The Diversity of Extremely Red Objects

Abstract

We present the results from a sensitive multiwavelength analysis of the properties of extremely red objects (EROs). Our analysis employs deep RIzJHK photometry of an 8<5 Â 8<5 region to select a sample of 68 EROs with ðRÀKÞ ! 5:3 and brighter than K ¼ 20:5 (5 ). We combine this photometric data set with an extremely deep 1.4 GHz radio map of the field obtained from the VLA. This map reaches a 1 limiting flux density of 3.5 lJy, making it the deepest 1.4 GHz map taken, and is sensitive enough to detect an active galaxy with L 1:4 e10 23 W Hz À1 at z > 1. If powered by a starburst, this radio luminosity is equivalent to a star formation rate of e25 M yr À1 for stars more massive than 5 M . We identify radio counterparts to 21 of the EROs in this field with radio fluxes above 12.6 lJy and resolve one-third of these with our 1>6 FWHM beam. The spectral energy distributions of the majority of these galaxies are consistent with those expected for dust-reddened starbursts at z $ 1. At these redshifts the radio luminosities of these galaxies indicate a median far-infrared luminosity of this population of L FIR e10 12 L , meaning that half of the radio-detected sample are ultraluminous infrared galaxies (ULIRGs). We conclude that e16% AE 5% of the ERO population brighter than K ¼ 20:5 are luminous infrared galaxies (LIRGs) at z $ 1. We also use photometric classification of the colors of the EROs to investigate the mix of dusty active and evolved passive systems in the remaining ERO population that is undetected in our radio map. Based on this we suggest that at least 30% and possibly up to $60% of all EROs with ðRÀKÞ ! 5:3 and K 20:5 are dusty, star-forming systems at ze1. Our best estimate of the star formation density in this highly obscured and optically faint (Re26) population is _ Ã ð0:1 100 M Þ ¼ 0:11 AE 0:03 M yr À1 Mpc À3 , comparable to estimates of that in H-emitting galaxies at z $ 1 and greater than the estimates from UV-selected samples at these epochs. This lends support to the claims of a strong increase in the contribution from obscured systems to the star formation density at high redshifts. Using the observed counts of the radio-detected ERO population, we model the apparent break in the K-band number counts of the whole ERO population at K $ 19 20 and propose that the passive ERO class dominates the total population in a relatively narrow magnitude range around Kd20, with dusty, active EROs making up the bulk of the population at fainter limits.

An overdensity of extremely red objects around faint mid-IR galaxies

“…For comparison with other dusty ERO searches (e.g., Smail et al 2002) this example case extreme dusty ERO corresponds to a star formation rate of SFR ≈ 600 M year −1 (as calculated from Mann et al 2002 using total IR luminosity). At K < 18 we have 16 IRTF EROs, and a maximum of two of them are detected in the mid-IR (the two ambigous cases).…”

“…First of all, the extreme red colours can be attributed also to dust reddened starburst galaxies thus contaminating the old distant early type samples (see e.g., recent papers by Cimatti et al 2003;Yan & Thompson 2003;Takata et al 2003, and references therein). According to Cimatti et al (2002) and Smail et al (2002) 30-60% of EROs at typical ERO brightnesses of K ∼ 18-20 might be dusty. At brighter levels, K < 17.5, we find a smaller percentage <20% of dusty EROs (Väisänen & Johansson 2004, hereafter Paper I).…”

On the colour–colour properties of the Extremely Red Objects

“…Filter set Configuration z max Colour (region) consistency for different classification methods Smail et al 2002 …”

“…In later surveys, the heterogeneity of the ERO population has been confirmed (e.g. Mannucci et al 2002;Smail et al 2002;Smith et al 2002a;Moustakas et al 2003;Yan & Thompson 2003) and the picture is further complicated by the finding by Smith et al (2002b) of an early-type disc ERO at z = 1.6.…”

An Imperfectly Passive Nature: Bright Submillimeter Emission from Dust-obscured Star Formation in the z = 3.717 “Passive” System, ZF 20115

“…These colors may arise due to the presence of a redshifted Balmer (3646 Å) or 4000 Å break in the spectral energy distribution (SED) of the source. However, the degeneracy between stellar age, redshift, and dust extinction means that proposed "passive" samples, selected on apparent colors, suffer high (80%) contamination from dusty interlopers at low and high redshift (Smail et al 2002b;Toft et al 2005;Boone et al 2011;Caputi et al 2012). Indeed, an early attempt to identify a z > 3 passive galaxy was presented by Mobasher et al (2005), who claimed the detection of a bright post-starburst galaxy at z ∼ 6.5.…”