THE METALLICITY DEPENDENCE OF THE CO → H2 CONVERSION FACTOR IN z ⩾ 1 STAR-FORMING GALAXIES

By R. Genzel, L. Tacconi, F. Combes, A. Bolatto, R. Neri, A. Sternberg, M. Cooper, N. Bouché, Frédéric Bournaud, A. Burkert and 13 more. 2011

187 citations recorded by the source corpus.

We use the first systematic samples of CO millimeter emission in z ⩾ 1 ‘‘main-sequence'' star-forming galaxies to study the metallicity dependence of the conversion factor αCO, from CO line luminosity to molecular gas mass. The molecular gas depletion rate inferred from the ratio of the star formation rate (SFR) to CO luminosity, is ∼1 Gyr−1 for near-solar metallicity galaxies with stellar masses above MS ∼ 1011 M☉. In this regime, the depletion rate does not vary more than a factor of two to three as a function of molecular gas surface density or redshift between z ∼ 0 and 2. Below MS the depletion rate increases rapidly with decreasing metallicity. We argue that this trend is not caused by starburst events, by changes in the physical parameters of the molecular clouds, or by the impact of the fundamental-metallicity–SFR–stellar mass relation. A more probable explanation is that the conversion factor is metallicity dependent and that star formation can occur in ‘‘CO-dark'' gas. The trend is also expected theoretically from the effect of enhanced photodissociation of CO by ultraviolet radiation at low metallicity. From the available z ∼ 0 and z ∼ 1–3 samples we constrain the slope of the log(αCO)–log (metallicity) relation to range between −1 and −2, fairly insensitive to the assumed slope of the gas–SFR relation. Because of the lower metallicities near the peak of the galaxy formation activity at z ∼ 1–2 compared to z ∼ 0, we suggest that molecular gas masses estimated from CO luminosities have to be substantially corrected upward for galaxies below MS.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/0b2e0684c76d79663ab40447adbf5c9685e3ef81

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. P. Hopkins
2. L. Hernquist
3. R. Somerville
4. R. Dav'e
5. D. Narayanan
6. T. Naab
7. M. Cappellari
8. E. Quataert

Citations in the corpus

• Cool Gas in High-Redshift Galaxies (2013, methodology): “Genzel et al. (2012) consider the effect of metallicity on the conversion factor α in a sample of CSG at z ∼ 1 to 2.”
• A fundamental relation between the metallicity, gas content, and stellar mass of local galaxies (2013, background): “…to model and to observationally constrain the relation between the CO/H2 conversion factor and gas-phase metallicity (e.g. Narayanan et al. 2011; Genzel et al. 2012; Bolatto et al. 2013), such a dependence is so strong that uncertainties in the metallicity and/or deviations from the nominal…”
• Introducing the Illustris Project: the evolution of galaxy populations across cosmic time (2014, methodology): “…by University of Cambridge user on 18 July 2018 For example, the observations we compare to here are based on CO measurements, which must rely on assumptions for the CO-to-H2 conversion factor to derive total molecular masses (Genzel et al. 2012).”
• Dusty Star Forming Galaxies at High Redshift (2014, background): “While the effect of CO photodissociationmay play a role in lower-metallicity galaxies at high-z (e.g. Genzel et al., 2012; Tan et al., 2013), for the typical dusty heavily starforming systems that are the subject of this review, we can expect that this will play little part in driving any…”
• SUBMILLIMETER GALAXIES AS PROGENITORS OF COMPACT QUIESCENT GALAXIES (2014, unknown): “2 − 4.51 (Erb et al. 2006; Genzel et al. 2012) 18.”
• Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, unknown): “2010; Genzel et al. 2012, Combes et al. 2013).”
• Feedback-regulated star formation in molecular clouds and galactic discs (2013, background): “…rises at low metallicities such as in the Small Magellanic Cloud (Blitz et al. 2007) and for high-redshift galaxies below a critical stellar mass (Genzel et al. 2012), probably because CO molecules are photo-dissociated by the UV radiation field as the attenuation by dust decreases (e.g., van…”
• An Overview of the Dwarf Galaxy Survey (2013, background): “Evidence is growing for molecular gas surface density, in particular, as the regulator for star formation in galaxies of the local Universe as well as at high-redshift, and not necessarily the total gas density (e.g., Kennicutt et al. 2007; Bigiel et al. 2008; Genzel et al. 2012).”
• MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, background): “First, the conversion factor needed to impose a fixed τmoldep depends more sharply on metallicity than the predictions from many theoretical models and some observational determinations (see plots in Krumholz et al. 2011; Schruba et al. 2012; Genzel et al. 2012).”
• Massive molecular outflows and evidence for AGN feedback from CO observations (2013, methodology): “Here we simply assume αCO(1−0) = 0.8 in ULIRGs (Downes & Solomon 1998), 1.2 in M 82 and M 82-like galaxies (Weiß et al. 2001) and 4.36 (rounded to 4.4 in Table 3) in the other galaxies (Milky Way value, Genzel et al. 2012).”
• A survey of molecular gas in luminous sub-millimetre galaxies (2012, background): “…and theoretical, dedicated to determining the value – and ascertaining the metallicity or environmental dependence – of α (e.g. Young & Scoville 1991; Solomon & Vanden Bout 2005; Liszt, Pety & Lucas 2010; Bolatto et al. 2011; Genzel et al. 2012; Narayanan et al. 2011; Papadopoulos et al. 2012).”
• COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, background): “Genzel et al. (2012) have combined the local (Leroy et al. 2011) and high-z empirical evidence for a second fitting function, 1.27 (12 log( / ) 8.67) (Z) = 10 (7).”
• LOW CO LUMINOSITIES IN DWARF GALAXIES (2012, methodology): “The recent study by Genzel et al. (2012) also applied the assumption of a constant SFE.”
• The evolution of the dust temperatures of galaxies in the SFR–M∗ plane up to z ~ 2 (2013, background): “(9) Over the range of stellar masses probed by our sample (i.e., ∼1.5 dex), we expect, based on the mass-metallicity relation, an increase in metallicity of ∼0.25 dex (∼0.4 dex) at z = 0 (z = 2) (Tremonti et al. 2004; z = 2, Erb et al. 2006; see also Genzel et al. 2012).”
• The ATLAS3D Project – XXVIII. Dynamically driven star formation suppression in early-type galaxies (2014, background): “This increased efficiency may be explained by a change in gas properties (e.g the high fraction of gas at high volume densities in starbursts), or may be an artefact of the imperfect methods we have of estimating star-formation rates, and tracing molecular hydrogen (Genzel et al. 2012).”
• REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “…to the contrary has been presented in numerous observational studies, the most recent of which are Leroy et al. (2011), Schruba et al. (2012), Genzel et al. (2012) and Rémy-Ruyer et al. (2014) who show that there is a tendency for αCO to decrease with metallicity both in local galaxies and…”
• A general model for the CO–H2 conversion factor in galaxies with applications to the star formation law (2011, methodology): “This has been noted both in observations (e.g. Leroy et al. 2011; Genzel et al. 2012) and in other numerical models (Feldmann, Gnedin & Kravtsov 2011; Krumholz, Leroy & McKee 2011; Shetty et al. 2011b).”
• PHIBSS: MOLECULAR GAS CONTENT AND SCALING RELATIONS IN z ∼ 1–3 MASSIVE, MAIN-SEQUENCE STAR-FORMING GALAXIES (2012, background): “The fewer z=2-2.5 points (crossed red squares) are consistent with the same relation but the scatter is larger, perhaps in part because of the metallicity dependence of the conversion factor for 5 of the 15 data points (Genzel et al. 2012).”
• VALIDATION OF THE EQUILIBRIUM MODEL FOR GALAXY EVOLUTION TO z ∼ 3 THROUGH MOLECULAR GAS AND DUST OBSERVATIONS OF LENSED STAR-FORMING GALAXIES (2013, methodology): “The z > 2 galaxies follow the inverse relation between αCO and metallicity seen in the z = 0 data and in the empirical relation of Genzel et al. (2012), although with a small systematic offset to lower values of αCO at fixed metallicity, the implications of which are discussed in §5.2.”