Work

THE SPECTRAL ENERGY DISTRIBUTIONS OF z ∼ 8 GALAXIES FROM THE IRAC ULTRA DEEP FIELDS: EMISSION LINES, STELLAR MASSES, AND SPECIFIC STAR FORMATION RATES AT 650 MYR

Year: 2013

Type: article

Abstract: Using new ultradeep Spitzer /InfraRed Array Camera (IRAC) photometry from the IRAC Ultra Deep Field program, we investigate the stellar populations of a sample of 63 Y -dropout galaxy candidates at z ∼ 8, only 650 Myr after the big bang. The sources are selected from HST /ACS+WFC3/IR data over the Hubble Ultra Deep Field (HUDF), two HUDF parallel fields, and wide area data over the CANDELS/GOODS-South. The new Spitzer /IRAC data increase the coverage in [3.6] and [4.5] to ∼120h over the HUDF reaching depths of ∼28 (AB,1σ). The improved depth and inclusion of brighter candidates result in direct ⩾3σ InfraRed Array Camera (IRAC) detections of 20/63 sources, of which 11/63 are detected at ⩾5σ. The average [3.6]–[4.5] colors of IRAC detected galaxies at z ∼ 8 are markedly redder than those at z ∼ 7, observed only 130 Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [O iii ] λλ4959, 5007 + Hβ) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z ∼ 7 and z ∼ 8 we estimate a rest-frame equivalent width of Å contributing mag to the [4.5] filter at z ∼ 8. The corresponding Å implies an average specific star formation rate of Gyr −1 and a stellar population age of Myr. Correcting the spectral energy distribution for the contribution of emission lines lowers the average best-fit stellar masses and mass-to-light ratios by ∼3 × , decreasing the integrated stellar mass density to Mpc −3 .

Source: The Astrophysical Journal Letters

DOI: 10.1088/2041-8205/777/2/l19