Star Formation in the Milky Way and Nearby Galaxies

“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).”

“More recent determinations over a wider range of masses, including brown dwarfs, indicate a clear flattening at lower masses, either as broken power laws (Kroupa et al. 1993) or a log-normal distribution (Miller & Scalo 1979; Chabrier 2003).”

“This uncertainty range does not include all possible systematic errors, arising for example from changes in X(CO) or the IMF with increasing surface density or SFR. Major systematic changes in either of these could easily change the derived value of N by as much as 0.2–0.3.”

“The High-density Regime The two regimes discussed above were able to reproduce all of the early observations of the large-scale star formation law by Kennicutt (1989, 1998b) and Martin & Kennicutt (2001).”

“The results cited here use the Starburst99 models of Leitherer et al. (1999), which are regularly updated in the on-line version of the package.”

“The High-density Regime The two regimes discussed above were able to reproduce all of the early observations of the large-scale star formation law by Kennicutt (1989, 1998b) and Martin & Kennicutt (2001).”

“This section focusses on the transformational results which have come from the Spitzer Space Telescope (Werner et al. 2004) and the Herschel Space Observatory (Pilbratt et al. 2010).”

“The mass dependence of the SSFR has been explored by numerous investigators (e.g., Brinchmann et al. 2004; Salim et al. 2007; Schiminovich et al. 2007; Lee et al. 2007).”

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

“The masses of embedded young clusters (Lada & Lada 2003), OB associations (McKee & Williams 1997), and massive open clusters (Elmegreen & Efremov 1997; Zhang & Fall 1999), follow a similar relation with γcluster ∼ 1.”

“The luminosities and masses of galaxies are well represented by an exponentially truncated power-law function (Schechter 1976): Φ(L) = (Φ∗/L∗)(L/L∗)αe(−L/L ∗) (3) The parameter α denotes the slope of the power-law function at low luminosities, and L∗ represents the luminosity above which the number…”

“ISM Basics The interstellar medium (ISM) is a complex environment encompassing a very wide range of properties (e.g., McKee & Ostriker 1977; Cox 2005).”

“In the diffuse medium, the reddening has been calibrated against atomic and molecular gas to obtainN(H I)+2N(H2) = 5.8× 10 21 cm−2E(B−V )(mag), not including helium (Bohlin et al. 1978).”

“Emission from small grains transiently heated by intense radiation fields (usually in or near star-forming regions) is important out to about 60 µm, whereas at longer wavelengths, emission from larger grains with steady state temperatures dominates (Draine 2003).”

“This suggestion led eventually to a picture of magnetically subcritical clouds that formed stars only via a redistribution of magnetic flux, commonly referred to as ambipolar diffusion (Shu et al. 1987; Mouschovias 1991), which resulted in cloud lifetimes about 10 times the free-fall time.”

“The warmer phases provide the raw material for the cold phases, perhaps through “cold” flows from the intergalactic medium (Dekel & Birnboim 2006).”

“Pictures of static clouds supported by magnetic fields are currently out of fashion (McKee & Ostriker 2007), but magnetic fields are almost certain to play a role in some way (for a current review, see Crutcher 2012, this volume).”

“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).”

“8.11 in Draine 2011).”

“The concept of a powerlaw relation between SFR density and gas density dates to Schmidt (1959, 1963), and relations of this kind are commonly referred to as “Schmidt laws.””

“The steep synchrotron spectrum makes this calibration strongly wavelength dependent, and most are referenced to 1.4GHz (e.g., Yun et al. 2001; Condon 1992; Bell 2003).”

“Other spaceborne instruments have also provided important datasets in this wavelength region, including the XMM Optical Monitor (Mason et al. 2001) and the Swift UV/Optical Telescope (Roming et al. 2005).”

“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…”

“The mass dependence of the SSFR has been explored by numerous investigators (e.g., Brinchmann et al. 2004; Salim et al. 2007; Schiminovich et al. 2007; Lee et al. 2007).”

“More recent determinations over a wider range of masses, including brown dwarfs, indicate a clear flattening at lower masses, either as broken power laws (Kroupa et al. 1993) or a log-normal distribution (Miller & Scalo 1979; Chabrier 2003).”

“The attenuation of the emission lines is found to be systematically higher than that of the continuum starlight at the same wavelengths (e.g., Calzetti et al. 1994), which presumably reflects the higher concentrations of dust in the young star-forming regions.”

“Using the YSO star counting method to get Ṁ∗ and extinction maps to get mean mass surface density of individual, nearby clouds, Evans et al. (2009) found that the local clouds all lay well above the K98 relation.”

“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 the area of the cloud defined roughly by the 1 K CO detection threshold, he found a median Σmol(cloud) = 42 M⊙ pc −2 for clouds, substantially less than originally found by Solomon et al. (1987), and only a few clouds have Σmol(cloud) > 100 M⊙ pc −2.”

“Portegies Zwart et al. (2010) have cataloged massive (M⋆ ≥ 10 4 M⊙), young (age ≤ 100 Myr) clusters (12) and associations (13), but none of these are more distant than the Galactic Center, so they are clearly undercounted.”

“This problem can be addressed partly by adopting a reddening law which compensates in part for these departures from a pure foreground scheme geometry (e.g., Charlot & Fall 2000).”

“…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).”

“…of the star formation history of a galaxy has been long recognized (e.g., Gavazzi & Scodeggio 1996), but data from SDSS and subsequent surveys have reshaped our picture of the population of star-forming galaxies (e.g., Kauffmann et al. 2003; Baldry et al. 2004, 2006; Brinchmann et al. 2004).”

“More recent determinations over a wider range of masses, including brown dwarfs, indicate a clear flattening at lower masses, either as broken power laws (Kroupa et al. 1993) or a log-normal distribution (Miller & Scalo 1979; Chabrier 2003).”

“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).”

“As mentioned earlier the fraction of star formation in LIRGs and ULIRGs increases sharply with redshift, but imaging and spectroscopy of these objects reveals a marked shift in the physical characteristics of the population.”

“Variations in the IMF with environment are plausible, but convincing evidence for variation remains elusive (Bastian et al. 2010).”

“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…”

“Blue circles denote measurements in the disk and center of the Milky Way, based on various virial, extinction, and isotopomeric analyses.”

“Studies of the role of turbulence show some promise in explaining this paradox (Krumholz & McKee 2005; Hennebelle & Chabrier 2011), and magnetic fields may yet play a role.”

“…of the star formation history of a galaxy has been long recognized (e.g., Gavazzi & Scodeggio 1996), but data from SDSS and subsequent surveys have reshaped our picture of the population of star-forming galaxies (e.g., Kauffmann et al. 2003; Baldry et al. 2004, 2006; Brinchmann et al. 2004).”

“This work has been enabled by large multi-wavelength surveys of nearby galaxies such as the Spitzer Infrared Nearby Galaxies Survey (SINGS) (Kennicutt et al. 2003), the GALEX Nearby Galaxies Survey (Gil de Paz et al. 2007), the Spitzer/GALEX Local Volume Legacy survey (LVL; Dale et al. 2009; Lee et al. 2011), and the…”

“Many of the most massive stars exist in tight (orbital periods of a few days) binaries (Zinnecker & Yorke 2007).”

“…progenitors of present-day normal galaxies, rather than by the transient merger-driven starbursts which dominate the present-day populations of ULIRGs, though examples of the latter are still found, especially in the population of submillimeter-luminous galaxies (SMGs) (e.g., Chapman et al. 2005).”

“Stage 0 sources have protostars, disks, jets, and envelopes with more mass in the envelope than in the star plus disk (Andre et al. 1993).”

“…morphologies in 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…”

“Earlier observations of bright starburst galaxies suggest a tight relation between the logarithmic IR/UV ratio (often termed the “IRX”) and UV color (e.g., Meurer et al. 1999), and indeed this “IRX–β relation” provided the primary means for calibrating the color versus attenuation relation.”

“One can then correlate N(CO) or its isotopes against extinction to determine a conversion factor (Dickman 1978; Frerking et al. 1982).”

“…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 regime.”

“For observations of normal galaxies where local attenuations are modest, integral-field mapping of galaxies offers the means to produce high-quality Hα maps corrected for dust using the Balmer decrement (e.g., Blanc et al. 2009, 2010; Sánchez et al. 2012).”

“Some clusters have measurements of stars down to very low masses (e.g., the Orion Nebula Cluster, Hillenbrand 1997), but most need corrections for unseen low-mass stars.”

“The final efficiency will depend on how long the cloud continues to form stars before being disrupted; cloud lifetimes are poorly constrained (McKee & Ostriker 2007; Bergin & Tafalla 2007), and they may differ for clouds where massive stars form.”

“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).”

“A conception of what the MW would look like from the outside (Churchwell et al. 2009) is shown in Figure 5.”

“Blue circles denote measurements in the disk and center of the Milky Way, based on various virial, extinction, and isotopomeric analyses.”

“However, molecular clouds are surrounded by atomic envelopes and a transition region in which the hydrogen is primarily molecular, but the carbon is mostly atomic (van Dishoeck & Black 1988).”

“…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 regime.”

“…dynamical time (Σgas/τdyn) shows a correlation that is nearly as tight as the conventional Schmidt law (Kennicutt 1998b), and it also removes the double sequence of disks and starbursts that results if X(CO) is systematically lower in the starbursts (§6, Daddi et al. 2010; Genzel et al. 2010).”

“A SFR calibration has not been derived from first principles, but observations have repeatedly confirmed a tight correlation between this non-thermal emission and the far-infrared emission of galaxies, which favors its application as a SFR tracer (e.g., Helou et al. 1985; Condon 1992).”

“The steep synchrotron spectrum makes this calibration strongly wavelength dependent, and most are referenced to 1.4GHz (e.g., Yun et al. 2001; Condon 1992; Bell 2003).”

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

“…of the star formation history of a galaxy has been long recognized (e.g., Gavazzi & Scodeggio 1996), but data from SDSS and subsequent surveys have reshaped our picture of the population of star-forming galaxies (e.g., Kauffmann et al. 2003; Baldry et al. 2004, 2006; Brinchmann et al. 2004).”

“Pictures of static clouds supported by magnetic fields are currently out of fashion (McKee & Ostriker 2007), but magnetic fields are almost certain to play a role in some way (for a current review, see Crutcher 2012, this volume).”

“…(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).”

“…observations of these high-redshift galaxies with IFU instruments, both with and without adaptive optics correction, have revealed many insights into the physical nature of this population of starburst galaxies (e.g., Genzel et al. 2008, 2011; Förster Schreiber et al. 2009 and references therein).”

“…of nearby galaxies such as the Spitzer Infrared Nearby Galaxies Survey (SINGS) (Kennicutt et al. 2003), the GALEX Nearby Galaxies Survey (Gil de Paz et al. 2007), the Spitzer/GALEX Local Volume Legacy survey (LVL; Dale et al. 2009; Lee et al. 2011), and the Herschel KINGFISH Survey (Kennicutt…”

“Theoretical studies include evolutionary tracks of premain-sequence stars (e.g., Chabrier & Baraffe 2000 for low mass stars and substellar objects).”

“This severe underestimate for Orion itself from Hα emission results because the Orion cluster is too small to fully populate the IMF; its earliest spectral type is O7 V, and so the star cluster produces relatively little ionizing luminosity relative to its total mass and SFR.”

“An alternative picture, called Competitive Accretion, developed by Klessen et al. (1998), Bate et al. (2003), and Bonnell et al. (2003), builds massive stars from the initial low-mass fragments.”

“An illustration of the effectiveness of the method is shown in Figure 3, taken from Kennicutt et al. (2009).”

“Three other space missions are beginning to influence this subject: the AKARImission (Murakami et al. 2007), the Wide-field Infrared Survey Explorer (WISE) mission (Wright et al. 2010), and the Planck mission (Planck Collaboration et al. 2011a).”

“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…”

“The neutral gas is subject to a thermal instability that predicts segregation into the warm neutral and cold neutral media (CNM), the latter with n > 10 cm−3 and TK < 100 K (Field et al. 1969).”

“…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., Dolphin 2002; Weisz et al. 2008).”

“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.”

“The MIPSGAL survey (Carey et al. 2009) should also provide a catalog of very opaque objects.”

“Rotation leads to flattening and a centrifugally supported disk (Terebey et al. 1984).”

“The isotopologues of CO (13CO and C18O) provide lower optical depths, but at the cost of weaker lines.”

“…dynamical time (Σgas/τdyn) shows a correlation that is nearly as tight as the conventional Schmidt law (Kennicutt 1998b), and it also removes the double sequence of disks and starbursts that results if X(CO) is systematically lower in the starbursts (§6, Daddi et al. 2010; Genzel et al. 2010).”

“Other spaceborne instruments have also provided important datasets in this wavelength region, including the XMM Optical Monitor (Mason et al. 2001) and the Swift UV/Optical Telescope (Roming et al. 2005).”

“The total gas surface density, with He, is Σgas = Σmol + µΣHI, where µ is the mean molecular weight per H atom; µ = 1.41 for MW abundances (Kauffmann et al. 2008), though a value of 1.36 is commonly used.”

“…has been given to measurements of the redshifted Lyα line (λrest = 121.6nm) as a tracer of star-forming galaxies, especially at high redshifts where it provides unique sensitivity to both low-mass star-forming galaxies and intergalactic gas clouds or “blobs” (e.g., Ouchi et al. 2009, 2010).”

“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).”

“…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).”

“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.”

“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.”

“…speaking this scaling relation only applies to the phase balance of cold gas rather than the SFR, but it can be recast into a predicted star formation law if assumptions are made about the scaling between the SFR and the molecular gas components (e.g., Blitz & Rosolowsky 2006; Leroy et al. 2008).”

“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…”

“One option is the cloud or clump crossing time (essentially the size over the velocity dispersion) (Elmegreen 2000), which may be particularly relevant in regions of triggered star formation.”

“Heyer et al. (2009) reanalyzed cloud properties in the inner Galaxy based on CO and 13CO surveys.”

“Including the atomic-molecular threshold, as treated in Krumholz et al. (2009b), clarifies that this “scale-free” picture implicitly recognizes the same three regimes discussed in §7.1.”

“The dark gas appears around AV = 0.4 mag, and is presumably the CO-poor, but H2-containing, outer parts of clouds (e.g., Wolfire et al. 2010), which can be seen in [CII] emission at 158 µm and in fluorescent H2 emission at 2 µm (e.g., Pak et al. 1998).”

“…component can be separated with multi-frequency radio measurements or high-frequency data (e.g., Israel & van der Hulst 1983; Niklas et al. 1997; Murphy et al. 2011), to provide a photoionization-based measure of the SFR, without the complications of dust attenuation which are encountered with…”

“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).”

“…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).”

“Surveys of H I from both northern (Taylor et al. 2003) and southern (McClure-Griffiths et al. 2005) hemispheres, along with a finer resolution survey of parts of the Galactic Plane (Stil et al. 2006) have given a much clearer picture of the atomic gas in the MW (for a review of H I surveys, see…”

“The masses of embedded young clusters (Lada & Lada 2003), OB associations (McKee & Williams 1997), and massive open clusters (Elmegreen & Efremov 1997; Zhang & Fall 1999), follow a similar relation with γcluster ∼ 1.”

“The radial distributions of atomic and molecular gas, along with the star formation rate surface density (here in units of M⊙ Gyr −1 pc−2), are plotted in Figure 7, along with a similar plot for NGC 6946 (Schruba et al. 2011).”

“Theoretical explanations for such thresholds can be found by considering magnetic support (Mouschovias & Spitzer 1976) or regulation by photo-ionization (McKee 1989).”

“Still higher initial masses (105-170 M⊙) for the stars in NGC3603 and even higher in R136 (165-320 M⊙) have been suggested (Crowther et al. 2010).”

“Lynds (1962) provided the mostly widely used catalog of “dark clouds” based on star counts, up to AV ∼ 6 mag, at which point the clouds were effectively opaque at visible wavelengths.”

“The characteristic depletion time (tdep §1.2) for the interstellar gas is 1–2 Gyr (e.g., Bigiel et al. 2011).”

“In the Milky Way, about half the volume is filled by a hot ionized medium (HIM) with n < 0.01 cm−3 and TK > 10 5 K, and half is filled by some combination of warm ionized medium (WIM) and warm neutral medium (WNM), with n ∼ 0.1 − 1 cm−3 and TK of several thousand K (Cox 2005).”

“This uncertainty range does not include all possible systematic errors, arising for example from changes in X(CO) or the IMF with increasing surface density or SFR. Major systematic changes in either of these could easily change the derived value of N by as much as 0.2–0.3.”

“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).”

“These regions are known as PDRs, which stands either for Photo-Dissociation Regions, or Photon-Dominated Regions (Hollenbach & Tielens 1997).”

“Today most workers calibrate SFR tracers using a modern IMF with a turnover below ∼1M⊙, for example the IMF of Kroupa & Weidner (2003), with a Salpeter slope (α∗ = −2.35) from 1–100M⊙ and α∗ = −1.3 for 0.1–1M⊙.”

“…formation, using tracers requiring higher densities (e.g., Plume et al. 1992; Beuther et al. 2002) or millimeter continuum emission from dust (e.g., Beuther et al. 2002; Mueller et al. 2002) have identified slightly smaller structures (r ∼ 0.5 pc) that are much denser, indeed denser on average…”

“Inclusion of realistic stellar feedback may obviate the need for artificial constraints on efficiency (e.g., Hopkins et al. 2011).”

“Stages I through III are usually associated with SED Classes I through III defined by SED slopes rising, falling slowly, and falling more rapidly from 2 to 25 µm (Lada & Wilking 1984).”

“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…”

“(6) Leroy et al. (2011) minimize the variation in δgdr to find the best fit to α′CO in several nearby galaxies.”

“Using the boundaries between classes from Greene et al. (1994), Evans et al. (2009) found that the combined Class 0/I phases last∼ 0.5 Myr, with a similar duration for Flat SEDs, assuming a continuous flow through the classes for at least the Class II duration of 2 ± 1 Myr (e.g., Mamajek 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…”

“Theoretical explanations for such thresholds can be found by considering magnetic support (Mouschovias & Spitzer 1976) or regulation by photo-ionization (McKee 1989).”

“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…”

“CO does emit beyond the area where 13CO is detectable and even, at a low level, beyond the usual detection threshold for CO. Stacking analysis of positions beyond the usual boundary of the Taurus cloud suggest a factor of two additional mass in this transition region (Goldsmith et al. 2008).”

“…(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).”

“For example, Krumholz et al. (2012) calculate tff from the mean density of the whole cloud (tff ∝ 1/〈ρ〉).”

“The concept of a powerlaw relation between SFR density and gas density dates to Schmidt (1959, 1963), and relations of this kind are commonly referred to as “Schmidt laws.””

“…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).”

“One can then correlate N(CO) or its isotopes against extinction to determine a conversion factor (Dickman 1978; Frerking et al. 1982).”

“Table 2 lists examples of dust attenuation corrections using combinations of FUV and Hα fluxes with various infrared and radio tracers, taken from Hao et al. (2011) and Kennicutt et al. (2009), respectively.”

“Direct evidence for this “downsizing” is seen in observations of the SSFR vs mass relation in high-redshift galaxies (e.g., Noeske et al. 2007).”

“…galaxies such as the Spitzer Infrared Nearby Galaxies Survey (SINGS) (Kennicutt et al. 2003), the GALEX Nearby Galaxies Survey (Gil de Paz et al. 2007), the Spitzer/GALEX Local Volume Legacy survey (LVL; Dale et al. 2009; Lee et al. 2011), and the Herschel KINGFISH Survey (Kennicutt et al. 2011).”

“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).”

“While there are still controversies, the data indicate that most clouds (or, more precisely, the parts of clouds with Zeeman measurements) are supercritical or close to critical, but not strongly subcritical (Crutcher et al. 2010).”

“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…”

“…galaxies such as the Spitzer Infrared Nearby Galaxies Survey (SINGS) (Kennicutt et al. 2003), the GALEX Nearby Galaxies Survey (Gil de Paz et al. 2007), the Spitzer/GALEX Local Volume Legacy survey (LVL; Dale et al. 2009; Lee et al. 2011), and the Herschel KINGFISH Survey (Kennicutt et al. 2011).”

“Complementary surveys of clouds have been done using mid-infrared extinction (e.g., Perault et al. 1996; Egan et al. 1998; Peretto & Fuller 2009), which can identify Infrared Dark Clouds (IRDCs) against the Galactic background emission.”

“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.”

“An alternative picture, called Competitive Accretion, developed by Klessen et al. (1998), Bate et al. (2003), and Bonnell et al. (2003), builds massive stars from the initial low-mass fragments.”

“…(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).”

“In fact, it is the clouds with the power-law tails that have active star formation (Kainulainen et al. 2009).”

“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).”

“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.”

“Recent analyses have become available (Kennicutt et al. 2007; Bigiel et al. 2008, 2010, 2011; Leroy et al. 2008; Blanc et al. 2009; Eales et al. 2010; Verley et al. 2010; Liu et al. 2011; Rahman et al. 2011, 2012; Schruba et al. 2011), and several other major studies are ongoing.”

“Including the atomic-molecular threshold, as treated in Krumholz et al. (2009b), clarifies that this “scale-free” picture implicitly recognizes the same three regimes discussed in §7.1.”

“…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 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).”

“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).”

“…0.5 Myr, with a similar duration for Flat SEDs, assuming a continuous flow through the classes for at least the Class II duration of 2 ± 1 Myr (e.g., Mamajek 2009), which is essentially the age when about half the stars in a dated cluster lack infrared excesses; hence all durations can be…”

“Indeed, Narayanan et al. (2012) use a grid of model galaxies to infer a smooth function of I(CO): αCO = min[6.3, 10.7 × I(CO) −0.32 ]Z ′−0.65, where Z ′ is the metallicity divided by the solar value.”

“…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).”

“…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 upper cut-off around 3-6× 106 M⊙ appears to be real, despite issues of blending of clouds (Williams & McKee 1997; Rosolowsky 2005).”

“…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 regime.”

“In nearby clouds, these correspond to dense cores (Enoch et al. 2009).”

“Deep GALEX imaging of E/S0 galaxies (e.g., Kaviraj et al. 2007), however, has revealed that approximately 30% of these early-type galaxies exhibit near-ultraviolet emission in excess of what could reasonably arise from an evolved stellar population (e.g., Kaviraj et al. 2007).”

“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).”

“The masses of embedded young clusters (Lada & Lada 2003), OB associations (McKee & Williams 1997), and massive open clusters (Elmegreen & Efremov 1997; Zhang & Fall 1999), follow a similar relation with γcluster ∼ 1.”

“Wu et al. (2005b) have subsequently shown that this linear relation extends down to the scales of individual star-forming molecular clouds and dense clumps in the Galaxy.”

“Simulations of turbulent gas with molecule formation also suggest that I(CO) can be a very poor tracer of AV (Glover et al. 2010).”

“First, no evidence has been found in well-studied molecular clouds for collapse at tff (§4.3, e.g., Zuckerman & Evans 1974).”

“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).”

“In a study on the inner Galaxy, Roman-Duval et al. (2010) concluded that 70% of molecular clouds (both in mass and number) were bound.”

“…form of this correction uses GALEX FUV (155 nm) and totalinfrared luminosities, hence: LFUV(corr) = LFUV(observed) + η LTIR (11) Other prescriptions sometimes adopt a higher order polynomial dependence on LIR (e.g., Buat et al. 2005), but for brevity we only discuss the linear combinations here.”

“Chomiuk & Povich (2011) have recently reviewed all methods of computing Ṁ∗ for the MW and conclude that they are consistent with Ṁ∗ = 1.9 ± 0.4 M⊙ yr −1.”

“The mass dependence of the SSFR has been explored by numerous investigators (e.g., Brinchmann et al. 2004; Salim et al. 2007; Schiminovich et al. 2007; Lee et al. 2007).”

“From surveys of emission from CO J = 1 → 0 in the outer galaxy, where confusion is less problematic, Heyer et al. (2001) found a size distribution of clouds, dN/dr ∝ r−3.2±0.1, with no sign of a break from the power-law form from 3 to 60 pc.”

“…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…”

“A turnover in the mass function appears at a mass about 3-4 times that seen in the stellar IMF (Alves et al. 2007; Enoch et al. 2008).”

“The effects of cloud temperature, density, metallicity, etc. on mass estimation from CO were discussed by Maloney & Black (1988), who concluded that large variations in the conversion factor are likely.”

“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.”

“The average surface density of molecular gas drops precipitously beyond 13.5 kpc, after a local peak in the Perseus arm (Heyer et al. 1998), as does the stellar density (Ruphy et al. 1996), thereby defining the far outer Galaxy, and the radius used in our whole Galaxy quantities (§5.1.2).”

“The masses of embedded young clusters (Lada & Lada 2003), OB associations (McKee & Williams 1997), and massive open clusters (Elmegreen & Efremov 1997; Zhang & Fall 1999), follow a similar relation with γcluster ∼ 1.”

“Krumholz & McKee (2008) have argued that a threshold clump surface density of 1 gm cm−2 is needed to suppress fragmentation, allowing the formation of massive stars.”

“…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).”

“The data shown in Figure 11 all come from observations of nearby galaxies (z < 0.03), but recently a number of studies have addressed the form of the molecular gas Schmidt law for starburst galaxies extending to redshifts z ≥ 2 (e.g., Bouché et al. 2007; Daddi et al. 2010; Genzel et al. 2010).”

“Complementary surveys of clouds have been done using mid-infrared extinction (e.g., Perault et al. 1996; Egan et al. 1998; Peretto & Fuller 2009), which can identify Infrared Dark Clouds (IRDCs) against the Galactic background emission.”

“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).”

“The dense clumps are generally found to be centrally condensed (Beuther et al. 2002; Mueller et al. 2002; van der Tak et al. 2000), which can also suppress over-fragmentation.”

“The second approach, which we could call a top-down picture, assumes that star formation is largely controlled by global dynamical phenomena, such as disk instabilities (e.g., Silk 1997), and the dynamical timescales in the parent galaxy.”

“Magnetic fields plus rotation lead to winds and jets (Shang et al. 2007; Pudritz et al. 2007), which drive molecular outflows from young stars of essentially all masses (Arce et al. 2007; Wu et al. 2004).”

“Dunham et al. (2011b) have compared a subset of clumps in the BGPS catalog (Aguirre et al. 2011; Rosolowsky et al. 2010) with kinematic distances to the Heiderman-Lada (HL) criterion for efficient star formation and to the Kauffman-Pillai (KP) criterion for massive star formation.”

“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⊙.”

“…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).”

“An alternative approach, based on counting likely YSOs in the GLIMPSE survey of the Galactic plane (Robitaille & Whitney 2010) is much less biased toward the most massive stars, but is limited by sensitivity, issues of identification of YSOs, and models of extinction.”

“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).”

“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…”

“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).”

“The surface and volume density criteria roughly agree in nearby clouds (Lada et al. 2012) but may not in other environments.”

“…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 metal abundances of approximately 1/4 to 1/3 Z⊙ (e.g., Madden…”

“In this picture, the offset of the turnover implies that about 0.2 to 0.3 of the core mass winds up in the star, consistent with simulations that include envelope clearing by outflows (e.g., Dunham et al. 2010).”

“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).”

“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).”

“…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).”

“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-”

“The warm, neutral phase is difficult to study, as lines become broad and weak, but it has been detected (Kulkarni & Heiles 1988).”

“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:…”