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.”

“The FUV conversion tabulated in Kennicutt & Evans (2012) (from Murphy et al. 2011), if rescaled from the Kroupa IMF to Salpeter, is very close to the z = 0, solar metallicity value of KFUV(z) in Figure 3, but our somewhat smaller value should be more representative for the peak era of cosmic star…”

“…radiation, and other microphysical processes in the turbulent, magnetized, multi-phase interstellar medium (McKee & Ostriker 2007; Ostriker 2009; Kennicutt & Evans 2012; Naab & Ostriker 2016); and the driving of galactic outflows of gas, energy, and heavy elements into the circum- and…”

“Consequently, H2 plays a central role in the evolution of galaxies and stellar systems (see the recent review by Kennicutt & Evans 2012).”

“6, respectively, which lies below the ‘knee’ of the star formation rate function of galaxies at these redshifts [213, 214].”

“…al. 2009; Conroy et al. 2009; Conroy & Gunn 2010) and improved global diagnostics of galactic star formation (Murphy et al. 2011; Hao et al. 2011; Kennicutt & Evans 2012 (KE12), and references within) have enabled the determination of key physical quantities of galaxies from these data:…”

“Since massive stars have short lifetimes, their ultraviolet (UV) radiation (reprocessed in various forms) is the most observable tracer of star formation and is used to calculate the star formation rates (Kennicutt 1998, Kennicutt & Evans 2012).”

“However, advances in our theoretical understanding of star formation as well as better observational characterization of key scaling relations (see Kennicutt & Evans 2012, for a review) have enabled the development of empirical recipes that smooth over much of the small-scale complexity.”

“This relationship has been reviewed recently for nearby galaxies by Kennicutt & Evans (2012).”

“The practice of measuring SFRs from monochromatic indicators (and combinations thereof) is well established (see e.g., the reviews of Kennicutt 1998, Kennicutt & Evans 2012).”

“In that paper we proposed that the lower than expected CO emission is the result of the more extensive UV photodissociation of CO molecules in lower metallicity environments, leading in effect in an increase of the conversion factor α by factors of >3 and >10 between solar and 0.5 and 0.25 times solar metallicity, res…”

“We refer the reader to the excellent recent review by Kennicutt and Evans (2012) for a more thorough summary of the star formation law in the Milky Way and nearby galaxies.”

“We also compare these to a pair of widely used SFRI calibrations for the same observational signatures (Kennicutt & Evans 2012; Murphy et al. 2011).”

“The star formation rate per unit area is probably also low in the outer disks (as is observed both in local and distant galaxies, e.g. Kennicutt & Evans 2012; Lehnert et al. 2013).”

“Most constraints on star formation efficiency for low mass galaxies come from studies of recent star formation (e.g., Hα, UV, 24 µm) and current gas content (e.g., Leroy et al. 2008; Salim et al. 2007; Kennicutt et al. 2008; Lee et al. 2011; Kennicutt & Evans 2012).”

“For additional background we refer the reader to the recent review by Kennicutt & Evans (2012).”

“We find that our Hα fluxes tend to be slightly higher than those predicted by Kennicutt & Evans (2012), e.g. by 0.08 to 0.19 dex for metallicities of 12 + log ( O H ) = 8.69 and 8.17 respectively at log10(U) = −3.0, which is consistent towithin the observed scatter on the relationship.”

“(5) Outflows through 1.0 Rvir are even more temporally offset from star formation, sometimes by a longer period (∼250 Myr) than common star formation diagnostics (Kennicutt & Evans 2012).”

“…of the form ΣSFR ∝ ΣNgas (albeit with significant scatter), similar in exponents N ≈ 1–2 to the KennicuttSchmidt relation (Schmidt 1959; Kennicutt 1998) and follow-up measurements for molecular gas (e.g., Wong & Blitz 2002; Gao & Solomon 2004; Bigiel et al. 2008; Kennicutt & Evans 2012).”

“%) in our Galaxy were made from gas that was added to the Galaxy, forming into stars within the system and only a very small fraction of the stellar mass comes from stars made in other galaxies that were added to our system via galactic mergers (Kennicutt & Evans 2012).”

“…and Astrophysics and elsewhere that discuss the molecular ISM in relation to star formation (McKee & Ostriker 2007, Hennebelle & Falgarone 2012, Kennicutt & Evans 2012, Krumholz 2014); interstellar turbulence (Scalo & Elmegreen 2004, Elmegreen & Scalo 2004); cold, dark clouds (Bergin & Tafalla…”

“However, this approach has since come under criticism (e.g. Ostriker & Shetty 2011, Kennicutt & Evans 2012, Krumholz, Dekel & McKee 2012) on the grounds that the resulting smaller depletion time scales then immediately introduce a bi-modal, and thus probably unphysical gas-star formation relation.”

“For this purpose one can also use the direct ratio of the galactic star formation rate SFRG ≃ 2M⊙ yr−1 [108, 353, 537] to the (dust-corrected) star formation rate in the local Universe SFRloc ≃ 0.”

“Star formation is an enormous topic with a large amount of work published (e.g., Kennicutt 1998; Kennicutt & Evans 2012; Madau & Dickinson 2014), and we thus limit our discussion to how star formation and galaxy structure are related.”

“Star formation is linked more closely to the molecular H2 gas (e.g. Kennicutt & Evans 2012; Saintonge et al. 2016; Catinella et al. 2018).”

“0 M per year, because we attribute a large fraction of the dust-corrected Hα luminosity (26) to the LINER component.”

“For comparison, we also plot SDSS galaxies with an exponential SB profile in Lackner & Gunn (2012) and the MilkyWay (Kennicutt & Evans 2012).”

“The adopted calibration means than an absolute magnitude of M1500 = −18.75 corresponds to a SFR of 1 M yr −1, consistent with the conversion given by Kennicutt & Evans (2012).”

“These tracers are all sensitive to high-mass star formation only, although the stellar mass range depends o the tracer, varying fromm & 5M⊙ for far-UV and far-IR, to m & 8M⊙ for non-thermal radio, andm & 20M⊙ for Hα (see Kennicutt & Evans 2012 for a recent review).”

“2019), and there is considerable scatter on scales below ∼100 pc (Kennicutt and Evans 2012; Kruijssen and Longmore 2014).”

“The typical range of power law indices isN = 1–2, whether Σ refers to dense or all gas, and across a range of spatial scales (Kennicutt 1998a,b; Bigiel et al. 2008; Liu et al. 2011; Ladaet al. ⋆ kruijssen@mpa-garching.mpg.de 2012; Kennicutt & Evans 2012).”

“…is not true in present-day spirals with tdep ∼ 1 Gyr,2 or even for large star-forming galaxies up to z ∼ 3 (Saintonge et al. 2013), though it 2However, Kennicutt & Evans (2012) point out that even the Milky might have been true for their progenitors at even higher redshift (Krumholz & Dekel 2012).”

“1 IN T RO D U C T I O N The gas-star formation cycle is central to the formation and evolution of galaxies (see e.g. Leroy et al. 2008; Lilly et al. 2013 and review by Kennicutt & Evans 2012).”

“Up to the present day, the most widely used SFR diagnostics have been based on continuum bands and optical/near-IR emission lines (see Kennicutt 1998; and Kennicutt & Evans 2012, for a detailed overview).”

“However, this is not observed (for an early discussion, see Zuckerman & Evans 1974; for more recent discussions, consult Kennicutt & Evans 2012).”

“Direct comparisons with SFR inferred from observations (e.g. Heiderman et al. 2010, Kennicutt and Evans, 2012) have been performed but given the relatively large scatter of the data, it seems hard to draw firm conclusions.”

“5.1 The broad-band emission from galaxies and AGNs The bulk of the emission from galaxies is produced at UV–submillimeter wavelengths (≈ 0.1–1000 µm; e.g., Kennicutt and Evans 2012) and is primarily due to the radiation produced by populations of stars as well as AGN activity, when present.”

“…a very rough estimate of the star-formation rate: We calculate the absolute luminosity in Hα, using the luminosity distance — see the plot of LHα against stellar mass in Figure 25 — and adopt log SFR = logLHα−41.27 (Kroupa IMF; from the literature compilation provided by Kennicutt & Evans 2012).”

“For the dust-based SFR estimates, we follow Kennicutt & Evans (2012): = ´ - - ( ) M L L SFR yr 1.49 10 .”

“The main estimator of star formation rate at our disposal is the extinctioncorrected Hα luminosity, which traces the instantenous star formation rate and is often used as the basis for calibrating other star formation rate indicators (Kennicutt 1998; Kennicutt & Evans 2012).”

“The HI recombination line luminosities and their ratios have been considered the “gold standard” for measuring SFRs and dust attenuation in local galaxies (Kennicutt & Evans 2012).”

“Despite its critical mportance, a fundamental physical understanding of star formation has not been achieved (McKee & Ostriker 2007; Kennicutt & Evans 2012; Krumholz 2014).”

“In order to estimate the total SFR across the CMZ, we apply several infrared luminosity-to-star formation rate (luminosity–SFR) relations to the bolometric and monochromatic infrared luminosities (see Kennicutt & Evans 2012, and references therein).”

“Following the notation above and in Kennicutt & Evans (2012), this is log10 C = −43.35 for FUV.”

“In theory, it is possible to modify the assumed SFH assumed in calculating SFRUV+IR to include more emission from old stars and reduce this bias (Kennicutt & Evans 2012).”

“…example, the Hα nebular emission line and UV continuum fluxes typically trace a galaxy’s SFR averaged over the last ' 10 and ' 200 Myr, respectively (Kennicutt & Evans 2012; 1 It is worth noting that the term ‘starburst’ is an ambiguous concept that is used in many ways in the literature; see…”

“Star formation rates used in this work are derived from the extinction corrected Hα luminosity inside the fibre, adopting the calibration proposed by Kennicutt & Evans (2012).”

“…this region therefore appears to be suppressed relative to the expectations from nearby starforming regions and nearby galaxy disks, where the star formation timescale decreases with increasing gas surface density (Kennicutt 1998; Kennicutt & Evans 2012; Leroy et al. 2013; Heiderman et al. 2010).”

“Based on the variability timescale of the most-massive range, the characteristic timescale for fluctuations in the galactic star formation for the ISM model adopted in Illustris has characteristic timescale of 500 Myr.”

“However, subsequent work has shown that it continues to hold even at high redshift (e.g. Bouché et al. 2007; Daddi et al. 2008, 2010a,b; Genzel et al. 2010; Tacconi et al. 2013), and on ∼1 kpc scales in the local Universe (Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008, 2013; Liu et al. 2011; Momose et…”

“Star-formation rates (SFR) were derived using a combination of GALEX FUV, IRAS and Herschel far-IR, and WISE and Spitzer mid-IR photometry following the prescriptions in Kennicutt & Evans (2012), as described further in A15.”

“Beyond this definition, an important advantage of the method used here is that the measured time-scales do not explicitly depend on the αCO conversion factor, which is uncertain in extragalactic environments (see e.g. Kennicutt & Evans 2012; Bolatto et al. 2013).”

“We calculate the host galaxy dust-corrected SFRs following Kennicutt & Evans (2012) and Hao et al. (2011): ( ) ( )= -Llog SFR log FUV 43.35, 3corr where ( ) ( ) ( ) ( )m= +L L LFUV FUV 3.89 25 m 4corr obs and luminosities are in units of erg s−1.”

“…includes binary evolution and an IMF extending to 300M predicts a factor of ≈ 3 larger Hα luminosity per solar mass of star formation after 100 Myr of constant star formation relative to that computed using the Kennicutt & Evans (2012) relation, which assumes a solar metallicity Starburst99 model.”

“G A ] 1 6 Fe b star-formation rate density (ρSFR; see Kennicutt & Evans 2012).”

“…tracing molecular gas in a galaxy, must be associated to a certain SFR. Assuming, for instance, that the observed line is CO(4-3) at z=1 and conservatively assuming the S-K law observed for local galaxies (Kennicutt & Evans 2012), one expects that the host galaxy should have a SFR ≈ 25M yr−1.”

“…to studying galaxy evolution, including galaxy stellar masses and star formation rates (SFRs).2 Thus, we are in a era in which robust determi- 2 Also empirically motivated diagnostics of galaxy SFR have been improved in the last few years; for a recent review, see Kennicutt & Evans (2012). c© 20??”

“In contrast, the volume-filling factor of molecular clouds in the inner Galaxy is ∼ 0.005 (RomanDuval et al. 2010; Kennicutt & Evans 2012) and is much lower in the rest of the Milky Way.”

“1 M yr−1 [41], corresponding to log10(sSFR/yr −1) = −10.”

“We use these data to roughly estimate star formation rates for these sources using standard prescriptions from Kennicutt & Evans (2012), and report the results in Table 2.”

“(see Kennicutt & Evans 2012, especially section 2.2, for a detailed discussion of H ii regions and their clumpiness).”

“2012; Kennicutt & Evans 2012).”

“…Milky Way, we are generally restricted to studies of global star formation properties integrated over entire star-forming complexes or even groups of complexes, which render it impossible to study either feedback processes or the environmental dependence of star formation (Kennicutt & Evans 2012).”

“Using the radio star formation rate calibration 59 , we infer SFR = 1 M (cid:12) yr − 1 .”

“…galaxies (e.g., Bigiel et al. 2008; Leroy et al. 2013) and somewhat shorter in galactic centers and starburst environments (e.g., Daddi et al. 2010; Leroy et al. 2015; Pereira-Santaella et al. 2016) as well as individual molecular clouds (e.g., Kennicutt & Evans 2012; Vutisalchavakul et al. 2016).”

“(2018) formation tracers span the full electromagnetic spectrum, from Xray and ultraviolet (UV) to radio wavelengths, via optical and infrared (IR) bands, and can be based on both continuum and line emissions (see the most recent review on the subject by Kennicutt & Evans 2012).”

“…important strength ofis method is that it is not hampered by the uncertain conversionfactors between gas tracer flux and gas mass that traditionally pl gue the inference of SF physics from galactic SF relations (see e.g. Kennicutt & Evans 2012), because it relies exclusively on trace flux ratios.”

“Indeed, the LIR is widely used as a proxy for the SFR (e.g., Scoville & Young 1983; Buat & Xu 1996; Kennicutt 1998; Kennicutt et al. 2009; Kennicutt & Evans 2012).”

“Our SFRs are higher than the ones found for these objects by Cardamone et al. (2009), despite our use of the Kennicutt & Evans (2012) relation, which leads to lower SFRs relative to the Kennicutt (1998) relation.”

“We then convert this to SFR using the indicator based on the total infrared luminosity of Kennicutt & Evans ( 2012 ) .”

“While there is a wide breadth of varying SFR calibrations for each observable in the literature (e.g. Kennicutt & Evans 2012), we limit ourselves to those used previously in GAMA, for the sake of clarity.”

“The details of this relation are not as important on the scale of a whole galaxy as they are for kpc scales, where observations still have some uncertainties (see the review in Kennicutt and Evans 2012).”

“Initially, the Hα line flux is calculated from the starformation rate of the stellar population template using the Kennicutt & Evans (2012) calibration, and the Hβ line flux is then calculated assuming case B recombination.”

“Finally, the dust-corrected luminosity was converted into a SFR estimate with the Kennicutt & Evans (2012) factor.”

“This Σ(SFR) is much lower than in the inner parts of the Galaxy or in many regions studied in other galaxies, but it is close to Electronic address: nje@astro.as.utexas.edu the average value over the inner 13.5 kpc of the Galaxy (Kennicutt and Evans 2012).”

“To convert Hα luminosity into SFR we use the relation by Kennicutt & Evans (2012).”

“In recent years it has become evident that theXCO conversion factor is not universal, but instead changes with the physical properties of the ISM (e.g. Kennicutt & Evans 2012 and the recent review of Bolatto et al. 2013).”

“SFRs are measured following Kennicutt & Evans (2012) where T17 use IRAS far-infrared flux measurements to estimate a total infrared luminosity.”

“We convert the extinction corrected Hα luminosity to SFR using the formula SFR(M /yr) = LHα(erg/s)×10−41.27 (Kennicutt & Evans 2012).”

“SFR is usually estimated from Hα, far-infrared, or UV luminosities (Kennicutt 1998; Kennicutt & Evans 2012; CatalánTorrecilla et al. 2015), which, despite their own caveats and lim- itations, get the job done with conveniently simple, one-line formulae.”