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Register a new userThe Statistical Properties Of The Very Weak Radio Source Population In The GOODS/ACS HDF-N Region
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Deep combination radio observations at 1.4GHz with the VLA and MERLIN have imaged a region 10 arcminutes square surrounding the Hubble Deep Field North (HDF-N). Initial studies of the weak radio source population have shown that the proportion of starburst systems increases with decreasing radio flux density with more than 70% of radio sources being starburst in nature at flux densities less than S1.4GHz)~70microJy. The recently published GOODS ACS field overlaps this area, and here we present the results of a follow-up statistical study of the very weak radio sources (S(1.4GHz)<40microJy) in an 8.5 arcminute square field centred on the HDF-N which contains the region of overlap. Radio emission at the level of a few microJy are statistically detected associated with ACS galaxies brighter than a z-band magnitude of 25. These very faint radio sources are extended starburst systems with average radii in the range 0.6 to 0.8 arcseconds and for those with measured redshifts, radio luminosities typically several times that of the nearby well-studies starburst galaxy M82.
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We present recent and ongoing results from extremely deep 18 day MERLIN + VLA 1.4GHz observations (rms: 3.3microJy/bm) of an 8.5-by-8.5 arcminute field centred upon the Hubble Deep Field North. This area of sky has been the subject of some of the deepest observations ever made over a wide range of frequencies, from X-rays to the radio. The results presented here use our deep, sub-arcsecond radio imaging of this field to characterise the radio structures of the several hundred GOODS Spitzer MIR sources in this field. These MIR sources primarily trace the luminous starburst sources. A significant proportion of the MIR sources are detected and resolved by our radio observations, allowing these observations to trace the IR/Radio correlation for galaxies over ~7 orders of magnitude, extending it to ever lower luminosities.
We use ultradeep 20 cm data from the Karl G. Jansky Very Large Array and 850 micron data from SCUBA-2 and the Submillimeter Array of an 124 arcmin^2 region of the Chandra Deep Field-north to analyze the high radio power (P_20cm>10^31 erg s^-1 Hz^-1) population. We find that 20 (42+/-9%) of the spectroscopically identified z>0.8 sources have consistent star formation rates (SFRs) inferred from both submillimeter and radio observations, while the remaining sources have lower (mostly undetected) submillimeter fluxes, suggesting that active galactic nucleus (AGN) activity dominates the radio power in these sources. We develop a classification scheme based on the ratio of submillimeter flux to radio power versus radio power and find that it agrees with AGN and star-forming galaxy classifications from Very Long Baseline Interferometry. Our results provide support for an extremely rapid drop in the number of high SFR galaxies above about a thousand solar masses per year (Kroupa initial mass function) and for the locally determined relation between X-ray luminosity and radio power for star-forming galaxies applying at high redshifts and high radio powers. We measure far-infrared (FIR) luminosities and find that some AGNs lie on the FIR-radio correlation, while others scatter below. The AGNs that lie on the correlation appear to do so based on their emission from the AGN torus. We measure a median radio size of 1.0+/-0.3 arcsecond for the star-forming galaxies. The radio sizes of the star-forming galaxies are generally larger than those of the AGNs.
(abridged) We present multiwavelength observations for a large sample of microjansky radio sources detected in ultradeep 1.4GHz maps centered on the Hubble Deep Field-North (HDF-N) and the Hawaii Survey Fields SSA13 and SSA22. Our spectroscopic redshifts for 169 radio sources reveal a flat median redshift distribution, and these sources are hosted by similarly luminous optical L* galaxies, regardless of redshift. This is a serious concern for radio estimates of the local star formation rate density, as a substantial fraction of the ultraviolet luminosity density is generated by sub-L* galaxies at low redshifts. From our submillimeter measurements for 278 radio sources, we find error-weighted mean 850micron fluxes of 1.72$pm$0.09 mJy for the total sample, 2.37$pm$0.13 mJy for the optically-faint (I>23.5) subsample, and 1.04$pm$0.13 mJy for the optically-bright (I<23.5) subsample. We significantly (>3sigma) detect in the submillimeter 50 of the radio sources, 38 with I>23.5. Spectroscopic redshifts for three of the I<23.5 submillimeter-detected radio sources are in the range z=1.0-3.4, and all show AGN signatures. Using only the submillimeter mapped regions we find that 69pm9% of the submillimeter-detected radio population are at I>23.5. We also find that 66pm7% of the S850>5 mJy (>4sigma) sources are radio-identified. We find that millimetric redshift estimates at low redshifts are best made with a FIR template intermediate between a Milky Way type galaxy and a starburst galaxy, and at high redshifts with an Arp220 template.
A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy, resolved by MERLIN+VLA at 0.2-2.0 resolution. 55 have Chandra X-ray counterparts including 18 with a hard X-ray photon index and high luminosity characteristic of a type-II (obscured) AGN. >70% of the radio sources have been classified as starbursts or AGN using radio morphologies, spectral indices and comparisons with optical appearance and MIR emission. Starbursts outnumber radio AGN 3:1. This study extends the VO methods previously used to identify X-ray-selected obscured type-II AGN to investigate whether very luminous radio and X-ray emission originates from different phenomena in the same galaxy. The high-redshift starbursts have typical sizes of 5--10 kpc and star formation rates of ~1000 Msun/yr. There is no correlation between radio and X-ray luminosities nor spectral indices at z>~1.3. ~70% of both the radio-selected AGN and the starburst samples were detected by Chandra. The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts, of which 11 are type-II AGN including 7 at z>1.5. This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray sources is correlated with X-ray hardness. Most extended radio starbursts at z>1.3 host X-ray selected obscured AGN. Radio emission from most of these ultra-luminous objects is dominated by star formation but it contributes less than 1/3 of their X-ray luminosity. Our results support the inferences from SCUBA and IR data, that at z>1.5, star formation is an order of magnitude more extended and more copious, it is closely linked to AGN activity and it is triggered differently, compared with star formation at lower redshifts.
We use ~88 arcmin**2 of deep (>~26.5 mag at 5 sigma) NICMOS data over the two GOODS fields and the HDF South to conduct a search for bright z>~7 galaxy candidates. This search takes advantage of an efficient preselection over 58 arcmin**2 of NICMOS H-band data where only plausible z>~7 candidates are followed up with NICMOS J-band observations. ~248 arcmin**2 of deep ground-based near-infrared data (>~25.5 mag, 5 sigma) is also considered in the search. In total, we report 15 z-dropout candidates over this area -- 7 of which are new to these search fields. Two possible z~9 J-dropout candidates are also found, but seem unlikely to correspond to z~9 galaxies. The present z~9 search is used to set upper limits on the prevalence of such sources. Rigorous testing is undertaken to establish the level of contamination of our selections by photometric scatter, low mass stars, supernovae (SNe), and spurious sources. The estimated contamination rate of our z~7 selection is ~24%. Through careful simulations, the effective volume available to our z>~7 selections is estimated and used to establish constraints on the volume density of luminous (L*(z=3), or -21 mag) galaxies from these searches. We find that the volume density of luminous star-forming galaxies at z~7 is 13_{-5}^{+8}x lower than at z~4 and >25x lower (1 sigma) at z~9 than at z~4. This is the most stringent constraint yet available on the volume density of >~L* galaxies at z~9. The present wide-area, multi-field search limits cosmic variance to <20%. The evolution we find at the bright end of the UV LF is similar to that found from recent Subaru Suprime-Cam, HAWK-I or ERS WFC3/IR searches. The present paper also includes a complete summary of our final z~7 z-dropout sample (18 candidates) identified from all NICMOS observations to date (over the two GOODS fields, the HUDF, galaxy clusters).
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