The milliJansky 12- m population: first follow-up

Abstract

We present the first results of our follow‐up programme of optical imaging and spectroscopy of a deep 12‐μm survey conducted with the ISO satellite. We find that the objects are typically of fairly low redshift (z∼0.1–0.2), but with a tail that extends to high redshifts. The highest redshift object is a previously unknown quasar at z=1.2. The sample of objects for which spectroscopy has been obtained forms a complete subsample of the original survey with an R‐band magnitude limit of 19.6. We are thus able to use accessible volume methods to determine the luminosity function. We find that the luminosity function is close to the low‐redshift 12‐μm luminosity function determined by IRAS, but with a possible excess at the highest luminosities, which also correspond to the highest redshifts. This excess is compatible with the rapid luminosity evolution of L ∝ (1+z)4.5 claimed by Xu and suggested by Elbaz et al. on the basis of number counts. These conclusions remain tentative because of small number statistics at the highest redshifts. Almost all of the galaxies in this survey are emission line objects. We use the spectra to determine emission line diagnostics for the underlying ionization source. We find that the majority of objects have H ii region‐like spectra (roughly 2/3) but there is a significant fraction (about 1/3) that contain an active galactic nucleus (AGN). This confirms suspicions that deep mid‐IR surveys will be a powerful way to find obscured AGN. Part of the survey area has been imaged deeply by the INT Wide Field Camera. Nearly all 12‐μm sources in this area, down to the 3σ sensitivity limit of the original ISO survey, have good optical identifications, the faintest being at R=23. The prospects are thus good for obtaining complete spectroscopy for this sample. With such data we should be able to make a clear measurement of the evolution rate of this population and determine the role of obscured AGN in the mid‐infrared.

Evolution of the far-infrared luminosity functions in the Spitzer Wide-area Infrared Extragalactic Legacy Survey

“…Saunders et al (1990) construct the 60 µm and 40 − 120 µm far-IR (FIR) LFs based on IRAS observations finding strong luminosity evolution (modelled as L * (z) ∝ (1 + z) αL , where L * is the characteristic luminosity and z is the redshift) with αL = 3 ± 1. Similar rates of positive evolution (αL = 3 − 5) are seen in LFs constructed from ISO surveys at 12 µm (Clements et al 2001), 90 µm (Serjeant et al 2004) and 170 µm (Takeuchi et al 2006). Pozzi et al (2004) determine the 15 µm LF of galaxies from the European Large Area ISO Survey (ELAIS) to find that the starburst population evolves both in luminosity, with αL = 3.5, and density (modelled as φ * (z) ∝ (1 + z) αD , where φ * is the characteristic number density) with αD = 3.8 being consistent with model predictions of source counts and redshift distribution.…”

Luminosity functions for galaxies and quasars in the Spitzer Wide-area Infrared Extragalactic Legacy Survey

“…Saunders et al (1990) used IRAS data to derive the 60‐ and 40–120 μm LFs, which were found to be indicative of strong evolution such that luminosity increases with redshift, perhaps ∝(1 + z ) 3±1 . Clements, Desert & Franceschini (2001) used deep 12‐μm ISO data with follow‐up optical imaging and spectroscopy to determine the mJy 12‐μm LF, finding that the excess (in comparison to the low‐redshift IRAS results) at high luminosities (higher redshift) was compatible with rapid luminosity evolution ∝(1 + z ) 4.5 . More recently, Serjeant et al (2004) used the optical–IR bandmerged European Large Area Infrared Survey (ELAIS) Final Analysis Catalogue of Rowan‐Robinson et al (2004) to calculate the ELAIS 90‐μm LF, finding that, for consistency with source counts, a luminosity evolution of (1 + z ) 3.4±1 was required – consistent with the evolution in comoving volume‐averaged SFR at z ≲ 1 derived from rest‐frame optical and ultraviolet (UV) surveys (e.g.…”

The 12 $\mathsf{\mu}$mISO-ESO-Sculptor and 24 $\mathsf{\mu}$mSpitzerfaint counts reveal a population of ULIRGs as dusty massive ellipticals

“…Note that our faintest bin (at ∼0.3 mJy) is very uncertain due to incompleteness, which may explain the downward shift for this last point. For clarity we do not include the 12 µm galaxy counts from Clements et al (1999), which show a large scatter, probably due to poor statistics (3−5 objects per bin). They may also suffer from contamination by a few stars at Fig.…”