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.”

“…understanding whether the use of a specific correction or calibration can introduce important systematic biases in the derived star formation rates (Kennicutt 1998; Kennicutt & Evans 2012; Kennicutt et al. 2009; Calzetti et al. 2007, 2010; Salim et al. 2007; Lee et al. 2009; Boselli et al. 2009).”

“Both are corrected to a Chabrier IMF using the conversion factors quoted in Kennicutt & Evans (2012).”

“We adopt this choice because for nearby nonstarburst galaxies it has emerged as a widely accepted estimator of ΣSFR (see Kennicutt & Evans 2012).”

“Following Kennicutt & Evans (2012) and Hao et al. (2011), we estimate dustcorrected SFRs as log SFR = log L(FUV)corr− 43.35, where L(FUV)corr = L(FUV)obs+3.89 L(25 µm) and the luminosities are in units of erg s−1.”

“…those needed to produce ionizing photons, the UV emission has a roughly 10-fold longer timescale than other star formation tracers such as H α ( e.g., Kennicutt & Evans 2012; Calzetti 2013 ) and can probe regions of very low SFR surface density, including the extended UV disks that are still the…”

“5HI,H2 (Kennicutt & Evans 2012).”

“We note that the LIR–SFR relation of Kennicutt (1998) and Kennicutt & Evans (2012) gives SFR ≈ 15 M⊙ yr −1 for the 1011L⊙.”

“Many local environmental metrics considered here are known to follow scaling relations (e.g., see reviews by Kennicutt & Evans 2012; Sánchez et al. 2021), and the same is true for the global galaxy properties (e.g., Saintonge & Catinella 2022).”

“In analogy to observations (e.g. Kennicutt & Evans 2012; Leroy et al. 2013; Haydon et al. 2018), we only use the stars which formed in the past 10 Myr to calculate the mean SFR over this time interval.”

“…ΣGMC evolves with time and epoch, we assume that ΣGMC scales with star formation-rate density ΣSFR(RGC, τ ) via the SchmidtKennicutt law with ΣGMC ∼ Σ1/αKSSFR and adopt αKS = 1.0, the measured Schmidt-Kennicutt index for the cold molecular clouds (Bigiel et al. 2008; Kennicutt & Evans 2012).”

“…(SFRs), we use far-infrared (FIR) fluxes obtained by IRAS (Rice et al. 1988; Moshir & et al. 1990; Surace et al. 2004; Serjeant & Hatziminaoglou 2009, see also corrections to Knapp et al. 1989 in NED by Knapp 1994) in conjunction with the methods to derive SFR described in Kennicutt & Evans (2012).”

“This can be an issue for a variety of tracers, including IR and UV emission (Kennicutt & Evans 2012).”

“These relations arise from the physical processes governing star formation in the interstellar medium (ISM) in galaxies near and far (see review by Kennicutt & Evans 2012).”

“In particular, the P − SFRS relation has much less scatter than the classical Kennicutt–Schmidt relationship (Kennicutt & Evans 2012) between gas surface density Σ and SFRS for the atomicdominated regime (e.g., Bigiel et al. 2008, 2010).”

“It provides no physical insight into the higher density gas (n 10 cmH 4 32 > - ) that is known to be found at the formation sites of individual stars (Kennicutt & Evans 2012).”

“Detailed discussion on the effect of different assumptions can be found in Kennicutt (1998) and Kennicutt & Evans (2012).”

“…the mass of the cold interstellar medium (ISM) and the internal physical conditions (temperature, density and radiation field) is therefore fundamental to our understanding of the process of star formation (see the reviews by McKee & Ostriker 2007; Kennicutt & Evans 2012; Carilli & Walter 2013).”

“L M10 erg s 1.62 SFR yr , 4 Ly SFR 42 1 1 for which we assume the Hα calibration relation (e.g., Kennicutt & Evans 2012) and the caseB recombination Lyα-to-Hα line ratio.”

“…there are two high-level mechanisms that shape the UV-FIR properties by which we trace galaxy evolution: star formation (SF, e.g. see review of Kennicutt & Evans 2012) and mergers (e.g. Bundy et al. 2004; Baugh 2006; Kartaltepe et al. 2007; Bundy et al. 2009; de Ravel et al. 2009; Jogee et…”

“We predict the cloud lifetimes for the Milky Way using the surface densities from Kennicutt & Evans (2012), the rotation curve from Bland-Hawthorn & Gerhard (2016), and the velocity dispersions from Heiles & Troland (2003), each of which vary with galac- MNRAS 000, 1–28 (2017) tocentric radius.”

“Therefore, the star formation rate (SFR) is an important factor in driving galaxy evolution (e.g., Kennicutt 1998a; McKee & Ostriker 2007; Kennicutt & Evans 2012).”

“Star formation rate is estimated by emission lines such as Hα, or ultra violet (UV) luminosity (see Kennicutt & Evans (2012) for a review). sSFR is estimated as SFR/M∗ when SFR and M∗ are both available.”

“Several studies provide empirical relations to estimate the SFR directly from the UV continuum, or from emission lines by atoms ionised by the UV emission (e.g. Kennicutt 1998; Kennicutt & Evans 2012; Madau & Dickinson 2014; Wilkins et al. 2019).”

“The implied SFRs from the dust emission, assuming the Kennicutt & Evans (2012) scaling (which uses an IMF from Kroupa & Weidner 2003), are 590+140−100 M yr −1 for component A and 220+150−70 M yr −1 for component B. Note that the total SFR fit to the aggregate SED (930+160−130 M yr −1) is lower…”

“To investigate this possibility in more detail, we considered two subgroups according to their SFR, as inferred from their LFIR (computed following Kennicutt & Evans (2012)), assuming that the bulk of the far-IR emission is associated with SF in the host galaxy: –”

“Based on the Kennicutt–Schmidt relation (Schmidt 1959; Kennicutt 1998; Kennicutt & Evans 2012) demonstrating empirically that the gas content of galaxies is one of the most essential quantities that govern star-formation activities in galaxies, better understanding the evolution of galaxies in…”

“…optical light and reemits it in the infrared (IR) (e.g. Calzetti et al. 2000; Buat et al. 2002; Takeuchi et al. 2012), a precise estimation of star formation rate (SFR) in galaxies requires correction for dust extinction (e.g. Steidel et al. 1999; Takeuchi et al. 2010; Kennicutt & Evans 2012).”

“…archaeology, i.e. measurements of stellar ages based on photometric or spectroscopic observations and stellar population synthesis models (Tinsley 1968; Gallagher et al. 1984; Sandage 1986; Kauffmann et al. 2003; Thomas et al. 2005; Kennicutt & Evans 2012; McDermid et al. 2015; Leja et al. 2017).”

“…± 0.01 −4.98± 0.24 1.77 ± 0.15 −3.28± 0.33 −2.70± 0.19 1.08± 0.11 aCorrected for stellar absorption based on a fit to the higher-order Balmer absorption series. bBased on the assumption that the observed Hα emission traces young stars and applying the scaling relation of Kennicutt & Evans (2012).”

“The electron density provides information on ISM conditions on scales much smaller than most of the measurements of gas surface density used in the context of the Kennicutt-Schmidt (KS) relation (Schmidt 1959; Kennicutt 1998; Kennicutt & Evans 2012).”

“These models have also been successful at understanding the relationship between gas density and the star formation rate (SFR), used to develop various star formation laws (Kennicutt & Evans 2012).”

“This gives an extinctioncorrected Hα flux of FHα,corr = 1.3×10−13 erg s−1 cm−2 corresponding to SFRHα = 3.9 M yr −1 (Kennicutt & Evans 2012).”

“In this paper, we investigate the dense gas mass fraction, fDG, for a large number of GMCs that reside in the inner Galaxy where most star formation takes place in the Milky Way.”

“For a constant SFR this cumulative fraction shows an almost linear rise in the FUV, as nearly equal amounts of the current FUV luminosity are contributed by stars in equal logarithmic age bins, up to∼100 Myr ages (see also Kennicutt & Evans 2012).”

“Most observations in the Milky Way and nearby galaxies reveal mass distributions in which the majority of the clouds by number are at low mass (γ < 0) but the majority of the mass is in high-mass clouds (γ > −2; e.g., Kennicutt & Evans 2012, and references therein).”

“As a result H2 plays a central role in the formation and evolution of galaxies (Kennicutt & Evans 2012).”

“…2 gives the SFR corresponding to the flux given in Column 1, using the Kennicutt (1998) SFR-luminosity relations, for the UV continuum or the Hα luminosity, respectively for BB of NB filters (using the recent recalibrations by Kennicutt & Evans 2012 detected SFRs would be ∼ 30 − 40 per cent lower.”

“The UV continuum magnitude (M1500,i) is primarily sensitive to the star-formation history of a galaxy over the last 10 - 100 Myr (Kennicutt & Evans 2012).”

“The relation of Shi et al. (2011) is also in excellent agreement with the more recent update of the SK relation by Kennicutt & Evans (2012).”

“To overcome it, some calibrations combine different wavelengths and account for all the star formation directly observed in ultraviolet (UV)optical or reprocessed in thermal infrared (IR; e.g., Hao et al. 2011; Kennicutt & Evans 2012; Calzetti 2012, and references therein).”

“…we assume a standard ΛCDM cosmology adopting the Planck-measured parameters, with H0= 67.4 km s −1Mpc−1 (Planck Collaboration et al. 2020), and where SFRs are mentioned, we assume a Kroupa initial mass function (Kroupa & Weidner 2003) and scaling relations drawn from Kennicutt & Evans (2012).”

“…at a value of approximately ∼ 1M yr−1, consistent with typically reported values of the MW total SFR derived from observations (∼ 2-3M yr−1, e.g. Kennicutt & Evans 2012) when we take into account that our simulated disc only extends to R' 5kpc, so the total gas mass in the simulation ('…”

“Specifically, we compare the location of Hα emission, a classic tracer of recent massive star formation (e.g., reviews by Kennicutt 1998b; Kennicutt & Evans 2012) to the location of CO emission, a tracer of molecular gas (e.g., reviews by Fukui, & Kawamura 2010; Bolatto et al. 2013; Heyer, & Dame…”

“The calibration from Kennicutt & Evans (2012) would give 14% smaller values.”

“Applying a scaling relation between LTIR and the SFR found in the local universe, log SFR (Me yr −1)=log LTIR (erg s −1)−43.41 (Kennicutt & Evans 2012), we find that the SFR in J0100+2802 is 1900Me yr −1.”

“If we think of the FIR emission in star formation dominated galaxies as a proxy for star formation activity (e.g., Kennicutt & Evans 2012), then the observed tight correlation between [C II]/FIR and ΣFIR is consistent with other studies of nearby, star-forming galaxies that find a tighter relation…”

“…out of the cold, dense regions of the interstellar medium (ISM) where conditions favour the presence of molecules, the low-level rotational transitions of 12CO are commonly used to study star formation properties in galaxies (e.g. Solomon & Sage 1988; Fukui & Kawamura 2010; Kennicutt & Evans 2012).”

“In addition, the Hαfluxes measured for all galaxies (both previously known and newly discovered in the local universe) will provide a recent SFR for each galaxy (i.e., the integrated SFR over the past ∼10Myr; e.g., Kennicutt 1998; Murphy et al. 2011; Kennicutt & Evans 2012).”

“In the inner parts of disks, this slope varies between unity for the molecular gas (Wong & Blitz 2002; Leroy et al. 2008; Bigiel et al. 2008) and 1.5 or so for the total gas (Kennicutt et al. 2007; see reviews in Kennicutt & Evans 2012 and Dobbs et al. 2013).”

“In our fiducial model, the SFE turns out to be ǫ⋆ = 10 −10/yr, similar to the present day SFE of the Milky Way (e.g., the review by Kennicutt & Evans 2012).”

“Simulations indicate that ULXs with neutron-star accretors and red giant or Hertzsprung-gap donors can appear several hundreds of Myrs after a star-formation episode (Wiktorowicz et al. 2017), and therefore their frequency in earlytype galaxies is expected to be strongly dependent on the SFHs. Calculated specific ULX…”

“For galaxies on the star formation main sequence (see reviews by Blanton & Moustakas 2009; Renzini & Peng 2015), the molecular gas mass (Mmol) and star formation rate (SFR) are tightly correlated (see reviews by Kennicutt & Evans 2012; Krumholz 2014).”

“See the review by Kennicutt & Evans (2012) for a more detailed discussion of observational results.”

“The second explanation involves the varying timescales associated with star formation and accretion onto the SMBH. Measuring the SFR from the IR luminosity results in an average SFR over nearly 100 Myr (Kennicutt & Evans 2012), while the X-ray luminosity is more aligned with the instantaneous AGN luminosity especially…”

“However, the difference can be due to using two different methods of measuring SFRs (Hαstar vs. UV+MIR; Kennicutt & Evans 2012).”

“…of available gas (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), as well as the ratio between Σgas and the mean freefall time tff (Krumholz et al. 2012; Federrath 2013; Krumholz 2014).”

“As the most abundant molecular species in the universe, molecular hydrogen (H2) is the main constituent of giant molecular clouds, the exclusive birthplaces of stars (e.g., Kennicutt & Evans 2012).”

“Adopting the star formation rate calibration of Kennicutt & Evans (2012), we estimate an unobscured SFR of≈ 4.3 M yr−1 for G1 which is roughly six times higher than the SFR inferred from the observed UV flux in the F275W band.”

“…a strong correlation has long been found between the amount of gas in the galaxy and the rate at which stars form (Schmidt 1959; Kennicutt 1989; Kennicutt & Evans 2012), whereby the rate of star formation per unit area in the galactic disc is correlated with the (total) surface density of gas…”

“Star forming galaxies have a meanLUV/LnHα = 120 (Kennicutt & Evans 2012).”

“The Hα and UV emission are sensitive to different star-formation timescales: the UV continuum is emitted by massive (OB) stars, and probes SF variations on ∼100 Myr timescales; Hα emission requires ionization from the most massive (O) stars, and probes 5-10 Myr timescales (Kennicutt & Evans 2012).”

“The SFRs can be converted to the calibrations in Kennicutt & Evans (2012) that follow a Kroupa IMF by multiplying SFRUV by a factor of 0.63 and multiplying SFRIR by a factor of 0.86.”

“For this, we calculated the AGN bolometric luminosity (LAGN,bol) using the rest-frame 7 μm dust continuum emission (see the Appendix for details) and the star formation luminosity based on the SFR values following LSF (Le)≈10 10×SFR(Me yr −1) (e.g., Kennicutt & Evans 2012).”

“Still with the focus of comparing our simulation to observable quantities, we estimate star formation rates (SFR) using the far-UV calibration from Kennicutt & Evans (2012).”

“…on the evolution of a galaxy is dramatic, because it will shape the galaxy dynamics, stellar evolution and chemical compositions in ways that are largely dictated by the intensity and the duration of the bursting event (i.e. Dekel & Silk 1986; Mac Low & Ferrara 1999; Kennicutt & Evans 2012).”

“The K-S law between ΣSFR and total gas surface density (Σgas=ΣHI+ΣH2 ) across entire galaxies and galactic nuclei, has a typical power-law index of ∼1.4-1.6 (Kennicutt 1998a,b; Kennicutt & Evans 2012).”

“The cold gas is a key property for the understanding of the evolution of galaxies since it is the fuel of the SF (e.g. Kennicutt & Evans 2012; Krumholz et al. 2012).”

“A full review of this field is beyond the scope of this paper, and we refer to the overview by Kennicutt & Evans (2012).”

“…IR luminosities (L8−1000µm) and SFRs listed in Table 7, the latter calculated using the calibration of Hao et al. (2011), Murphy et al. (2011) and Kennicutt & Evans (2012), with a Salpeter IMF (although see Romano et al. 2017, for a cautionary tale regarding the IMF in such starbursts).”

“Indeed, well-studied starforming clouds in the Gould Belt appear to have molecular tdep much lower—perhaps by an order of magnitude—than values indicated by kiloparsec-scale extragalactic studies (Kennicutt & Evans 2012).”

“…relationship, we note that observations of galaxies over a large range in surface densities show that the SFR surface density is well-correlated with the gas surface density (Kennicutt & Evans 2012), ΣSFR ∝ ΣNg , when the star formation rate and gas surface densities measured over ∼ 1 kpc scales.”

“The closest giant molecular cloud systems are of course in our own Milky Way, but it is not possible to probe the entire Galaxy.”

“Observational work by a large number of authors, summarised in the comprehensive reviews of Kennicutt (1998) and Kennicutt & Evans (2012) has demonstrated convincingly that on large scales (∼ kpc or larger) there is a clear power-law relationship between the star formation rate surface density,…”

“7 by Kennicutt & Evans (2012).”

“Almost all of them have a fundamentally empirical calibration (e.g., see Murphy et al. 2011; Leroy et al. 2012; Kennicutt & Evans 2012; Calzetti 2013).”

“As discussed in that paper, these agree well with previous calibrations using similar bands (e.g., Salim et al. 2007; Kennicutt & Evans 2012; Leroy et al. 2012; Janowiecki et al. 2017), usually within ∼0.1 dex.”

“The leaky galaxies are more extraordinary in this regard when compared to typical low-z star-forming galaxies, which have an SFR/area of only ∼10−2Me yr −1 kpc−2 (Kennicutt & Evans 2012).”

“The conversion factor is from Kennicutt & Evans (2012) and has an uncertainty of 0.3 dex.”

“If we assume the LIR − SFRIR relation presented in Kennicutt & Evans (2012), the infrared luminosity translates into an upper limit on the dust obscured SFRIR 4 M yr −1.”

“…Hα and 8µm star formation rates is likely to be driven by a combination of the difference in the timescales probed by the 8µm and Hα emission (see e.g. Kennicutt & Evans 2012), inaccuracies in accounting for dust extinction, and the 0.19 dex scatter in the L8µm-LHα relation (Roussel et al. 2001).”

“There the global efficiency of star formation ΣSFR/Σgas is relatively low and uncorrelated with Σgas (Bigiel 2008; Kennicutt & Evans 2012).”

“In both cases, most of the X-ray obscuration probably arises in very dense gas surrounding the AGN accretion disk, since the typical gas mass surface densities of galaxy disks generally cannot account for the high values of NH (e.g., Kennicutt & Evans 2012).”

“On the other hand, the widely adopted calibration used for local and low-redshift galaxies 34 predicts a higher SFR of 22 ± 5 M ⊙ yr −1 , mainly due to the absence of these low-metallicity and binary stars.”

“…the global star formation rates (SFRs) of the dwarf galaxies via their far-ultraviolet (FUV; 1528Å) and midinfrared (MIR; 25 μm) luminosities (Kennicutt & Evans 2012; Hao et al. 2011) using the following equations: ( [ ]) ( ( ) ( ) ( ) ( ) ( ) M L L L L log SFR yr log FUV 43.35 FUV FUV 3.89…”

“The bolometric, thermal IR luminosity (LTIR) is one of the most reliable tracers of dust-obscured star formation (Kennicutt 1998; Pérez-González et al. 2006; Kennicutt et al. 2009; Kenni- cutt & Evans 2012).”

“…ejecta over the stellar IMF, and where the star formation rate (SFR) surface density, Σsfr(t), is given by the Schmidt-Kennicutt relation (Kennicutt & Evans 2012): Σsfr(t) = Σsfr(t0) [ Σg(t) Σg(t0) ]k , (2) where Σsfr(t0) Σg(t0) are, respectively, the SFR and gas mass surface densities…”

“Taking a disk scale length of rs ≈ 8 kpc, which is typical of nearby elliptical/S0 galaxies (see e.g., de Jong et al. 2004), we estimate that the disk component, if present, would have added < 35% of the total light. limit in the F275W bandpass and the SFR calibrator from Kennicutt & Evans (2012).”