THE STAR FORMATION EFFICIENCY IN NEARBY GALAXIES: MEASURING WHERE GAS FORMS STARS EFFECTIVELY

By A. Leroy, F. Walter, Elias Brinks, F. Bigiel, W. D. de Blok, B. Madore, M. Thornley. 2008

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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 0.2r25–0.25r25. 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.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

• A COMPARISON OF METHODS FOR DETERMINING THE MOLECULAR CONTENT OF MODEL GALAXIES (2010, background): “…nearby galaxies in the last decade have unequivocally established that the star formation rate in a galaxy correlates much more strongly with its molecular content than with its total gas content (Wong & Blitz 2002; Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008; Blanc et al. 2009).”
• CO excitation of normal star forming galaxies out to $z=1.5$ as regulated by the properties of their interstellar medium (2014, methodology): “For ΣSFR we use sizes measured by Leroy et al (2008) for spirals, for (U)LIRGs we use measurements from Kennicutt (1998), Scoville (1994), and Downes & Solomon (1998), and for BzK galaxies we use the optical sizes (D10; Table 3).”
• THE BARYONIC TULLY–FISHER RELATION OF GAS-RICH GALAXIES AS A TEST OF ΛCDM AND MOND (2011, background): “Leroy et al. (2008) use the observed (low) star formation in dwarf Irregular galaxies to infer the amount of molecular gas present.”
• EVIDENCE FOR UBIQUITOUS COLLIMATED GALACTIC-SCALE OUTFLOWS ALONG THE STAR-FORMING SEQUENCE AT z ∼ 0.5 (2013, background): “Studies of the ISM in nearby star-forming galaxies (e.g., the THINGS survey; Leroy et al. 2008; Bagetakos et al. 2011) show that interstellar H I gas and the galaxies’ stellar components have comparable scale heights and are typically cospatial, although the H I often extends to much larger radii.”
• The ATLAS3D project - XIII. Mass and morphology of H I in early-type galaxies as a function of environment (2011, background): “Therefore, the ISM in these galaxies has similar properties as that in the central region of spirals (Leroy et al. 2008), confirming that in many of them H I is being turned into H2 efficiently.”
• THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, methodology): “We supplement our sample with a compilation of local disks (Leroy et al. 2008), as well as with a sample of local ULIRGs (Solomon et al. 1997, Rodŕıguez Zauŕın et al., 2010).”
• THE STAR FORMATION HISTORIES OF LOCAL GROUP DWARF GALAXIES. I. HUBBLE SPACE TELESCOPE/WIDE FIELD PLANETARY CAMERA 2 OBSERVATIONS (2014, background): “Most constraints on star formation efficiency for low mass galaxies come from studies of recent star formation (e.g., Hα, UV, 24 µm) and current gas content (e.g., Leroy et al. 2008; Salim et al. 2007; Kennicutt et al. 2008; Lee et al. 2011; Kennicutt & Evans 2012).”
• THE UNIVERSAL RELATION OF GALACTIC CHEMICAL EVOLUTION: THE ORIGIN OF THE MASS–METALLICITY RELATION (2014, methodology): “Leroy et al. (2008) combine FUV GALEX and 24 µm Spitzer imaging to measure the SFR profile.”
• GALAXIES IN ΛCDM WITH HALO ABUNDANCE MATCHING: LUMINOSITY–VELOCITY RELATION, BARYONIC MASS–VELOCITY RELATION, VELOCITY FUNCTION, AND CLUSTERING (2010, methodology): “Here, we use the recent observational samples of Stark et al. (2009) and Leroy et al. (2008), along with Verheijen (2001) and the Geha et al. (2006) sample used for the LV relation.”
• A BUDGET AND ACCOUNTING OF METALS AT z ∼ 0: RESULTS FROM THE COS-HALOS SURVEY (2013, background): “In Figure 3 we show the atomic plus molecular gas mass measurements from McGaugh (2005, 2012) and Leroy et al. (2008), adding star-forming (NUV − r < 4) galaxies from the COLDGASS sample (Saintonge et al. 2011).”
• STAR FORMATION IN DISK GALAXIES. I. FORMATION AND EVOLUTION OF GIANT MOLECULAR CLOUDS VIA GRAVITATIONAL INSTABILITY AND CLOUD COLLISIONS (2008, background): “(1) & (2) are found (Wong & Blitz 2002, Kennicutt et al. 2007, Leroy et al. 2008, Bigiel et al. 2008), although there is still debate as to whether it is the total or just the molecular gas mass that is most fundamental for controlling the star formation rate (Blitz & Rosolowsky 2006, Leroy et al.…”
• COMBINED CO AND DUST SCALING RELATIONS OF DEPLETION TIME AND MOLECULAR GAS FRACTIONS WITH COSMIC TIME, SPECIFIC STAR-FORMATION RATE, AND STELLAR MASS (2014, background): “In the IRAM HERACLES survey Bigiel et al. (2008, 2011), Leroy et al. (2008, 2013) and Schruba et al. (2011) studied the spatial distribution of CO 2-1 emission on subgalactic scales (resolution ~1 kpc) in 30 local disk and dwarf star forming galaxies near the main sequence.”
• The SILCC project - III. Regulation of star formation and outflows by stellar winds and supernovae (2016, background): “We thank M. Anderson, C. Federrath, J. Mackey, M. M. Mac Low, E. Pellegrini, and X. Shi for useful discussions and Leroy et al. (2008) and Ekström et al. (2012) for making their data publicly available.”
• Physical Processes in the Interstellar Medium (2014, background): “Work by a number of groups has shown that on large scales, there is a tight correlation between the surface density of molecular gas and the surface density of star formation in spiral galaxies (Wong & Blitz, 2002; Leroy et al., 2008; Bigiel et al., 2008; Bigiel et al., 2011; Schruba et al., 2011).”
• The lifecycle of molecular clouds in nearby star-forming disc galaxies (2019, background): “…a gas depletion time tdep ≡ Mgas/SFR, which is observed to be tdep ≈ 2 Gyr in nearby star-forming galaxies (Bigiel et al. 2008; Bigiel et al. 2011; Leroy et al. 2008, 2013; Blanc et al. 2009; Schruba et al. 2011) and represents the time necessary to convert the entire reservoir of molecular…”
• EXTENDING THE NEARBY GALAXY HERITAGE WITH WISE: FIRST RESULTS FROM THE WISE ENHANCED RESOLUTION GALAXY ATLAS (2012, methodology): “18a below), Bell et al. (2003), Leroy et al. (2008) and de Blok et al. (2008), due to the Zhu et al. (2010) formulation which uses “reference” stellar masses (see also Kannappan & Gawiser 2007).”
• The CO-to-H2 Conversion Factor (2013, background): “By assuming that these galaxies obey the same gas surface-density to star formation relation observed in local disks (e.g., Leroy et al. 2008), Genzel et al. estimate H2 masses and derive a metallicity-dependent calibration for XCO at high redshift.”
• HERACLES: THE HERA CO LINE EXTRAGALACTIC SURVEY (2009, methodology): “The plot underscores the well-established close association between CO emission and stellar light in disk galaxies (e.g., Young & Scoville 1991; Young et al. 1995; Regan et al. 2001) and the similar match between the distributions of CO and star formation (e.g., Leroy et al. 2008).”