The Star Formation in Radio Survey: 3–33 GHz Imaging of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions

Abstract

We present 3, 15, and 33 GHz imaging toward galaxy nuclei and extranuclear star-forming regions using the Karl G. Jansky Very Large Array as part of the Star Formation in Radio Survey.With 3-33 GHz radio spectra, we measured the spectral indices and corresponding thermal (free-free) emission fractions for a sample of 335 discrete regions having significant detections in at least two radio bands.After removing 14 likely background galaxies, we find that the median thermal fraction at 33 GHz is 92%±0.8% with a median absolute deviation of 11%, when a two-component power-law model is adopted to fit the radio spectrum. Limiting the sample to 238 sources that are confidently identified as star-forming regions and not affected by potential AGN contamination (i.e., having galactocentric radii r G 250 pc) results in a median thermal fraction of 93%±0.8% with a median absolute deviation of 10%. We further measure the thermal fraction at 33 GHz for 163 regions identified at 7″ resolution to be 94%±0.8% with a median absolute deviation of 8%. Together, these results confirm that free-free emission dominates the radio spectra of star-forming regions on scales up to ∼500 pc in normal star-forming galaxies. We additionally find a factor of ∼1.6 increase in the scatter of the measured spectral index and thermal fraction distributions as a function of decreasing galactocentric radius. This trend is likely reflective of the continuous star formation activity occurring in the galaxy centers, resulting in a larger contribution of diffuse nonthermal emission relative to star-forming regions in the disk.

LeMMINGs – II. The e-MERLIN legacy survey of nearby galaxies. The deepest radio view of the Palomar sample on parsec scale

“…However, this interpretation does not preclude the possibility that the radio emission arises from compact radio emission from circumnuclear SF (e.g. Linden et al 2020) or circumnuclear disc (e.g Carilli et al 1998).…”

Where’s the Dust?: The Deepening Anomaly of Microwave Emission in NGC 4725 B

“…Radio data were obtained as part of multiple campaigns using the VLA covering the S-(2-4 GHz), Ku-(12-18 GHz), K-(18-26.5 GHz), Ka-(26.5-40 GHz), and Q-(40-50 GHz) bands, which are summarized in Table 1. Details on the data reduction and imaging procedures can be found in Murphy et al (2018bMurphy et al ( , 2018c and Linden et al (2020). To summarize, we followed standard calibration procedures, using the VLA calibration pipeline built on the Common Astronomy Software Applications (CASA; McMullin et al 2007) versions 4.6.0 and 4.7.0.…”

“…Although seemingly a common phenomenon from Galactic sources, AME has been observed in surprisingly few extragalactic sources, with only NGC 6946 (Murphy et al 2010;Scaife et al 2010;Hensley et al 2015), NGC 4725 (Murphy et al 2018c), and M31 (Battistelli et al 2019) having firm detections. This dearth of extragalactic detections likely arises from the lack of deep, widefield observations of nearby extragalactic sources at ∼30 GHz, the insensitivity of interferometers to large angular scale emission, and the need for multifrequency data (Hensley et al 2015;Linden et al 2020).…”

“…In this Letter, we revisit the recent detection of AME from an optically faint region within the star-forming galaxy NGC 4725 from 33 GHz Karl G. Jansky Very Large Array (VLA) observations carried out as part of the Star Formation in Radio Survey (SFRS; Murphy et al 2018b;Linden et al 2020). This source, NGC 4725 B, was studied by Murphy et al (2018c), who concluded that it was consistent with a highly embedded (A V >5 mag) nascent star-forming region, in which young (3 Myr) massive stars are still enshrouded by their natal cocoons of gas and dust, lacking enough supernovae to produce measurable synchrotron emission.…”

Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H i, He i, and He ii

“…Optical emission lines, e.g., through BPT diagrams (Baldwin et al 1981;Veilleux & Osterbrock 1987), are often used to indicate the presence of an underlying star-forming population. The total production rates of ionizing EUV photons by young star clusters are often determined from visible wavelength Hα and Hβ emission line fluxes or from radio free-free continuum emission (e.g., Murphy et al 2012;Linden et al 2020;Murphy et al 2017). As a result, quantitative relationships between line fluxes and the underlying EUV fluxes are often made difficult by extinction uncertainties.…”