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Register a new userMolecular Line Emission as a Tool for Galaxy Observations (LEGO). I. HCN as a tracer of moderate gas densities in molecular clouds and galaxies
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Authors
Jens Kauffmann
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Trends observed in galaxies, such as the Gao & Solomon relation, suggest a linear relation between the star formation rate and the mass of dense gas available for star formation. Validation of such relations requires the establishment of reliable methods to trace the dense gas in galaxies. One frequent assumption is that the HCN ($J=1$--0) transition is unambiguously associated with gas at $rm{}H_2$ densities $gg{}10^4~rm{}cm^{-3}$. If so, the mass of gas at densities $gg{}10^4~rm{}cm^{-3}$ could be inferred from the luminosity of this emission line, $L_{rm{}HCN,(1text{--}0)}$. Here we use observations of the Orion~A molecular cloud to show that the HCN ($J=1$--0) line traces much lower densities $sim{}10^3~rm{}cm^{-3}$ in cold sections of this molecular cloud, corresponding to visual extinctions $A_Vapprox{}6~rm{}mag$. We also find that cold and dense gas in a cloud like Orion produces too little HCN emission to explain $L_{rm{}HCN,(1text{--}0)}$ in star--forming galaxies, suggesting that galaxies might contain a hitherto unknown source of HCN emission. In our sample of molecules observed at frequencies near 100~GHz (also including $rm{}^{12}CO$, $rm{}^{13}CO$, $rm{}C^{18}O$, CN, and CCH), $rm{}N_2H^+$ is the only species clearly associated with rather dense gas.
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It has been recently argued that the HCN J=1--0 line emission may not be an unbiased tracer of dense molecular gas ($rm nga 10^4 cm^{-3}$) in Luminous Infrared Galaxies (LIRGs: $rm L_{FIR}> 10^{11} L_{odot}$) and HCO$^+$ J=1--0 may constitute a better tracer instead (Gracia-Carpio et al. 2006), casting doubt into earlier claims supporting the former as a good tracer of such gas (Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J=1--0 transition from either species a poor proxy of its mass. Moreover the well-known sensitivity of the HCO$^+$ abundance on the ionization degree of the molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO$^+$ as dense gas tracers) may severely reduce the HCO$^+$ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result to the decreasing HCO$^+$/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and casts doubts on the HCO$^+$ rather than the HCN as a good dense molecular gas tracer. Multi-transition observations of both molecules are needed to identify the best such tracer, its relation to ongoing star formation, and constrain what may be a considerable range of dense gas properties in such galaxies.
We report column densities of molecular gas in the W5 star-forming region as traced with OH 18-cm emission in a grid survey using the Green Bank Telescope. OH appears to trace a greater column density than does CO in 8 out of 15 cases containing OH emission detections; the two molecules trace the same column densities for the other 7 cases. OH and CO trace a similar morphology of molecular gas with a nearly one-to-one correspondence. The mass of molecular gas traced by OH in the portion of the survey containing OH emission is $1.7$ (+ 0.6 or - 0.2) $times 10^4 M_{odot}$, whereas the corresponding CO detections trace $9.9 times 10^3 M_{odot} (pm 0.7) times 10^3$. We find that for lines observed in absorption, calculations assuming uniform gas and continuum distributions underestimate column density values by 1 to 2 orders of magnitude, making them unreliable for our purposes. Modeling of this behavior in terms of OH cloud structure on a scale smaller than telescopic resolution leads us to estimate that the filling factor of OH gas is a few to 10 percent. Consideration of filling factor effects also results in a method of constraining the excitation temperature values. The total molecular gas content of W5 may be approximately two to three times what we report from direct measurement, because we excluded absorption line detections from the mass estimate.
We use a sample of 36 galaxies from the KINGFISH (Herschel IR), HERACLES (IRAM CO), and THINGS (VLA HI) surveys to study empirical relations between Herschel infrared (IR) luminosities and the total mass of the interstellar gas (H2+HI). Such a comparison provides a simple empirical relationship without introducing the uncertainty of dust model fitting. We find tight correlations, and provide fits to these relations, between Herschel luminosities and the total gas mass integrated over entire galaxies, with the tightest, almost linear, correlation found for the longest wavelength data (SPIRE500). However, we find that accounting for the gas-phase metallicity (affecting the dust-to-gas ratio) is crucial when applying these relations to low-mass, and presumably high-redshift, galaxies. The molecular (H2) gas mass is found to be better correlated with the peak of the IR emission (e.g. PACS160), driven mostly by the correlation of stellar mass and mean dust temperature. When examining these relations as a function of galactocentric radius we find the same correlations, albeit with a larger scatter, up to a radius of 0.7 r_25 (within which most of the galaxys baryonic mass resides). However, beyond this radius the same correlations no longer hold, with the gas mass (predominantly HI) increasing relative to the infrared emission. The tight relations found for the bulk of the galaxys baryonic content suggest that the total gas masses of disk-like (non-merging) galaxies can be inferred from far-infrared continuum measurements in situations where only the latter are available, e.g. in ALMA continuum observations of high-redshift galaxies.
In a pilot project to study the relationship between star formation and molecular gas properties in nearby normal early-type galaxies, we used the IRAM 30m telescope to observe the 13CO(J=1-0), 13CO(J=2-1), HCN(J=1-0) and HCO+(J=1-0) line emission in the four galaxies of the SAURON sample with the strongest 12CO emission. We report the detection of 13CO emission in all four SAURON sources and HCN emission in three sources, while no HCO+ emission was found to our detection limits in any of the four galaxies. We find that the 13CO/12CO ratios of three SAURON galaxies are somewhat higher than those in galaxies of different Hubble types. The HCN/12CO and HCN/13CO ratios of all four SAURON galaxies resemble those of nearby Seyfert and dwarf galaxies with normal star formation rates, rather than those of starburst galaxies. The HCN/HCO+ ratio is found to be relatively high (i.e., >1) in the three SAURON galaxies with detected HCN emission, mimicking the behaviour in other star-forming galaxies but being higher than in starburst galaxies. When compared to most galaxies, it thus appears that 13CO is enhanced (relative to 12CO) in three out of four SAURON galaxies and HCO+ is weak (relative to HCN) in three out of three galaxies. All three galaxies detected in HCN follow the standard HCN-infrared luminosity and dense gas fraction-star formation efficiency correlations. As already suggested by 12CO observations, when traced by infrared radiation, star formation in the three SAURON galaxies thus appears to follow the same physical laws as in galaxies of different Hubble types. The star formation rate and fraction of dense molecular gas however do not reach the high values found in nearby starburst galaxies, but rather resemble those of nearby normal star-forming galaxies.
We present synthetic Hi and CO observations of a simulation of decaying turbulence in the thermally bistable neutral medium. We first present the simulation, with clouds initially consisting of clustered clumps. Self-gravity causes these clump clusters to form more homogeneous dense clouds. We apply a simple radiative transfer algorithm, and defining every cell with <Av> > 1 as molecular. We then produce maps of Hi, CO-free molecular gas, and CO, and investigate the following aspects: i) The spatial distribution of the warm, cold, and molecular gas, finding the well-known layered structure, with molecular gas surrounded by cold Hi, surrounded by warm Hi. ii) The velocity of the various components, with atomic gas generally flowing towards the molecular gas, and that this motion is reflected in the frequently observed bimodal shape of the Hi profiles. This conclusion is tentative, because we do not include feedback. iii) The production of Hi self-absorption (HISA) profiles, and the correlation of HISA with molecular gas. We test the suggestion of using the second derivative of the brightness temperature Hi profile to trace HISA and molecular gas, finding limitations. On a scale of ~parsecs, some agreement is obtained between this technique and actual HISA, as well as a correlation between HISA and N(mol). It quickly deteriorates towards sub-parsec scales. iv) The N-PDFs of the actual Hi gas and those recovered from the Hi line profiles, with the latter having a cutoff at column densities where the gas becomes optically thick, thus missing the contribution from the HISA-producing gas. We find that the power-law tail typical of gravitational contraction is only observed in the molecular gas, and that, before the power-law tail develops in the total gas density PDF, no CO is yet present, reinforcing the notion that gravitational contraction is needed to produce this component. (abridged)
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