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.”

“09 36 7v 1 2 formation occurs over a large range in physical scale, from pc-sized molecular clouds to large kpc-sized starforming regions in the disks and nuclei of galaxies (e.g. Kennicutt & Evans 2012).”

“This can be because of the different gravitational potential adopted, and/or the scale height of SNe; Σgas is not a major factor since these simulations generally have their ΣgasΣSFR along the Kennicutt-Schmidt relation (Kennicutt & Evans 2012).”

“A variety of indicators have been calibrated to estimate the SFR in star-forming galaxies, ranging from ultraviolet, infrared, and radio continuum emission to emission lines tracing photoionized or photodissociated regions (for a review, see Kennicutt & Evans 2012).”

“…not just for averages of local galaxies, but also for ∼ kpc-sized patches in nearby spiral galaxies (e.g. Bigiel et al. 2008; Schruba et al. 2011; Kennicutt & Evans 2012; Leroy et al. 2013), and for whole galaxies out to high redshift (e.g. Daddi et al. 2010; Genzel et al. 2010; Tacconi et al.…”

“The SFR surface density is calculated from the mass of young star cluster particles with the age of star cluster particles younger than tbin = 10 Myr (roughly equivalent to SFRs traced by Hα emissions; Kennicutt & Evans 2012).”

“Observationally, it is well known that the surface density of SFR ( ∑ SFR) strongly correlates with that of the gas (e.g., Kennicutt 1998; Wong & Blitz 2002; Bigiel et al. 2008; Leroy et al. 2008; Kennicutt & Evans 2012), which is often referred to as the Kennicutt-Schmidt (KS) relation.”

“However, due to the lacking dipole moment and the low mass of the molecule, it is hard to study H2 in emission at high redshift (z & 2), because the rotational transitions require high temperatures to be excited (Kennicutt & Evans 2012).”

“…taking the sum of the two as the total SFR: • FUV+CE01: SFRUV is obtained directly from the GALEX FUV luminosity using the calibration presented in Kennicutt & Evans (2012) and SFRIR is obtained by fitting the templates of Chary & Elbaz (2001) for star-forming galaxies to all photometric data…”

“11 of Kennicutt & Evans 2012).”

“For this purpose we adopt a conversion (Kennicutt & Evans 2012) SFR LIR = 1.5 × 10−10 M yr−1/L .”

“…from empirical fitting between Bruzual & Charlot (2003; BC03) and the spectral energy distribution (SED) fitting with the FAST code.20 The star-formation rate (SFR) is converted using the method described in Kennicutt & Evans (2012) from the Hα flux that is measured from the NB filter excess.”

“Finally, we computed the star formation rate of the source with the conversion from luminosity given by Kennicutt & Evans (2012) which uses a Kroupa (Kroupa 2001) initial mass function (IMF).”

“…average SFR of cluster-core and field galaxies from Figure 5 of Alberts et al. (2014) (assuming a linear correspondence between average SFR and molecular gas mass fraction, Kennicutt & Evans 2012); and 4) 85% of the total accumulated gas from the merging companion galaxy is available to form stars.”

“We analytically derive SFR surface densities and stellar mass surface densities, dividing SFR and M∗ by the projected area of the galaxy by assuming: ΣSFR = SFR 2 πre2 (5) ΣM∗ = M∗ 2 πre2 · (6) Gas surface density and total gas mass are then computed assuming that galaxies follow the Kennicutt-Schmidt (KS) law (Kennic…”

“Giant molecular clouds (hereafter GMCs) are massive and cold molecular gas reservoir for star formation (& 104M and 10 − 100 pc; see Williams et al. 2000; Kennicutt & Evans 2012) and are thus essential to study star formation and subsequent galaxy evolution.”

“Diverse topics such as planet, star & galaxy formation, the initial stellar mass function, the distance scale and supernovae are just some examples of areas that benefit from realistic models of star formation and evolution (Kennicutt & Evans 2012; Stassun et al. 2014).”

“The atomic to molecular hydrogen transition is a prerequisite process for star formation through the collapse of molecular clouds and therefore has important implications for the evolution of galaxies (e.g. Kennicutt & Evans 2012).”

“Major progress has been made in the measure of the SFR inside the disk of nearby galaxies with strong relation found between SFR and molecular gas content (e.g., Bigiel et al. 2008; Leroy et al. 2008; Kennicutt & Evans 2012, for a review), and Article number, page 1 of 15 ar X iv :1 50 4.”

“Furthermore, SFR measurements at 50 pc scales are problematic using indirect tracers like Hα and dust emission for many reasons, including sampling of the stellar population (others include drift of stars between pixels, star formation history, and others outlined in Kennicutt & Evans 2012).”

“In general, the largest systematic errors are dust attenuation and sensitivity to the population of the upper IMF in regions with low absolute SFRs (Kennicutt & Evans 2012).”

“This is worrisome as the UV is one of the most widely used star formation tracers (Kennicutt 1998; Kennicutt & Evans 2012) and is observable by ground-based surveys for galaxies with z & 1.”

“…estimators of galaxies’ star formation rates (SFRs, e.g., Moustakas 2006; Madau & Dickinson 2014) over short timescales (10Myr, e.g., Kennicutt 1998; Kennicutt & Evans 2012) because it tightly correlates with the production of ionizing photons by OB stars, it does not depend on the metallicity,…”

“00 02 0v 1 [ as tr o- cal time ΣgasΩ, to the star formation surface density ΣSFR, respectively, to probe the connections between local gas masses and star formation (Kennicutt, Robert C. 1989, and see Kennicutt & Evans 2012 for a recent review).”

“Star formation rates, ionized gas mass loading factors and feedback efficiency From LHα we derive star formation rates (SFRs) ac- cording to Kennicutt & Evans (2012), log(SFR) (M yr −1) = log LHα − log CHα (3) (with CHα = 41.27) and, together with R90, the star formation rate surface density, ΣSFR.”

“…all Herschel-detected starbursts with spectroscopic redshifts, SFRs are determined from the total infrared (TIR) luminosity LTIR and the calibration of Kennicutt & Evans (2012) to fully account for the obscured component that dominates the emission from starbursts (∼ 90%; Rodighiero et al. 2011).”

“…of star formation rate (SFR) and that of total gas, including atomic and molecular gas (traced by the H I 21 cm line and rotational lines of CO, respectively), is characterized by a power-law index of n≈1.4 ( nSFR gasS µ S ; see Kennicutt 1998; Kennicutt & Evans 2012, and references therein).”

“Thus, IR turns out to be a very good observational tracer for star formation (e.g. Kennicutt & Evans 2012).”

“Naturally, given their selection, the most common tracer of LBGs’ SFRs has traditionally been through their rest-frame UV stellar continuum emission (e.g. Kennicutt & Evans 2012).”

“…these methods involve significant uncertainties inherent in both the determination of the cosmic star formation rate density, and more importantly, the assumed ’star formation law’ relating CO luminosity to FIR luminosity, or to star formation rate (Kennicutt & Evans 2012; Carilli & Walter 2013).”

“We confirmed that this was not the case for the CO-derived molecular gas mass, which is also convertible to SFR (Kennicutt & Evans 2012).”

“This is because the FUV component measures star formation on a longer timescale and both the FUV and IR have contributions not related to recent star formation (Kennicutt & Evans 2012).”

“…budget is dominated by star formation (i.e. re-processed ultraviolet emission from massive stars), then the implied star formation rate of 9io9 is Ṁ⋆ ≈ 2500M⊙ yr−1 (Kennicutt & Evans (2012), assuming a Chabrier initial mass function), a thousand times the star formation rate of the Milky Way.”

“The latter, however, traces the obscured SFR, and it is virtually unaffected by foreground extinction, although it depends on the extinction level in the star-forming regions (e.g., objects with no extinction do not emit in the IR; Kennicutt & Evans 2012).”

“The conclusion is that stars form in regions rich in cold, dense, molecular material (Kennicutt 1998; Kennicutt & Evans 2012).”

“The average SFR within the PHATTER footprint within the past 100Myr is useful to compare with SFR measurements involving broadband FUV flux (see Kennicutt & Evans 2012).”

“The median ratio between SFR and LIR is found to be 10−10.1 M yr −1 L−1 for both the main samples of ALCS and ASPECS, which is only offset from the conversion factor in Kennicutt & Evans (2012) by 0.1 dex (assuming Chabrier IMF).”

“Assuming that dust heating is dominated by young stars, the infrared luminosity can be converted to a SFR using the Kennicutt (1998) and Kennicutt & Evans (2012) conversions, giving SFR= (1830 ± 595) − (2090±680)M yr−1, respectively.”

“This results in a 0.42 dex systematic uncertainty for the star formation rate values, from adding in quadrature the systematic uncertainties from the SED fitting and the 0.3 dex systematic uncertainty on the conversion between LIR and SFR (Kennicutt & Evans 2012).”

“The star-formation rates (SFRs) of these high redshift galaxies are mostly estimated from ultra-violet (UV) measurements emitted by short-lived massive stars (e.g. Kennicutt & Evans 2012).”

“The H ii region has an equivalent radius of 142 pc, a velocity dispersion of 50 km s−1, and an Hα luminosity of 6.3 × 1039 erg s−1, which corresponds to a local SFR of ∼0.034 M yr−1, adopting the calibration of Kennicutt & Evans (2012).”

“…remains work to be done in converging on conversion factors between luminosities or line widths, and SFRs and gas masses but it is unlikely that these factors randomly vary by ∼2 dex in neighbouring kpc-patches of ISM (Kennicutt & Evans 2012; Narayanan et al. 2012; Bolatto, Wolfire & Leroy 2013).”

“This gas phase is regarded as the immediate fuel from which stars form, and can be used to test the SB-QSO evolutionary sequence (e.g. Kennicutt & Evans 2012; Leroy et al. 2008).”

“Throughout the Galaxy, giant molecular clouds (GMCs) host the majority of star formation (Kennicutt & Evans 2012).”

“…limit inclination imin = 25 deg, (b) derived from C IV FWHM and the 1350 Å continuum luminosity, (c) derived from rest-frame 1450 Å magnitude and assuming a bolometric correction Lbol = 4.2 L1450 (Runnoe et al. 2012a,b) and (d) derived from LFIR (from W13) and the Kennicutt & Evans (2012) relation.”

“Paper I speculated that the universality of Σmax might owe to stellar feedback processes.1 After all, it is widelyrecognized that feedback plays an important role regulating star formation (SF) in cold, dense molecular clouds (see Kennicutt & Evans 2012, for a review).”

“If we consider that NGC 796 formed from a cloud of similar mass, it must have star formation efficiencies around ∼10%, which is much higher than that of most known star-forming regions (Kennicutt & Evans 2012), suggesting that the parent cloud must have been much larger.”

“…IMF by multiplying by The GALEX/S4G radial profiles catalog 33 bIMF = 0.67, or by bIMF = 0.63 for a Chabrier IMF, as reviewed and prescribed in Kennicutt & Evans (2012); Madau & Dickinson (2014): SFR(M yr −1) = 1.4× 10−28 · bIMF · Lν (ergs · s−1Hz−1) (B1) and with the following expression of…”

“6 (see Kennicutt & Evans 2012, for a complete review).”

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

“Furthermore, star formation in the Milky Way is not uniquely slow, lying near the Kennicutt–Schmidt relation between the SFR and gas surface densities (Figure 11 in Kennicutt & Evans 2012).”

“…the rest-frame UV light from young massive stars, or to measure the nebular emission line fluxes such as Hα (λ=6563Å) line associated with young starforming regions, applying a proper dust attenuation correction (e.g. Kennicutt 1998; Kennicutt et al. 2009; Hao et al. 2011; Kennicutt & Evans 2012).”

“(26) More generally, one can write ( SFR100 M⊙ yr −1 ) ≡ kX ( LintrX L⊙ ) , (27) where kX is the conversion from the intrinsic luminosity of band X to the average SFR over the last 100Myr (Kennicutt 1998; Kennicutt & Evans 2012; Murphy et al. 2011).”

“The latter measures the ratio of Hα flux, and hence Star Formation Rate (SFR, Kennicutt & Evans 2012), to the stellar continuum.”

“As a nascent fuel for star formation, molecular gas plays an important role in the evolution of galaxies (e.g., Kennicutt & Evans 2012).”

“(2016) FUV luminosity νLν, fuv/(erg s −1) are related through the calibration Cfuv (Kennicutt & Evans 2012), log10(SFR/M yr −1) = log10(νLν, fuv/erg s −1)− log10 Cfuv.”

“To better track total star formation, rest-frame mid-far IR observations, which indicate the amount of dust heated by young, massive stars, are essential (e.g., Kennicutt 1998; Kennicutt & Evans 2012).”

“3.2), while multiplying all star formation rate surface densities by a factor 0.63 (Kennicutt & Evans 2012).”

“In a recent study of 70 galaxies in the Herschel Spectroscopic Survey of Warm Molecular Gas in Local Luminous Infrared Galaxies, Zhao et al. (2013) found that the SFR determined from the TIR luminosity via the relationship in Kennicutt & Evans (2012) correlated with the [N] 205 µm line luminosity.”

“In particular, the rate of star formation varies by orders of magnitude within galaxies, and even more between galaxies of different types ([25]).”

“Given that the current and likely for the last many Gyrs SFR in the Milky Way is ∼ 2 M⊙ yr −1 (Chomiuk & Povich 2011; Kennicutt & Evans 2012)4, the SD SNe Ia rate in our simulation is 2.0 × 10−4yr−1 at t = 10 Gyr.”

“If these cores are the result of clump fragmentation that lead to starformation (Evans 1999; Lada & Lada 2003; Kennicutt & Evans 2012), then a relationship between the characteristic fragmentatio scale and the IMF is natural.”

“To understand the physical processes responsible for the IR activity, it is thus important to understand the physical parameters affecting the star formation rates (SFRs) of galaxies (see McKee & Ostriker 2007; Kennicutt & Evans 2012 for a review).”

“The use of UV-based SFRs derived using the conversion factor from Kennicutt & Evans (2012) does not change the distribution of our final spectroscopic sample on Figure 6, which is expected, since the Prospector -based SFRs largely rely on the rest-UV photometry.”

“…difference with the former comes from the SFH used for the fit (a delayed SFH with a peak 1 Gyr before and no recent burst with a burst fraction equal to 10−7) versus the simple assumption of a constant SFH over 100 Myr to calculate SFR(IR)+SFR(UV) (e.g., Kennicutt & Evans 2012; Buat et al. 2008).”

“We fit a standard modified blackbody to this photometry to measure LIR and translate this to an obscured SFR using standard conversions (e.g., Kennicutt & Evans 2012), yielding SFR≈400 M yr −1.”

“Deriving SFRs from observations is also non-trivial, and several indicators exist and probe different timescales for star formation (Calzetti 2013; Kennicutt & Evans 2012; Calzetti et al. 2007; Salim et al. 2007).”

“The Lyα and Hα 10 Note that we neglect dust extinction, which is typically between 0 and 1 mag at Hα (Kennicutt & Evans 2012).”

“…complex and not well understood baryonic processes, such as the formation and evolution of stars (Kennicutt 1998; McKee & Ostriker 2007; Heber 2009; Kennicutt & Evans 2012; Duchêne & Kraus 2013), the accretion and feedback of supermassive black holes (Melia & Falcke 2001; Merloni 2004; Fabian…”

“We used the conversion from Kennicutt & Evans (2012) assuming a Kroupa & Weidner (2003) IMF while Östlin et al. (2001) used their own conversion with a Salpeter (1955) IMF.”

“This is a real possibility for the solar neighborhood, since observations of nearby gas-embedded clusters tell us that low-SFE embedded clusters are more common than high-SFE ones (Lada & Lada 2003; Evans et al. 2009; Peterson et al. 2011; Kennicutt & Evans 2012; Kainulainen et al. 2014).”

“The interested reader can find in Kennicutt (1998), McKee & Ostriker (2007), Kennicutt & Evans (2012) and Bland-Hawthorn & Gerhard (2016) a large collection of critically revised estimates and galaxy modelling techniques.”

“rmation rate (SFR) in starbursts is considered to be out of equilibrium, with gasconsumption timescales of 1Gyr or shorter, and typical timescales for the starburst episode of a few 100Myrs (see e.g. [1, 2] and references therein). Cosmic rays (CRs) are considered to be an important star-formation regulator since they…”

“For the assumed shape of the intrinsic SED, we adopted our 2007 version of the Bruzual & Charlot (2003) model for a system that has been forming stars at a constant rate for 100 Myr.”

“While much progress has been made in understanding selection effects and uncertainties in SFR andM∗ measurements (Kennicutt & Evans 2012; Renzini & Peng 2015), the measurements of SFMS from different works based on multi-wavelength deep surveys surprisingly reached a good agreement when calibrated…”

“…Gas in Local Luminous Infrared Galaxies (LIRGs; PI: N. Lu), and found a relationship between the measured luminosity of [NII] 205 µm and the SFR, where the SFR in Zhao et al. (2013) was calibrated from the total infrared luminosity by using the relationship given in Kennicutt & Evans (2012).”

“Finally, we use the calibrations in Table 1 of Kennicutt & Evans (2012) to calculate the SFRs.”

“…variables to avoid confusion with other papers in the literature and rely on context to distinguish them. of -2.35, which is comparable to the “typical” slope for a cluster mass function (Kennicutt & Evans 2012), and the other with a slope of -1.59, the slope derived above for the entire sample.”

“Yet, the more massive IR-bright starbursts may have very different population properties in terms of the IMF, specific star formation intensity, and their different halo masses may affect their relation to the circumgalactic gas (Kaviraj 2009; Howell et al. 2010; Kennicutt & Evans 2012).”

“The problem of diffuse emission affecting star formation rate indicators is discussed in numerous papers, for recent discussion see Leroy et al. (2012); Kennicutt & Evans (2012).”

“In regions of high molecular gas surface densities, the star formation rates (SFRs) are higher than expected for a simple scaling of the Kennicutt-Schmidt relation in more typical conditions (e.g., Kennicutt & Evans 2012).”

“In particular, embedded high-mass stars create strong radiation fields that enhance the mid-IR emission so that the mid-IR can be used as a tracer for star formation (Calzetti et al. 2007; Kennicutt & Evans 2012).”

“…that with compact morphology, originates from dust excitation by radiation from surrounding massive stars and empirically exhibits a correlation with tracers of massive star formation (Kennicutt & Evans 2012; Galliano et al. 2018; Hassani et al. 2022; Leroy et al. 2022; Thilker et al. 2023).”