CI and CO in nearby galaxy centers

Israel, F. P.

doi:10.1051/0004-6361/200811586

Cited by 53 publications

(51 citation statements)

References 75 publications

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“…We note also that the outer regions are marginally affected by the choice of X CO , and they are still consistent with a "standard" X CO values for normal disk galaxies, while the inner regions need a lower value to reproduce a decreasing dust-to-gas mass ratio. This is in agreement with the evidence found for a lower X CO in galaxy centers, e.g., Sodroski et al (1994), Israel et al (2006), Israel (2009), Watanabe et al (2011. On the other hand, the dust-to-gas mass ratio obtained with X CO = 1.8 × 10 20 cm −2 and X CO = 4.0 × 10 20 cm −2 tend to be flatter or positive, especially with the larger value of X CO .…”

Section: Can We Constrain X Co ?supporting

confidence: 91%

TheHerschelVirgo Cluster Survey

Magrini

Bianchi

Corbelli

et al. 2011

A&A

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Context. Using Herschel data from the open time key project the Herschel Virgo Cluster Survey (HeViCS), we investigated the relationship between the metallicity gradients expressed by metal abundances in the gas phase as traced by the chemical composition of HII regions, and in the solid phase, as traced by the dust-to-gas mass ratio. Aims. We derived the radial gradient of the dust-to-gas mass ratio for all galaxies observed by HeViCS whose metallicity gradients are available in the literature. They are all late type Sbc galaxies, namely NGC 4254, NGC 4303, NGC 4321, and NGC 4501. Methods. We fitted PACS and SPIRE observations with a single-temperature modified blackbody, inferred the dust mass, and calculated two dimensional maps of the dust-to-gas mass ratio, with the total mass of gas from available HI and CO maps. HI moment-1 maps were used to derive the geometric parameters of the galaxies and extract the radial profiles. We examined different dependencies on metallicity of the CO-to-H 2 conversion factor (X CO ), used to transform the 12 CO observations into the amount of molecular hydrogen. Results. We found that in these galaxies the dust-to-gas mass ratio radial profile is extremely sensitive to choice of the X CO value, since the molecular gas is the dominant component in the inner parts. We found that for three galaxies of our sample, namely NGC 4254, NGC 4321, and NGC 4501, the slopes of the oxygen and of the dust-to-gas radial gradients agree up to ∼0.6−0.7 R 25 using X CO values in the range 1/3−1/2 Galactic X CO . For NGC 4303 a lower value of X CO ∼ 0.1 × 10 20 is necessary. Conclusions. We suggest that such low X CO values might be due to a metallicity dependence of X CO (from close to linear for NGC 4254, NGC 4321, and NGC 4501 to superlinear for NGC 4303), especially in the radial regions R G < 0.6−0.7 R 25 where the molecular gas dominates. On the other hand, the outer regions, where the atomic gas component is dominant, are less affected by the choice of X CO , and thus we cannot put constraints on its value there.

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Section: Can We Constrain X Co ?supporting

confidence: 91%

TheHerschelVirgo Cluster Survey

Magrini

Bianchi

Corbelli

et al. 2011

A&A

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“…Our assumption is that 12 CO J = 3-2 traces pre-star formation molecular gas that is in general denser than 12 CO J = 1-0. In Paper I we used a constant ratio of 0.6, representative of ratios seen in Galactic and extragalactic GMCs (based on the results of Leroy et al 2008;Wilson et al 1999;Israel 2009aIsrael , 2009b, essentially focusing on the molecular gas closest to the star formation (unlike the above median 12 CO J = 3-2/J = 1-0 ratios for the three galaxies, which are more representative of the global molecular gas population). The limitation of this assumption is that the cause of the differences between different molecular gas tracers is degenerate, thus higher 12 CO J = 3-2 values could indicate warmer rather than denser gas (as found by Oka et al 2007).…”

Section: Resultsmentioning

confidence: 99%

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. Ii. Warm Molecular Gas and Star Formation in Three Field Spiral Galaxies

Warren

Wilson

Israel

et al. 2010

ApJ

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We present the results of large-area 12 CO J = 3-2 emission mapping of three nearby field galaxies, NGC 628, NGC 3521, and NGC 3627, completed at the James Clerk Maxwell Telescope as part of the Nearby Galaxies Legacy Survey. These galaxies all have moderate to strong 12 CO J = 3-2 detections over large areas of the fields observed by the survey, showing resolved structure and dynamics in their warm/dense molecular gas disks. All three galaxies were part of the Spitzer Infrared Nearby Galaxies Survey sample, and as such have excellent published multiwavelength ancillary data. These data sets allow us to examine the star formation properties, gas content, and dynamics of these galaxies on sub-kiloparsec scales. We find that the global gas depletion time for dense/warm molecular gas in these galaxies is consistent with other results for nearby spiral galaxies, indicating this may be independent of galaxy properties such as structures, gas compositions, and environments. Similar to the results from The H i Nearby Galaxy Survey, we do not see a correlation of the star formation efficiency with the gas surface density consistent with the Schmidt-Kennicutt law. Finally, we find that the star formation efficiency of the dense molecular gas traced by 12 CO J = 3-2 is potentially flat or slightly declining as a function of molecular gas density, the 12 CO J = 3-2/J = 1-0 ratio (in contrast to the correlation found in a previous study into the starburst galaxy M83), and the fraction of total gas in molecular form.

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“…α CO can be converted to X CO units by multiplying by a factor of 4.6 × 10 19 . et al 1987;Wilson 1995;Bolatto et al 2008;Donovan Meyer et al 2012;Wei et al 2012;Gratier et al 2012), and modeling multiple molecular gas lines with varying optical depths and critical densities (e.g., Weiß et al 2001;Israel 2009aIsrael , 2009b. In general, few of these techniques are effective for constraining α CO in galaxies outside the Local Group due to the difficulty of obtaining the necessary observations (i.e., γ -ray maps or CO mapping at <100 pc resolution) or doubts about fundamental assumptions (e.g., that CO traces the full extent of molecular gas in the clouds; that the clouds lack contributions to virial balance from magnetic or pressure forces; that simple radiative transfer models can reproduce molecular gas excitation on kiloparsec scales).…”

Section: Introductionmentioning

confidence: 99%

THE CO-TO-H₂CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

Sandström

Leroy

Walter

et al. 2013

ApJ

488

439

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We present ∼kiloparsec spatial resolution maps of the CO-to-H 2 conversion factor (α CO ) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for α CO and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H i column density to solve for both α CO and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, 12 CO J = (2-1) maps from the IRAM 30 m large program HERACLES, and H i 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our α CO results on the more typically used 12 CO J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for α CO and the DGR. On average, α CO = 3.1 M pc −2 (K km s −1 ) −1 for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of α CO as a function of galactocentric radius. However, most galaxies exhibit a lower α CO value in the central kiloparsec-a factor of ∼2 below the galaxy mean, on average. In some cases, the central α CO value can be factors of 5-10 below the standard Milky Way (MW) value of α CO,MW = 4.4 M pc −2 (K km s −1 ) −1 . While for α CO we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate α CO for studies of nearby galaxies.

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CI and CO in nearby galaxy centers

Cited by 53 publications

References 75 publications

TheHerschelVirgo Cluster Survey

TheHerschelVirgo Cluster Survey

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. Ii. Warm Molecular Gas and Star Formation in Three Field Spiral Galaxies

THE CO-TO-H₂CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

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CI and CO in nearby galaxy centers

Cited by 53 publications

References 75 publications

TheHerschelVirgo Cluster Survey

TheHerschelVirgo Cluster Survey

The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey. Ii. Warm Molecular Gas and Star Formation in Three Field Spiral Galaxies

THE CO-TO-H2CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES

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THE CO-TO-H₂CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES