The Physics of Galaxy Formation by Claudia Del P. Lagos

Author:Claudia Del P. Lagos
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

5.1 Introduction

The connection between molecular gas and star formation (SF) is a fundamental but poorly understood problem in galaxy formation. Observations have shown that the star formation rate (SFR) correlates with the abundance of cold, dense gas in galaxies, suggesting that molecular gas is needed to form stars. A variety of observational evidence supports this conclusion, such as the correlation between the surface densities of SFR and (hereafter CO) emission and between the CO and infrared (IR) luminosities (e.g. Solomon and Vanden Bout 2005; Bigiel et al. 2008). The CO luminosity traces dense gas in the interstellar medium (ISM), which is dominated by molecular hydrogen (). The IR luminosity approximates the total luminosity emitted by interstellar dust, which, in media that are optically thick to ultraviolet (UV) radiation, is expected to correlate closely with the SFR in star-forming galaxies.

In the local Universe, high-quality, spatially resolved CO data show a tight and close to linear correlation between the surface density of the SFR and the surface density of CO emission, that extends over several orders of magnitude and in very different environments: from low-metallicity, atomic-dominated gas to high-metallicity, molecular-dominated gas (e.g. Wong and Blitz 2002; Leroy et al. 2008; Bigiel et al. 2008, 2011; Schruba et al. 2011; Rahman et al. 2012; see Chap. 1). This suggests that SF proceeds in a similar way in these different environments. Support for this also comes from the IR-CO luminosity relation, in which high-redshift submillimeter galaxies (SMGs) and quasi-stellar objects (QSOs) falling on a similar relation to luminous IR galaxies (LIRGs) and ultra-luminous IR galaxies (ULIRGs) in the local Universe (see Chap. 1).

The CO emission from galaxies is generally assumed to be a good indicator of molecular gas content. However, to infer the underlying mass from CO luminosity it is necessary to address how well CO traces mass. This relation is usually parametrised by the conversion factor, , which is the ratio between the column density and the integrated CO line intensity. Large efforts have been made observationally to determine the value of for the transition, and it has been inferred directly in a few galaxies, mainly through virial estimates and measurements of dust column density. Typical estimates for normal spiral galaxies lie in the range (e.g. Young and Scoville 1991; Boselli et al. 2002; Blitz et al. 2007; Bolatto et al. 2011). However, systematic variations in the value of have been inferred in galaxies whose ISM conditions differ considerably from those in normal spiral galaxies, favouring a larger in low-metallicity galaxies and a smaller in starburst galaxies (e.g. Leroy et al. 2007, 2011; Magdis et al. 2011; see Solomon and Vanden Bout 2005 for a review).

Theoretically, most studies of are based on Photon Dominated Region (PDR; e.g. Bell et al. 2006) or large velocity gradient (LVG; e.g. Weiß et al. 2005) models. Such models have been shown to be an excellent theoretical tool, reproducing the emission of various chemical species coming from regions where the CO emission dominates (i.e. in giant molecular clouds,

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