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.”

“However, assuming a constant star formation rate, the Hα luminosity per unit star formation rate reaches a steady state after only a few Myr (as stars arriving on the zero age main sequence are balanced by ones that evolve toward supernovae; Krumholz & Tan 2007; Kennicutt & Evans 2012).”

“Thus, there is a huge gap between galactic and extragalactic molecular cloud studies that needs to be bridged (Kennicutt & Evans 2012); so far, this has not yet been done even for CO, let alone for other molecules.”

“Notwithstanding the above, possible effects associated with, for example, diffuse ionised gaseous emission, geometry, or galaxy inclination should be considered (e.g. Relaño et al. 2006; Kennicutt et al. 2009; Kennicutt & Evans 2012; van der Wel et al. 2014).”

“…Row: Bolometric stellar flux in units of erg s−1 cm−2 Å−1; SFR in units of M yr−1 derived from extinction corrected Hα flux using the calibration from Kennicutt & Evans (2012); Ionization parameter q in units of the cm s−1 calculated as described in § 4.4; Middle and Bottom Row: Gas phase…”

“This assumption will be especially justified if the star formation efficiency is small, which is notably the case in low-surface-brightness and dwarf irregular galaxies (e.g., Van der Hulst et al. 1993; Schombert et al. 2001; Boissier et al. 2008; Wyder et al. 2009; Kennicutt & Evans 2012).”

“However, we cannot necessarily assume that other assumptions are fulfilled: age effects may be strong and star–forming regions may be individually resolved (Kennicutt & Evans 2012; Kruijssen & Longmore 2014).”

“…underlay our understandings of star-formation processes have been scalefree relationships like the Salpeter (1955) stellar initial mass function (IMF) and the Kennicutt–Schmidt law relating global galactic star-formation rate to interstellar material (Schmidt 1959; Kennicutt & Evans 2012).”

“We note that their observation reaches to a dust-free SFR of ∼6 M yr−1 (Kennicutt & Evans 2012) and lower SFR galaxies will not be matched.”

“…abundances of 10Be, a short-lived radionuclide uniquely synthesized by spallation reactions, in CAIs recording uniform initial 26Al/27Al values indicate that no more than ∼2.5% of the solar system’s 26Al inventory may have been produced by local irradiation processes (Kennicutt & Evans 2012).”

“UV SFRs are derived from the best-fi t UV continuum fl ux densities with conversion factor in Kennicutt & Evans ( 2012 ) and are not corrected for dust attenuation. the template overpredicts the 2 – 3 mm fl ux densities, which is caused by the large dust emissivity index ( β ∼ 2.5 ) of HDF850.”

“Furthermore, if we were to instead use UV-based SFRs derived with the conversion factor from Kennicutt & Evans (2012) alongside measurements of the rest-UV magnitudes derived as νL ν at λ rest = 1500˚A, it would not change the distribution of our final spectroscopic sample in Figure 4.”

“…we calculate the NUV star-formation rates (Kennicutt & Evans 2012; Hao et al. 2011; Murphy et al. 2011) and corrected NUV starformation rates (Kennicutt & Evans 2012; Hao et al. 2011) respectively as log10(SFRNUV ) = log10(νLν) − 43.17 (8) and log10(SFRNUVcorr ) = log10(νLνcorrected) − 43.17.”

“The solid blue line shows the Murphy et al. (2011) calibration between LIR and the SFR (as reported by Kennicutt & Evans 2012).”

“The corresponding time-scale is ∼10 Myr (e.g. Kennicutt & Evans 2012), which is comparable to the one adopted in the simulations.”

“These measurements provided by the newer stellar population synthesis models of FSPS provide updates to those from Kennicutt & Evans (2012) and Hao et al. (2011), who find that the corresponding values for the mean stellar ages contributing to emission for Hα and the NUV are 3Myr and 10Myr,…”

“…from Izotov et al. (2016) and Izotov et al. (2018a) have had their stellar masses and SFRs converted from the Salpeter (1955) IMF used in these studies to the Chabrier (2003) IMF used in this work using conversion factors found in the literature (e.g. Kennicutt & Evans 2012; Speagle et al. 2014).”

“Hα probes the star formation activity for galaxies in a composite SED (e.g., Kennicutt & Evans 2012).”

“4SFR 2 1 5 gas 2 1.5 S = ´ S - - - - ⎛ ⎝⎜ ⎞ ⎠⎟ ( ) This result was shown in Paper I to agree in both slope and intercept with the observations in Kennicutt & Evans (2012), which are for average SFRs in the main disks of spiral galaxies (the average rate per unit area in a galaxy is a good…”

“…rate (SFR; e.g. Kennicutt 1998; Jansen, Franx & Fabricant 2001; Brinchmann et al. 2004; Kewley, Geller & Jansen 2004; Moustakas & Kennicutt 2006; Kennicutt & Evans 2012), AGN bolometric luminosity (e.g. Heckman et al. 2004; Lamastra et al. 2009), and the metallicity (e.g. Pagel et al. 1979;…”

“…bound molecular clouds denominated giant molecular clouds (GMCs, typical sizes of ∼50 pc, molecular gas masses of Mmol > 104 M , see for example Kennicutt & Evans 2012), which dominate the ISM mass within a few kpc from the galaxy centre (Scoville & Solomon 1975; Solomon et al. 1979; Larson…”

“This value for the Hα luminosity converts into a S FR = 0.76 M yr−1, using the calibration by Kennicutt & Evans (2012).”

“Star formation is one of the key factors in shaping the gas environment in galaxies (Kennicutt & Evans 2012).”

“Most studies of the molecular gas in nearby galaxies have focused on the lower-J carbon monoxide (CO) transitions, as these are the brightest molecular lines in other galaxies and are essential to characterise their total molecular masses and their dynamical properties (Kennicutt & Evans 2012).”

“The star formation in galaxies can be inferred by lines like Lyα and Hα, and by continuum emission in the ultraviolet (UV), infrared (IR), radio and X-ray bands (see Kennicutt & Evans 2012 for a review).”

“In fact, we know from studies of our own galaxy that star formation can occur in heavily obscured regions (for review Evans, 1999; Kennicutt & Evans, 2012), and therefore much of the light may be missed in optical observing campaigns.”

“Finally a significant contribution to the FIR may come from dust heated by old stars (Kennicutt & Evans 2012; Lutz 2014, e.g.) which emit fewer UV/optical photons capable to excite the PAHs.”

“The slope N is a critical parameter of various proposed theories of star formation (see e.g. Mac Low & Klessen 2004; McKee & Ostriker 2007; Kennicutt & Evans 2012, and references therein).”

“Using the SFRLIR calibration given in Kennicutt & Evans (2012), we can also build a direct relation between SFR and L[NII], namely log SFR (M⊙ yr −1) = (−5.31±0.32)+(0.95±0.05) logL[NII] (L⊙) (2) Comparing to the IR luminosity, the [N ii] 205 µm emission could be a more convenient SFR indicator in…”

“However, in the case of the Milky Way, which is a Sbc-type spiral galaxy, there is convincing evidence for a ‘quiescent’ evolution at quasi-constant pace over billions of years (Kennicutt & Evans 2012).”

“Star formation in the Milky Way typically proceeds in molecular clouds with sizes of 10–100 pc, leading to loose conglomerations of stars that slowly disperse within the Galaxy (Kennicutt & Evans 2012).”

“5 is adopted from Leroy et al. (2008), and it is functionally similar to the prescription in Kennicutt & Evans (2012).”

“In order to measure the SFR in distant galaxies, several possible tracers have been and are being studied: ultraviolet (UV) radiation, infrared (IR) radiation, emission from polycyclic aromatic hydrocarbons (PAHs), atomic and molecular lines (e.g., Kennicutt 1998; Kennicutt & Evans 2012).”

“SFRs are the same as derived by Fang et al. (2017) based on the luminosity at 2800 Å, after the correction of dust extinction at 2800 Å (Calzetti et al. 2000; Kennicutt & Evans 2012): SFR[M yr−1] = 2.59 × 10−10LNUV,corrected[L ].”

“Using the calibration by [46], the corresponding star formation rate (SFR) is ' 3 × 104M yr−1 (the typical SFR of 3200M yr−1 mentioned in the [44] paper is a misprint, cf.”

“Hydrogen Balmer emission lines are considered to be another gold standard for tracing robust SFRs in local galaxies (Moustakas et al. 2006; Hao et al. 2011; Kennicutt & Evans 2012, among many others).”

“…noteworthy that, despite their low SFR surface density ( ∼ 1 × 10−3 M yr−1 kpc−2), ∼3 times lower than the average value in the Milky Way and comparable to that in LSB dwarf irregulars (cf Kennicutt & Evans 2012), the detected spiral-like features have to contain a high-mass tail of ionizing stars.”

“From Kennicutt & Evans (2012): log SFR[M yr−1] = log LHα [erg s −1] − 41.27 (4) which, divided by the CO(1–0) beam solid angle Ω, gives the SFR surface density ΣSFR.”

“We followed Kennicutt & Evans (2012) and attributed an error of 0.3 dex to these data.”

“…distribution of the class I and class II objects supports their assumption that the spatial distribution of YSOs is primordial. total mass in YSOs (e.g. Kennicutt & Evans 2012), with average gas surface (mass) densities three or four times the surface (mass) density of YSOs (Gutermuth et al. 2009).”

“…(2018) (see also Scholtz et al. 2020), is remarkably symmetric around the quasar except for a narrow plume of emission that juts out ∼1 1 27 For comparison, we derive an upper limit of SFR = 25 Me yr −1 from the total Hα flux in the nebula using the SFR–LHα relation from Kennicutt & Evans (2012).”

“With these values we then calculate star-formation rates (SFRs) utilizing calibrators from the literature (Kennicutt & Evans 2012 with references to Murphy et al. 2011; Hao et al. 2011 therein) and obtain SFR estimates of 830+34−190 M yr −1 for MORA-5 and 200+250−60 M yr −1 for MORA-9.”

“All molecular gas masses quoted in this paper include the 36% contribution of helium (Kennicutt & Evans 2012; Bolatto et al. 2013).”

“…at any redshift are hardly enough to sustain their star formation rates (SFR) over a time scale of a few Gyr (Genzel et al. 2010; Daddi et al. 2010; Kennicutt & Evans 2012; Tacconi et al. 2013; Leroy et al. 2013; Scoville et al. 2016); (ii) The observed metallicity distribution of old stars in the…”

“It is Article published by EDP Sciences A10, page 1 of 24 estimated that a significant fraction of stars are formed in embedded clusters; these can count tens to hundreds of members, spanning a variety of evolutionary stages (see Lada & Lada 2003; Kennicutt & Evans 2012).”

“(4) This is the same as the relation observed for many types of galaxies by Kennicutt & Evans (2012), as shown in Figure 1a.”

“It is essential to understand these issues because the midIR is frequently used as a proxy for the star formation rate (SFR) in galaxies (e.g., Kennicutt & Evans 2012).”

“Because of its symmetry, molecular hydrogen (H2) has no permanent dipole moment and its infrared transitions require high excitation conditions, thus the H2 emission is not able to trace molecular clouds (e.g.,Kennicutt & Evans 2012).”

“We know on kiloparsec scales that the formation of stars scales with the density of gas, and that the phase and motions of the gas are affected by the subsequent evolution of the stars (e.g. Kennicutt & Evans 2012, and references therein).”

“…spectrum of a galaxy, we can discern chemical abundances and metallicities (e.g., Lequeux et al. 1979), star formation rates (e.g., Kennicutt 1998; Kennicutt & Evans 2012), nebular dust attenuation estimates (e.g., Groves et al. 2012; Buat et al. 2002), temperatures and densities of the…”

“The UV flux is coming from the stellar photospheres of young stars and directly traces the star formation over the past 100-200 Myr (Kennicutt & Evans 2012), while the optical flux at redder wavelengths traces more evolved stellar populations.”

“From the Hα detection, we estimate a dust-uncorrected star formation rate (SFR) of 3.9 ± 0.4 M yr−1 (assuming a Kroupa initial mass function; Kennicutt & Evans 2012).”

“We are assuming, as we did in §4.4, that the starformation rate is proportional to the gas mass rather than to a different power of the gas mass (there is evidence for both in the literature - Kennicutt and Evans 2012).”

“…is in principle governed by three factors: (1) The molecular gas reservoir of the galaxy and its molecular clouds (Gao & Solomon 2004), (2) the dynamics and kinematics of the galactic disk (Elmegreen 2011), and (3) the star formation efficiency and rate at various scales (Kennicutt & Evans 2012).”

“However, the inferred SFRs of galaxies are sensitive to the choice of time-scale over which the SFR is averaged (Kennicutt & Evans 2012; Hopkins et al. 2014; Sparre et al. 2015b).”

“This gives an estimate of the star-formation rate (SFR) by applying the formula of Kennicutt & Evans (2012): log 10 ( SFR ) = log 10 L (H α ) − 41.27.”

“IMF, most of the cluster mass range, and upper end of the GMC mass functions are reasonably approximated by power-laws, with similar indices (−2±0.3, Kennicutt & Evans 2012).”

“The relation between the TIR luminosity and the SFR is given by SFR = LTIR/6.7 × 109 L (Kennicutt & Evans 2012).”

“There are various indicators proposed for star-formation activity in galaxies, including rest-frame UV continuum light, nebular emission lines (such as Hα), mid- to far-infrared dust thermal emission, and radio continuum luminosity (see review by Kennicutt 1998; Kennicutt & Evans 2012).”

“…UV light in galaxies, therefore measuring the total IR light from dust provides insights into the current ( 100 Myr) star formation rate (SFR) (e.g. Kennicutt & Evans 2012), although for very passive galaxies where the current SFR is much lower than it was earlier, IR emission can be an…”

“Even though H i does not directly foster star formation (Kennicutt & Evans 2012), it has been shown that the mass of the stars and the atomic hydrogen (H i) mass are highly correlated (Cortese et al. 2011; Huang et al. 2012).”

“(10): Chi-squared and degrees of freedom. our targets have star formation rates similar to our Galaxy (∼2M⊙yr −1; Kennicutt & Evans 2012).”

“Similarly, variations in N have been reported for even smaller scales of 100–500 pc (see reviews by Kennicutt & Evans 2012 for details).”

“…(Myers et al. 1986; Mooney & Solomon 1988; Williams & McKee 1997; Evans et al. 2009; Heiderman et al. 2010; Lada, Lombardi & Alves 2010; Murray 2011; Kennicutt & Evans 2012; Lee, Miville-Deschênes & Murray 2016), possibly due to stellar feedback disrupting the molecular cloud once a certain…”

“Despite the prominent role that massive stars have for the evolution of their host galaxies (Kennicutt & Evans 2012), the physical processes connected to their formation are still far from being clear.”

“However, it is often assumed that the far-IR and submm part of the broadband SED, which in star-forming galaxies is a robust tracer of the SFR (e.g. Kennicutt 1998; Kennicutt & Evans 2012), has but a minor contribution from the AGN and hence directly traces star-formation even in AGN hosts.”

“These IR luminosities, if produced solely by SF, are equivalent to star formation rates (SFR) of >15 and >150 M yr−1 , respectively (see Table 1 of Kennicutt & Evans 2012).”

“Our extrapolations predict that the (likely incomplete) younger, clustered population represents >60% of the total stellar population, hence we do not expect that our reported global SFR is significantly overestimated due to our assumption of a standard IMF.”

“The KS law was originally applied to global properties, but has also been demonstrated (Bigiel et al. (2008) and Kennicutt & Evans (2012)) to describe the local relation between SF surface density and gas density within galaxies.”

“This is partially because in Paper I we have adopted an old SFR calibrator from Kennicutt (1998) which gives a SFR∼17% higher than the recent result (Kennicutt & Evans 2012), and partially because the SFR based on total IR luminosity (as adopted in Paper I) is also systematically higher than that…”

“In addition, empirically constrained proxies for star formation rates ba ed on a variety of observational tracers are in common use (see Buatet al. 2010, Kennicutt & Evans 2012, Calzetti et al. 2013 and references therein.).”

“…et al. 2012), which corresponds to a NUV luminosity of LNUV = (1.08±0.04)×10 40 erg/s. Assuming that all of this emission is stellar in origin, we estimate the star formation rate (SFR) using the relation of log Ṁ⋆/(M⊙ yr −1) = log LNUV/(erg s −1)−43.17 (Kennicutt & Evans 2012).”

“Global SFRs can be inferred using emission lines or broad-band fluxes (see Kennicutt & Evans 2012, for a review on SFR tracers).”

“In contrast, the CO J = 1–0 line shows a superlinear relation with far-infrared luminosity (sometimes converted into star formation rate) and with the total (H I + H2) gas surface density with a power-law index of 1.4 (see Kennicutt & Evans 2012 for review).”

“From the total infrared luminosity we derive SFRTIR of 4700 M yr −1 using the relation in Kennicutt & Evans (2012).”

“It is therefore known that molecular clouds play an important role in star formation and galaxy evolution (see the review by Kennicutt & Evans 2012).”

“Although we applied the standard conversion factor to every location in each galaxy, the factor is not constant, and could depend on the physical and chemical conditions of molecular gas (e.g., Kennicutt & Evans 2012; Bolatto et al. 2013).”

“However, at these wavelengths the dust absorption is very important and correcting the UV luminosities from the dust attenuation is not trivial (Lagache et al. 2005; Kennicutt & Evans 2012).”

“…the dust continuum flux and size (Figure 5), probing the physical drivers of star formation more directly, i.e., the Kennicutt-Schmidt law (Kennicutt 1998; Leroy et al. 2008; Bigiel et al. 2008; Daddi et al. 2010a; Bigiel et al. 2011; Schruba et al. 2011; Kennicutt & Evans 2012; Leroy et al. 2013).”

“It has long been suggested that the XCO factor may depend on a variety of ISM parameters, such as metallicity and density (for a review, see Kennicutt & Evans 2012 and Bolatto et al. 2013).”

“It should be noted that the global star formation properties, such as the star formation rate and Schmidt–Kennicutt relation (Kennicutt & Evans 2012, and references therein), are insensitive to the adopted value of C∗ and are instead primarily controlled by the global evolution of the ISM from…”

“Nevertheless, they reproduce the CO column densities relatively well over a limited column density range (e.g. Kennicutt & Evans 2012), if corrections for the depth dependent 12CO/13CO isotope ratio are applied (see Sections 3.3 and 3.4 in Szűcs et al. 2014).”

“We can also change the power-law Salpeter IMF to a more realistic Kroupa & Weidner (2003) IMF with a turnover below ∼ 1 M⊙ and use the updated IR star formation calibration in Kennicutt & Evans (2012).”

“Although galaxy scale star formation rates (SFRs) can be derived at many wavelengths, ranging from the X-ray to the radio (see Kennicutt 1998 and Kennicutt & Evans 2012 for reviews), the most common techniques (at low redshifts) use rest-frame ultraviolet (UV) and optical diagnostics.”

“…(∼ 10− 100Myr), the thermal far-infared emission has been widely used as a star formation rate indicator (Kennicutt 1998; Calzetti et al. 2010; Kennicutt & Evans 2012), with the integrated dust emission frequently estimated from sparsely-sampledphotometry by assuming or deriving a model…”

“While a sub-linear (N < 1) slope could reduce the magnitude of the observed drop in XCO towards the central regions, this scenario is not currently supported by observations (Calzetti et al. 2012; Kennicutt & Evans 2012, and references within).”