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Register a new userSuperCLASS -- II: Photometric Redshifts and Characteristics of Spatially-Resolved $mu$Jy Radio Sources
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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.
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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.
Upcoming imaging surveys, such as LSST, will provide an unprecedented view of the Universe, but with limited resolution along the line-of-sight. Common ways to increase resolution in the third dimension, and reduce misclassifications, include observing a wider wavelength range and/or combining the broad-band imaging with higher spectral resolution data. The challenge with these approaches is matching the depth of these ancillary data with the original imaging survey. However, while a full 3D map is required for some science, there are many situations where only the statistical distribution of objects (dN/dz) in the line-of-sight direction is needed. In such situations, there is no need to measure the fluxes of individual objects in all of the surveys. Rather a stacking procedure can be used to perform an `ensemble photo-z. We show how a shallow, higher spectral resolution survey can be used to measure dN/dz for stacks of galaxies which coincide in a deeper, lower resolution survey. The galaxies in the deeper survey do not even need to appear individually in the shallow survey. We give a toy model example to illustrate tradeoffs and considerations for applying this method. This approach will allow deep imaging surveys to leverage the high resolution of spectroscopic and narrow/medium band surveys underway, even when the latter do not have the same reach to high redshift.
(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%.
We investigate the spatially-resolved star formation relation using a galactic disk formed in a comprehensive high-resolution (3.8 pc) simulation. Our new implementation of stellar feedback includes ionizing radiation as well as supernova explosions, and we handle ionizing radiation by solving the radiative transfer equation rather than by a subgrid model. Photoheating by stellar radiation stabilizes gas against Jeans fragmentation, reducing the star formation rate. Because we have self-consistently calculated the location of ionized gas, we are able to make spatially-resolved mock observations of star formation tracers, such as H-alpha emission. We can also observe how stellar feedback manifests itself in the correlation between ionized and molecular gas. Applying our techniques to the disk in a galactic halo of 2.3e11 Msun, we find that the correlation between star formation rate density (estimated from mock H-alpha emission) and molecular hydrogen density shows large scatter, especially at high resolutions of <~ 75 pc that are comparable to the size of giant molecular clouds (GMCs). This is because an aperture of GMC size captures only particular stages of GMC evolution, and because H-alpha traces hot gas around star-forming regions and is displaced from the molecular hydrogen peaks themselves. By examining the evolving environment around star clusters, we speculate that the breakdown of the traditional star formation laws of the Kennicutt-Schmidt type at small scales is further aided by a combination of stars drifting from their birthplaces, and molecular clouds being dispersed via stellar feedback.
Since its launch in 1999, the XMM-textit{Newton} mission has compiled the largest catalogue of serendipitous X-ray sources, with the 3XMM being the third version of this catalogue. This is because of the combination of a large effective area (5000 $rm cm^2$ at 1 keV) and a wide field of view (30 arcmin). The 3XMM-DR6 catalogue contains about 470,000 unique X-ray sources over an area of 982 $rm deg^2$. A significant fraction of these (100,178 sources) have reliable optical, near/mid-IR counterparts in the SDSS, PANSTARRS, VIDEO, UKIDSS and WISE surveys. In a previous paper we have presented photometric redshifts for these sources using the TPZ machine learning algorithm. About one fourth of these (22,677) have adequate photon statistics so that a reliable X-ray spectrum can be extracted. Obviously, owing to both the X-ray counts selection and the optical counterpart constraint, the sample above is biased towards the bright sources. Here, we present XMMFITCAT-Z: a spectral fit catalogue for these sources using the Bayesian X-ray Analysis (BXA) technique. As a science demonstration of the potential of the present catalogue, we comment on the optical and mid-IR colours of the 765 X-ray absorbed sources with $N_mathrm{H} > 10^{22},mathrm{cm}^{-2}$. We show that a considerable fraction of X-ray selected AGN would not be classified as AGN following the mid-IR W1-W2 vs. W2 selection criterion. These are AGN with lower luminosities, where the contribution of the host galaxy to the MIR emission is non-negligible. Only one third of obscured AGN in X-rays present red colours or r-W2 > 6. Then it appears that the r-W2 criterion, often used in the literature for the selection of obscured AGN, produces very different X-ray absorbed AGN samples compared to the standard X-ray selection criteria.
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