Two phase galaxy formation: the gas content of normal galaxies

M. Cook 1 2 C. Evoli 2 E. Barausse 0 2 G. L. Granato 1 4 A. Lapi 2 3 4 0 Centre for Fundamental Physics, University of Maryland , College Park, MD 20742-4111 , USA 1 INAF, Osservatorio Astronomico di Padova , Vicolo dell' Osservatorio 5, I-35122 Padova , Italy 2 Astrophysics Sector, SISSA/ISAS , Via Beirut 2-4, I-34151 Trieste , Italy 3 Department of Physics, Univ. di Roma 'Tor Vergata' , Via della Ricerca Scientifica 1, I-00133 Rome , Italy 4 INAF, Osservatorio Astronomico di Trieste , Via G.B. Tiepolo 11, I-34131 Trieste , Italy A B S T R A C T We investigate the atomic (H I) and molecular (H2) Hydrogen content of normal galaxies by combining observational studies linking galaxy stellar and gas budgets to their host dark matter (DM) properties, with a physically grounded galaxy formation model. This enables us to analyse empirical relationships between the virial, stellar and gaseous masses of galaxies, and explore their physical origins. Utilizing a semi-analytic model (SAM) to study the evolution of baryonic material within evolving DM haloes, we study the effects of baryonic infall and various star formation and feedback mechanisms on the properties of formed galaxies using the most up-to-date physical recipes. We find that in order to significantly improve the agreement with observations of low-mass galaxies, we must suppress the infall of baryonic material and exploit a two-phase interstellar medium, where the ratio of H I to H2 is determined by the galactic disc structure. Modifying the standard Schmidt-Kennicutt star formation law, which acts upon the total cold gas in galaxy discs and includes a critical density threshold, and employing a star formation law which correlates with the H2 gas mass results in a lower overall star formation rate. This, in turn, allows us to simultaneously reproduce stellar, H I and H2 mass functions of normal galaxies. 1 I N T R O D U C T I O N Neutral atomic hydrogen is the most abundant element in the Universe and plays a fundamental role in galaxy formation, principally as the raw material from which stars form. Within galaxies, the interstellar medium (ISM) acts as a temporally evolving baryonic component; competing processes cause the accumulation (through external infall from the intergalactic medium and stellar evolution) and depletion (through star formation and various feedback mechanisms) of hydrogen. Thus, observational determinations and theoretical predictions of the hydrogen budget within galaxies of various masses and morphologies are of central importance to constraining the physics of galaxy formation (see Kauffmann, White & Guilderoni 1993; Benson et al. 2003; Yang, Mo & van den Bosch 2003; Mo et al. 2005; Kaufmann et al. 2009) Moreover, within the ISM hydrogen comprises the majority of the cold gas mass, and when non-ionized exists within two-phases, atomic H I and molecular H2. A large body of observational analysis has shown that within galaxies, H I generally follows a smooth, diffuse distribution whereas H2 regions are typically dense, optically thick clouds which act as the birthplaces for newly formed stars (Drapatz & Zinnecker 1984; Wong & Blitz 2002; Blitz & Rosolowski 2004; Krumholz & McKee 2005; Wu et al. 2005; Blitz & Rosolowsky 2006). Due to the distinct differences in these phases, and the central importance of ISM physics to the evolution of galaxies, cosmological simulations have begun to include both phases (see Gnedin, Tassis & Kravstov 2009 and references therein), and observations have begun focusing on simultaneous measurements of both H I and H2 (see Obreschkow & Rawlings 2009). The distinction between these two phases has recently been shown to be of crucial importance to constrain the physics of galaxy formation. In particular, resolved spectroscopy using Galaxy Evolution Explorer (GALEX) showing obscured star-forming regions in nearby galaxies (Kennicutt et al. 2003, 2007; Calzetti et al. 2007; Gil de Paz et al. 2007), and various observational surveys providing maps of gas in galaxies at high-resolutions (Helfer et al. 2003; Walter et al. 2008; Leroy et al. 2009), have revealed a deeper level of complexity on subgalactic scales. These studies allowed theoretical models for the ISM and star formation to be constrained and further developed. Furthermore, due to the constant replenishment and depletion of hydrogen in either H I or H2 phases, and to their separate yet interlinked properties, at any epoch, measurements of the fraction of H I and H2 are highly constraining for the processes of molecular cloud formation, star formation, baryonic infall and various feedbacks. Therefore, simultaneously predicting the stellar and gas mass functions of normal galaxies is a major challenge for any physically motivated galaxy formation model, requiring an accurate depiction of all of the aforementioned processes (see Mo et al. 2005 for a detailed discussion). These issues manifest most clearly within the largely successful cold dark matter (CDM) paradigm within the lowest mass systems, where it still remains unclear whether strongly non-linear feedback mechanisms, lower star formation efficiencies or suppression of initial infall on to dark matter (DM) haloes is the dominant driver for the suppression of luminous structure formation (Mo et al. 2005). It is more than likely that a combination of the above-mentioned effects will go a long way to alleviating current tensions between models and observations, since current semi-analytical models (SAMs) incorporate several processes in order to generate a deficiency of stellar mass in DM haloes; many of which operate most effectively at low masses (Benson et al. 2003; De Lucia, Kauffmann & White 2004). Observationally, Zwaan et al. (2005) used the catalogue of 4315 extragalactic H I 21-cm emission line detections from the H I Parkes All Sky Survey (HIPASS; Barnes et al. 2001) and obtained the most accurate measurement of the H I mass function of galaxies to date. The H I mass function (HIMF) is fitted with a Schechter function with a faint-end slope of 1.37 0.03. The sensitivity of this survey was so high that they were able to extend their analysis well down to H I masses of 107.2 M , hence this is the most complete analysis so far. Using these statistical constraints, it has now become possible to make stringent comparisons between theoretical models and observations even in low-mass galaxies. The physics of cold gas becomes increasingly relevant for constraining galaxy formation models at relatively low masses (dominated by late-type galaxies), where the presence of gas becomes substantial and therefore may break the degeneracies between feedback, star formation and infall processes. Moreover, within the CDM scenario the H I and H2 mass budgets in galaxies are determined by an intricate offset between several competing processes, all of which have strong mass dependencies. Thus, the present H I and H2 fractions are strong functions of host (...truncated)