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.”

“We therefore “observe” our model SED and apply several recipes from Kennicutt & Evans (2012) to convert flux to a SFR.”

“& 0.3 in starbust galaxies (e.g., Bigiel et al. 2008; Kennicutt & Evans 2012).”

“Various SFR indicators of galaxies have been used in the literature over the years: for recent reviews, see Kennicutt & Evans (2012) and Calzetti (2013).”

“Because LIR is proportional to the star formation rate (e.g. Kennicutt & Evans 2012), the ratio between LIR and Mgas is thus a good representative of SFE as defined by SFE ≡ SFR/Mgas, (8) representing the SFR per unit molecular gas mass.”

“…star formation in galaxies less sensitive to the ambiguity in extinction corrections that plague far-ultraviolet measures, and free from UV-to-IR light reprocessing assumptions (e.g., contributions from “warm” versus “cool” components) inherent in far-infrared measures (cf. Kennicutt & Evans 2012).”

“Characterizing the physical properties of the molecular gas phase in galaxies, the key phase for star formation (review by Kennicutt & Evans 2012), is among the main drivers in the studies of nearby galaxies.”

“(4) Note that Kennicutt & Evans (2012) have recently provided a little lower conversion factor between the SFR and the observed Hα or FUV flux, what is mainly the result of a different initial mass function and updated stellar population models.”

“…2.95±0.05 0.16±0.04 0.43±0.01 - +7.72 0.08 0.07 Notes. a Rest-frame. b Extinction-corrected [O III]λ5007/[O II]λ3727 ratio. c Extinction-corrected [O I]λ6300/Hβ ratio. d Corrected for Milky Way extinction. e Derived from Hα following Kennicutt & Evans (2012). f Derived using the direct…”

“The total infrared luminosity can be obtained by integrating the dust spectral energy distribution between the rest-frame wavelengths of 3 and 1100µm (Kennicutt & Evans 2012).”

“…Observatory, Spitzer, and Herschel (Kessler et al. 2003; Werner et al. 2004; Pilbratt et al. 2010), mapped emission from star formation and dust with increasing resolution and coverage across the mid- and far-infrared (e.g., Soifer et al. 1987, 2008; Kennicutt & Evans 2012, and references therein).”

“The solar metallicity (Z∗ = 0.02) conversion is only different by 0.05 dex from the conversion factors of Hao et al. (2011) and Kennicutt & Evans (2012), once corrected for IMF differences (see Table 2 of Theios et al. 2019).”

“The SFR predictions from the SAM have been averaged over 100 Myr for an appropriate comparison.”

“…luminosities for a given SFR. Indirect SFR tracers, such as Hα and IR luminosities, can also be biased by emission from diffuse gas and dust, which may be located far from actual regions of star formation; these can affect measurements on the scale of hundreds of parsecs (Kennicutt & Evans 2012).”

“The radio continuum emission of SFGs is tightly correlated with the star formation rate (SFR; e.g. Kennicutt & Evans 2012, and references therein); hence the redshift-dependent radio luminosity function of SFGs can be derived from the evolving SFR function.”

“After correcting the NUV luminosity by this, we converted the NUV luminosity to SFR using the Kennicutt & Evans (2012) calibration: SFRUV,corr [M yr −1] = 2.59 × 10−10LNUV,corr [L ], (1) where the calibration constant assumes a Kroupa (2001) IMF (essentially identical to the Chabrier IMF used by…”

“…18, Mrk 209, NGC 4861, UM 461, VII Zw 403), hence appear either more gas-rich, with part of their H i gas probably less connected to the star formation activity, or more quiescent than expected, where the TIR luminosity might slightly underestimate the star formation rate (Kennicutt & Evans 2012).”

“The most comprehensive measurements of the Schmidt law are those of Kennicutt (1998 and references therein, see also Kennicutt & Evans 2012) who compiled galaxy-averaged measurements of the SFR and total gas (i.e., atomic & molecular) surface densities for a large sample of star forming galaxies…”

“Furthermore, the summation of star formation over these regions would gradually decrease as internal pressure decreases and more individual regions slowly end star formation over time (McKee & Ostriker 2007; Kennicutt & Evans 2012).”

“In particular, rest-frame optical lines can reveal the star formation rate (Kennicutt & Evans 2012), nebular dust attenuation (Buat et al. 2002; Groves et al. 2012), active galactic nucleus (AGN) content (Baldwin et al. 1981; Veilleux & Osterbrock 1987), and the metallicity (Lequeux et al. 1979;…”

“Converting to SFR using the (Kennicutt & Evans 2012) conversion, which does not implicitly assume a SFH, results in a larger estimated SFR= 540M /yr, but consistent within the 1σ uncertainties from SED fitting.”

“The resulting time-scales after feedback is included agree well with observational estimates (Sancisi et al. 2008; Bigiel et al. 2011; Kennicutt & Evans 2012; Leroy et al. 2013; Putman 2017).”

“Kennicutt 1998; Daddi et al. 2010; Kennicutt & Evans 2012).”

“Subsequent large-scale analyses have found indices from 1.0-1.4 for the molecular gas and higher values, 1.4-3.1, for the total gas surface density (Kennicutt & Evans 2012).”

“UV continuum arises from hot, massive (M∗ > 3M ) O and B stars, and as such is a good tracer of more recent SF in galaxies (e.g. Kennicutt & Evans 2012), with luminosity-weighted mean ages for a constant SFR predicted to be ∼28 Myr for the GALEX FUV band, and ∼80 Myr for GALEX NUV (Grootes et al.…”

“100 Myr, (Kennicutt 1998; Kennicutt & Evans 2012; Madau & Dickinson 2014). averaged over shorter timescales of ∼ 107 years (Hopkins et al. 2014; Sparre et al. 2015; Domı́nguez et al. 2015).”

“Star formation indicators are similarly subject to a number of caveats, including uncertainties in calibrations, the types of populations used as tracers, and the dust corrections (e.g., Brinchmann et al. 2004; Salim et al. 2007, 2009; Lee et al. 2009, 2011; Kennicutt & Evans 2012; Calzetti 2013).”

“Therefore, the presence of neutral gas byitself does not guarantee star formation (see Kennicutt & Evans 2012 for a recent review).”

“To this end we used the expression (cf. Kennicutt & Evans 2012) log Ṁ = log L(Hβ) − 40.81, (19) where Ṁ is the star-formation rate in M yr−1 and L(Hβ) is the Hβ luminosity in erg s−1.”

“10 three of the SFR indicators listed in table 1 of Kennicutt & Evans (2012), calculated for mock observations of our EAGLE galaxies, to the intrinsic SFR provided in the public EAGLE data base (McAlpine et al. 2016).”

“We note that this calibration produces SFR estimates ∼ 10% lower than the recent calibration of Kennicutt & Evans (2012) and ∼ 10% higher than the calibration of Madau, Pozzetti & Dickinson (1998), when both are corrected to a Chabrier IMF.”

“Further aspects of multiwavelength methods to determine star formation rates, including updated calibrations, are covered in the recent review by Kennicutt & Evans (2012).”

“The main contributor to the 0.24 dex scatter may be the different evolutionary stages of the molecular clouds within galaxies and their dynamical environment (e.g. Lada et al. 2010; Lombardi et al. 2010; Onodera et al. 2010; Schruba et al. 2010; Murray et al. 2010; Murray 2011; Zamora-Avilés et al. 2012; Zamora-Avilés…”

“One of the major challenges in studies of the main sequence is to measure the SFRs of galaxies with sufficient accuracy over a wide dynamic range in stellar mass and redshift (see Kennicutt & Evans 2012 and Madau & Dickinson 2014 for recent reviews of SFR estimators).”

“…tracer for molecular hydrogen (H2) and star-forming regions in both our Milky Way and other galaxies (e.g., Bolatto et al. 2008, 2013; Leroy et al. 2008; Roman-Duval et al. 2010; Saintonge et al. 2011a; Kennicutt & Evans 2012; Tacconi et al. 2013; Dobbs et al. 2014; Heyer & Dame 2015).”

“TO–FUV RATIO SFRHβ andSFR 1500Å are calculated3 through the formula of Kennicutt & Evans (2012)4 with an intrinsic Hα/Hβ ratio of 2.86 (added as the last term in the right-hand side of Equation (1) below): log(SFRHβ) = log(LHβ)− 41.27 + log(2.86) (1) log(SFR 1500Å ) = log(L 1500Å )− 43.35, (2) 2…”

“This fit has the form: log10 ΣSFR Σmol = −2.07 + 0.93 log10 Σdense Σmol , (19) where ΣSFR/Σmol has units of Myr −1.”

“Orr et al. (2018) demonstrate that their treatment of H I and H2 gas yields reasonable agreement with the observed Kennicutt Law (Kennicutt 1998; Kennicutt & Evans 2012).”

“Over the past decades, star formation within molecular clouds has been studied in extensive detail in the solar neighbourhood, the Galactic disc, and the Magellanic Clouds (see e.g. Kennicutt & Evans 2012 for a ? kruijssen@uni-heidelberg.de review).”

“…models (Eldridge et al. 2017) used in this paper, the Hα luminosity per unit solar mass of stars formed per year is larger by a factor of ∼ 2 (once differences in IMF have been accounted for) compared to the canonical conversion often used in the literature (Kennicutt 1998; Kennicutt & Evans 2012).”

“7 from Kennicutt & Evans 2012) and ΩMW ' 0.029 Myr−1 (Section 6.4.2 in Bland-Hawthorn & Gerhard 2016).”

“…USA 4 Fermilab Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, IL 60510-0500 USA ? semenov@uchicago.edu duce the observed relation between gas surface density and star formation rate (Schmidt 1959, 1963; Kennicutt 1998; Bigiel et al. 2008; Kennicutt & Evans 2012).”

“…ow nloaded from https://academ ic.oup.com /m nras/article-abstract/454/2/2067/2892482 by C ardiff U niversity user on 19 Septem ber 2019 from molecular gas, having masses ≥104 M , and sizes ∼20– 50 pc, whose properties are noticeably different from the ambient medium (Kennicutt & Evans 2012).”

“Dust in the galaxy is heated by UV photons from recently formed OB stars, which then re-emit in the mid-to-far-infrared regime (Draine 2003) creating a strong correlation between the FIR luminosity and star formation rate (SFR) of a galaxy (Kennicutt 1998; Kennicutt & Evans 2012).”

“From Kennicutt & Evans (2012), for a stellar population that fully samples the initial mass function (IMF) and the stellar age distribution, the bolometric luminosity Lbol,IMF is related to the Hα luminosity LHα by the expression Lbol,IMF = 138LHα.”

“…by guest on 27 July 2018 observed to be extremely slow, in the sense that molecular clouds undergoing free-fall collapse should have star formation rates ∼100 times higher than the observed rate (e.g. Zuckerman & Evans 1974; Krumholz & Tan 2007; Evans et al. 2009; Kennicutt & Evans 2012).”

“Stars and star clusters form due to the gravitational collapse of (parts of) GMCs (Kennicutt and Evans 2012; Krumholz et al. 2019).”

“2009; Kennicutt and Evans 2012).”

“…origin of the cosmic star formation history (i.e., the rate of star formation taking place per unit comoving volume), we need to quantify the mass of cold, dense gas in galaxies as a function of cosmic time, because this †rp462@cornell.edu gas phase controls star formation (Kennicutt & Evans 2012).”

“We have estimated the star-formation rate in the disk from the intrinsic far-infrared luminosity, which is the method suggested by Kennicutt & Evans (2012) for estimating the star-formation rate in a highly obscured galaxy.”

“…et al. 2005) and WISE 22µm observations using the scaling relation from Buat et al. (2011): SFR = SFRUV + 0.83SFRIR, (2) where SFRUV is calculated using the scaling relation from the GALEX UV flux (Kennicutt & Evans 2012) and SFRIR is calculated from the WISE 22 µm luminosity (Jarrett et al. 2011).”

“In the lower-SFR regimes, corrections for dust extinction are generally low or moderate, and SED-based SFRs are generally reliable (e.g., Wuyts et al. 2011; Kennicutt & Evans 2012).”

“This is comparable to the typical SFR surface density seen in local starbursts (excluding the high end tail) and Giant Molecular Clouds in the Milky Way, but much higher than that seen in local blue compact dwarfs (see Figure 9 in Kennicutt & Evans 2012).”

“This arises from complexity in the physical processes of the system which produce scatter in the data around the simplified models used to describe SFR as functions of luminosity in bands such as UV or IR (Kennicutt & Evans 2012).”

“In the present work, we determine αn at a smaller beam size of ∼0.5 kpc (the scale of the giant SF regions in NGC6946 Kennicutt & Evans 2012) and without any assumption on the ISM density and/or cooling mechanism of CREs.”

“Observations have revealed strong power-law correlations between the star formation rate ΣSFR and gas surface densities in disk galaxies, now referred to as the Schmidt or Kennicutt-Schmidt (hereafter KS) law (Schmidt 1959; Kennicutt 1989, 1998; Kennicutt & Evans 2012).”

“We verified that the densities that are reached when the gas is forming stars are realistic and that SFR follow a Kennicutt–Schmidt law SFR ∝ ngas with n = 1.4 (see Kennicutt & Evans 2012 for a recent review) and lie within observational data (Bigiel et al. 2008) Fig.”

“We are considering the following standard SFR tracers (see Kennicutt 1998; Kennicutt & Evans 2012; Davies et al. 2016): • Hα: Hα photons arise from the gas ionized by young MNRAS 000, 1–24 (2019) (< 20 Myr) stars.”

“A commonly used approach to measure the SFR of galaxies accounts for the ultraviolet emission from short-lived massive stars (Kennicutt & Evans 2012).”

“As bright Hii regions make up 40-70% of the Hα flux in galaxies (Oey et al. 2007; Kennicutt & Evans 2012), this relation would be an appropriate choice for integrated spectra of star-forming galaxies.”

“These calibrations, which update those found in Kennicutt (1998) and have been adopted by Kennicutt & Evans (2012), were calculated using Starburst99 (Leitherer et al. 1999) for a common IMF so that each diagnostic can be compared fairly.”

“Stars form in the dense, molecular phase of the interstellar medium (ISM; see, e.g., reviews in Kennicutt & Evans 2012; Carilli & Walter 2013; Dobbs et al. 2014; Combes 2018; Tacconi et al. 2020, and Hodge & da Cunha 2020).”

“The use of the UV emission provides mean SFRs over the most recent ∼100 Myr timescale (Kennicutt & Evans 2012; Calzetti 2013).”

“Using the SFR–LHα formula of Kennicutt & Evans (2012) log SFR (M⊙ yr −1) = logLHα (erg s −1)− 41.27, (3) the corresponding summed SFR is ∼ 3.9×10−3 M⊙ yr−1 assuming a typical 1 mag extinction correction for Hα.”

“The survey is designed with the following science goals: • Study the process of star-formation on galactic scales (Kennicutt & Evans 2012, and references therein), including the correlations between the SFR and the star formation efficiency (SFE) with other parameters like gas and stellar surface…”

“…star formation proceeds much slower than the freefall rate (εff= 1 or tdep? tff) (e.g., Zuckerman et al. 1974; Williams & McKee 1997; Krumholz & Tan 2007; Kennicutt & Evans 2012; Evans et al. 2014; Lee et al. 2016; Barnes et al. 2017; Leroy et al. 2017; Ochsendorf et al. 2017; Utomo et al. 2018).”

“The latter is calculated using a SFR of 1.9 M yr −1 (Chomiuk & Povich 2011) and a radius of 13.5 kpc for the star forming disk of the MW (Kennicutt & Evans 2012).”

“However, our understanding of the physical processes of molecular cloud and star formation is still limited (see reviews by McKee & Ostriker 2007; Kennicutt & Evans 2012; Tan et al. 2014).”

“Among different diagnostics, emission lines such as Hα, Hβ, and [O II] represent the most recent 0–10Myr SFR, best matching the life of LGRB progenitor stars with >30M☉ (Kennicutt & Evans 2012).”

“For the Pop IIc population, therefore, the specific SFR is rather small, and comparable to the SFR (∼ 10−10 yr−1) in the present Galaxy (e.g., the review by Kennicutt & Evans 2012).”

“The IR luminosity LIR is estimated by integrating Equation (10) over the solid angle and between 3 and 1100 μm in the rest frame (e.g., Kennicutt & Evans 2012).”

“We estimated νSN=0.43 yr −1 from the total infrared luminosity of the galaxy LIR;10 45 erg s−1 (between 8 and 1000 μm, Ackermann et al. 2012), and assuming a Kroupa initial mass function (Kroupa 2001; Kennicutt & Evans 2012), yielding Lkin SF=1.4×1043 erg s−1.”

“Additionally, the observed correlation between the star formation rate density and electron density may ensue as a result of the direct scaling between the gas surface density (Σgas) and ΣSFR (e.g. Kennicutt & Evans 2012; Kennicutt 1998).”

“The slightly sublinear fits and the larger discrepancies at low SFR can result from the fact that the radio emission is more sensitive to the upper end of the IMF (as is also true for Hα or any diagnostic requiring ionized gas), as can be seen from Table 1 of Kennicutt & Evans (2012).”

“Yet, despite its considerable importance, measuring star formation accurately remains a challenging task (see Kennicutt & Evans 2012, for a recent review).”

“…GHz, e.g., Condon 1992; Tabatabaei et al. 2017) from supernovae and their remnants, and a thermal flat-spectrum element from warm HII regions; these two physical processes render radio continuum emission a robust SFR tracer on timescales of 0-100 Myr (Murphy et al. 2011; Kennicutt & Evans 2012).”

“However, if we assume that the FIR emission is a reliable tracer of the star formation activity (e.g., Kennicutt & Evans 2012), then the robustness of this tracer is limited by the large variations observed in the [C II] to FIR ratio.”

“In Sections 5.2.1 and 5.2.3, we compare our results with extragalactic studies that use Hα and far IR indicators sensitive to the presence of massive young stars, and we thus adopt tsf = 10 Myr (see, e.g., Table 1 in Kennicutt & Evans 2012).”

“17 MPG-Fellow at MPE. relation (Schmidt 1959; Kennicutt 1998a, K98; Kennicutt & Evans 2012), 〈Σ̇star form〉 = 〈 ξΣNgas 〉 , (1) where N = 1.4 (±0.15) if Σgas = ΣHI+H2 (Kennicutt 1998a, 1998b, 2007), above a threshold of ∼10 M pc−2.”

“The star formation rate surface density implied by these obsevations is ΣSFR & 40 M /yr/kpc2 — a range reached only by the most intense blue compact dwarfs and ultraluminous infrared galaxies in the local Univeres (e.g. Kennicutt & Evans 2012).”

“The 5 red points, however, were flagged as having bad SFR fits by the MPA-JHU catalogue so instead we used the Kennicutt & Evans (2012) formalism and the Hα emission line flux to give an SFR.”

“This calibration also assumes that over timescales > 6 Myr, star formation remains constant, and no information about the previous star formation history is given (see Kennicutt & Evans 2012 and references therein).”

“For those outflows showing evidence for star formation, we can estimate the star formation rate by using the broad component of Hα as a tracer of star formation (Kennicutt & Evans 2012).”

“For the SINGS comparison sample (Kennicutt et al. 2009) we update the SFR to the calibration of Kennicutt & Evans (2012).”

“Among the first ones are the processes related to stellar evolution (e.g. the star formation activity Kennicutt 1998; Kennicutt & Evans 2012; Madau & Dickinson 2014, supernovae explosions, Burrows 2000; France et al. 2010; Marasco et al. 2015; Fielding et al. 2017), nuclear activity (accretion on a…”

“Finally, the SFRFUV,corr is calculated according to the prescription of Kennicutt & Evans (2012) by first correcting the FUV luminosity with the IR luminosity and then converting to a SFR. n = + ´ -m n - -”