The Star Formation Rate and Dense Molecular Gas in Galaxies

By Yu Gao, P. Solomon. 2003

771 citations recorded by the source corpus.

HCN luminosity is a tracer of dense molecular gas, n(H2) ≳ 3 × 104 cm-3, associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies, including nine ultraluminous infrared galaxies (ULIGs, LIR ≳ 1012 L☉), 22 luminous infrared galaxies (LIGs, 1011 L☉ < LIR ≲ 1012 L☉), and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity, and the observations are reported in Paper I. This paper analyzes the relationships between the total far-IR luminosity (a tracer of the star formation rate), the global HCN line luminosity (a measure of the total dense molecular gas content), and the CO luminosity (a measure of the total molecular content). We find a tight linear correlation between the IR and HCN luminosities LIR and LHCN (in the log-log plot) with a correlation coefficient R = 0.94, and an almost constant average ratio LIR/LHCN = 900 L☉ (K km s-1 pc2)-1. The IR-HCN linear correlation is valid over 3 orders of magnitude including ULIGs, the most luminous objects in the local universe. The direct consequence of the linear IR-HCN correlation is that the star formation law in terms of dense molecular gas content has a power-law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas. The HCN-CO correlation is also much tighter than the IR-CO correlation. We suggest that the nonlinear correlation between LIR and LCO may be a consequence of the stronger and perhaps more physical correlations between LIR and LHCN and between LHCN and LCO. Thus, the star formation rate indicated by LIR depends on the amount of dense molecular gas traced by HCN emission, not the total molecular gas traced by CO emission. One of the main arguments in favor of an active galactic nucleus (AGN) as the power source in ULIGs is the anomalously high ratio LIR/LCO or LIR/M(H2) or high star formation rate per M☉ of gas, compared with that from normal spiral galaxies. This has been interpreted as indicating that a dust-enshrouded AGN is required to produce the very high luminosity. Viewed in terms of the dense gas mass the situation is completely different. The ratio LIR/LHCN or LIR/Mdense, a measure of the star formation rate per solar mass of dense gas, is essentially the same in all galaxies including ULIGs. The ratio LIR/Mdense is virtually independent of galaxy luminosity and on average LIR/Mdense ≈ 90L☉/M☉, about the same as in GMC cores but much higher than in GMCs. We find that ULIGs simply have a large quantity of dense molecular gas and thus produce a prodigious starburst that heats the dust, produces the IR, and blocks all or most optical radiation. The HCN global luminosity may be used as an indicator of the star formation rate in high-redshift objects including hyperluminous galaxies. The HCN/CO ratio is an indicator of the dense molecular gas fraction and gauges the globally averaged molecular gas density. We find that the HCN/CO ratio is a powerful starburst indicator. All galaxies in our sample with a high dense gas mass fraction indicated by LHCN/LCO > 0.06 are LIGs or ULIGs. Normal spirals all have similar and low dense gas fractions LHCN/LCO = 0.02-0.05. The global star formation efficiency depends on the fraction of the molecular gas in a dense phase.

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/a2f25fcab94efd06dec19d88f46ecae016be8f3a

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Spectroscopy

Citations in the corpus

The properties of the interstellar medium within a star-forming galaxy at z= 2.3 (2010, methodology): “Using our 3-σ upper limit on the HCN(1–0) luminosity we derive a limit on the dense gas mass of Mdense(H2)= αHCN(1−0) L ′ HCN ≪ 4.5× 10 10 M⊙ with αHCN ≪ 10 (Gao & Solomon 2004).”
Theory of Star Formation (2007, background): “In particular, Wu et al. (2005) show that the ratio LHCN/LIR ∝ Mdense clumps/Ṁ∗ measured in Milky Way star-forming regions agrees with the same values measured in high-redshift galaxies (Gao & Solomon, 2004), for which there is only one order of magnitude scatter.”
A UNIVERSAL, LOCAL STAR FORMATION LAW IN GALACTIC CLOUDS, NEARBY GALAXIES, HIGH-REDSHIFT DISKS, AND STARBURSTS (2011, positive): “Based on observations of the linear relation between mass of gas traced by HCN and star formation rate (Gao & Solomon 2004; Wu et al. 2005, 2010; however, see Gao et al. 2007 and Krumholz & Thompson 2007 for evidence that the relation is not strictly linear), in these models it is generally assumed…”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “…star formation using HCN, HCO+, higher rotational transitions CO, and other molecules that have critical densities ranging from nH2 ∼ 104 − 108 cm−3 (Gao & Solomon 2004a,b; Narayanan et al. 2005; Graciá-Carpio et al. 2006; Gao et al. 2007; Bussmann et al. 2008; Baan et al. 2008; Juneau et al.…”
Slow Star Formation in Dense Gas: Evidence and Implications (2006, background): “Gao & Solomon (2004a) perform radiative transfer calculations under a variety of assumptions about the temperature and distribution of the HCN-emitting gas, and arrive at conversion factors of 10Y25.”
Highly-excited CO emission in APM 08279+5255 at z = 3.9 (2007, methodology): “L′HCN relation observed in local galaxies (Gao & Solomon 2004; recalculated to FIR-HCN by Carilli et al. 2005).”
A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the Herschel Gould Belt survey (2015, methodology): “…is affected by large uncertainties because different tracers of dense gas and star formation were used by Lada et al. (2010) on the one hand and Gao & Solomon (2004) on the other, these results suggest that there may be a quasi-universal “star formation law” within dense gas above the (column)…”
GeV OBSERVATIONS OF STAR-FORMING GALAXIES WITH THE FERMI LARGE AREA TELESCOPE (2012, methodology): “In order to select a sample of galaxies with unambiguous ongoing star formation, we base our sample of galaxies on the HCN survey of Gao & Solomon (2004a).”
What controls star formation in the central 500 pc of the Galaxy (2013, background): “Dense molecular gas as traced by HCN is found to be correlated with star formation tracers (e.g. Gao & Solomon 2004; Wu et al. 2005).”
On the frequency, intensity, and duration of starburst episodes triggered by galaxy interactions and mergers (2008, result): “This parametrization is consistent with the observational evidence that on global scales the SFR in disk galaxies is well represented by a Schmidt law with an exponent 1.4 (see Kennicutt 1998a,b, but also Wong & Blitz 2002; Boissier et al. 2003; Gao & Solomon 2004).”
COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale (2011, background): “…inferred from these fluxes is an excellent measurement of the total luminosity from young stars that has been reprocessed by dust and is known to correlate well with the total CO luminosity of the galaxies (e.g. Sanders & Mirabel 1985; Sanders, Scoville & Soifer 1991; Gao & Solomon 2004).”
The CO-to-H2 Conversion Factor (2013, background): “Similarly, a positive correlation exists between gas density and star formation rate (e.g., Gao & Solomon 2004).”
THE STAR FORMATION RATE OF TURBULENT MAGNETIZED CLOUDS: COMPARING THEORY, SIMULATIONS, AND OBSERVATIONS (2012, background): “…of the form ΣSFR ∝ ΣNgas (albeit with significant scatter), similar in exponents N ≈ 1–2 to the KennicuttSchmidt relation (Schmidt 1959; Kennicutt 1998) and follow-up measurements for molecular gas (e.g., Wong & Blitz 2002; Gao & Solomon 2004; Bigiel et al. 2008; Kennicutt & Evans 2012).”
On the origin of the galaxy star‐formation‐rate sequence: evolution and scatter (2009, background): “…Blitz & Rosolowsky (2006) we assume that star formation takes place in dense molecular gas, traced by the hydrogen cyanide molecule (HCN), with a constant star formation efficiency: SFR [M pc−2 Gyr−1] = ̃SF [Gyr−1] mol,HCN [M pc−2] , (A1) where ̃SF 10−13 Gyr−1 (Gao & Solomon 2004; Wu et al. 2005).”
The evolution of massive black holes and their spins in their galactic hosts (2012, background): “…that star formation in galactic disks only happensinside dense molecular clouds, which are well traced by the HCN luminosity (Wu et al. 2005; Gao & Solomon 2004), we follow 4 The reservoir’s geometry is needed for instance to calculate the velocity Vc of the composite system, entering e.g.…”
Connecting Dense Gas Tracers of Star Formation in our Galaxy to High-z Star Formation (2005, methodology): “A recent survey of HCN J = 1− 0 emission in 65 normal spiral and starburst galaxies found that the star formation rate, as traced by the infrared luminosity, has a tight and linear correlation with the luminosity of HCN (Gao & Solomon 2004a, 2004b).”
HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies (2003, background): “In a companion paper (Gao & Solomon 2004, hereafter Paper II), we fully discuss the interpretation and implications of our HCN survey.”
COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, methodology): “…easily accessible to global measurements of the standard tracers of star formation and gas (i.e. stellar and infrared luminosity, CO 1-0, 2-1, 3-2 line luminosity, HI mass, dust mass) in a large number of galaxies (e.g. Young & Scoville 1991, Solomon & Sage 1988, Gao & Solomon 2004, Scoville 2013).”
THE STAR FORMATION RATE AND GAS SURFACE DENSITY RELATION IN THE MILKY WAY: IMPLICATIONS FOR EXTRAGALACTIC STUDIES (2010, methodology): “…correlated with the total IR luminosity in clumps as long as LIR > 10 4.5 L⊙ (Wu et al. 2005), as well as in both normal spiral and starburst galaxies (Gao & Solomon 2004a,b), we can use it to compare gas and star formation from the total IR luminosity in both the Milky Way and external galaxies.”
Global hierarchical collapse in molecular clouds. Towards a comprehensive scenario (2019, background): “…between the fraction of clumps associated with massive stars and their peak column density (e.g., Urquhart et al. 2018) or between the surface density of dense gas and of the star formation rate (e.g., Gao & Solomon 2004; Lada et al. 2010), as shown by Vázquez-Semadeni et al. (2018, see also Secs.”