A Spectroscopic Follow-up Program of Very Massive Galaxies at 3 < z < 4: Confirmation of Spectroscopic Redshifts, and a High Fraction of Powerful AGNs

Abstract

We present the analysis and results of a spectroscopic follow-up program of a mass-selected sample of six galaxies at z 3 4 < < using data from Keck-NIRPSEC and VLT-Xshooter. We confirm the z 3 > redshifts for half of the sample through the detection of strong nebular emission lines, and improve the z phot accuracy for the remainder of the sample through the combination of photometry and spectra. The modeling of the emission-line-corrected spectral energy distributions (SEDs) adopting improved redshifts confirms the very large stellar masses of the sample (M M 1.5 4 10 11 *~´ -) in the first 2 Gyr of cosmic history, with a diverse range in stellar ages, starformation rates, and dust content. From the analysis of emission-line luminosities and widths, and far-infrared (FIR) fluxes, we confirm that 80%  of the sample are hosts to luminous hidden active galactic nuclei (AGNs), with bolometric luminosities of ∼10 44-46 erg s −1 . We find that the MIPS24 μm photometry is largely contaminated by AGN continuum, rendering the SFRs derived using only 24 μm photometry to be severely overestimated. By including the emission from the AGN in the modeling of the UV-to-FIR SEDs, we confirm that the presence of the AGN does not considerablybias the stellar masses ( 0.3 < dex at 1σ). We show evidence for a rapid increase of the AGN fraction from ∼30% to ∼60%-100% over the 1Gyr between z 2 and z 3 . Although we cannot exclude some enhancement of the AGN fraction for our sample due to selection effects, the small measured [O III] contamination to the observed K-band fluxes suggests that our sample is not significantly biased toward massive galaxies hosting AGNs.

The COSMOS2015 galaxy stellar mass function

“…Most of the MIPS-bright galaxies are fit by templates with SFR > 100 M yr −1 , although in reality their IR emission may be caused not only by heated dust, but also (at least partly) by an active galactic nucleus (e.g. Casey et al 2014b;Marsan et al 2015Marsan et al , 2017. Figure A.2 summarises the changes described above, showing as a function of z the fraction of galaxies either excluded from the SMF computation (i.e.…”

“…10). At z > 3, massive galaxies can also host an active black hole and disregarding its contribution in the SED fitting may cause a stellar mass overestimate of 0.1−0.3 dex (Hainline et al 2012;Marsan et al 2017). The impact of different AGN populations on the SMF shall be discussed in a future publication (Delvecchio et al, in prep.).…”

Automated Mining of the ALMA Archive in the COSMOS Field (A3COSMOS). I. Robust ALMA Continuum Photometry Catalogs and Stellar Mass and Star Formation Properties for ∼700 Galaxies at z = 0.5–6

“…Spectroscopic redshifts also exist for ∼ 7.1 × 10 4 galaxies, from the latest compilation by M. Salvato et al (version Sept. 1st, 2017; available internally in the COSMOS collaboration), which includes almost all spectroscopic observations in the COSMOS field: Lilly et al (2007Lilly et al ( , 2009; zCOSMOS Survey; with VLT/VI-MOS); Fu et al (2010; with Spitzer/IRS); Casey et al (2012Casey et al ( , 2017 with Keck II/DEIMOS); Comparat et al (2015; with VLT/FORS2); Le Fèvre et al (2015) and Tasca et al (2017) (VUDS Survey; with VLT/VMOS); Hasinger et al (2018; with Keck II/DEIMOS); Kriek et al (2015;MOSDEF Survey; with Keck I/MOSFIRE); Marsan et al (2017; with Keck II/NIRSPEC); Masters et al (2017; with Keck II/DEIMOS); Nanayakkara et al (2016; with Keck I/MOSFIRE); Silverman et al (2015a;FMOS-COSMOS Survey; with Subaru/FMOS); van der Wel et al (2016; LEGA-C Survey; with VLT/VMOS); Yun et al (2015; with LMT/RSR) (listed only references whose spectroscopic redshifts are used in this work).…”

Candidate massive galaxies atz ∼ 4 in the Dark Energy Survey

“…Let us finally comment on the number counts of these extreme systems. D13 calculated the predicted galaxy number counts for a variety of approaches, including fully theoretical cosmological simulations (as per the Millennium Simulation), semi-empirical predictions obtained by passively evolving the z = 0 and 0.5 < z < 0.7 published mass functions to higher redshift, available high-z mass functions (Marchesini et al 2010), and also results from abundance-matching techniques (Behroozi et al 2013). For example, empirical mass functions do forecast the presence of ∼ 7000 galaxies with log(M * /M ) ≥ 12.0 within the whole DES.…”

“…In order to bridge the fossil record with the formation event and trace galaxy evolution over cosmic time, many works have attacked the problem in a statistical sense, by probing number density evolution as a function of galaxy mass. Studies of the galaxy mass function over the past decade reached the uniform conclusion that the abundance of the most massive galaxies (M/M > 10 11.5 ) hardly evolves since z ∼ 1 (Cimatti, Daddi & Renzini 2006;Wake et al 2006;Pozzetti et al 2010;Marchesini et al 2010;Muzzin et al 2013;Gonzalez-Perez et al 2009;Mortlock et al 2015). One caveat to these studies has been that the observational database was drawn from small area, deep surveys, which carry the problem of cosmic variance.…”