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Register a new userThe GOODS-N Jansky VLA 10 GHz Pilot Survey: Sizes of Star-Forming $mu$Jy Radio Sources
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(Abridged) Our sensitive ($sigma_{rm n} = 572,{rm nJy,beam}^{-1}$), high-resolution (FWHM $theta_{1/2} = 220,{rm mas} approx2mathrm{,kpc~at~}zgtrsim1$) 10$,$GHz image covering a single Karl G.~Jansky Very Large Array (VLA) primary beam (FWHM $Theta_{1/2} = 4.25$) in the GOODS-N field contains 32 sources with $S_{rm p}gtrsim2,mu{rm Jy~beam}^{-1}$ and optical and/or near-infrared (OIR) counterparts. Most are about as large as the star-forming regions that power them. Their median FWHM major axis is $langletheta_{rm M} rangle=167pm32,{rm mas} approx 1.2pm0.28,{rm kpc}$ with rms scatter 91 was $approx$ 0.79 kpc. In units of the effective radius $r_{rm e}$ that encloses half their flux, these radio sizes are $langle r_{rm e}rangle = 69pm13{rm mas} approx pm114mathrm{,pc}$ and have rms scatter $38mathrm{,mas}approx324mathrm{,pc}$. These sizes are smaller than those measured at lower radio frequencies, but agree with dust emission sizes measured at mm/sub-mm wavelengths and extinction-corrected H$alpha$ sizes. We made a low-resolution ($theta_{1/2}=1.0$) image with $approx10times$ better brightness sensitivity to detect extended sources and measure matched-resolution spectral indices $alpha_{1.4}^{10}$. It contains 6 new sources with $S_{rm p}gtrsim3.9,mu{rm Jy~beam}^{-1}$ and OIR counterparts. The median redshift of all 38 sources is $1.24pm0.15$. The 19 sources with 1.4$,$GHz counterparts have median spectral index $-0.74pm0.10$ with rms scatter $0.35$. Including upper limits on $alpha$ for sources not detected at 1.4$,$GHz flattens the median to $gtrsim-0.61$, suggesting that the $mu$Jy radio sources at higher redshifts, and hence selected at higher rest-frame frequencies, may have flatter spectra. If the non-thermal spectral index is -0.85, the median thermal fraction at rest-frame frequency 20$,$GHz is $gtrsim$48%.
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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 optical and near-infrared imaging covering a $sim$1.53 deg$^2$ region in the Super-Cluster Assisted Shear Survey (SuperCLASS) field, which aims to make the first robust weak lensing measurement at radio wavelengths. We derive photometric redshifts for $approx$176,000 sources down to $i^prime_{rm AB}sim24$ and present photometric redshifts for 1.4 GHz $e$-MERLIN and VLA detected radio sources found in the central 0.26 deg$^{2}$. We compile an initial catalog of 149 radio sources brighter than S$_{1.4}>75$ $mu$Jy and find their photometric redshifts span $0<z_{rm phot}<4$ with radio luminosities between $10^{21}-10^{25}$ W Hz$^{-1}$, with medians of $langle z rangle =0.55$ and $langle L_{1.4}rangle =1.9times10^{23}$ W Hz$^{-1}$ respectively. We find 95% of the uJy radio source sample (141/149) have SEDs best fit by star-forming templates while 5% (8/149) are better fit by AGN. Spectral indices are calculated for sources with radio observations from VLA and GMRT at 325 MHz, with an average spectral slope of $alpha=0.59pm0.04$. Using the full photometric redshift catalog we construct a density map at the redshift of the known galaxy clusters, $z=0.20pm0.08$. Four of the five clusters are prominently detected at $>7 sigma$ in the density map and we confirm the photometric redshifts are consistent with previously measured spectra from a few galaxies at the cluster centers.
We construct the average radio spectral energy distribution (SED) of highly star-forming galaxies (HSFGs) up to z~4. Infrared and radio luminosities are bound by a tight correlation that is defined by the so-called q parameter. This infrared-radio correlation provides the basis for the use of radio luminosity as a star-formation tracer. Recent stacking and survival analysis studies find q to be decreasing with increasing redshift. It was pointed out that a possible cause of the redshift trend could be the computation of rest-frame radio luminosity via a single power-law assumption of the star-forming galaxies (SFGs) SED.To test this, we constrained the shape of the radio SED of a sample of HSFGs. To achieve a broad rest-frame frequency range, we combined previously published VLA observations of the COSMOS field at 1.4 GHz and 3 GHz with unpublished GMRT observations at 325 MHz and 610 MHz by employing survival analysis to account for non-detections in the GMRT maps. We selected a sample of HSFGs in a broad redshift range (0.3<z<4,SFR>100M0/yr) and constructed the average radio SED. By fitting a broken power-law, we find that the spectral index changes from $alpha_1=0.42pm0.06$ below a rest-frame frequency of 4.3 GHz to $alpha_2=0.94pm0.06$ above 4.3 GHz. Our results are in line with previous low-redshift studies of HSFGs (SFR>10M0/yr) that show the SED of HSFGs to differ from the SED found for normal SFGs (SFR<10M0/yr). The difference is mainly in a steeper spectrum around 10 GHz, which could indicate a smaller fraction of thermal free-free emission. Finally, we also discuss the impact of applying this broken power-law SED in place of a simple power-law in K-corrections of HSFGs and a typical radio SED for normal SFGs drawn from the literature. We find that the shape of the radio SED is unlikely to be the root cause of the q-z trend in SFGs.
We investigate the linear radio size properties of the $mu$Jy populations of radio-selected active galactic nuclei (AGN) and star-forming galaxies (SFGs) using a multi-resolution catalog based on the original VLA-COSMOS 3,GHz 0farcs75 resolution mosaic and its convolved images (up to a resolution of 2farcs2). The final catalog contains 6,399 radio sources above a 3,GHz total flux density of $S_T>20$ $mu$Jy (median $<S_T>=37$ $mu$Jy), with redshift information (median $<z>=1.0$), and multi-wavelength classification as SFGs, radio-excess AGN (RX-AGN), or non-radio-excess AGN (NRX-AGN). RX-AGN are those whose radio emission exceeds the star formation rate derived by fitting the global spectral energy distribution. We derive the evolution with redshift and luminosity of the median linear sizes of each class of objects. We find that RX-AGN are compact, with median sizes of $sim$ 1-2 kpc and increasing with redshift, corresponding to an almost constant angular size of 0farcs25. NRX-AGN typically have radio sizes a factor of 2 larger than the RX-AGN. The median radio size of SFGs is about 5 kpc up to $zsim 0.7$, and it decreases beyond this redshift. Using luminosity-complete subsamples of objects, we separately investigate the effect of redshift and luminosity dependance. We compare the radio sizes of SFGs with those derived in the rest-frame far-infrared (FIR) and UV bands. We find that SFGs have comparable sizes (within 15%) in the radio and rest-frame FIR, while the sizes measured in the UV-band are systematically larger than the radio sizes.
We combine a deep 0.5~deg$^2$, 1.4~GHz deep radio survey in the Lockman Hole with infrared and optical data in the same field, including the SERVS and UKIDSS near-infrared surveys, to make the largest study to date of the host galaxies of radio sources with typical radio flux densities $sim 50 ;mu$Jy. 87% (1274/1467) of radio sources have identifications in SERVS to $ABapprox 23.1$ at 3.6 or 4.5$mu$m, and 9% are blended with bright objects (mostly stars), leaving only 4% (59 objects) which are too faint to confidently identify in the near-infrared. We are able to estimate photometric redshifts for 68% of the radio sources. We use mid-infrared diagnostics to show that the source population consists of a mixture of star forming galaxies, rapidly accreting (cold mode) AGN and low accretion rate, hot mode AGN, with neither AGN nor starforming galaxies clearly dominating. We see the breakdown in the $K-z$ relation in faint radio source samples, and show that it is due to radio source populations becoming dominated by sources with radio luminosities $sim 10^{23};{rm WHz^{-1}}$. At these luminosities, both the star forming galaxies and the cold mode AGN have hosts with stellar luminosities about a factor of two lower than those of hot mode AGN, which continue to reside in only the most massive hosts. We show that out to at least $zsim 2$, galaxies with stellar masses $>10^{11.5}, M_{odot}$ have a radio-loud fraction up to $sim 30$%. This is consistent with there being a sufficient number of radio sources that radio-mode feedback could play a role in galaxy evolution.
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