Abstract
We perform 3-D dust radiative transfer (RT) calculations on hydrodynamic simulations of isolated and merging disk galaxies in order to quantitatively study the dependence of observed-frame submillimeter (submm) flux density on galaxy properties. We find that submm flux density and star formation rate (SFR) are related in dramatically different ways for quiescently star-forming galaxies and starbursts. Because the stars formed in the merger-induced starburst do not dominate the bolometric luminosity and the rapid drop in dust mass and more compact geometry cause a sharp increase in dust temperature during the burst, starbursts are very inefficient at boosting submm flux density (e.g., a $\ga16$x boost in SFR yields a $\la 2$x boost in submm flux density). Moreover, the ratio of submm flux density to SFR differs significantly between the two modes; thus one cannot assume that the galaxies with highest submm flux density are necessarily those with the highest bolometric luminosity or SFR. These results have important consequences for the bright submillimeter-selected galaxy (SMG) population. Among them are: 1. The SMG population is heterogeneous. In addition to merger-driven starbursts, there is a subpopulation of galaxy pairs, where two disks undergoing a major merger but not yet strongly interacting are blended into one submm source because of the large ($\ga 15$", or $\sim 130$ kpc at $z = 2$) beam of single-dish submm telescopes. 2. SMGs must be very massive ($M_{\star} \ga 6 \times 10^{10} \msun$). 3. The infall phase makes the SMG duty cycle a factor of a few greater than what is expected for a merger-driven starburst. (Abridged.)
Highlights
Submillimeter-selected galaxies (SMGs; Smail et al 1997; Barger et al 1998; Hughes et al 1998; Eales et al 1999; see Blain et al 2002 for a review) are extremely luminous, high-redshift (Chapman et al 2005) galaxies powered primarily by star formation rather than AGN (Alexander et al 2005a,b, 2008; Valiante et al 2007; Menendez-Delmestre et al 2007, 2009; Pope et al 2008; Younger et al 2008, 2009b)
Because the stars formed in the merger-induced starburst do not dominate the bolometric luminosity and the rapid drop in dust mass and more compact geometry cause a sharp increase in dust temperature during the burst, starbursts are very inefficient at boosting submm flux density (e.g., a 16x boost in star formation rate (SFR) yields a 2x boost in submm flux density)
Md decreases as a result of the decrease in gas mass, but only by ∼ 40% because the decrease in gas mass is partially mitigated by metal enrichment of the gas from star formation, as the metallicity doubles over the course of the simulation
Summary
Submillimeter-selected galaxies (SMGs; Smail et al 1997; Barger et al 1998; Hughes et al 1998; Eales et al 1999; see Blain et al 2002 for a review) are extremely luminous (bolometric luminosity Lbol ∼ 1012 − 1013L⊙; e.g., Kovacs et al 2006), high-redshift (Chapman et al 2005) galaxies powered primarily by star formation rather than AGN (Alexander et al 2005a,b, 2008; Valiante et al 2007; Menendez-Delmestre et al 2007, 2009; Pope et al 2008; Younger et al 2008, 2009b). Semi-analytic models predict that the SMG population is dominated by merger-induced starbursts rather than quiescent star formation (Baugh et al 2005; Fontanot et al 2007; Swinbank et al 2008; Lo Faro et al 2009; Fontanot & Monaco 2010; Gonzalez et al 2011; but cf. Granato et al 2004)
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