Star Formation in the Milky Way and Nearby Galaxies

“…picture is a self-regulated star formation model: the disk adjusts to an equilibrium in which feedback from massive star formation acts to balance the hydrostatic pressure of the disk or to produce an equilibrium porosity of the ISM (e.g., Cox 1981; Dopita 1985; Silk 1997; Ostriker et al. 2010).”

“Several important studies have also been published over the past decade from observations made with the Ultraviolet Imaging Telescope on the ASTRO missions (Stecher et al. 1997; Marcum et al. 2001).”

“Simulations of unstable large clumps in fact tend to fragment so strongly as the mean density increases that they overproduce brown dwarfs, but no massive stars (e.g., Klessen et al. 1998; Martel et al. 2006).”

“…extreme circumnuclear starburst galaxies or individual star-forming regions, nearly all of the dust heating arises from young stars, whereas in evolved galaxies with low specific SFRs, the fraction can be as low as ∼10% (e.g., Sauvage & Thuan 1992; Walterbos & Greenawalt 1996; Cortese et al. 2008).”

“Using a YSO-counting method for five GMCs in the N159 and N44 regions in the LMC, Chen et al. (2010) were able to reach stellar masses of about 8 M⊙, below which they needed to extrapolate.”

“Other authors however have reported steeper dependences (N = 1.2–1.7), closer to those seen in integrated measurements (e.g., Kennicutt et al. 2007; Verley et al. 2010; Rahman et al. 2011; Liu et al. 2011; Momose 2012).”

“…of the star formation law derived in this way have been carried out by numerous authors (e.g., Kennicutt 1989; Martin & Kennicutt 2001; Wong & Blitz 2002; Boissier et al. 2003; Heyer et al. 2004; Komugi et al. 2005; Schuster et al. 2007; Leroy et al. 2008; Schruba et al. 2011; Gratier et al. 2010).”

“The value of αCO appears to decrease by factors of 2-5 in nearby galaxy nuclei (e.g., Meier & Turner 2004; Meier et al. 2008), and by a factor of about 5-6 in local ULIRGs (Downes & Solomon 1998; Downes et al. 1993), and probably by a similar factor in high-redshift, molecule-rich galaxies (Solomon…”

“…picture is a self-regulated star formation model: the disk adjusts to an equilibrium in which feedback from massive star formation acts to balance the hydrostatic pressure of the disk or to produce an equilibrium porosity of the ISM (e.g., Cox 1981; Dopita 1985; Silk 1997; Ostriker et al. 2010).”

“…have calibrated monochromatic SFR in- dices, usually tuned to one of the Spitzer or Herschel bands, including 24µm (e.g., Wu et al. 2005a; AlonsoHerrero et al. 2006; Calzetti et al. 2007; Relaño et al. 2007; Rieke et al. 2009, and references therein), and 70 and 160µm (Calzetti et al. 2010a).”

“A key ingredient is to determine the distances, using spectral lines and methods to break the distance ambiguity in the inner galaxy; initial work is underway (Russeil et al. 2011; Schlingman et al. 2011; Dunham et al. 2011b; Foster et al. 2011).”

“Furthermore the observed 2–10 keV fluxes of galaxies are observed to be strongly correlated with their infrared and non-thermal radio continuum fluxes (e.g., Bauer et al. 2002; Ranalli et al. 2003; Symeonidis et al. 2011), strengthening the link to the SFR.”

“Interestingly, the distribution of denser gas, traced by millimeter-wave dust continuum emission (Beuther et al. 2012), is also symmetric on kpc scales, but asymmetries appear within the CMZ region (Bally et al. 2010).”

“Studies of clusters in nearby galaxies have also found γcluster = 1± 0.1, with a possible upper mass cutoff or turn-over around 1− 2× 106 M⊙ (Gieles et al. 2006), perhaps dependent on the galaxy (but see Chandar et al. 2011).”

“In contrast, the most massive known open clusters in the MW appear to have a mass of (6 − 8)× 104 M⊙ (Portegies Zwart et al. 2010; Davies et al. 2011; Homeier & Alves 2005).”

“Various objections and caveats to this appealing picture have been registered (e.g., Swift & Williams 2008; Reid et al. 2010; Clark et al. 2007; Hatchell & Fuller 2008).”

“…FUV–NUV color or the UV spectral slope (usually denoted β) can be used to estimate the dust attenuation, and numerous calibrations have been published (e.g., Calzetti et al. 1994; Kong et al. 2004; Seibert et al. 2005; Johnson et al. 2007; Salim et al. 2007; Treyer et al. 2007; Hao et al. 2011).”

“The cloud mass function of 246 clouds in the far outer galaxy has a power-law slope of αcloud = −1.88, similar to but slightly steeper than that found inside the solar circle (§2.5) and a maximum mass of about 104 M⊙ (Snell et al. 2002).”

“In contrast, the most massive known open clusters in the MW appear to have a mass of (6 − 8)× 104 M⊙ (Portegies Zwart et al. 2010; Davies et al. 2011; Homeier & Alves 2005).”

“Simulations including radiative feedback, acting on a global scale, have shown that fragmentation can be suppressed and massive stars formed (e.g., Bate 2009; Urban et al. 2010; Krumholz et al. 2010).”

“The value of αCO appears to decrease by factors of 2-5 in nearby galaxy nuclei (e.g., Meier & Turner 2004; Meier et al. 2008), and by a factor of about 5-6 in local ULIRGs (Downes & Solomon 1998; Downes et al. 1993), and probably by a similar factor in high-redshift, molecule-rich galaxies (Solomon…”

“…have calibrated monochromatic SFR in- dices, usually tuned to one of the Spitzer or Herschel bands, including 24µm (e.g., Wu et al. 2005a; AlonsoHerrero et al. 2006; Calzetti et al. 2007; Relaño et al. 2007; Rieke et al. 2009, and references therein), and 70 and 160µm (Calzetti et al. 2010a).”

“A similar energy-balancing approach has been applied to derive dust-corrected emission-line luminosities of galaxies (Calzetti et al. 2007; Zhu et al. 2008; Kennicutt et al. 2009; Treyer et al. 2010).”

“Other studies have found similar thresholds for efficient star formation or the presence of dense cores (Onishi et al. 1998; Enoch et al. 2007; Johnstone et al. 2004; André et al. 2010; Li et al. 1997; Lada 1992).”

“A key ingredient is to determine the distances, using spectral lines and methods to break the distance ambiguity in the inner galaxy; initial work is underway (Russeil et al. 2011; Schlingman et al. 2011; Dunham et al. 2011b; Foster et al. 2011).”

“Using a threshold of C18O emission to define clumps, Higuchi et al. (2009) found that the star formation efficiency (ǫ) in clumps varies widely but averages 10%, about 4 times that in clouds as a whole (§4.1).”

“…similar to the cloud-counting argument offered above for CO. Collectively, we refer to these lines as “dense gas tracers”, but the effective densities increase with J and vary among species (see Evans 1999; Reiter et al. 2011b for definitions and tables of effective and critical densities).”

“…in the SFR. Subsequent comparisons of UV and Hα profiles appear to confirm the presence of radial turnovers in the SFR in most disks, however, even in cases where lower levels of star formation persist to much larger radii (e.g., Thilker et al. 2007; Christlein et al. 2010; Goddard et al. 2010).”

“A hybrid picture, in which a star forms initially from its parent core, but then continues to accrete from the surrounding clump, has been advanced (Myers 2009, 2011), with some observational support (e.g., Longmore et al. 2011).”

“…leads to the formation of a first hydrostatic core in a small region where the dust continuum emission becomes optically thick; this short-lived core is still molecular and grows in mass by continued infall from the outer layers (Larson 1969; Boss & Yorke 1995; Omukai 2007; Stahler & Palla 2005).”

“The pros and cons of these two models are discussed in a joint paper by the leading protagonists (Krumholz & Bonnell 2009).”

“Various objections and caveats to this appealing picture have been registered (e.g., Swift & Williams 2008; Reid et al. 2010; Clark et al. 2007; Hatchell & Fuller 2008).”

“A hybrid picture, in which a star forms initially from its parent core, but then continues to accrete from the surrounding clump, has been advanced (Myers 2009, 2011), with some observational support (e.g., Longmore et al. 2011).”

“If the IMF itself were completely blind to the SFR, we would expect such effects to become apparent below Hα luminosities of order 1038 ergs s−1, or SFRs of order 0.001 M⊙ yr −1 (e.g., Cerviño et al. 2003).”

“The star formation rate, measured from LFIR, does not, however, reflect this asymmetry; in fact, it is larger in quadrant IV than in I, suggesting a star formation efficiency up to twice as high, despite the lower peak Σmol (Bronfman et al. 2000).”

“Aspects of the top-down picture date back to early dynamical models of the ISM and the first observations of star formation thresholds in disks (e.g., Quirk & Tinsley 1973; Larson 1987; Zasov & Stmakov 1988; Kennicutt 1989; Elmegreen 1991; Silk 1997).”

“Just as the UV and visible tracers miss radiation that has been attenuated by dust, the infrared emission misses the starlight that is not absorbed by dust (e.g., Hirashita et al. 2001).”

“…similar to the cloud-counting argument offered above for CO. Collectively, we refer to these lines as “dense gas tracers”, but the effective densities increase with J and vary among species (see Evans 1999; Reiter et al. 2011b for definitions and tables of effective and critical densities).”

“A major breakthrough in recent years has been the use of mid-infrared spectroscopy to distinguish dust heated by massive stars from that heated by a buried AGN (e.g., Genzel et al. 1998; Laurent et al. 2000; Dale et al. 2006; Armus et al. 2007).”

“With the Spitzer and Herschel observatories, it has been possible to extend imaging and spectroscopy of the most luminous infrared-emitting galaxies to intermediate redshifts (e.g., Elbaz et al. 2005; Yan et al. 2007).”

“In fact, the other molecules are more sensitive to photodissociation than is CO, so they will be even more dependent on metallicity (e.g., Rosolowsky et al. 2011).”

“At wavelengths with strong, diffuse Galactic background emission, such as the mid-infrared, dust absorption against the continuum can be used in a way parallel to extinction maps (e.g., Bacmann et al. 2000; Stutz et al. 2007).”

“In the most extreme starburst galaxy environments, if standard values of X(CO) are used, the average surface densities of gas (virtually all molecular) reach 1000 to 104 M⊙ pc −2, and the volume filling factor of clumps could reach unity (Wu et al. 2009).”

“Figure 10 shows a recent determination of this distribution from Bothwell et al. (2011), based on flux-limited UV and TIR samples of galaxies and SFRs derived using the methods of Hao et al. (2011), and corrections for AGN contamination following Wu et al. (2010b, 2011).”

“…in the SFR. Subsequent comparisons of UV and Hα profiles appear to confirm the presence of radial turnovers in the SFR in most disks, however, even in cases where lower levels of star formation persist to much larger radii (e.g., Thilker et al. 2007; Christlein et al. 2010; Goddard et al. 2010).”

“A survey of the inner part of the MW in 13CO (Jackson et al. 2006) has helped with some of problems caused by optical depth in the main isotopologue.”

“…applications of this method were largely restricted to luminous starburst galaxies and star-forming regions, for which UV and IR data could be obtained prior to the advent of GALEX, Spitzer, and Herschel (e.g., Buat et al. 1999; Meurer et al. 1999; Gordon et al. 2000), yielding values of η ∼ 0.6.”

“Reddening and extinction of starlight provided early evidence of the existence of interstellar clouds (Barnard 1908).”

“The FUV conversion tabulated in Kennicutt & Evans (2012) (from Murphy et al. 2011), if rescaled from the Kroupa IMF to Salpeter, is very close to the z = 0, solar metallicity value of KFUV(z) in Figure 3, but our somewhat smaller value should be more representative for the peak era of cosmic star…”

“…radiation, and other microphysical processes in the turbulent, magnetized, multi-phase interstellar medium (McKee & Ostriker 2007; Ostriker 2009; Kennicutt & Evans 2012; Naab & Ostriker 2016); and the driving of galactic outflows of gas, energy, and heavy elements into the circum- and…”

“Consequently, H2 plays a central role in the evolution of galaxies and stellar systems (see the recent review by Kennicutt & Evans 2012).”

“6, respectively, which lies below the ‘knee’ of the star formation rate function of galaxies at these redshifts [213, 214].”

“…al. 2009; Conroy et al. 2009; Conroy & Gunn 2010) and improved global diagnostics of galactic star formation (Murphy et al. 2011; Hao et al. 2011; Kennicutt & Evans 2012 (KE12), and references within) have enabled the determination of key physical quantities of galaxies from these data:…”