F.PHYSICIST
10th September 2011, 09:26 PM
THE STAR FORMATION EFFICIENCY IN NEARBY GALAXIES: MEASURING WHERE GAS FORMS STARS EFFECTIVELY
We measure the star formation efficiency (SFE), the star formation rate (SFR) per unit of gas, in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds. We use H I maps from The H I Nearby Galaxy Survey (THINGS) and derive H2 maps of CO measured by HERA CO-Line Extragalactic Survey and Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies. We estimate the SFR by combining Galaxy Evolution Explorer (GALEX) far-ultraviolet maps and the Spitzer Infrared Nearby Galaxies Survey (SINGS) 24 μm maps, infer stellar surface density profiles from SINGS 3.6 μm data, and use kinematics from THINGS. We measure the SFE as a function of the free fall and orbital timescales, midplane gas pressure, stability of the gas disk to collapse (including the effects of stars), the ability of perturbations to grow despite shear, and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at (5.25 ± 2.5) × 10–10 yr–1 (equivalent to an H2 depletion time of 1.9 × 109 yr) as a function of all of these variables at our 800 pc resolution. Where the interstellar medium (ISM) is mostly H I, however, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2r 25-0.25r 25. We interpret this decline as a strong dependence of giant molecular cloud (GMC) formation on environment. The ratio of molecular-to-atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe—phase balance in the H I, H2 formation and destruction, and stellar feedback—governs the formation of GMCs from H I.
Keywords
galaxies: evolution (http://iopscience.iop.org/searchaaskeyword?code=11%2F13&code_desc=galaxies%3A+evolution&source=aas&type=kwd_group); galaxies: ISM (http://iopscience.iop.org/searchaaskeyword?code=11%2F22&code_desc=galaxies%3A+ISM&source=aas&type=kwd_group); radio lines: galaxies (http://iopscience.iop.org/searchaaskeyword?code=12%2F14&code_desc=radio+lines%3A+galaxies&source=aas&type=kwd_group); stars: formation (http://iopscience.iop.org/searchaaskeyword?code=8%2F23&code_desc=stars%3A+formation&source=aas&type=kwd_group)
PACS
98.62.Ai Origin, formation, evolution, age, and star formation (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.62.Ai)
97.10.Pg Radii (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=97.10.Pg)
98.52.Wz Dwarf galaxies (elliptical, irregular, and spheroidal) (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.52.Wz)
98.58.Db Molecular clouds, H2 clouds, dense clouds, and dark clouds (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.58.Db)
98.58.Ge H I regions and 21-cm lines; diffuse, translucent, and high-velocity clouds (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.58.Ge) Subjects
Atomic and molecular physics (http://iopscience.iop.org/search?searchType=category&categorys=Atomic+and+molecular+physics)
Astrophysics and astroparticles (http://iopscience.iop.org/search?searchType=category&categorys=Astrophysics+and+astroparticles) Dates
Issue 6 (2008 December)
Received 2008 March 17, accepted for publication 2008 September 25
Published 2008 November 18
We measure the star formation efficiency (SFE), the star formation rate (SFR) per unit of gas, in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds. We use H I maps from The H I Nearby Galaxy Survey (THINGS) and derive H2 maps of CO measured by HERA CO-Line Extragalactic Survey and Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies. We estimate the SFR by combining Galaxy Evolution Explorer (GALEX) far-ultraviolet maps and the Spitzer Infrared Nearby Galaxies Survey (SINGS) 24 μm maps, infer stellar surface density profiles from SINGS 3.6 μm data, and use kinematics from THINGS. We measure the SFE as a function of the free fall and orbital timescales, midplane gas pressure, stability of the gas disk to collapse (including the effects of stars), the ability of perturbations to grow despite shear, and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at (5.25 ± 2.5) × 10–10 yr–1 (equivalent to an H2 depletion time of 1.9 × 109 yr) as a function of all of these variables at our 800 pc resolution. Where the interstellar medium (ISM) is mostly H I, however, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2r 25-0.25r 25. We interpret this decline as a strong dependence of giant molecular cloud (GMC) formation on environment. The ratio of molecular-to-atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe—phase balance in the H I, H2 formation and destruction, and stellar feedback—governs the formation of GMCs from H I.
Keywords
galaxies: evolution (http://iopscience.iop.org/searchaaskeyword?code=11%2F13&code_desc=galaxies%3A+evolution&source=aas&type=kwd_group); galaxies: ISM (http://iopscience.iop.org/searchaaskeyword?code=11%2F22&code_desc=galaxies%3A+ISM&source=aas&type=kwd_group); radio lines: galaxies (http://iopscience.iop.org/searchaaskeyword?code=12%2F14&code_desc=radio+lines%3A+galaxies&source=aas&type=kwd_group); stars: formation (http://iopscience.iop.org/searchaaskeyword?code=8%2F23&code_desc=stars%3A+formation&source=aas&type=kwd_group)
PACS
98.62.Ai Origin, formation, evolution, age, and star formation (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.62.Ai)
97.10.Pg Radii (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=97.10.Pg)
98.52.Wz Dwarf galaxies (elliptical, irregular, and spheroidal) (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.52.Wz)
98.58.Db Molecular clouds, H2 clouds, dense clouds, and dark clouds (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.58.Db)
98.58.Ge H I regions and 21-cm lines; diffuse, translucent, and high-velocity clouds (http://iopscience.iop.org/search?searchType=selectedPacsMscCode&primarypacs=98.58.Ge) Subjects
Atomic and molecular physics (http://iopscience.iop.org/search?searchType=category&categorys=Atomic+and+molecular+physics)
Astrophysics and astroparticles (http://iopscience.iop.org/search?searchType=category&categorys=Astrophysics+and+astroparticles) Dates
Issue 6 (2008 December)
Received 2008 March 17, accepted for publication 2008 September 25
Published 2008 November 18