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.”

“(12) in Kennicutt & Evans (2012).”

“After converting the SFRs into radio emission using the Kennicutt & Evans (2012) relation, we find 94% (139/148) of the sample have R −5 (full range: −6.6 R −4.8).”

“…did not take into account data at far UV (FUV) and mid infrared wavelengths (MIR), which are crucial for the calculation of physical parameters such as stellar mass and SFR and which depend critically on the properties and distribution of the dust (Bell et al. 2003; Kennicutt & Evans 2012).”

“An empirical study by Leroy et al. (2008) found a tight relation between the surface density of star formation and the surface density of molecular clouds (Kennicutt & Evans 2012).”

“We use the SFR scaling factor from Hao et al. (2011)/Murphy et al. (2011) (as compiled in Kennicutt & Evans 2012) scaled to a Kroupa (2001) initial mass function.”

“Our updated SFR measurements are generally around a factor of two lower (see Kennicutt & Evans 2012, their Table 1).”

“Although little is known about the small scale details of the SF process, such as the collapse and onset of GMC fragmentation or the impact of turbulent and magnetic pressure on the gas clouds, the larger scale galactic effects are well studied and understood (Kennicutt & Evans 2012).”

“Following Kennicutt & Evans (2012) again, SFRIR/M yr−1 = 1.49 × 10−10 LIR/L (2) (Murphy et al. 2011).”

“8Formally, tcr ≡ / , where = 4.1 × 1017 erg g−1 ≈ 6.7 × 109 L (M yr−1)−1 is the light-to-mass ratio for a stellar population in statistical equilibrium between star formation and stellar death (Kennicutt & Evans 2012).”

“If we assume S FR = 2000 − 3000 M⊙ yr−1 , we can estimate the expected 2-10 keV luminosity using the Ranalli et al. (2003) rela- Article number, page 8 of 11 tion (updated by Kennicutt & Evans 2012) and obtain LS F2−10 ≈ 1.2×1043 erg s−1 , which is ∼10% of the estimated scattered flux (Table 1).”

“In short, stars form in dense molecular gas according to a volumetric Schmidt (1959) relation, with an imposed star formation efficiency per free fall time of ∗ = 0.02, as motivated by observations (Kennicutt 1998; Kennicutt & Evans 2012; Narayanan et al. 2008, 2012; Hopkins et al. 2013).”

“It is true that the concept of SFR at small spatial scales may not be valid for the following main reasons (Kennicutt & Evans 2012): incomplete sampling of the Initial Mass Function for SFR below ∼0.01 M yr−1, the breakdown of the assumption of continuous star formation implicit in the SFR…”

“…of comet- or planet-sized solid H2 bodies may help to explain the observed low efficiency of star formation (McKee & Ostriker 2007; Draine 2011; Kennicutt & Evans 2012) if a large fraction of molecular cloud cores (the low mass tail of the Salpeter law) condenses first as small bodies and then…”

“Salomé et al. (2016) have calculated the SFR in the outer filament of Cen A using three tracers; FUV and TIR (using SFR relations described by Kennicutt & Evans 2012) and FIR (using SFR relations described by Kennicutt 1998b).”

“Böker et al. (2001) reports the detection of very deep Calcium triplet (CaT) in absorption based on their optical échlette spectra of the nucleus of NGC 4449 and hence predicts a younger cluster age between 5–20 Myr, which is in relatively good agreement with our results of the main emission region whose average age…”

“2+625911 estimated from infrared luminosity (Kennicutt & Evans 2012), 1.4 GHz radio luminosity (Murphy et al. 2011), and Hα+24 μm luminosities are 240 Me yr −1, 387 Me yr −1 and 128 Me yr −1, respectively.”

“Most assume a fully populated IMF and/or a Solar metallicity (Kennicutt 1998a; Salim et al. 2007; Hao et al. 2011; Kennicutt & Evans 2012).”

“The majority of the optical spectra are taken from Moustakas & Kennicutt (2006), Moustakas et al. (2010), Gavazzi et al. (2004), and Kennicutt (1992), while the mid-IR spectra are from Spitzer Space Telescope’s Infrared Spectrograph (IRS; Houck et al. 2004) and the Akari infrared camera (IRC; Onaka et al. 2007).”

“The factor of 0.87 in the equation converts from the Kennicutt & Evans (2012) SFR calibration, which uses a Kroupa & Weidner (2003) IMF, to the Kennicutt (1998a) calibration and the Chabrier (2003) IMF used in § 2.1.”

“Indeed, the emission of the warm dust is directly correlated with the star formation rate, as it is heated by young stars (Kennicutt 1998; Kennicutt & Evans 2012).”

“Although both the UV continuum and Lyα are generated by the flux from hot, young stars, Lyα is excited by the far-UV emission of stars with M 15 M , whereas the UV light originates in the atmospheres of M 5 M objects (e.g., Kennicutt & Evans 2012).”

“Regarding the Schmidt–Kennicutt slope (exponent denoted by N for surface densities, ΣSFR ∝ Σ N mol), Kennicutt (1998) found a slope of 1.4±0.15 (for a recent review, see Kennicutt & Evans 2012).”

“…PdBI, and the Submillimeter Array, SMA) and to theSpitzer andHerschel space telescopes, much progress has been achieved towards und rstanding the formation and structure of prestellar dense cores, and constraining the evolutionary stages of star-forming regions (e.g., Kennicutt & Evans 2012).”

“For the 70 µm-to-SFR conversion quoted by Kennicutt & Evans (2012), the bar end has an S FR of ≈0.23 M yr−1 inside the 8′′ Gaussian beam.”

“The SFRs are estimated from the infrared (IR) luminosity following (Kennicutt & Evans 2012; Hao et al. 2011; Murphy et al. 2011 We use the WISE W4 band (22 µ m) luminosity in place of the IRAS 25 µ m luminosity.”

“Even for our fiducial result of resolved ζ, the rest-UV and Hα light profiles are limited to a surface brightness of νLν ≳ 1041.8 erg s−1 kpc−2 and LHα ≳ 1039.6 erg s−1 kpc−2, corresponding to a SFR of ≳ 10−1.7 M⊙ yr−1 kpc−2 according to the standard (Kennicutt & Evans 2012) calibration.”

“We use the relation between Hα luminosity and star-formation rate from Kennicutt & Evans (2012) to calculate the starformation rate assuming that all Hα arises from star formation (which need not be true; e.g., Yan 2018; Belfiore et al. 2022).”

“We find an obscured SFR∼ 10Me yr−1 with a large systematic uncertainty due to the unknown Tdust.”

“Compression from supernova (SN) explosions can trigger further star formation (Kennicutt & Evans 2012).”

“The IR emission is directly related to the of a source due to its nature as reprocessed light coming from UV photons of newborn stars and reemitted at longer wavelengths (Kennicutt 1998; Kennicutt & Evans 2012)13 [ ] [ ] ( )M Lyr 2.84 10 erg s .”

“…tend to have a high fraction of circumnuclear star-forming rings at more or less than 1 kpc from the core (Deo et al. 2006; Hennig et al. 2018), while it is still very difficult to reach a star formation surface density of - -M10 yr kpc1 2 (see Kormendy & Kennicutt 2004; Kennicutt & Evans 2012).”

“However, Kennicutt & Evans (2012) shows that the H2 surface density drops faster with radius than the SFR distribution derived using HII regions (Misiriotis et al. 2006; Sofue 2017).”

“Using a Chabrier (2003) initial mass function (IMF), the calibration of the far-IR star formation rate (SFR) from Kennicutt & Evans (2012) gives a relation of SFR = 1.4 × 10−10 (LIR/L ) M yr−1 (see also Murphy et al. 2011b).”

“SFRs are obtained from IR luminosities using Kennicutt & Evans (2012)’s conversion formula: SFR/M⊙yr −1 = 1.49 × 10−10LIR/L⊙.”

“In this work, we have used the calibration given by Kennicutt & Evans (2012), using values from Hao et al. (2011) and Murphy et al. (2011), integrating the greybody fit from 3-1100µm: log10(SFRTIR) = log10(LTIR) − 43.41, (4) where LTIR is in ergs s−1.”

“The physical processes occuring at the formation phase of stars, yield observational hints in different wavelength ranges that can be used as tracers of star formation (Kennicutt & Evans 2012; Calzetti 2013).”

“(8) predicts that Z will vary as SFR1/N , with 1/N ∼ 0.71 (Kennicutt & Evans 2012).”

“Then we convert the estimated Hα luminosities to SFRs using the equation from Kennicutt & Evans (2012); log(SFRHα/M yr −1) = log(LHα/erg s −1)− 41.27.”

“(15) is re-writing the expression in terms of the Star Formation Rate (SFR) estimated using the prescription by Kennicutt & Evans (2012)12, SFR 1M⊙yr−1 ≃ £Hα 1.86× 1041 erg s−1 , (20) which depends only on the total Hα flux of the region, £Hα.”

“See Kennicutt & Evans (2012) for a recent review.”

“Numerous studies have characterized the relationship between star-formation activity, gas conditions, local environment, and galaxy morphology (e.g., Kennicutt 1998; Balogh et al. 2004; Kennicutt & Evans 2012).”

“Global and kiloparsec-scale correlations between star formation (SF) activity, gas content, and galactic dynamical properties have been observed (e.g., Kennicutt & Evans 2012).”

“…at higher redshifts, the ? email: jzwart@uwc.ac.za star-formation rate (SFR) is often determined in one of the following ways (Calzetti 2012, Kennicutt & Evans 2012 and Kurczynski et al. 2012 give excellent reviews): (i) UV observations (e.g. Lilly et al. 1996, Steidel et al. 1999, Wilson…”

“Finally, we convert the Hα luminosities to starformation rates using the formula from Kennicutt & Evans (2012).”

“The starburst luminosity is therefore log10(LSB)=13.5±0.1 which translates to a star formation rate of ∼4500±900 M⊙yr−1 using the Kennicutt & Evans (2012) relation:SFR = 3.89×10−44×LSB(ergs−1) Despite the AGN dominating the infrared dust emission, the SED fitting indicatesthat this AGN cannot…”

“In dust-rich galaxies, infrared SFRs are more reliable than optical SFRs, since much of the optical emission is attenuated ( Kennicutt 1998; Calzetti et al. 2007; Kennicutt & Evans 2012; Boquien et al. 2016 ) .”

“The oxygen we breathe was generated in the cores of massive stars [1] and expelled into the interstellar medium in violent explosions [2] before mixing with the molecular cloud that formed our solar system [3].”

“Molecular gas and star formation are observed to be tightly correlated (e.g. Kennicutt 1998; Kennicutt & Evans 2012; Bolatto et al. 2013) in our Galaxy as well as in local and high-redshift galaxies, with the possible exception of star-formation in extremely metal-poor environments where stars may…”

“M L L L L log SFR yr log FUV 43.35 FUV FUV 3.89 25 m 1 1 corr corr obs ( ) ( ) ( ) ( ) ( ) ( ) (Kennicutt & Evans 2012; Hao et al. 2011).”

“The SFR scaling relation from LIR is taken from Kennicutt & Evans (2012): SFR [M⊙/yr] = 1.47× 10 −10 LIR [L⊙] (6) We calibrate this SFR with LH2O using LIR/LFIR values from the SED fit and Equation 5.”

“10 Myr) rate of star formation (e.g., Kennicutt 1998; Kennicutt & Evans 2012).”

“The total infrared luminosity (LIR) is known to be a good tracer of the SFR, with the canonical relation given by Kennicutt (1998) (updated Kennicutt & Evans 2012).”

“Moreover, while the global rate of star formation in galaxies and the positions of galaxies in the Schmidt–Kennicutt diagram (e.g., Kennicutt & Evans 2012) are likely controlled by macroscopic phenomena such as cosmic accretion, large-scale feedback, and large-scale turbulence (Sánchez Almeida et…”

“…of this gas to form a cool neutral phase, the formation of molecular clouds, the fragmentation and accretion of this molecular gas to form progressively denser structures such as clumps and cores, and the subsequent contraction of the cores to form stars and planets (Kennicutt & Evans 2012).”

“It also closely follows the Schmidt-Kennicutt relation (Kennicutt & Evans 2012; Krumholz et al. 2012).”

“From their light we extrapolate an entire initial mass function worth of stellar mass in those galaxies (Kennicutt & Evans 2012).”

“Using the luminosity measured from the SDSS spectrum, LHα = 1.4 × 10 41 erg s−1, the SFR from Kennicutt & Evans (2012) is then SFR = 5.37 × 10−42 LHα = 0.75 M yr −1.”

“Giant molecular clouds (GMCs) are believed to be the parental structure of hydrogen molecules (H2) forming stars, which have a typical mass >∼ 10 4M⊙, where M⊙ is the solar mass, and >∼ 10 parsec (pc) (e.g., Williams et al. 2000; Kennicutt & Evans 2012).”

“SFRs come from rest-frame NUV luminosities at λ ≈ 2800Å that are corrected for extinction by assuming a Calzetti law (A2800 ≈ 1.79AV): SFRNUV,cor[M⊙yr −1] = 2.59 × 10−10LNUV,cor[L⊙] (Kennicutt & Evans 2012).”

“Using the calibration by Kennicutt & Evans (2012), the corresponding star formation rate (SFR) is ' 3 × 104 M yr −1.”

“Following the star formation rate calibration in Kennicutt (1998) and Kennicutt & Evans (2012), we can estimate the SFR by using the relation SFR(LFIR)/(M /yr) = 1.46×LFIR/(1010 L ).”

“…by Kennicutt (1998), hereafter K98, ΣSFR correlates with Σgas (Schmidt 1959; Kennicutt 1998; Bigiel et al. 2008; Leroy et al. 2008; Daddi et al. 2010; Schruba et al. 2011; Renaud et al. 2012; Kennicutt & Evans 2012), which can be approximated by an empirical power law with exponent n, ΣSFR ∝ Σngas.”

“…and specifically on duration and age of the burst (see Efstathiou et al. 2000; Bressan et al. 2002); the standard value adopted here is the average for a continuous star formation over 100 Myr, the age at which 90% of emission has been contributed (see Kennicutt & Evans 2012, their Table 1).”

“A typical size scale for a region where stars are born is within “cores” sizes of 0.03 − 0.2 pc with a mean density of 104−105 cm−3 (Kennicutt & Evans 2012; Bergin & Tafalla 2007; Ward-Thompson et al. 2007).”

“In particular, it is well-known that the gas surface density is strongly correlated to star formation activity, as reflected in the Kennicutt-Schmidt relation (Schmidt 1959; Kennicutt 1998; Kennicutt et al. 2008; Walter et al. 2008; Bigiel et al. 2011; Kennicutt & Evans 2012).”

“In both figures, the KS law is plotted (magenta solid line; Kennicutt & Evans 2012), as well as the observed HI relation for the Faint Irregular Galaxies GMRT Survey (FIGGS; yellow solid line; Roychowdhury et al. 2014), and the observed (H2+HI) relation for starburst galaxies (purple solid line;…”

“…together with the generally higher molecular cloud temperatures (Pattle et al. 2015; Liseau et al. 2015), imply larger pressures which may accelerate the formation of denser cores and the rate of star formation (e.g. Kennicutt & Evans 2012) compared to the other nearby molecular cloud complexes.”

“…average distance of our galaxies, each spaxel subtends 25 pc on a side and Hα based SFR estimations have been shown to be unreliable on scales less than 100–1000 pc (Kennicutt & Evans 2012), or when the luminosity drops below 1038–1039 erg s−1 (which corresponds to an SFR of ∼0.001–0.01 Me yr −1).”

“Table 2 in Kennicutt & Evans (2012) gives the conversion between K12 and K98 calibrations.”

“Kennicutt & Evans (2012) showed that most normal disk galaxies tend to lie in a tight region within this plane, while the starbursts are spread all over, mainly covering the upper two to three decades of total SFR and ΣSFR.”

“With the measured flux of Hα line, we obtain the SFR(Hα) using the canonical Hα-SFR calibration relation (listed in Column 11 of Table 2; Hao et al. 2011; Murphy et al. 2011; Kennicutt & Evans 2012).”

“In SIDES, LIR is converted to SFR using the Kennicutt (1998) equation, while in this work we use the relationship from Kennicutt & Evans (2012), which produces SFRs roughly 24% lower than the former.”

“For example, measurements of nebular emission lines can give insights on the star formation rates within galaxies (Kennicutt 1998; Kennicutt & Evans 2012), the primary excitation mechanisms responsible for energising the ISM (Baldwin et al. 1981; Kewley et al. 2001; Steidel et al. 2014; Shapley et…”

“The library uses Girardi et al. (2000) evolutionary tracks, except for the youngest ages between 1 Myr and 3 Myr, which are based on Geneva tracks (Schaller et al. 1992; Schaerer et al. 1993a,b; Charbonnel et al. 1993).”

“We derive SFR from the intrinsic Hα luminosity following log(SFR/M yr−1)=log(LHα) − 41.27 given in Kennicutt & Evans (2012).”

“In particular, Kennicutt & Evans (2012) showed that star formation is relatively smoothly and uniformly distributed in the thin disk, which has a small typical scale height of 0.2 kpc (Mackereth et al. 2017).”

“…Gullberg et al. (2015) were corrected using themagnification estimates from Hezaveh et al. (2013) and Spilker et al. (2016), available for 17 out of the 20 sources; for the other three sources we used the median value µmedian = 7.4. dSFRs were converted into LIR following Kennicutt & Evans (2012).”