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Register a new userClustering of Star-forming Galaxies Near a Radio Galaxy at z=5.2
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We present HST/ACS observations of the most distant radio galaxy known, TN J0924-2201 at z=5.2. This radio galaxy has 6 spectroscopically confirmed Lya emitting companion galaxies, and appears to lie within an overdense region. The radio galaxy is marginally resolved in i_775 and z_850 showing continuum emission aligned with the radio axis, similar to what is observed for lower redshift radio galaxies. Both the half-light radius and the UV star formation rate are comparable to the typical values found for Lyman break galaxies at z~4-5. The Lya emitters are sub-L* galaxies, with deduced star formation rates of 1-10 Msun/yr. One of the Lya emitters is only detected in Lya. Based on the star formation rate of ~3 Msun/yr calculated from Lya, the lack of continuum emission could be explained if the galaxy is younger than ~2 Myr and is producing its first stars. Observations in V_606, i_775, and z_850 were used to identify additional Lyman break galaxies associated with this structure. In addition to the radio galaxy, there are 22 V-break (z~5) galaxies with z_850<26.5 (5sigma), two of which are also in the spectroscopic sample. We compare the surface density of 2/arcmin^2 to that of similarly selected V-dropouts extracted from GOODS and the UDF Parallel fields. We find evidence for an overdensity to very high confidence (>99%), based on a counts-in-cells analysis applied to the control field. The excess is suggestive of the V-break objects being associated with a forming cluster around the radio galaxy.
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We present near-infrared and optical spectroscopic observations of a sample of 450$mu$m and 850$mu$m-selected dusty star-forming galaxies (DSFGs) identified in a 400 arcmin$^2$ area in the COSMOS field. Thirty-one sources of the 102 targets were spectroscopically confirmed at $0.2<z<4$, identified primarily in the near-infrared with Keck MOSFIRE and some in the optical with Keck LRIS and DEIMOS. The low rate of confirmation is attributable both to high rest-frame optical obscuration in our targets and limited sensitivity to certain redshift ranges. The high-quality photometric redshifts available in the COSMOS field allow us to test the robustness of photometric redshifts for DSFGs. We find a subset (11/31$approx35$%) of DSFGs with inaccurate ($Delta z/(1+z)>0.2$) or non-existent photometric redshifts; these have very distinct spectral energy distributions from the remaining DSFGs, suggesting a decoupling of highly obscured and unobscured components. We present a composite rest-frame 4300--7300AA spectrum for DSFGs, and find evidence of 200$pm$30 km s$^{-1}$ gas outflows. Nebular line emission for a sub-sample of our detections indicate that hard ionizing radiation fields are ubiquitous in high-z DSFGs, even more so than typical mass or UV-selected high-z galaxies. We also confirm the extreme level of dust obscuration in DSFGs, measuring very high Balmer decrements, and very high ratios of IR to UV and IR to H$alpha$ luminosities. This work demonstrates the need to broaden the use of wide bandwidth technology in the millimeter to the spectroscopic confirmations of large samples of high-z DSFGs, as the difficulty in confirming such sources at optical/near-infrared wavelengths is exceedingly challenging given their obscuration.
We present the clustering properties of a complete sample of 968 radio sources detected at 1.4 GHz by the VLA-COSMOS survey with radio fluxes brighter than 0.15 mJy. 92% have redshift determinations from the Laigle et al. (2016) catalogue. Based on their radio-luminosity, these objects have been divided into two populations of 644 AGN and 247 star-forming galaxies. By fixing the slope of the auto-correlation function to gamma=2, we find r_0=11.7^{+1.0}_{-1.1} Mpc for the clustering length of the whole sample, while r_0=11.2^{+2.5}_{-3.3} Mpc and r_0=7.8^{+1.6}_{-2.1} Mpc (r_0=6.8^{+1.4}_{-1.8} Mpc if we restrict our analysis to z<0.9) are respectively obtained for AGN and star-forming galaxies. These values correspond to minimum masses for dark matter haloes of M_min=10^[13.6^{+0.3}_{-0.6}] M_sun for radio-selected AGN and M_min=10^[13.1^{+0.4}_{-1.6}] M_sun for radio-emitting star-forming galaxies (M_min=10^[12.7^{+0.7}_{-2.2}] M_sun for z<0.9). Comparisons with previous works imply an independence of the clustering properties of the AGN population with respect to both radio luminosity and redshift. We also investigate the relationship between dark and luminous matter in both populations. We obtain <M*>/M_halo<~10^{-2.7} for AGN, and <M*>/M_halo<~10^{-2.4} in the case of star-forming galaxies. Furthermore, if we restrict to z<~0.9 star-forming galaxies, we derive <M*>/M_halo<~10^{-2.1}, result which clearly indicates the cosmic process of stellar build-up as one moves towards the more local universe. Comparisons between the observed space density of radio-selected AGN and that of dark matter haloes shows that about one in two haloes is associated with a black hole in its radio-active phase. This suggests that the radio-active phase is a recurrent phenomenon.
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We study the relationship between stellar mass, star formation rate (SFR),ionization state, and gas-phase metallicity for a sample of 41 normal star-forming galaxies at $3 lesssim z lesssim 3.7$. The gas-phase oxygen abundance, ionization parameter, and electron density of ionized gas are derived from rest-frame optical strong emission lines measured on near-infrared spectra obtained with Keck/MOSFIRE. We remove the effect of these strong emission lines in the broad-band fluxes to compute stellar masses via spectral energy distribution fitting, while the SFR is derived from the dust-corrected ultraviolet luminosity. The ionization parameter is weakly correlated with the specific SFR, but otherwise the ionization parameter and electron density do not correlate with other global galaxy properties such as stellar mass, SFR, and metallicity. The mass-metallicity relation (MZR) at $zsimeq3.3$ shows lower metallicity by $simeq 0.7$ dex than that at $z=0$ at the same stellar mass. Our sample shows an offset by $simeq 0.3$ dex from the locally defined mass-metallicity-SFR relation, indicating that simply extrapolating such relation to higher redshift may predict an incorrect evolution of MZR. Furthermore, within the uncertainties we find no SFR-metallicity correlation, suggesting a less important role of SFR in controlling the metallicity at high redshift. We finally investigate the redshift evolution of the MZR by using the model by Lilly et al. (2013), finding that the observed evolution from $z=0$ to $zsimeq3.3$ can be accounted for by the model assuming a weak redshift evolution of the star formation efficiency.
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