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Register a new userThe Star Formation in Radio Survey: 3 - 33 GHz Imaging of Nearby Galaxy Nuclei and Extranuclear Star-forming Regions
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We present 3, 15, and 33 GHz imaging towards 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 $92 pm 0.8%$ 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 $r_{rm G} geq 250$ pc), results in a median thermal fraction of $93 pm 0.8 %$ 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 $94 pm 0.8 %$ 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 $sim$500 pc in normal star-forming galaxies. We additionally find a factor of $sim$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 a larger contribution of diffuse nonthermal emission relative to star-forming regions in the disk.
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We present 33,GHz photometry of 103 galaxy nuclei and extranuclear star-forming complexes taken with the Green Bank Telescope (GBT) as part of the Star Formation in Radio Survey (SFRS). Among the sources without evidence for an AGN, and also having lower frequency radio data, we find a median thermal fraction at 33GHz of ~76% with a dispersion of ~24%. For all sources resolved on scales <0.5kpc, the thermal fraction is even larger, being >90%. This suggests that the rest-frame 33GHz emission provides a sensitive measure of the ionizing photon rate from young star-forming regions, thus making it a robust star formation rate indicator. Taking the 33GHz star formation rates as a reference, we investigate other empirical calibrations relying on different combinations of warm 24mu m dust, total infrared (IR; 8-1000mu m), Halpha line, and far-UV continuum emission. The recipes derived here generally agree with others found in the literature, albeit with a large dispersion that most likely stems from a combination of effects. Comparing the 33GHz to total IR flux ratios as a function of the radio spectral index, measured between 1.7 and 33GHz, we find that the ratio increases as the radio spectral index flattens which does not appear to be a distance effect. Consequently, the ratio of non-thermal to total IR emission appears relatively constant, suggesting only moderate variations in the cosmic-ray electron injection spectrum and ratio of synchrotron to total cooling processes among star-forming complexes. Assuming that this trend solely arises from an increase in the thermal fraction sets a maximum on the scatter of the non-thermal spectral indices among the star-forming regions of sigma_alpha^{NT} < 0.13.
We present 33 GHz imaging for 112 pointings towards galaxy nuclei and extranuclear star-forming regions at $approx$2 resolution using the Karl G. Jansky Very Large Array (VLA) as part of the Star Formation in Radio Survey. A comparison with 33 GHz Robert C. Byrd Green Bank Telescope single-dish observations indicates that the interferometric VLA observations recover $78pm4 %$ of the total flux density over 25 regions ($approx$ kpc-scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the $approx30-300$ pc scales sampled by our VLA observations, the bulk of the 33 GHz emission is recovered and primarily powered by free-free emission from discrete HII regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii $r_{rm G} geq 250$ pc) and detected at $>3sigma$ significance at 33 GHz and in H$alpha$. Assuming a typical 33 GHz thermal fraction of 90 %, the ratio of optically-thin 33 GHz-to-uncorrected H$alpha$ star formation rates indicate a median extinction value on $approx30-300$ pc scales of $A_{rm Halpha} approx 1.26pm0.09$ mag with an associated median absolute deviation of 0.87 mag. We find that 10 % of these sources are highly embedded (i.e., $A_{rm Halpha}gtrsim3.3$ mag), suggesting that on average HII regions remain embedded for $lesssim1$ Myr. Finally, we find the median 33 GHz continuum-to-H$alpha$ line flux ratio to be statistically larger within $r_{rm G}<250$ pc relative the outer-disk regions by a factor of $1.82pm0.39$, while the ratio of 33 GHz-to-24 $mu$m flux densities are lower by a factor of $0.45pm0.08$, which may suggest increased extinction in the central regions.
We present the first results of a high-resolution Karl G. Jansky Very Large Array (VLA) imaging survey of luminous and ultra-luminous infrared galaxies (U/LIRGs) in the Great Observatories All-Sky LIRG Survey (GOALS). From the full sample of 68 galaxies, we have selected 25 LIRGs that show resolved extended emission at sufficient sensitivity to image individual regions of star-formation activity beyond the nucleus.~With wideband radio continuum observations, which sample the frequency range from $3-33$ GHz, we have made extinction-free measurements of the luminosities and spectral indicies for a total of 48 individual star-forming regions identified as having de-projected galactocentric radii ($r_{G}$) that lie outside the 13.2$mu$m core of the galaxy.~The median $3-33$ GHz spectral index and 33 GHz thermal fraction measured for these extranuclear regions is $-0.51 pm 0.13$ and $65 pm 11%$ respectively.~These values are consistent with measurements made on matched spatial scales in normal star-forming galaxies, and suggests that these regions are more heavily-dominated by thermal free-free emission relative to the centers of local ULIRGs.~Further, we find that the median star-formation rate derived for these regions is $sim 1 M_{odot}$ yr$^{-1}$, and when we place them on the sub-galactic star-forming main sequence of galaxies (SFMS), we find they are offset from their host galaxies globally-averaged specific star-formation rates (sSFRs).~We conclude that while nuclear starburst activity drives LIRGs above the SFMS, extranuclear star-formation still proceeds in a more extreme fashion relative to what is seen in local spiral galaxies.
Multi-epoch radio-interferometric observations of young stellar objects can be used to measure their displacement over the celestial sphere with a level of accuracy that currently cannot be attained at any other wavelength. In particular, the accuracy achieved using carefully calibrated, phase-referenced observations with Very Long Baseline Interferometers such as NRAOs Very Long Baseline Array is better than 50 micro-arcseconds. This is sufficient to measure the trigonometric parallax and the proper motion of any radio-emitting young star within several hundred parsecs of the Sun with an accuracy better than a few percent. Using that technique, the mean distances to Taurus, Ophiuchus, Perseus and Orion have already been measured to unprecedented accuracy. With improved telescopes and equipment, the distance to all star-forming regions within 1 kpc of the Sun and beyond, as well as their internal structure and dynamics could be determined. This would significantly improve our ability to compare the observational properties of young stellar objects with theoretical predictions, and would have a major impact on our understanding of low-mass star-formation.
We present a study of molecular outflows using six molecular lines (including 12CO/13CO/C18O/HCO+(J = 1-0) and SiO/CS(J = 2-1)) toward nine nearby high-mass star-forming regions with accurate known distances. This work is based on the high-sensitivity observations obtained with the 14-m millimeter telescope of Purple Mountain Observatory Delingha (PMODLH) observatory. The detection rate of outflows (including 12CO, 13CO, HCO+, and CS) is 100%. However, the emission of SiO was not detected for all sources. The full line widths ($Delta V$) at 3$sigma$ above the baseline of these molecular lines have the relationship $Delta V_{rm ^{12}CO} > Delta V_{rm HCO^{+}} > Delta V_{rm CS} approx Delta V_{rm ^{13}CO} > Delta V_{rm ^{18}CO}$. 12CO and HCO+ can be used to trace relatively high-velocity outflows, while 13CO and CS can be employed to trace relatively low-velocity outflows. The dynamical timescales of the 13CO and CS outflows are longer than those of the 12CO and HCO+ outflows. The mechanical luminosities, masses, mass-loss rates and forces of all outflows (including 12CO, 13CO, HCO+, and CS) are correlated with the bolometric luminosities of their central IRAS sources.
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