ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES

“We correct the maps for the effect of Galactic extinction following Schlegel et al. (1998).”

“We also use UV and Hα, corrected for extinction based on astrophysical expectations, e.g., by the “Balmer decrement” or IRX-β approach (Storey & Hummer 1995; Osterbrock & Ferland 2006; Kennicutt 1998; Cortese et al. 2008).”

“This value is intermediate among recent determinations of the Milky Way αCO (e.g., Strong & Mattox 1996; Dame et al. 2001; Heyer et al. 2009; Abdo et al. 2010) and represents a commonly adopted ”best single value” for work on nearby disk galaxies (e.g., Wong & Blitz 2002; Leroy et al. 2008).”

“…Moustakas et al. (2010), supplement these with extinctions estimated from the gas column following Wong & Blitz (2002, their “hybrid” model) and extinctions inferred from comparing thermal radio continuum flux estimates from Niklas et al. (1995, 1997) to our Hα maps (following Wong & Blitz 2002).”

“Adding UV and IR emission has an established pedigree (Buat 1992; Meurer et al. 1995, 1999; Cortese et al. 2008), while the combination of Hα and IR has been explored more recently (Calzetti et al. 2007; Kennicutt et al. 2007, 2009).”

“To calculate χ2 we use uncertainties estimated from the scatter in the convolved maps away from the galaxy or 10% of the observed intensity, whichever is higher (this approach follows Draine et al. 2007).”

“Assuming the Paα to trace the true SFR, Calzetti et al. (2007) found: ΣSFR [ M⊙ yr −1 kpc−2 ] = 634 IHα [ erg s−1 sr−1 ] +(10) 13 AHα refers to the extinction (in magnitudes) of Hα emission by dust.”

“Note that the Calzetti et al. (2010) results are partially anchored to the Kennicutt et al. (2009) results, which we consider somewhat uncertain for these targets.”

“This value is intermediate among recent determinations of the Milky Way αCO (e.g., Strong & Mattox 1996; Dame et al. 2001; Heyer et al. 2009; Abdo et al. 2010) and represents a commonly adopted ”best single value” for work on nearby disk galaxies (e.g., Wong & Blitz 2002; Leroy et al. 2008).”

“It is ≈ 20% lower than the more recent calculation by Murphy et al. (2011), which like Kennicutt (1998) considers a theoretical population that has been continuously star-forming for ∼ 100 Myr.”

“These robust, extinction-free independent tracers of ionizing photons suggest wHα ≈ 1.3, similar to the wHα = 1.23 found for H II knots in M51 by Kennicutt et al. (2007).”

“Not much is known about the behavior of the DGR on kpc scales, but changes in metallicity do tend to be weak over such scales (e.g., Rosolowsky et al. 2008; Moustakas et al. 2010).”

“…by the CO J = 1 → 0 flux measured by Young et al. (1995), Helfer et al. (2003), or Kuno et al. (2007). of metallicities (e.g., Moustakas et al. 2010) and observational evidence suggests that the CO-to-H2 conversion factor varies as a function of metallicity (e.g., see summary in Leroy et al. 2011).”

“Many studies of nearby galaxies have revealed a strong empirical correlation between 24µm emission and sites of active star formation (e.g., Calzetti et al. 2005; Alonso-Herrero et al. 2006; Pérez-González et al. 2006; Prescott et al. 2007) and it has been used extensively to investigate the…”

“Whenever possible, we draw maps from the LVL survey.”

“Proposals to identify and subtract cirrus emission tend to be morphological, defining the cirrus as a smooth background and removing it via image processing techniques such as iterative median filtering (e.g., see Greenawalt et al. 1998; Thilker et al. 2000; Dale et al. 2007; Rahman et al. 2010).”

“For a steeper relationship like the ΣSFR ∼ Σ2HI found in the outer parts of galaxies (Bigiel et al. 2010) the implied uncertainty is ∼ 0.3.”

“We recover the expected time sensitivity of a few Myr for Hα (Vacca et al. 1996; McKee & Williams 1997) while FUV covers a wide range of times with a characteristic value of ∼ 10–30 Myr and most emission gone by 65 Myr.”

“Note that the Calzetti et al. (2010) results are partially anchored to the Kennicutt et al. (2009) results, which we consider somewhat uncertain for these targets.”

“See Muñoz-Mateos et al. (2009a) for more details.”

“…of nearby galaxies have revealed a strong empirical correlation between 24µm emission and sites of active star formation (e.g., Calzetti et al. 2005; Alonso-Herrero et al. 2006; Pérez-González et al. 2006; Prescott et al. 2007) and it has been used extensively to investigate the correlations…”

“This value is intermediate among recent determinations of the Milky Way αCO (e.g., Strong & Mattox 1996; Dame et al. 2001; Heyer et al. 2009; Abdo et al. 2010) and represents a commonly adopted ”best single value” for work on nearby disk galaxies (e.g., Wong & Blitz 2002; Leroy et al. 2008).”

“Over a square kpc, we expect to average several populations so that the scatter in intensity should be lower than in Table 3 by ∼ √ N , where N is the number of independent populations that we average (e.g., see Schruba et al. 2010).”

“See Muñoz-Mateos et al. (2009a) for more details.”

“…a strong empirical correlation between 24µm emission and sites of active star formation (e.g., Calzetti et al. 2005; Alonso-Herrero et al. 2006; Pérez-González et al. 2006; Prescott et al. 2007) and it has been used extensively to investigate the correlations between gas and star formation…”

“…et al. 2005; Alonso-Herrero et al. 2006; Pérez-González et al. 2006; Prescott et al. 2007) and it has been used extensively to investigate the correlations between gas and star formation (e.g., Kennicutt et al. 2007; Bigiel et al. 2008; Leroy et al. 2008; Wilson et al. 2009; Rahman et al. 2010).”

“…between 24µm emission and sites of active star formation (e.g., Calzetti et al. 2005; Alonso-Herrero et al. 2006; Pérez-González et al. 2006; Prescott et al. 2007) and it has been used extensively to investigate the correlations between gas and star formation (e.g., Kennicutt et al.…”

“Proposals to identify and subtract cirrus emission tend to be morphological, defining the cirrus as a smooth background and removing it via image processing techniques such as iterative median filtering (e.g., see Greenawalt et al. 1998; Thilker et al. 2000; Dale et al. 2007; Rahman et al. 2010).”

“Robust estimates of the magnitude of this effect remain scarce, but Inoue (2001) and Inoue et al. (2001) studied star-forming regions in the Milky Way and Local Group galaxies and suggested the dust may absorb as many as half of the ionizing photons.”

“Small-Scale Scatter in Scaling Relations: High resolution observations and simulations have begun to consider the scatter in the ratios of SFR tracers to gas as a function of scale (Schruba et al. 2010; Onodera et al. 2010; Feldmann et al. 2011).”

“Calculations by Dopita et al. (2003) also suggest plausible magnitudes of several times 10% with the effect strongest for ultracompact H II regions (see also Groves et al. 2008).”

“Such a field would drive an equilibrium dust temperature ∼ 1 K lower than a U = 1 field, consistent with the ∼ 16–19 K observed in H I and the outskirts of molecular clouds in the Solar Neighborhood (e.g., Boulanger et al. 1996; Schnee et al. 2008; Lee et al. 2011).”

“In any case, our understanding of IR cirrus emission will benefit greatly from achieving good IR SED coverage at very high spatial resolution (e.g., Lawton et al. 2010).”

“Leroy et al. (2008) adopted an R-band stellar-to-nebular extinction ratio of AHα/AR ≈ 2 (Calzetti et al. 1994; Roussel et al. 2005) and an extinction law where AFUV/AR = 8.24/2.33 (Cardelli et al. 1989; Wyder et al. 2007, using their filter definitions for R and the GALEX FUV band).”

“Law et al. (2011) found typically ∼ 7% of 24µm luminosity in these galaxies, on average, to come from old stars.”

“Star Formation Rate and Stellar Mass Surface Density Maps The star formation rate (SFR) surface densities (ΣSFR) are calculated from Hα and 24 µm maps using the Hα maps and procedure described in Leroy et al. (2012).”

“While Leroy et al. (2008, 2012, 2013) argue in favor of a more or less universal gas depletion time of ∼ 2Gyr, Shetty et al. (2013) and Shetty et al. (2014) find that the depletion time varies from galaxy to galaxy, and as a general trend, increases with surface density.”

“Also see Leroy et al. (2012) for an extensive discussion.”