Star Formation in the Milky Way and Nearby Galaxies

“Assumptions about accretion and initial conditions may also have substantial consequences for the usual methods of determining ages of young stars (Baraffe et al. 2009; Baraffe & Chabrier 2010; Hosokawa et al. 2011).”

“Evidence for variations in star formation rate on short timescales is discussed by Yusef-Zadeh et al. (2009).”

“…the different dust emission components translate into considerable variations in the dust SEDs within and between galaxies (e.g., Dale & Helou 2002; Dale et al. 2005; Smith et al. 2007), and as a consequence the conversion of infrared luminosities into SFRs must change for different IR wavelengths.”

“Numerous surveys of the MW have been obtained in CO J = 1 → 0 (e.g., Dame et al. 2001; Bronfman et al. 1988; Clemens et al. 1988).”

“Recent work by Ostriker et al. (2010) and Ostriker & Shetty (2011) provides a theoretical explanation for these relations.”

“In such regions that emission at all wavelengths will be dominated by a very young population with ages of typically a few Myr, which will be shorter than the averaging times assumed for all but the emission-line tracers.”

“Evidence for a higher value of αcloud has been found for the LMC (Fukui et al. 2001; Wong et al. 2011) and M33 (Engargiola et al. 2003; Rosolowsky 2005).”

“Several ongoing studies are quantifying this parameter by comparing Lyα fluxes of galaxies with independent SFR tracers such as Hα or the UV continuum (e.g., Atek et al. 2009; Scarlata et al. 2009; Blanc et al. 2011; Hayes et al. 2011).”

“…for tracers of star formation have been made in radio continuum (free-free) emission (Altenhoff et al. 1970) and recombination lines (Anderson et al. 2011; Lockman 1989), in water masers (Cesaroni et al. 1988; Walsh et al. 2011), and methanol masers (Pestalozzi et al. 2005; Green et al. 2009).”

“Very similar results were obtained from studies of millimeter continuum emission from dust (Faúndez et al. 2004) toward a sample of southern hemisphere sources surveyed in CS J = 2 → 1 (Bronfman et al. 1996).”

“…this capability is mainly limited to nearby galaxies, but with the expansion of far-infrared to millimeter surveys of high-redshift galaxies, opportunities to apply multiwavelength diagnostics to distant galaxies should expand in the coming decade (e.g., Overzier et al. 2011; Reddy et al. 2012).”

“These surveys in turn have led to large spinoff surveys in H I (e.g., THINGS, Walter et al. 2008; FIGGS, Begum et al. 2008; Local Volume H I Survey; Koribalski 2010; LITTLE THINGS, Hunter et al. 2007) and in CO (e.g., BIMA-SONG, Helfer et al. 2003; IRAM HERACLES, Leroy et al. 2009; JCMT NGLS, Wilson et al. 2009; CARMA…”

“The line luminosities of other dense gas tracers also show strong correlations with both virial mass (Wu et al. 2010a) and the mass derived from dust continuum observations (Reiter et al. 2011b).”

“In most extragalactic studies, individual clouds are not resolved, and the luminosity of CO is essentially a cloud counting technique (Dickman et al. 1986).”

“In a model that accounts for this, Hennebelle & Chabrier (2011) reproduce at some level the observations of surface density thresholds for efficient star formation observed in nearby clouds (Lada et al. 2010; Heiderman et al. 2010).”

“Inside about 13 kpc, the atomic surface density is roughly constant at 10-15 M⊙ pc −2, while the average molecular surface density increases sharply, passing 1 M⊙ pc −2 somewhere near the solar neighborhood (Dame et al. 2001), to a local maximum around Rgal = 4-5 kpc (Nakanishi & Sofue 2006).”

“Prestellar cores are centrally condensed, and can be modeled as Bonnor-Ebert spheres (Ward-Thompson et al. 1999; Evans et al. 2001; Kirk et al. 2005), which have nearly power-law (n(r) ∝ r−2) envelopes around a core with nearly constant density.”

“Calibrations of the SFR in terms of X-ray luminosity and stellar mass have been published by Colbert et al. (2004) and Lehmer et al. (2010).”

“…and/or spectroscopy reveals extended disks of HII regions that trace the UV emission in most cases (e.g., Zaritsky & Christlein 2007; Christlein et al. 2010; Goddard et al. 2010), confirming the earlier detection of HII regions at large radii (e.g., Ferguson et al. 1998; Ryan-Weber et al. 2004).”

“…in low-SFR environments could cause Hα to substantially underestimate the actual SFR. Interestingly a systematic deficit of Hα emission in dwarf galaxies with low SFRs and in low SFR density regions is observed (Sullivan et al. 2000; Bell & Kennicutt 2001; Lee et al. 2009a; Meurer et al. 2009).”

“Zuckerman & Palmer (1974) pointed out that such a picture would produce stars at 30 times the accepted average recent rate of star formation in the Milky Way (§5.1) if stars formed with high efficiency.”

“This suggests that most of the decrease in the cosmic SFR in recent epochs has been driven by downsiz- ing in the level of steady-state star formation (e.g., Bell et al. 2005; Jogee et al. 2009).”

“…the picture at this point is that measuring the extent of the brick wall will not tell you anything about particularly deep or dense extensions of the building, but if the buildings are similar, the extent of the wall is a rough guide to the total mass of the building (cf. Shetty et al. 2011b).”

“Early work on this problem (e.g., Kennicutt 1989; Martin & Kennicutt 2001; Boissier et al. 2003) was based on radial profiles in Hα, and some questions have been raised about whether these thresholds resulted from breakdowns in the SFR versus Hα calibration in low surface brightness regimes, as…”

“Assumptions about accretion and initial conditions may also have substantial consequences for the usual methods of determining ages of young stars (Baraffe et al. 2009; Baraffe & Chabrier 2010; Hosokawa et al. 2011).”

“Surveys for tracers of star formation have been made in radio continuum (free-free) emission (Altenhoff et al. 1970) and recombination lines (Anderson et al. 2011; Lockman 1989), in water masers (Cesaroni et al. 1988; Walsh et al. 2011), and methanol masers (Pestalozzi et al. 2005; Green et al.…”

“The properties of the Plume et al. (1992) sample have been studied in a series of papers (Plume et al. 1997; Mueller et al. 2002; Shirley et al. 2003), with the most recent summary in Wu et al. (2010a).”

“Studies of molecular cloud properties in other galaxies have been recently reviewed (Fukui & Kawamura 2010; Blitz et al. 2007).”

“…of these galaxies reveals significant detections of molecular gas in a large fraction of the galaxies with detected star formation, with molecular gas masses of 107− 1010M⊙ (Combes et al. 2007; Crocker et al. 2011), and with SFRs roughly consistent with the Schmidt law seen in normal galaxies.”

“Based on a model including dark matter (Kalberla & Kerp 2009; Kalberla et al. 2007; Kalberla & Dedes 2008), the MW mass within 60 kpc of the center is M(tot) = 4.6× 1011 M⊙, with M(baryon) = 9.5× 10 10 M⊙.”

“For low-mass stars, outflows provide the primary feedback (e.g., Li & Nakamura 2006) while high-mass stars add radiation pressure and expanding H II regions (e.g., Murray 2011).”

“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).”

“…between characteristic dust attenuation in a star-forming galaxy and the SFR itself, with the consequence that galaxies with the highest absolute SFRs are nearly all dusty infrared-luminous and ultraluminous galaxies (e.g., Wang & Heckman 1996; Martin et al. 2005b; Bothwell et al. 2011).”

“A recent study of the SMC (∼ 1/5Z⊙) by Bolatto et al. (2011) offers clues to how these results may change in low-metallicity environments.”

“For example, Kewley et al. (2002) showed that for a sample of spiral galaxies with integrated emission-line spectra, the TIR-based calibration of K98 for dusty starburst galaxies was in good agreement with SFRs based on attenuation-corrected Hα line fluxes.”

“Evidence for a higher value of αcloud has been found for the LMC (Fukui et al. 2001; Wong et al. 2011) and M33 (Engargiola et al. 2003; Rosolowsky 2005).”

“These have been estimated by several authors using evolutionary synthesis models (e.g., Smith et al. 2002; Raiter et al. 2010), but the precise dependences are sensitive to the details of the input stellar models, in particular how effects of stellar rotation are modelled.”

“A recent, careful study of CO and 13CO toward the Taurus molecular cloud shows that N(CO) traces AV up to 10 mag in some regions but only up to 4 mag in other regions, and other issues cause problems below AV = 3 mag (Pineda et al. 2010b).”

“The topic of clusters is connected to the topic of massive stars because 70% of O stars reside in young clusters or associations (Gies 1987).”

“The nearest young cluster that has formed O stars is 415-430 pc away in Orion (Menten et al. 2007; Hirota et al. 2007), and most lie at much larger distances, making study of the entire IMF difficult.”

“…for tracers of star formation have been made in radio continuum (free-free) emission (Altenhoff et al. 1970) and recombination lines (Anderson et al. 2011; Lockman 1989), in water masers (Cesaroni et al. 1988; Walsh et al. 2011), and methanol masers (Pestalozzi et al. 2005; Green et al. 2009).”

“Abdo et al. (2010) also inferred the existence of “dark gas”, which is not traced by either H I or CO, surrounding the CO-emitting region in the nearby Gould Belt clouds, with mass of about half of that traced by CO. Planck Collaboration et al. (2011b) have also indicated the existence of “dark…”

“A TIR luminosity of 4× 108 L⊙ (Sodroski et al. 1997) for the CMZ would imply Ṁ∗ ∼ 0.06 M⊙ yr −1 using the conversion in Table 1, roughly consistent with other recent estimates (Immer et al. 2012), based on analysis of point infrared sources, but new data from the surveys mentioned above should…”

“We can expect continuing improvements in the model of the MW as VLBI astrometry improves distance determinations of star forming regions across the MW (Reid et al. 2009; Brunthaler et al. 2011).”

“One analysis found that clouds forming stars with masses over 10 M⊙ satisfied the following relation: m(r) ≥ 870M⊙(r/pc) 1.33 (Kauffmann et al. 2010; Kauffmann & Pillai 2010), where m(r) is the enclosed mass as a function of radius.”

“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).”

“Even larger variations in L[CII]/LIR were found in a more diverse sample of starforming galaxies by Malhotra et al. (1997).”

“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).”

“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 similar non-linear dependence is observed for total SFR (as opposed to SFR surface density) versus total molecular gas mass (e.g., Solomon & Sage 1988; Gao & Solomon 2004), and presumably is another manifestation of the same underlying physical correlation.”

“The Milky Way as a Whole The Milky Way is a barred spiral galaxy (Burton 1988; Dame et al. 2001; Benjamin et al. 2005).”

“Gutermuth et al. (2011) find a continuation of the steep increase in Σ(SFR) to higher Σgas in a study including embedded clusters.”

“…a serious issue for the formation of massive stars in spherical geometries, but more realistic, aspherical simulations show that the radiation is channeled out along the rotation axis, allowing continued accretion through a disk (Yorke & Sonnhalter 2002; Krumholz et al. 2009a; Kuiper et al. 2011).”

“A second picture, exemplified by Dobbs et al. (2011), is that most clouds and most parts of clouds are not gravitationally bound but are transient objects.”

“Cloud projected boundaries, defined by either dust or CO emission or extinction, can be characterized by a fractal dimension around 1.5 (Falgarone et al. 1991; Scalo 1990; Stutzki et al. 1998), suggesting an intrinsic 3-”

“Deeper surveys of nearby clouds (Ward-Thompson et al. 2007b), the Galactic Plane, and extragalactic fields are planned with SCUBA-2.”

“…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).”

“In exceptional cases (e.g., NGC5236=M83, NGC4625) the star formation extends to 3–4 times the normal R25 optical radius, to the edge of the HI disk (Thilker et al. 2005; Gil de Paz et al. 2005).”

“Various authors (Krumholz & Thompson 2007; Narayanan et al. 2008; Juneau et al. 2009) have pointed out that lines like those of HCN J = 1 → 0 are not thermalized at lower densities, so can result in a linear relation even if the underlying star formation relation is a local version of a non-linear…”

“The accuracy of these pre- scriptions rests heavily on the presumed (but uncertain) intrinsic colors, the shape of the dust extinction curve, and the complicated effects of geometry and scattering when averaging over a large physical region (e.g., Gordon et al. 2001).”

“The relation derived for diffuse gas is often applied in molecular clouds, but grain growth flattens the extinction curve, leading to larger values of RV , and a much flatter curve in the infrared (Flaherty et al. 2007; Chapman et al. 2009).”

“A recent, careful study of CO and 13CO toward the Taurus molecular cloud shows that N(CO) traces AV up to 10 mag in some regions but only up to 4 mag in other regions, and other issues cause problems below AV = 3 mag (Pineda et al. 2010b).”

“Several ongoing studies are quantifying this parameter by comparing Lyα fluxes of galaxies with independent SFR tracers such as Hα or the UV continuum (e.g., Atek et al. 2009; Scarlata et al. 2009; Blanc et al. 2011; Hayes et al. 2011).”

“Recent measurements with Herschel show that this is mainly a physical change, and not an artifact of lower spatial resolution at longer wavelengths (Boquien et al. 2011).”

“…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 observed rotational H2 lines originate in surfaces of clouds, probing 1% to 30% of the gas in a survey of galaxies (e.g., Roussel et al. 2007).”

“…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 & Vanden Bout 2005).”

“Lee et al. (2009b) analyzed the statistics of the SFRs in more depth and confirmed a larger fraction of stars formed in bursts in low-mass galaxies (∼25%), but these still represent only a small fraction of the total stars formed– even in the dwarfs the majority of stars appear to form in extended…”

“A Spitzer study (Gutermuth et al. 2009) of 2548 YSOs in 39 nearby (d < 1.7 kpc), previously known (primarily from the compilation by Porras et al. 2003) young clusters, but excluding Orion and NGC2264, found the following median properties: 26 members, core radius of 0.39 pc, stellar surface…”

“Recent modelling of the statistics of Hα and UV emission by Weisz et al. (2012) and Fumagalli et al. (2011) as well as analysis of resolved stellar populations (e.g., Weisz et al. 2008) confirm the importance of fluctuations in the SFR.”

“The star formation rate of the MW has been estimated from counting H II regions, which can be seen across the MW and extrapolating to lower mass stars (Mezger 1987; McKee &Williams 1997; Murray & Rahman 2010).”

“…for tracers of star formation have been made in radio continuum (free-free) emission (Altenhoff et al. 1970) and recombination lines (Anderson et al. 2011; Lockman 1989), in water masers (Cesaroni et al. 1988; Walsh et al. 2011), and methanol masers (Pestalozzi et al. 2005; Green et al. 2009).”

“The suggestion by Bok & Reilly (1947) that these could be the sites of star formation was seminal.”

“By the same token, much of the Hα and dust emission in galaxies, typically 30–60%, is emitted by diffuse ionized gas and dust located hundreds of parsecs or more from any young stars (e.g., Oey et al. 2007; Dale et al. 2007).”

“Herschel surveys will be providing higher resolution [25′′ at 350 µm (Griffin et al. 2010) and 6′′ at 70 µm (Poglitsch et al. 2010)] images of the nearby clouds (André et al. 2010), the Galactic Plane (Molinari et al. 2010), and many other galaxies (e.g., Skibba et al. 2011; Wuyts et al. 2011).”

“…(ideally volume-limited) selection criteria, and studies of this kind (based on UV, visible, and/or Hα observations) have been carried out by several groups (e.g., Karachentsev et al. 2004; Blanton et al. 2005; Meurer et al. 2006; Kennicutt et al. 2008; Dalcanton et al. 2009; Hunter et al. 2010).”

“Multi-transition studies, along with realistic modeling will help to refine the interpretation of line luminosities of dense gas tracers (e.g., Graciá-Carpio et al. 2008; Juneau et al. 2009).”

“Despite their conclusion thatX(CO) is lower, Oka et al. (1998) argue that the CMZ has a molecular mass of 2-6× 107 M⊙.”

“Still more massive clusters can be found in other galaxies, and a possible precursor (Mcloud > 1× 10 5 M⊙ within a 2.8 pc radius) has recently been identified near the center of the MW (Longmore et al. 2012).”

“Subsequent analyses extending to normal star-forming galaxies (e.g., Hirashita et al. 2003; Kong et al. 2004; Burgarella et al. 2005; Buat et al. 2005; Treyer et al. 2010; Hao et al. 2011) typically produce values of η which are somewhat lower (e.g., 0.46 for Hao et al. 2011), which almost…”

“A number of studies have shown that the PAH luminosity scales relatively well with the SFR in metalrich luminous star-forming galaxies (e.g., Roussel et al. 2001; Peeters et al. 2004; Förster Schreiber et al. 2004; Wu et al. 2005a; Farrah et al. 2007; Calzetti et al. 2007), but the PAH bands…”

“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).”

“…the mass function appears to be steeper in the interarm regions of M51 than in the spiral arms (E. Schinnerer and A. Hughes, personal communication), possibly caused by the aggregation of clouds into larger structures within arms and disaggregation as they leave the arms (Koda et al. 2009).”

“The properties of the Plume et al. (1992) sample have been studied in a series of papers (Plume et al. 1997; Mueller et al. 2002; Shirley et al. 2003), with the most recent summary in Wu et al. (2010a).”

“One approach, exemplified by Vázquez-Semadeni et al. (2011) argues that clouds are formed in colliding flows of the warm, neutral medium.”

“…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).”

“…can be understood if most gas disks lie near the limit of gravitational stability (Q∼ 1; Kennicutt 1989), and the critical column densities for the formation of cold gas phase, molecule formation, and gravitational instability lie close to each other (e.g., Elmegreen & Parravano 1994; Schaye 2004).”

“…the picture at this point is that measuring the extent of the brick wall will not tell you anything about particularly deep or dense extensions of the building, but if the buildings are similar, the extent of the wall is a rough guide to the total mass of the building (cf. Shetty et al. 2011b).”

“Freeze-out of CO and conversion to CO2 ice causes further complications in cold regions of high column density (Lee et al. 2003; Pontoppidan et al. 2008; Whittet et al. 2007), but other issues likely dominate the interpretational uncertainties for most extragalactic CO observations.”

“Recent modelling of the statistics of Hα and UV emission by Weisz et al. (2012) and Fumagalli et al. (2011) as well as analysis of resolved stellar populations (e.g., Weisz et al. 2008) confirm the importance of fluctuations in the SFR.”

“…phase balance, and feedback are providing deeper insights into the physical nature of the star formation scaling laws and thresholds and the life cycles of molecular clouds (e.g., Robertson & Kravtsov 2008; Dobbs 2008; Dobbs & Pringle 2010; Tasker & Tan 2009; Tasker 2011; Brooks et al. 2011).”

“Estimates of Ṁ∗ from a model of the total far-infrared emission of the MW (e.g., Misiriotis et al. 2006) are somewhat less sensitive to the high end of the IMF (§3) and average over a longer time.”

“…this capability is mainly limited to nearby galaxies, but with the expansion of far-infrared to millimeter surveys of high-redshift galaxies, opportunities to apply multiwavelength diagnostics to distant galaxies should expand in the coming decade (e.g., Overzier et al. 2011; Reddy et al. 2012).”

“Evidence for overall inward flow in clumps is difficult; evidence for it is found in some surveys (Wu & Evans 2003; Fuller et al. 2005; Reiter et al. 2011a), but not in others (Purcell et al. 2006).”

“For example Dopita (1985) and Dopita & Ryder (1994) proposed a scaling between the SFR density and the product of gaseous and stellar surface densities; the latter scales with the disk hydrostatic pressure and hence bears some relation to the picture of Blitz & Rosolowsky (2006).”

“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).”

“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).”

“…(§2.3) have identified what might be called “dense clumps”, which do appear to have a mass function similar to that of clusters (Shirley et al. 2003; Beltrán et al. 2006; Reid & Wilson 2005), The comparison of mass functions supports the idea (McKee & Ostriker 2007) that cores are the…”

“Larson (1982) concluded that his correlation could be due to sampling.”

“…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).”

“Theories of feedback-regulated star formation show promise in explaining the low efficiency per orbit in normal galaxies (Kim et al. 2011), and they may also explain the less effective role of negative feedback in merger-driven starbursts (e.g., Ostriker & Shetty 2011).”

“Recent modelling of the statistics of Hα and UV emission by Weisz et al. (2012) and Fumagalli et al. (2011) as well as analysis of resolved stellar populations (e.g., Weisz et al. 2008) confirm the importance of fluctuations in the SFR.”

“The probability distribution function of surface densities, as mapped by extinction, can be fitted to a lognormal function for low extinctions, AV ≤ 2 − 5 mag (Lombardi et al. 2010), with a power law tail to higher extinctions, at least in some clouds.”

“A number of studies have shown that the PAH luminosity scales relatively well with the SFR in metalrich luminous star-forming galaxies (e.g., Roussel et al. 2001; Peeters et al. 2004; Förster Schreiber et al. 2004; Wu et al. 2005a; Farrah et al. 2007; Calzetti et al. 2007), but the PAH bands weaken dramatically below…”

“Evidence for overall inward flow in clumps is difficult; evidence for it is found in some surveys (Wu & Evans 2003; Fuller et al. 2005; Reiter et al. 2011a), but not in others (Purcell et al. 2006).”

“Major concentrations of molecular clouds with very active star formation appear to be associated with regions of low shear (Luna et al. 2006), and with the junction of the bar and spiral arms (Nguyen Luong et al. 2011).”

“Ho & Keto (2007) compiled data from ISO and Spitzer on the [Ne II]12.8µm and [Ne III]15.6µm lines, and found that the sum of the line fluxes correlates well with hydrogen recombination line fluxes, with a scatter of ∼0.3 dex.”

“…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.”

“For more massive stars, it is essential to include accretion in the evolutionary calculations (Palla & Stahler 1992) and high accretion rates strongly affect the star’s evolution (Zinnecker & Yorke 2007; Hosokawa & Omukai 2009).”

“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).”

“Confirmation of the star formation has come from visible-wavelength IFU spectroscopy, as exploited for example by the SAURON survey (Shapiro et al. 2010).”

“Assumptions about accretion and initial conditions may also have substantial consequences for the usual methods of determining ages of young stars (Baraffe et al. 2009; Baraffe & Chabrier 2010; Hosokawa et al. 2011).”

“…2005a; Farrah et al. 2007; Calzetti et al. 2007), but the PAH bands weaken dramatically below metal abundances of approximately 1/4 to 1/3 Z⊙ (e.g., Madden 2000; Engelbracht et al. 2005; Calzetti et al. 2007; Smith et al. 2007), rendering them problematic as quantitative SFR tracers in this…”

“Numerous surveys of the MW have been obtained in CO J = 1 → 0 (e.g., Dame et al. 2001; Bronfman et al. 1988; Clemens et al. 1988).”

“Subsequent observations of a wider range of galaxies, however, has revealed a much larger scatter in the relation (e.g., Boquien et al. 2012).”

“In contrast, Liszt et al. (2010) argue that X(CO) stays the same even in diffuse gas.”

“One analysis found that clouds forming stars with masses over 10 M⊙ satisfied the following relation: m(r) ≥ 870M⊙(r/pc) 1.33 (Kauffmann et al. 2010; Kauffmann & Pillai 2010), where m(r) is the enclosed mass as a function of radius.”

“Interestingly PAH emission appears to correlate the most strongly with the longer-wavelength component of the thermal dust emission (e.g., Bendo et al. 2008), though it often also appears as resolved shells around the young star-forming regions (Helou et al. 2004).”

“A detailed study of the IMF in the nearest massive young cluster, Orion, with stars from 0.1 to 50 M⊙ with a mean age of 2 Myr, shows a peak between 0.1 and 0.3 M⊙, depending on evolutionary models, and a deficit of brown dwarfs relative to the field IMF (Da Rio et al. 2012).”

“Gamma-rays from decay of pions produced from high-energy cosmic rays interacting with baryons in the ISM directly trace all the matter if one knows the cosmic ray flux (e.g., Bloemen 1989).”

“Various authors (Krumholz & Thompson 2007; Narayanan et al. 2008; Juneau et al. 2009) have pointed out that lines like those of HCN J = 1 → 0 are not thermalized at lower densities, so can result in a linear relation even if the underlying star formation relation is a local version of a non-linear…”

“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).”

“Persic et al. (2004) derived an alternate calibration in terms of the hard X-ray binary luminosity alone, which is useful for nearby resolved galaxies.”

“More recently Shi et al. (2011) show that the scatter in the star formation law is minimized with a relation of the form ΣSFR ∝ Σgas Σ 0.5 ∗ .”

“These provide a homogeneous body of deep measurements of the star formation and the nature of the star formation law at low surface densities (§6; Wyder et al. 2009).”

“Pflamm-Altenburg et al. (2007, 2009) have investigated the effect of an IGIMF (§2.5) on SFR tracers, and shown that the systematic depletion of massive stars in low-SFR environments could cause Hα to substantially underestimate the actual SFR. Interestingly a systematic deficit of Hα emission in…”

“…of these galaxies reveals significant detections of molecular gas in a large fraction of the galaxies with detected star formation, with molecular gas masses of 107− 1010M⊙ (Combes et al. 2007; Crocker et al. 2011), and with SFRs roughly consistent with the Schmidt law seen in normal galaxies.”

“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.”

“…across all of the SFR regimes described above, and which even extends to star-forming clouds in the Milky Way (Gao & Solomon 2004; Wu et al. 2005b; Gao et al. 2007; Lada et al. 2012), However, there is still some evidence of bimodality even if tracers of dense gas are used (Garćıa-Burillo et al.…”

“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).”

“Pflamm-Altenburg et al. (2007, 2009) have investigated the effect of an IGIMF (§2.5) on SFR tracers, and shown that the systematic depletion of massive stars in low-SFR environments could cause Hα to substantially underestimate the actual SFR. Interestingly a systematic deficit of Hα emission in…”

“Steeper IMFs in massive elliptical galaxies have been suggested by van Dokkum & Conroy (2011).”

“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 term “starburst galaxy” was introduced by Weedman et al. (1981), but nowadays the term is applied to a diverse array of galaxy populations.”

“Freeze-out of CO and conversion to CO2 ice causes further complications in cold regions of high column density (Lee et al. 2003; Pontoppidan et al. 2008; Whittet et al. 2007), but other issues likely dominate the interpretational uncertainties for most extragalactic CO observations.”

“14 in Pak et al. 1998).”

“High-resolution UV imaging with HST often reveals extended star-forming rings or spiral arms in these galaxies (Salim & Rich 2010).”

“At this point, it seems that the mass function of cores is clearly steeper than that of clouds, much closer to that of stars (Motte et al. 1998; Enoch et al. 2008; Sadavoy et al. 2010).”

“Another important factor may be the excitation of the CO tracer used (e.g., Juneau et al. 2009).”

“There are several alternative quantitative definitions of the qualitative terms used here (Robitaille et al. 2006; Crapsi et al. 2008).”

“Further studies indicate that the power-law tail begins at Σmol ∼ 40− 80 M⊙ pc −2, and that the material in the power-law tail lies in unbound clumps with mean density about 103 cm−3 (Kainulainen et al. 2011).”

“…phase balance, and feedback are providing deeper insights into the physical nature of the star formation scaling laws and thresholds and the life cycles of molecular clouds (e.g., Robertson & Kravtsov 2008; Dobbs 2008; Dobbs & Pringle 2010; Tasker & Tan 2009; Tasker 2011; Brooks et al. 2011).”

“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).”

“Further modeling with different metallicities (Shetty et al. 2011a; Feldmann et al. 2012) has begun to provide some perspective on the range of behaviors likely in other galaxies.”

“…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).”

“…what might be called “dense clumps”, which do appear to have a mass function similar to that of clusters (Shirley et al. 2003; Beltrán et al. 2006; Reid & Wilson 2005), The comparison of mass functions supports the idea (McKee & Ostriker 2007) that cores are the progenitors of stars and…”

“…phase balance, and feedback are providing deeper insights into the physical nature of the star formation scaling laws and thresholds and the life cycles of molecular clouds (e.g., Robertson & Kravtsov 2008; Dobbs 2008; Dobbs & Pringle 2010; Tasker & Tan 2009; Tasker 2011; Brooks et al. 2011).”

“K98 published a single SFR calibration for the line, but subsequent analyses (e.g., Jansen et al. 2001; Kewley et al. 2004; Moustakas et al. 2006) have shown that the systematic effects must be removed or at least calibrated to first order for reliable measurements.”

“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).”

“…in low-SFR environments could cause Hα to substantially underestimate the actual SFR. Interestingly a systematic deficit of Hα emission in dwarf galaxies with low SFRs and in low SFR density regions is observed (Sullivan et al. 2000; Bell & Kennicutt 2001; Lee et al. 2009a; Meurer et al. 2009).”

“These surveys in turn have led to large spinoff surveys in H I (e.g., THINGS, Walter et al. 2008; FIGGS, Begum et al. 2008; Local Volume H I Survey; Koribalski 2010; LITTLE THINGS, Hunter et al. 2007) and in CO (e.g., BIMA-SONG, Helfer et al. 2003; IRAM HERACLES, Leroy et al. 2009; JCMT NGLS, Wilson et al. 2009; CARMA…”

“A piece of a spiral arm at Rgal = 15 kpc has been recently identified in H I and CO, containing a molecular cloud with M = 5× 104 M⊙ (Dame & Thaddeus 2011).”

“The distribution of SSFRs shows a marked increase in dispersion for galaxies with MB > −14.5 or circular velocity of 50 km s−1 (Lee et al. 2007; Bothwell et al. 2009).”

“A third measuring bias will set in if the resolution of the measurements becomes smaller than the Strömgren diameters of H II regions or the corresponding dust emission nebulae, which tend to match or exceed those of the H II regions (e.g., Watson et al. 2008; Prescott et al. 2007).”

“However, we see stars down to the hydrogen-burning limit and a continuous distribution of brown dwarfs below that, extending down even to masses lower than those seen in extrasolar planets (Allers et al. 2006).”

“The most massive star with a dynamical mass (NGC 3603- A1) weighs in at 116 ± 31 M⊙ and is a 3.77 day binary with a companion at 89 ± 16 M⊙ (Schnurr et al. 2008).”

“The distribution of SSFRs shows a marked increase in dispersion for galaxies with MB > −14.5 or circular velocity of 50 km s−1 (Lee et al. 2007; Bothwell et al. 2009).”

“Some observations of massive dense clumps have found substructure on the scale of cores (Beuther et al. 2007b; Brogan et al. 2009), but the most massive cores identified in these works are 50-75 M⊙.”

“For extragalactic studies, this subject has been revolutionised by the launch of the Galaxy Evolution Explorer (GALEX) mission (Martin et al. 2005a).”

“…phase balance, and feedback are providing deeper insights into the physical nature of the star formation scaling laws and thresholds and the life cycles of molecular clouds (e.g., Robertson & Kravtsov 2008; Dobbs 2008; Dobbs & Pringle 2010; Tasker & Tan 2009; Tasker 2011; Brooks et al. 2011).”

“Evidence for overall inward flow in clumps is difficult; evidence for it is found in some surveys (Wu & Evans 2003; Fuller et al. 2005; Reiter et al. 2011a), but not in others (Purcell et al. 2006).”

“Relative motions of cores within clumps appear to be very low (< 0.1 /kms), challenging the Competitive Accretion model (Walsh et al. 2004).”

“…and/or spectroscopy reveals extended disks of HII regions that trace the UV emission in most cases (e.g., Zaritsky & Christlein 2007; Christlein et al. 2010; Goddard et al. 2010), confirming the earlier detection of HII regions at large radii (e.g., Ferguson et al. 1998; Ryan-Weber et al. 2004).”

“Several ongoing studies are quantifying this parameter by comparing Lyα fluxes of galaxies with independent SFR tracers such as Hα or the UV continuum (e.g., Atek et al. 2009; Scarlata et al. 2009; Blanc et al. 2011; Hayes et al. 2011).”

“…108 L⊙ (Sodroski et al. 1997) for the CMZ would imply Ṁ∗ ∼ 0.06 M⊙ yr −1 using the conversion in Table 1, roughly consistent with other recent estimates (Immer et al. 2012), based on analysis of point infrared sources, but new data from the surveys mentioned above should allow refinement of…”

“Recently, Krumholz (2011) has derived a very general expression for a characteristic mass of 0.15 M⊙, with only a very weak dependence on pressure.”

“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).”

“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).”

“…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).”

“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).”

“Many of these use the distribution of blue helium-burning stars in the CMD (e.g., Dohm-Palmer et al. 2002), but more recent analyses fit the entire upper CMDs to synthetic stellar populations to derive estimates of spatially resolved stellar age distributions with formal uncertainties (e.g.,…”

“Astrometric studies are beginning to be able to constrain these motions (Rochau et al. 2010), along with cleaner separation of cluster members from field objects.”

“Based on these surveys, properties of the cool atomic clouds have been analyzed by Dickey et al. (2003).”

“Evidence for a higher value of αcloud has been found for the LMC (Fukui et al. 2001; Wong et al. 2011) and M33 (Engargiola et al. 2003; Rosolowsky 2005).”

“These surveys in turn have led to large spinoff surveys in H I (e.g., THINGS, Walter et al. 2008; FIGGS, Begum et al. 2008; Local Volume H I Survey; Koribalski 2010; LITTLE THINGS, Hunter et al. 2007) and in CO (e.g., BIMA-SONG, Helfer et al. 2003; IRAM HERACLES, Leroy et al. 2009; JCMT NGLS, Wilson et al. 2009; CARMA…”

“For individual clouds, ǫ ranges from 2% to 8% (Evans et al. 2009; Peterson et al. 2011).”

“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 a similar vein, Boselli et al. (2002) and RodriguezFernandez et al. (2006) have investigated the applicability of the [C II]158µm line as a SFR measure.”

“A re- view in this journal of extragalactic science from Spitzer can be found in Soifer et al. (2008).”

“Although star formation had been detected occasionally in nearby elliptical and S0 galaxies (e.g., Pogge & Eskridge 1993), most of these galaxies have historically been regarded as being “red and dead” in terms of recent star formation.”

“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).”

“Another CARMA-based study by Rahman et al. (2011), however, suggests that the effects of diffuse emission may not be sufficient to account for all of the differences between different analyses.”

“Follow-up deep Hα imaging and/or spectroscopy reveals extended disks of HII regions that trace the UV emission in most cases (e.g., Zaritsky & Christlein 2007; Christlein et al. 2010; Goddard et al. 2010), confirming the earlier detection of HII regions at large radii (e.g., Ferguson et al. 1998;…”

“There is evidence of continued grain growth in denser regions, which particularly affects the opacity at longer wavelengths, such as the mm/submm regions (Shirley et al. 2011).”

“The line luminosities of other dense gas tracers also show strong correlations with both virial mass (Wu et al. 2010a) and the mass derived from dust continuum observations (Reiter et al. 2011b).”

“Major concentrations of molecular clouds with very active star formation appear to be associated with regions of low shear (Luna et al. 2006), and with the junction of the bar and spiral arms (Nguyen Luong et al. 2011).”

“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:…”