CALIBRATING EXTINCTION-FREE STAR FORMATION RATE DIAGNOSTICS WITH 33 GHz FREE–FREE EMISSION IN NGC 6946

By E. Murphy, J. Condon, E. Schinnerer, Robert C. Kennicutt Jr., D. Calzetti, Lee Armus, G. Helou, J. Turner, G. Aniano, P. Beirão and 13 more. 2011

450 citations recorded by the source corpus.

Using free–free emission measured in the Ka band (26–40 GHz) for 10 star-forming regions in the nearby galaxy NGC 6946, including its starbursting nucleus, we compare a number of star formation rate (SFR) diagnostics that are typically considered to be unaffected by interstellar extinction. These diagnostics include non-thermal radio (i.e., 1.4 GHz), total infrared (IR; 8–1000 μm), and warm dust (i.e., 24 μm) emission, along with hybrid indicators that attempt to account for obscured and unobscured emission from star-forming regions including Hα + 24 μm and UV + IR measurements. The assumption is made that the 33 GHz free–free emission provides the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24 μm, Hα + 24 μm, and UV + IR SFR calibrations are in good agreement with the 33 GHz free–free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimates the nuclear SFR by a factor of ∼2 relative to the 33 GHz free–free SFR. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the 33 GHz free–free SFRs for the nucleus and underestimate the SFRs from the extranuclear star-forming regions by an average factor of ∼2 and ∼4–5 before and after subtracting local background emission, respectively. This result likely arises from the cosmic-ray (CR) electrons decaying within the starburst region with negligible escape, whereas the transient nature of star formation in the young extranuclear star-forming complexes allows for CR electrons to diffuse significantly further than dust-heating photons, resulting in an underestimate of the true SFR. Finally, we find that the SFRs estimated using the total 33 GHz flux density appear to agree well with those estimated using free–free emission due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33 GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/e71004d07fefa96681c9f9743798008c4d1f276a

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

In Nuclear and High Energy Physics

Citations in the corpus

A TURNOVER IN THE GALAXY MAIN SEQUENCE OF STAR FORMATION AT M* ∼ 1010 M☉ FOR REDSHIFTS z < 1.3 (2015, methodology): “Star Formation Rate Calculations There are many methods for estimating a galaxy’s SFR based on observations at various wavelengths (for a review, see Kennicutt 1998; Murphy et al. 2011).”
Binary Population and Spectral Synthesis Version 2.1: Construction, Observational Verification, and New Results (2017, result): “We also compare these to a pair of widely used SFRI calibrations for the same observational signatures (Kennicutt & Evans 2012; Murphy et al. 2011).”
ESTIMATING THE STAR FORMATION RATE AT 1 kpc SCALES IN NEARBY GALAXIES (2012, methodology): “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.”
A deep ALMA image of the Hubble Ultra Deep Field (2016, methodology): “…of each of the ALMA sources, exploring the impact of both including and excluding the 24-µm photometry to yield realistic uncertainties, and then converting to SFR using the conversion of Murphy et al. (2011) with a final minor scaling applied to convert from a Kroupa to a Chabrier IMF.”
THE ERA OF STAR FORMATION IN GALAXY CLUSTERS (2013, methodology): “We convert the total infrared luminosities to star formation rates using the relation given in Murphy et al. (2011).”
THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS). I. A CLOUD-SCALE/MULTI-WAVELENGTH VIEW OF THE INTERSTELLAR MEDIUM IN A GRAND-DESIGN SPIRAL GALAXY (2013, background): “The thermal radio continuum should trace the same population of stars responsible for the ionization of the HII regions, however, it is basically unaffected by the presence of dust (Condon 1992; Murphy et al. 2011, 2012).”
The applicability of far-infrared fine-structure lines as star formation rate tracers over wide ranges of metallicities and galaxy types (2014, methodology): “For all other galaxies in the literature sample, we estimate the star formation rates based on the TIR luminosity (LTIR10, 8−1000 µm) and the SFR calibration reported in Hao et al. (2011); Murphy et al. (2011) (see also Table A.1).”
Pōniuā‘ena: A Luminous z = 7.5 Quasar Hosting a 1.5 Billion Solar Mass Black Hole (2020, methodology): “The SFRTIR is estimated as ∼ 700 M /yr by applying the local scaling relation from Murphy et al. (2011).”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “…component can be separated with multi-frequency radio measurements or high-frequency data (e.g., Israel & van der Hulst 1983; Niklas et al. 1997; Murphy et al. 2011), to provide a photoionization-based measure of the SFR, without the complications of dust attenuation which are encountered with…”
A HIGHLY CONSISTENT FRAMEWORK FOR THE EVOLUTION OF THE STAR-FORMING “MAIN SEQUENCE” FROM z ∼ 0–6 (2014, methodology): “For additional discussion on the nature of SFR indicators and the assumptions used to derive them, see Hao et al. (2011), Murphy et al. (2011), Murphy et al. (2012), KE12.”
THE EMISSION BY DUST AND STARS OF NEARBY GALAXIES IN THE HERSCHEL KINGFISH SURVEY (2011, background): “…CIB/(COB+CIB)≈ 40− 60%), which indicates that the IR output of galaxies evolves with time and was larger in the past, consistent with studies of the evolution of IR luminosity functions (e.g., Le Floc’h et al. 2005; Chary & Pope 2010; Murphy et al. 2011a; cf., models in Fontanot & Somerville 2010).”
A z = 0 Multiwavelength Galaxy Synthesis. I. A WISE and GALEX Atlas of Local Galaxies (2019, background): “These have mostly been developed for smaller samples of local galaxies, often focusing on cases where “gold standard” estimates of M? or SFR are available (e.g., Calzetti et al. 2007; Murphy et al. 2011; Meidt et al. 2014; Hunt et al. 2019).”