Abstract
We examine in detail the recent proposal that extreme Cosmic-Ray-Dominated-Regions (CRDRs) characterize the ISM of galaxies during events of high-density star formation, fundamentally altering its initial conditions (Papadopoulos 2010). Solving the coupled chemical and thermal state equations for dense UV-shielded gas reveals that the large cosmic ray energy densities in such systems (U_{CR} (few)x(10^3-10^4) U_{CR,Gal}) will indeed raise the minimum temperature of this phase (where the initial conditions of star formation are set) from ~10K (as in the Milky Way) to (50-100)K. Moreover in such extreme CRDRs the gas temperature remains fully decoupled from that of the dust, with T_{kin} >> T_{dust}, even at high densities (n(H_2)~10^5--10^6 cm^{-3}), quite unlike CRDRs in the Milky Way where T_k T_{dust} when n(H_2) >= 10^5 cm^{-3}. These dramatically different star formation initial conditions will: a) boost the Jeans mass of UV-shielded gas regions by factors of ~10--100 with respect to those in quiescent or less extreme star forming systems, and b) "erase" the so-called inflection point of the effective equation of state (EOS) of molecular gas. Both these effects occur across the entire density range of typical molecular clouds, and may represent {\it a new paradigm for all high-density star formation in the Universe}, with cosmic rays as the key driving mechanism, operating efficiently even in the high dust extinction environments of extreme starbursts...
Highlights
Much of the stellar mass in the Universe forms in starbursts (e.g. Blain et al 1999; Genzel et al 2001; Smail et al 2002), spectacular events during which the star formation rate (SFR) of a galaxy rises from few solar masses per year, typical of spirals such as the Milky-Way (Mckee & Williams 1997), to several hundred solar masses per year as in the local Ultra Luminous Infrared Galaxies (ULIRGs) (e.g. Sanders & Mirabel 1996; Genzel et al 1998; Sanders & Ishida 2004) where merger-driven starbursts take place in very compact (D∼100–300 pc) dense gas disks (e.g. Downes & Solomon 1998; Sakamoto et al 2008)
Using inappropriate PDR-type initial conditions of star formation in numerical models: a) would make them hard-pressed to account even for the near-invariant initial mass function (IMF) found in the Galaxy, b) omits the significant, Cosmic rays (CRs)-induced, thermal decoupling between gas and dust (Figures 2, 3)
We have conducted new, detailed calculations on the thermal balance of UV-shielded dense gas in Cosmic-Ray-Dominated-Regions (CRDRs), the ISM phase where the initial conditions of star formation are set in galaxies, in order to examine in detail the effects of extreme CRDRs on the mass scale of young stars and the IMF recently suggested by Papadopoulos (2010)
Summary
Much of the stellar mass in the Universe forms in starbursts (e.g. Blain et al 1999; Genzel et al 2001; Smail et al 2002), spectacular events during which the star formation rate (SFR) of a galaxy rises from few solar masses per year, typical of spirals such as the Milky-Way (Mckee & Williams 1997), to several hundred solar masses per year as in the local Ultra Luminous Infrared Galaxies (ULIRGs) (e.g. Sanders & Mirabel 1996; Genzel et al 1998; Sanders & Ishida 2004) where merger-driven starbursts take place in very compact (D∼100–300 pc) dense gas disks (e.g. Downes & Solomon 1998; Sakamoto et al 2008). A recent study has shown that the average CR energy densities (UCR) in compact starbursts typical in ULIRGs (recently found in an SMG at z∼2.3, Swinbank et al 2010), will be boosted by the tremendeous factors of UCR∼(few)×(103– 104) UCR,Gal, potentially transforming their ISM into extreme Cosmic-Ray-Dominated Regions (CRDRs) This will dramatically alter the thermal and ionization state of UV-shielded cores, and the initial conditions of star-formation, throughout their considerable molecular gas reservoirs (∼109-1010 M⊙). In this new context and throughout this work SFR and ρsfr will denote these quantities only for the massive stars (which define UCR and power the IR/cm continuum of the ISM), with their total values necessarily remaining uncertain, depending on the exact IMF, which will no longer be an invariant
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