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Register a new userThe Star Formation Rate - Dense Gas Relation in the Nuclei of Nearby Galaxies
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We investigate the relationship between the star formation rate (SFR) and dense molecular gas mass in the nuclei of galaxies. To do this, we utilize the observed 850 micron luminosity as a proxy for the infrared luminosity and SFR, and correlate this with the observed CO (J=3-2) luminosity. We find tentative evidence that the LIR-CO (J=3-2) index is similar to the Kennicutt-Schmidt (KS) index (N ~ 1.5) in the central ~1.7 kpc of galaxies, and flattens to a roughly linear index when including emission from the entire galaxy. This result may imply that the volumetric Schmidt relation is the underlying driver behind the observed SFR-dense gas correlations, and provides tentative confirmation for recent numerical models. While the data exclude the possibility of a constant LIR-CO (J=3-2) index for both galaxy nuclei and global measurements at the ~80% confidence level, the considerable error bars cannot preclude alternative interpretations.
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We present observations made with the 10m Heinrich Hertz Sub-Millimeter Telescope of HCN (3-2) emission from a sample of 30 nearby galaxies ranging in infrared luminosity from 10^10 - 10^12.5 L_sun and HCN (3-2) luminosity from 10^6 - 10^9 K km s^-1 pc^2. We examine the correlation between the infrared luminosity and HCN (3-2) luminosity and find that the best fit linear regression has a slope (in log-log space) of 0.74+/-0.12. Including recently published data from Gracia-Carpio et al. tightens the constraints on the best-fit slope to 0.79+/-0.09. This slope below unity suggests that the HCN (3-2) molecular line luminosity is not linearly tracing the amount of dense gas. Our results are consistent with predictions from recent theoretical models that find slopes below unity when the line luminosity depends upon the average gas density with a power-law index greater than a Kennicutt-Schmidt index of 1.5.
We model the star formation relation of molecular clumps in dependence of their dense-gas mass when their volume density profile is that of an isothermal sphere, i.e. $rho_{clump}(r) propto r^{-2}$. Dense gas is defined as gas whose volume density is higher than a threshold $rho_{th}=700,M_{odot}.pc^{-3}$, i.e. HCN(1-0)-mapped gas. We divide the clump into two regions: a dense inner region (where $rho_{clump}(r) geq rho_{th}$), and low-density outskirts (where $rho_{clump}(r) < rho_{th}$). We find that the total star formation rate of clumps scales linearly with the mass of their dense inner region, even when more than half of the clump star formation activity takes place in the low-density outskirts. We therefore emphasize that a linear star formation relation does not necessarily imply that star formation takes place exclusively in the gas whose mass is given by the star formation relation. The linearity of the star formation relation is strengthened when we account for the mass of dense fragments (e.g. cores, fibers) seeding star formation in the low-density outskirts, and which our adopted clump density profile $rho_{clump}(r)$ does not resolve. We also find that the star formation relation is significantly tighter when considering the dense gas than when considering all the clump gas, as observed for molecular clouds of the Galactic plane. When the clumps have no low-density outskirts (i.e. they consist of dense gas only), the star formation relation becomes superlinear and progressively wider.
An imaging survey of CO(1-0), HCN(1-0), and HCO$^+$(1-0) lines in the centers of nearby Seyfert galaxies has been conducted using the Nobeyama Millimeter Array and the RAINBOW interferometer. Preliminary results reveal that 3 Seyferts out of 7 show abnormally high HCN/CO and HCN/HCO$^+$ ratios, which cannot occur even in nuclear starburst galaxies. We suggest that the enhanced HCN emission originated from X-ray irradiated dense obscuring tori, and that these molecular line ratios can be a new diagnostic tool to search for ``pure AGNs. According to our HCN diagram, we suggest that NGC 1068, NGC 1097, and NGC 5194 host ``pure AGNs, whereas Seyfert nuclei of NGC 3079, NGC 6764, and NGC 7469 may be ``composite in nature.
We present an analysis of the relation between star formation rate (SFR) surface density (sigmasfr) and mass surface density of molecular gas (sigmahtwo), commonly referred to as the Kennicutt-Schmidt (K-S) relation, at its intrinsic spatial scale, i.e. the size of giant molecular clouds (10-150 pc), in the central, high-density regions of four nearby low-luminosity active galactic nuclei (AGN). We used interferometric IRAM CO(1-0) and CO(2-1), and SMA CO(3-2) emission line maps to derive sigmahtwo and HST-Halpha images to estimate sigmasfr. Each galaxy is characterized by a distinct molecular SF relation at spatial scales between 20 to 200 pc. The K-S relations can be sub-linear, but also super-linear, with slopes ranging from 0.5 to 1.3. Depletion times range from 1 and 2Gyr, compatible with results for nearby normal galaxies. These findings are valid independently of which transition, CO(1-0), CO(2-1), or CO(3-2), is used to derive sigmahtwo. Because of star-formation feedback, life-time of clouds, turbulent cascade, or magnetic fields, the K-S relation might be expected to degrade on small spatial scales (<100 pc). However, we find no clear evidence for this, even on scales as small as 20 pc, and this might be because of the higher density of GMCs in galaxy centers which have to resist higher shear forces. The proportionality between sigmahtwo and sigmasfr found between 10 and 100 Msun/pc2 is valid even at high densities, 10^3 Msun/pc2. However, by adopting a common CO-to-H2 conversion factor (alpha_CO), the central regions of the galaxies have higher sigmasfr for a given gas column than those expected from the models, with a behavior that lies between the mergers/high-redshift starburst systems and the more quiescent star-forming galaxies, assuming that the first ones require a lower value of alpha_CO.
It is well-established that a gas density gradient inside molecular clouds and clumps raises their star formation rate compared to what they would experience from a gas reservoir of uniform density. This effect should be observed in the relation between dense-gas mass $M_{dg}$ and star formation rate $SFR$ of molecular clouds and clumps, with steeper gas density gradients yielding higher $SFR/M_{dg}$ ratios. The content of this paper is two-fold. Firstly, we build on the notion of magnification factor introduced by Parmentier (2019) to redefine the dense-gas relation (i.e. the relation between $M_{dg}$ and $SFR$). Not only does the $SFR/M_{dg}$ ratio depend on the mean free-fall time of the gas and on its (intrinsic) star formation efficiency per free-fall time, it also depends on the logarithmic slope $-p$ of the gas density profile and on the relative extent of the constant-density region at the clump center. Secondly, we show that nearby molecular clouds follow the newly-defined dense-gas relation, provided that their dense-gas mass is defined based on a volume density criterion. We also find the same trend for the dense molecular clouds of the Central Molecular Zone (CMZ) of the Galaxy, although this one is scaled down by a factor of $10$ compared to nearby clouds. The respective locii of both nearby and CMZ clouds in the $(p, SFR/M_{dg})$ parameter space is discussed.
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