Star-formation laws in luminous infrared galaxies. New observational constraints on models

By S. García-Burillo, A. Usero, A. Alonso-Herrero, J. Graciá-Carpio, M. Pereira-Santaella, L. Colina, P. Planesas, S. A. OAN-Spain, CAB-CSIC-Spain, IFCA-CSIC-Spain and 3 more. 2011

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The observational study of star formation relations in gala xies is central to unraveling the related physical processe s that are at work on both local and global scales. It is still debated whether s tar formation can be described by a universal law that remains valid in different populations of galaxies. We wish to expand the sample of extreme starbursts, represented by local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs), with high quality observations in the 1‐0 line of HCN, which is taken as a proxy for the dense molecular gas content. The new data presented in this work allow us to enlarge in particular the number of LIRGs studied in HCN by a factor 3 compared to previous works. The chosen LIRG sample has a range of HCN luminosities that partly overlaps with that of the normal galaxy population. We study if a universal law can account for the star formation relations observed for the dense molecular gas in normal star forming galaxies and extreme starbursts and explore the validity of different theoretical prescriptions of the star formation law. We have used the IRAM 30m telescope to observe a sample of 19 LIRGs in the 1‐0 lines of CO, HCN and HCO + . The galaxies have been extracted from a sample of local LIRGs with available high-quality and high-resolution images obtained at optical, near and mid IR wavelengths, which probe the star formation activity. We have thus derived the star f ormation rates using different tracers and determined the sizes of the star forming regions in all the targets. The analysis of the new data proves that the effi ciency of star formation in the dense molecular gas (SFEdense) of extreme starbursts is a factor 3‐4 higher compared to normal galaxies. Kennicutt-Schmidt (KS) power laws have also been derived. We find a dua lity in KS laws that is further reinforced if we account for the likel y different conversion factor for HCN (α HCN ) in extreme starbursts and for the unobscured star formation rate in normal galaxies. This result extends to the higher molecular densities probed by HCN lines the more extreme bimodal behavior of star formation laws, derived from CO molecular lines by two recent surveys. We have confronted our observations with the predictions of theoretical models in which the effi ciency of star formation is determined by the ratio of a constant star formation rate per free-fall time (SFRff) to the local free-fall time (tff). We find that it is possible to fit the observed differences in the SFEdense between normal galaxies and LIRGs/ULIRGs using a common constant SFRff and a set of physically acceptable HCN densities, but only if SFRff∼0.005‐0.01 and/or ifα HCN is a factor of∼a few lower than our favored values. Star formation recipes that explicitly depend on the galaxy global dynamical time scales do not significantly improve th e fit to the new HCN data presented in this work.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/1c7524bf4b4dcabf680b6b83038fcbd91f1b7402

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Citations in the corpus

• PHIBSS: Unified Scaling Relations of Gas Depletion Time and Molecular Gas Fractions (2017, methodology): “90 CO 1-0 detections with the IRAM 30m of z=0.002-0.09 luminous and ultra-luminous IRgalaxies (LIRGs and ULIRGs) from the GOALS survey (Armus et al. 2009), from the work of Gao & Solomon (2004), Gracia-Carpio et al. (2008, 2009, and priv. comm.), and Garcia-Burillo et al. (2012).”
• CO excitation of normal star forming galaxies out to $z=1.5$ as regulated by the properties of their interstellar medium (2014, background): “Evidence from 5 We note, however, that Garcia-Burillo et al. (2012) emphasise that (U)LIRGs have a systematically higherLIR to HCN luminosity ratio by about a factor of two than normal galaxies, and suggest a slightly nonlinear correlation between HCN andLIR.”
• 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): “2009), from the work of Gao & Solomon (2004), Gracia-Carpio et al. (2008, 2009, and priv. comm.), and Garcia-Burillo et al. (2012); 3.”
• Dusty Star Forming Galaxies at High Redshift (2014, result): “Even the results from HCN (J = 1–0) alone provide a somewhat confusing picture as some groups have found a super-linear relationship between LIR and HCN (J = 1–0) in local galaxies (García-Burillo et al., 2012).”