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

By I. Labbé, P. Oesch, R. Bouwens, G. Illingworth, D. Magee, V. González, C. Carollo, M. Franx, M. Trenti, P. V. van Dokkum and 5 more. 2012

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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.

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Citations in the corpus

Ultraviolet emission lines in young low-mass galaxies at z ≃ 2: physical properties and implications for studies at z > 7 (2014, background): “While t e presence of large equivalent width [OIII]+Hβ emission inz ≃ 7−8 galaxies (e.g., Labbé et al. 2013, Smit et al. 2013) requires a nonnegligible gas-phase abundance of oxygen at early times, itcould take much longer for carbon to reach the abundance required to produce prominent CIII]…”
A CENSUS OF STAR-FORMING GALAXIES IN THE Z ∼ 9–10 UNIVERSE BASED ON HST+SPITZER OBSERVATIONS OVER 19 CLASH CLUSTERS: THREE CANDIDATE Z ∼ 9–10 GALAXIES AND IMPROVED CONSTRAINTS ON THE STAR FORMATION RATE DENSITY AT Z ∼ 9.2 (2012, methodology): “Here we make use of the Mophongo software (Labbé et al. 2006, 2010a, 2010b, 2013) to do photometry on sources in our fields, given the confusion.”
Lyα EMISSION FROM A LUMINOUS z = 8.68 GALAXY: IMPLICATIONS FOR GALAXIES AS TRACERS OF COSMIC REIONIZATION (2015, background): “…but, given Y -band observations are not yet available over the full CANDELS field, an alternative selection criterion was adopted that takes advantage of red IRAC [3.6]-[4.5] colors, indicating prominent [O III]+Hβ emission within the 4.5 µm band (see also Labbé et al. 2013; Smit et al. 2015).”
A SPECTROSCOPIC REDSHIFT MEASUREMENT FOR A LUMINOUS LYMAN BREAK GALAXY AT z = 7.730 USING KECK/MOSFIRE (2015, background): “This is lower than the Lyα emitter criterion EW0 > 25 Å set in recent analyses that use the Lyα fraction among LBGs to constrain the reionization process (e.g. Stark et al. 2011; Treu et al. 2013).”
UV LUMINOSITY FUNCTIONS AT REDSHIFTS z ∼ 4 TO z ∼ 10: 10,000 GALAXIES FROM HST LEGACY FIELDS (2014, methodology): “…[OIII] emission lines in z ∼ 4-8 galaxies (Schaerer & de Barros 2009; Shim et al. 2011; Stark et al. 2013; Schenker et al. 2013; González et al. 2012; Labbé et al. 2013; Smit et al. 2013; González et al. 2014), it is probable that the adopted conversion factors underestimate the actual SFRs…”
An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source (2015, background): “…of candidate of strong Hβ+[O iii] emitters has been identified at much higher redshift (z > 6.5) by looking at the Spitzer/IRAC excess in channels 1 or 2 (depending on redshift; Schaerer & de Barros 2010; Ono et al. 2012; Labbé et al. 2013; Smit et al. 2014, 2015; Roberts-Borsani et al. 2015).”
The galaxy stellar mass function at 3.5 ≤z ≤ 7.5 in the CANDELS/UDS, GOODS-South, and HUDF fields (2015, background): “Further studies (e.g. Stark et al. 2013; Labbé et al. 2013; Oesch et al. 2013a; Schenker et al. 2013) have shown that the nebular contribution can also be important at z > 3−4, when the [OIII] and Hβ lines enter the IRAC 3.6 μm filter.”
THE EVOLUTION OF THE GALAXY STELLAR MASS FUNCTION AT z = 4–8: A STEEPENING LOW-MASS-END SLOPE WITH INCREASING REDSHIFT (2015, methodology): “…integration time of 23–46 hr per pointing (Dickinson et al., in preparation); a 5′×5′ region in the ERS observed to 100 hr depth (PI Fazio); and the IRAC Ultra Deep Field 2010 program, which observed the HUDF and its two parallel fields to 120, 50, and 100 hr, respectively (Labbé et al. 2013).”
The Frontier Fields: Survey Design and Initial Results (2016, methodology): “Deep imaging with the Spitzer IRAC 3.6 and 4.5 micron bands are also required to improve photometric redshifts, measure stellar masses and specific star-formation rates, and rule out low-redshift interlopers (e.g. Labbé et al. 2013; Bradăc et al. 2014).”
Lyα EMISSION FROM GREEN PEAS: THE ROLE OF CIRCUMGALACTIC GAS DENSITY, COVERING, AND KINEMATICS (2015, background): “As a comparison, inferred emission line contamination of Spitzer/ IRAC photometry provides tentative evidence for similar WHα in star-forming galaxies at z ∼ 4–7 (Shim et al. 2011; Labbé et al. 2013; Stark et al. 2013; Smit et al. 2014).”