CO excitation of normal star-forming galaxies out toz= 1.5 as regulated by the properties of their interstellar medium

Abstract

We investigate the CO excitation of normal (near-IR selected BzK) star-forming (SF) disk galaxies at z = 1.5 using IRAM Plateau de Bure observations of the CO[2-1], CO , and CO[5-4] transitions for four galaxies, including VLA observations of CO[1-0] for three of them, with the aim of constraining the average state of H 2 gas. By exploiting previous knowledge of the velocity range, spatial extent, and size of the CO emission, we measure reliable line fluxes with a signal-to-noise ratio >4-7 for individual transitions. While the average CO spectral line energy distribution (SLED) has a subthermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO emission is four times stronger than assuming MW excitation. This demonstrates that there is an additional component of more excited, denser, and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is lower than in local (ultra-)luminous infrared galaxies (ULIRGs) and high-redshift starbursting submillimeter galaxies, however, and appears to be closely correlated with the average intensity of the radiation field U and with the star formation surface density, but not with the star formation efficiency. The luminosity of the CO transition is found to be linearly correlated with the bolometric infrared luminosity over four orders of magnitudes. For this transition, z = 1.5 BzK galaxies follow the same linear trend as local spirals and (U)LIRGs and high-redshift star-bursting submillimeter galaxies. The CO[5-4] luminosity is thus empirically related to the dense gas and might be a more convenient way to probe it than standard high-density tracers that are much fainter than CO. We see excitation variations among our sample galaxies that can be linked to their evolutionary state and clumpiness in optical rest-frame images. In one galaxy we see spatially resolved excitation variations, where the more highly excited part of the galaxy corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high-excitation CO emission in high-redshift disk-like galaxies.

The Alma Spectroscopic Survey in the Hubble Ultra Deep Field: Molecular Gas Reservoirs in High-Redshift Galaxies

“…Additionally, we compare with the average template for high-z main-sequence galaxies by Daddi et al (2015) and with the theoretical predictions based on the SFR surface density by Narayanan & Krumholz (2014) (see Table 1 and the discussion in Section 5.2.2). In no case do we find starburst-like CO excitation that would be comparable with Figure 5.…”

“…30 The uncertainty in ¢ -L CO 1 0 ( ) accounts for both the measured flux uncertainty and the standard deviation in the r J1 values in the sample studied by Daddi et al (2015). The molecular gas masses are then derived as…”

“…Table 3 lists the values of molecular gas masses that we derive for each source. We then combine these measurements or limits on the molecular gas mass with properties of the galaxies inferred from the SED Table 2 CO Lines in the Galaxies of our Sample 30 Daddi et al (2015) do not measure CO(4-3) in the galaxies in their sample. The value of r 41 adopted here is extrapolated from their measurements of r 31 and r 51 , in the case of a CO ladder that peaks around » J 5 (see their Figure 10, left).…”

“…We then generated mock velocity maps for these three cases, assuming that the CO light traces the mass distribution; and we Daddi et al (2015). (6) Molecular gas mass, assuming a = 3.6…”

“…Atz 0, the conversion factor that is typically used is ∼4  M (Kkm s −1 pc 2 ) −1 for "normal" * > M 10 9  M starforming galaxies with metallicities close to solar (see Bolatto et al 2013, for a recent review). If other CO transitions are observed instead of the J=1  0 ground state transition, a further factor is required to account for the CO excitation (see, e.g., Weiß et al 2007;Daddi et al 2015). Tacconi et al (2010) and Daddi et al (2010a) investigated the molecular gas content of highly star-forming galaxies atz 1.2 andz 2.3 via the CO(3-2) transition.…”