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.”

“MIR emission comes primarily from hot dust, heated by short-wavelength photons emitted from newborn stars, and traces star formation on time scales < 100 Myr (Calzetti et al., 2005; Kennicutt & Evans, 2012).”

“UV scaling relations are widely used because of their relative simplicity and because they have been consistently calibrated against each other (e.g., Kennicutt & Evans 2012, and references therein).”

“…by Universidade Federal do Rio Grande do Sul user on 08 March 2018 which is based on the calibration from Kennicutt & Evans (2012), in the limit that all the radiation from the young stars of a continuous star-forming population with a Chabrier IMF is…”

“(24) Calibrations of XCO against metallicity in the literature show a large variation between different authors (see Bolatto et al. 2008; Leroy et al. 2011; Kennicutt & Evans 2012, for summaries).”

“Kennicutt & Evans (2012) have reviewed this subject extensively in the context of both Galactic and extragalactic star formation.”

“Given the tight correlation between LIR and SFR, the IR data have been extensively used as proxy for SFR, assuming that most of galaxy star formation is obscured by dust (Kennicutt 1998; Kennicutt & Evans 2012).”

“Second, using the conversion between SFR and dust continuum measurement / total infrared luminosity (e.g., Kennicutt & Evans 2012).”

“0.3 M yr−1 (Kennicutt & Evans 2012).”

“We use the most recent GALEX FUV and Spitzer 24 μm fluxes from the literature and the formulae given in Kennicutt & Evans (2012) to derive an extinctioncorrected FUV-based SFR (by including the 24 μm flux to correct for attenuation by dust) and the 24 μm flux measurements alone to determine an…”

“…the Milky Way SFR range from around 0.68 − 4 M yr−1 (e.g., Diehl et al. 2006; Misiriotis et al. 2006; Murray & Rahman 2010; Robitaille & Whitney 2010; Kennicutt & Evans 2012), which combined with our estimate of f (t < 1010yr) = 0.31 − 0.33 gives an event rate from quadruple systems of ∼ 0.01 −…”

“We assign locations of CC SNRs in M33 using FUV (Gil de Paz et al. 2007) and 24 µm (Dale et al. 2009) images, which trace recent star formation (Kennicutt & Evans 2012).”

“There is a power-law correlation between the surface densities of gas (Σgas) and SFR (ΣSFR), with numerous studies attempting to constrain the power-law index via global quantities from galaxy samples and resolved observations of individual systems (see Kennicutt & Evans 2012, for a review).”

“It is possible to turn this line flux into an approximation of the SFR for this galaxy, assuming an intrinsic Lyα/Hα ratio of 8.7 (from case B recombination theory; see Osterbrock 1989) and the Kennicutt & Evans (2012) relationship between Hα luminosity and SFR.”

“Finally, we convert the inferred dust-corrected Hα luminosities to SFRs using a scaling relation between Hα and SFR log10 ( SFR M yr−1 ) = log10 ( L(Hα) ergs−1 ) −41.27, (3) (Murphy et al. 2011; Hao et al. 2011; Kennicutt & Evans 2012).”

“In this work we use extinction-corrected Hα (Hαcorr) as our SFR reference indicator following the recipe given by Kennicutt & Evans (2012).”

“The smoothed definition of the SFR is therefore more in line with what is measured observationally through UV or nebular emission line SFR tracers (e.g., see Table 1 of Kennicutt & Evans 2012).”

“We note that, if we use the SFR estimated from the UV+FIR luminosity (Kennicutt & Evans 2012), which is the same method used for the other [CII] emitters from the literature (e.g. Matthee et al. 2019) and for the ALPINE survey (Schaerer et al. 2020), our sources are in agreement with the local…”

“The SFR of a galaxy is directly proportional to the intrinsic UV luminosity; however, the latter can be significantly absorbed by even a modest amount of dust and re-radiated mostly at far-IR/(sub)mm wavelengths (e.g., Kennicutt & Evans 2012).”

“The Kennicutt-Schmidt law implies an associated surface gas density (e.g. Kennicutt & Evans 2012, Fig.11 there) of > 104 − 105 M⊙ pc −2, or a total associated gas mass Mgas > 3× 10 10 − 3× 1011 M⊙ at the core.”

“Star formation occurs exclusively within molecular clouds, which are local overdensities with respect to the molecular gas density distribution (for a recent review see Kennicutt & Evans 2012).”

“In all three cases, the classical Kennicutt-Schmidt law (Kennicutt 1998; Kennicutt & Evans 2012), where the total gas surface density is on the x-axis, shows a superlinear scaling, as observed in nearby star forming galaxies.”

“Derived distances to and physical properties of these objects may answer several outstanding questions about massive star formation (Kennicutt & Evans 2012).”

“The Hα, TIR, FIR, and 24 µm SFR were calculated following Kennicutt & Evans (2012), Kennicutt (1998), and Calzetti et al. (2007).”

“Our result, in essence, corroborates the common idea - supported by the fact that a variety of approaches leads to similar values - that the current SFR of the Milky Way lies in the range 1.5-2.5 M⊙ yr −1, which is consistent with the values found for spiral galaxies (e.g., Kennicutt & Evans 2012).”

“…et al. 2010; Gutermuth et al. 2011) between the molecular gas mass surface density (Σ ) and the star formation rate surface density (ΣSFR), which is observed to have a power law form of ΣSFR ∝ ΣN with N = 1–1.5 (e.g. Kennicutt 1998; Bigiel et al. 2008; Kennicutt and Evans 2012; Leroy et al. 2013).”

“Despite calculating almost identical dust emission luminosities, the SFR we derive using our chosen calibration: that presented in Murphy et al. (2011) and recommended by the review of Kennicutt & Evans (2012); is a factor of 3 higher than value reported by Negrello et al. (2014).”

“These leakage may be associated with short and extreme bursts of star formation as proposed to explain the intrinsically blue rest frame UV slope observed in dusty, star forming galaxies (SFR>50 M yr −1) up to z∼5 (e.g., Casey et al. 2014).”

“The FIR luminosity here is defined between 40 and 300 µ m and we use the Kennicutt & Evans (2012) relation to convert this FIR luminosity to a star formation rate: Once again, these values can be found in Table 3 and correspond to the average of the T d = 47 K, β = 1.6 and T d = 41 K, β = 1.95 grey…”

“The luminosity of Hα can be used to measure the SFR for a galaxy, following Kennicutt & Evans (2012): ( [ ]) ( ( ) [ ]) ( )☉ a= -- -”

“…including flux measurements, corrections made for attenuation by dust, variations in the SFR, uncertainties in the calibration used to convert the measured fluxes to SFR as discussed in the review by Kennicutt & Evans (2012), and the leakage of Lyman continuum photons from the parent galaxy.”

“The global SFR for our sources is calculated from the extinctioncorrected Hα luminosity, which is converted to SFR assuming the Kennicutt & Evans (2012) relation and applying a scaling factor of 1.06 to convert from Kroupa, Tout & Gilmore (1993) to Chabrier (2003) IMF.”

“> 20M , (O & B spectral type) whose lifetimes are 10-20 Myr or shorter (Kennicutt 1998; Kennicutt & Evans 2012).”

“…line originates primarily from gas ionised by young and massive OB stars; it is thus an excellent tracer of ongoing star formation (Kennicutt 1998; Boselli et al. 2001, 2009; Kennicutt & Evans 2012), sensitive to stellar populations much younger than those detected in the GALEX UV bands (Hα.”

“Moreover, excitation energies are very high and require very high temperatures or strong ultraviolet (UV) fields to excite its rotational levels (Kennicutt & Evans 2012).”

“One of the most studied correlations is the powerlaw relationship between the gas surface density, Σgas, and the SFR surface density, ΣSFR (Schmidt 1959; Kennicutt 1998; see Kennicutt & Evans 2012 for a recent review).”

“NGC 6946 has long been thought to be a Milky Way counterpart (e.g., Kennicutt & Evans 2012).”

“The SFR (or SFR surface density) is observed to correlate with MH2 (or MH2 surface density) with a power-law index of N≈1–2 (Kennicutt & Evans 2012).”

“Corresponding authors: H.E.S.S. Collaboration, e-mail: contact.hess@hess-experiment.eu † Deceased. starburst phase typically lasts for a few hundred million years (see e.g. Kennicutt & Evans 2012; Krumholz 2014, and references therein).”

“The total IR luminosity (LIR) is a good tracer of the SF in dusty environments such as the nuclei of ULIRGs (e.g., Kennicutt & Evans 2012).”

“We calculated the FIR star formation rate using the calibration from Kennicutt & Evans (2012), correcting to a Chabrier (2003) IMF.”

“While the uncertainties for the SFRs are inhomogeneously de- 19 We assume a CO-to-H2 conversion factor of αCO = 0.8 M (K km s−1 pc2)−1 and determine the SFRs from LFIR using the equations from Kennicutt & Evans (2012) (note that we do not correct to the 3-1100µm LTIR used there).”

“This offset is due to an age difference: the gas is tracing star formation yet to begin up to ∼ 1 Myr, while the infrared emission traces older (0-100 Myr, with a peak at 5 Myr; Kennicutt & Evans 2012) star formation.”

“Stars form exclusively in molecular clouds, although the question of whether molecular gas is a prerequisite or a byproduct of star formation is currently under debate (e.g., Glover & Clark 2012; Kennicutt & Evans 2012; Krumholz 2012).”

“This latter takes into account only core-collapse SNe, A70, page 13 of 27 as they can be directly related to the recent star formation traced by FUV emission from massive stars younger than 100 Myr (e.g. Kennicutt & Evans 2012).”

“The only SN Ibn with a direct progenitor detection (albeit in outburst) was SN 2006jc; Tominaga et al. (2008) estimated that this SN came from a MZAMS = 40 M star, which would have a lifetime of ≈5 Myr.”

“Moreover, if there were no gas replenishment from the outside, the observed rate of star formation is so high that most star-forming galaxies in the present-day Universe would soon run out of gas and cease to form stars (Kennicutt & Evans 2012).”

“The resulting LIR–based star formation rates (Kennicutt & Evans 2012) are 125±14 M yr−1 (J2100–”

“A star formation rate (SFR) is then estimated by multiplying the Hα luminosity by 5.3 × 10−42M yr−1 (erg s−1)−1 as in Murphy et al. (2011); Hao et al. (2011); Kennicutt & Evans (2012).”

“For a detailed discussion of these SFR indicators and their calibration, we refer the reader to Kennicutt et al. (2009), Kennicutt & Evans (2012), and references therein.”

“The relatively small Hα and UV half–light radii, ∼100 pc and ∼160 pc, respectively (Calzetti et al. 2004), imply a high star formation rate density, ΣSFR ∼3.5 M yr−1 kpc−2, confirming the star bursting nature of the galaxy (Kennicutt & Evans 2012).”

“Other common calibrations, which assume a Kroupa IMF and different stellar evolutionary models, would give SFRs that are lower by a factor of ∼0.68 (see Kennicutt & Evans 2012).”

“Extensive reviews can be found in K98b and Kennicutt & Evans (2012).”

“An additional complication is that the slope varies across the range of surface densities probed, with a steeper slope (N ∼ 2) at low (Σ 10 M pc−2) and high (Σ 200 M pc−2) gas surface densities (e.g. Bigiel et al. 2008; Daddi et al. 2010; Genzel et al. 2010; Kennicutt and Evans 2012), It was suggested by Krumholz et a…”

“Dust-corrected ultraviolet (UV) continuum emission and optical line emission ([O II] λ3727, Hα) have been used frequently to probe star formation on timescales of a few to ∼200Myr (e.g., Gallagher et al. 1989; Kennicutt 1998; Hao et al. 2011; Kennicutt & Evans 2012).”

“For models with constant star formation histories, we compute the range of expected UV-to-Hα ratios at an age of 100Myr (the generic assumption in SFR conversions, e.g., Kennicutt & Evans (2012)).”

“For a comprehensive review of the most recent SF relation studies on sub-kpc scales, we refer to Kennicutt & Evans (2012).”

“We compared the SED-derived SFRs for objects in our sample to SFRs estimated from dustcorrected Hβ emission measured in the grism data (assuming 6 Case-B recombination and the calibration of Kennicutt & Evans 2012).”

“The Hβ SFR is found by following the Kennicutt & Evans (2012) SFR relation for Hα and Hα/Hβ = 2.86 (assuming Case B recombination, T = 104 K, and ne = 104 cm−3; Osterbrock 1989): log(SFR)[M /yr] = log[L(Hβ)] − 40.82 (1) This SFR is not corrected for dust attenuation, and so is likely a lower limit…”

“…molecular clouds are involved in a number of physical processes occurring in the Milky Way, such as star formation (e.g., McKee & Ostriker 2007; Kennicutt & Evans 2012) and large-scale Galactic structure development (e.g., Dame et al. 1986; RomanDuval et al. 2010), many key mechanisms of…”

“We adopt as a prototype the Milky Way Global Model (MWGB) developed by Strong et al. (2010) where the HE gamma–ray flux has a spectral slope ∼ 2.7.”

“For this reason, combinations of Hα (or UV) and IR measurements have been extensively explored to calibrate the dust absorption (as well as dust attenuation of recombination emission lines) and are widely adopted in Galactic and extragalactic studies (see Kennicutt & Evans 2012 for review).”

“In the present paper we adopt the flat cosmology indicated by the Planck Collaboration XIII (2016) data, with round parameters: Hubble constant H0 = 100 h km s −1 Mpc−1 for h = 0.67, matter density ΩM = 0.32, baryon density Ωb = 0.05, and mass variance σ8 = 0.83 on a scale of 8 h−1 Mpc. Galaxy stellar masses and lumin…”

“It is interesting to note that the rms scatter around the bestfitting power law is ±0.4dex, This is similar to the rms observed (±0.3dex) in the SK-relation of integrated ‘normal’ spiral galaxies (Kennicutt & Evans 2012).”

“We also note the possibility of missing nebu-lar emission coming from optically-thick dust-enshrouded star-forming regions (Kennicutt & Evans 2012).”

“A correlation between star formation and gas surface density has been well-studied on galaxy scales (see Kennicutt & Evans, 2012, for a review), and lately extended to individual local clouds using star counts and the masses or surface densities of the host cloud (e.g., Lada et al., 2010; Heiderman…”

“Following Kennicutt & Evans (2012), these luminosities translate into UV– based SFRs of 11, 13, and 25 M yr −1, respectively.”

“We note that large systematics may affect these determinations, due to uncertainties in the density and geometry of the outflows and because they only refer to a specific phase of the ISM.”

“For a fully sampled IMF, in turn, the ionising photon production rate is proportional to the rate of recent star formation (e.g. Kennicutt 1998; Kennicutt & Evans 2012).”

“For HMXBs, their luminosity function scales with the star formation rate (Mineo et al. 2012), which itself depends on the Hα luminosity, SFR ≈ 10−41.27× LHα (erg s −1) M yr −1(Kennicutt & Evans 2012).”

“5 0.1 10 4 ´ -( ) ☉ yr−1 (Kennicutt & Evans 2012).”

“Numerous studies have shown that in metal-rich SFGs, polycyclic aromatic hydrocarbon (PAH) luminosity scales relatively well with SFR (Calzetti 2011; Kennicutt & Evans 2012 (KE12) and references within).”

“…to which galaxies adhere: star formation rate versus stellar mass or gas density (Daddi et al. 2007; Elbaz et al. 2007; Noeske et al. 2007; Kennicutt & Evans 2012), rotational velocity versus luminosity or baryonic mass for disks (Courteau et al. 2007; McGaugh 2012; Lelli et al. 2016),…”

“Kennicutt & Evans (2012) reviewed star formation in the Milky Way and nearby galaxies and described that there might be a breakdown of SFR laws for small size regions.”

“Thus the inner parts of galaxies will become gas-depleted and cease star formation unless fresh gas is supplied at an equal rate, which for L∗ galaxies is a few M yr −1 (Kennicutt & Evans 2012).”

“Uncertainties in this parameter are substantial (compare also Kennicutt & Evans 2012).”

“Compared with Hα, FUV estimates of star formation are more likely to provide correct estimates of the SFR as they rely on both O and B stars instead of only those with M & 15 M (Lee et al. 2011; Kennicutt & Evans 2012).”

“…21 cm observations for NGC 1365, so we assume it to have a uniform neutral gas surface density of 8Me pc −2 everywhere, based on the observed flatness of the atomic gas distribution in the disks of galaxies (e.g., Schruba et al. 2011; Bigiel & Blitz 2012; Kennicutt & Evans 2012; Wong et al. 2013).”

“Based on the observed flatness of the atomic gas surface density over the inner parts of galaxy disks (e.g., Schruba et al. 2011; Bigiel & Blitz 2012; Kennicutt & Evans 2012; Wong et al. 2013), we assume that the atomic gas has a flat distribution with ΣHI= 8Me pc −2 (including helium).”

“Before dust absorption and other attenuation mechanisms, the Lyα line is 8.7 times stronger (for case B recombination) than Hα, a frequently chosen emission-line tracer of star formation activity(Kennicutt & Evans 2012).”

“Although this is quite high compared to nearby galaxies (see Kennicutt & Evans (2012) and references therein), it falls in the range of Mach numbers associated with starburst galaxies (Gao & Solomon 2004; Bouché et al. 2007; Cresci et al. 2009; Förster Schreiber et al. 2009; Tacconi et al. 2010).”