrared luminosities in excess of 10 13 L with implied star-formation rates of thousands of solar masses per year. They represent the most intense starbursts in the Universe, yet many are completely optically obscured. Their easy detection at submm wavelengths is due to dust heated by ultraviolet radiation of newly forming stars. When summed up, all of the dusty, star-forming galaxies in the Universe produce an infrared radiation field that has an equal energy density as the direct starlight emission from all galaxies visible at ultraviolet and optical wavelengths. The bulk of this infrared extragalactic background light emanates from galaxies as diverse as gas-rich disks to mergers of intense starbursting galaxies. Major advances in far-infrared instrumentation in recent years, both spacebased and ground-based, has led to the detection of nearly a million DSFGs, yet our understanding of the underlying astrophysics that govern the start and end of the dusty starburst phase is still in nascent stage. This review is aimed at summarizing the current status of DSFG studies, focusing especially on the detailed characterization of the bestunderstood subset (submillimeter galaxies, who were summarized in the last review of this field over a decade ago, Blain et al. 2002), but also the selection and characterization of more recently discovered DSFG populations. We review DSFG population statistics, their physical properties including dust, gas and stellar contents, their environments, and current theoretical models related to the formation and evolution of these galaxies.

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Section: Deriving H 2 Gas Masses From High-redshift Galaxiesmentioning

confidence: 99%

Dusty star-forming galaxies at high redshift

2014

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“…Recent studies have also shown that a CO in low-metallicity galaxies can be substantially higher than the Galactic value (e.g., Leroy et al 2011;Schruba et al 2012;Bolatto et al 2013) and hence we restricted our sample to galaxies with an oxygen abundance value of [ ]  + 12 log O H 8.4, using the average of their characteristic metallicities derived from a theoretical calibration and an empirical calibration as recommended by Moustakas et al (2010).…”

Section: Model-extrapolated Lower Temperature T ℓmentioning

confidence: 99%

“…Since H 2 can self-shield from UV photons in regions where CO is photodissociated (Wolfire et al 2010), at low metallicity, not only is the CO abundance reduced, but as dust opacity is reduced and ionized regions become hotter and more porous, a CO,Gal can severely underestimate the molecular gas mass. Considerable effort has been invested in detecting and interpreting CO emission at metallicity 50 times lower than the solar metallicity, [ ] + 12 log O H ∼7.0, to assess the molecular gas content (Leroy et al 2011;Schruba et al 2012;Cormier et al 2014;Rémy-Ruyer et al 2014). Dust emission can be used to estimate the molecular gas mass in the ISM, assuming a constant DGR; however, it is essential to know the change in DGR with metallicity.…”

Section: Effect Of Dust Temperature On the Warm H 2 Fractionmentioning

confidence: 99%

LIGHTING THE DARK MOLECULAR GAS: H₂ AS A DIRECT TRACER

Togi

Smith

2016

ApJ

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Robust knowledge of molecular gas mass is critical for understanding star formation in galaxies. The H 2 molecule does not emit efficiently in the cold interstellar medium, hence the molecular gas content of galaxies is typically inferred using indirect tracers. At low metallicity and in other extreme environments, these tracers can be subject to substantial biases. We present a new method of estimating total molecular gas mass in galaxies directly from pure mid-infrared rotational H 2 emission. By assuming a power-law distribution of H 2 rotational temperatures, we can accurately model H 2 excitation and reliably obtain warm (T100 K) H 2 gas masses by varying only the power law's slope. With sensitivities typical of Spitzer/IRS, we are able to directly probe the H 2 content via rotational emission down to ∼80 K, accounting for ∼15% of the total molecular gas mass in a galaxy. By extrapolating the fitted power-law temperature distributions to a calibrated single lower cutoff temperature, the model also recovers the total molecular content within a factor of ∼2.2 in a diverse sample of galaxies, and a subset of broken powerlaw models performs similarly well. In ULIRGs, the fraction of warm H 2 gas rises with dust temperature, with some dependency on α CO . In a sample of five low-metallicity galaxies ranging down to [ ] + = 12 log O H 7.8, the model yields molecular masses up to ∼100×larger than implied by CO, in good agreement with other methods based on dust mass and star formation depletion timescale. This technique offers real promise for assessing molecular content in the early universe where CO and dust-based methods may fail.

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“…As is usually done for nearby galaxies 7,15 the GDR in the star-forming clumps is taken as the ratio of atomic gas to dust in the diffuse region of the disk. This works because (1) the atomic gas dominates the total gas mass in the diffuse regions, and (2) the GDR is roughly constant in star-forming disks after removing the metallicity gradients 15,21 .…”

mentioning

confidence: 99%

Inefficient star formation in extremely metal poor galaxies

Shi

Armus

Hélou

et al. 2014

Nature

175

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The first galaxies contain stars born out of gas with little or no metals. The lack of metals is expected to inhibit efficient gas cooling and star formation 1, 2 but this effect has yet to be observed in galaxies with oxygen abundance relative to hydrogen below a tenth of that of the Sun 2-4 . Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon Monoxide (CO) emission is unreliable as tracers of gas at low metallicities 5-7 , and while dust has been used to trace gas in lowmetallicity galaxies 5, 8-10 , low-spatial resolution in the far-infrared has typically led to large uncertainties 9, 10 . Here we report spatiallyresolved infrared observations of two galaxies with oxygen abundances below 10 per cent solar, and show that stars form very inefficiently in seven star-forming clumps of these galaxies. The star formation efficiencies are more than ten times lower than found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.The two galaxies that are the focus of this study are Sextans A, a dwarf irregular at 1.4 Mpc with oxygen abundance of 7% Solar 11,12 , and ESO 146-G14, a low-surface-brightness galaxy at 22.5 Mpc with 9% solar oxygen abundance 11,13 . Their metallicities may be similar to that of gas out of which Population II stars form in the early Universe around redshift from 7 to 12 14 . An effective way to estimate the total gas content in extremely metal poor galaxies is to employ spatially resolved maps of the far-infrared (IR) emitting dust as tracers of the atomic and molecular gas by multiplying with an appropriate gas-todust ratio (GDR) that is inferred from regions with little or no active star formation, a methodology that has been extensively demonstrated in relatively metal rich galaxies 7, 15 .The infrared observations described in this paper were carried out at 70, 160, 250, 350 and 500 µm with the Photodetector Array Camera and Spectrometer (PACS) 16 and Spectral and Photometric Imaging REceiver (SPIRE) 17 onboard the Herschel Space Observatory. We complement our far-IR data with mid-IR images from the Spitzer Space Telescope to construct the full IR spectral energy distributions (SED). Far-ultraviolet (UV) images from the GALEX Space Telescope archive are used to trace un-obscured star formation. Maps of atomic gas are available in the literature for Sextans A . Figure 1 shows the multi-wavelength images of our sample galaxies. Based on the far-UV image we defined the star-forming disk as an ellipse to closely follow the 10-σ (∼ 26 AB-mag/arcsec 2 ) contour of the far-UV emission as shown in the Fig. 1 gions with elevated (> 3σ) emission relative to local disk backgrounds in both far-UV and 160 µm bands after smoothing images to 28 arcsec resolutions. The diffuse emission is measured by subtracting the total emission of all star-forming clumps in the disk from the integrated disk emission. For Se...

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THE CO-TO-H₂CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP

Cited by 584 publications

References 79 publications

Dusty star-forming galaxies at high redshift

Dusty star-forming galaxies at high redshift

LIGHTING THE DARK MOLECULAR GAS: H₂ AS A DIRECT TRACER

Inefficient star formation in extremely metal poor galaxies

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THE CO-TO-H2CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP

Cited by 584 publications

References 79 publications

Dusty star-forming galaxies at high redshift

Dusty star-forming galaxies at high redshift

LIGHTING THE DARK MOLECULAR GAS: H2 AS A DIRECT TRACER

Inefficient star formation in extremely metal poor galaxies

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Product

Resources

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THE CO-TO-H₂CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP

LIGHTING THE DARK MOLECULAR GAS: H₂ AS A DIRECT TRACER