EVIDENCE FOR LOW EXTINCTION IN ACTIVELY STAR-FORMING GALAXIES ATz> 6.5

Abstract

We present a search for the [C II] 158µm fine structure line (a main cooling line of the interstellar medium) and the underlying far-infrared (FIR) continuum in three high-redshift (6.6<z<8.2) star-forming galaxies using the IRAM Plateau de Bure interferometer. We targeted two Lymanα-selected galaxies (Lyman-Alpha-Emitters, LAEs) with moderate UV-based star formation rates (SFR∼20 M ⊙ yr −1 ; Himiko at z=6.6 and IOK-1 at z=7.0) and a Gamma Ray Burst (GRB) host galaxy (GRB 090423 at z∼8.2). Based on our 3σ rest-frame FIR continuum limits, previous (restframe) UV continuum measurements and spectral energy distribution (SED) fitting, we rule out SED shapes similar to highly obscured galaxies (e.g. Arp 220, M 82) and less extreme dust-rich nearby spiral galaxies (e.g. M 51) for the LAEs. Conservatively assuming a SED shape typical of local spiral galaxies we derive upper limits for the FIR-based star formation rates (SFRs) of ∼70 M ⊙ yr −1 , ∼50 M ⊙ yr −1 and ∼40 M ⊙ yr −1 for Himiko, IOK-1 and GRB 090423, respectively. For the LAEs these limits are only a factor ∼3 higher than the published UV-based SFRs (uncorrected for extinction). This indicates that the dust obscuration in the z>6 LAEs studied here is lower by a factor of a few than what has recently been found in some LAEs at lower redshift (2<z<3.5) with similar UV-based SFRs. A low obscuration in our z>6 LAE sample is consistent with recent rest-frame UV studies of z∼7 Lyman-Break-Galaxies (LBGs).

ALMA SPECTROSCOPIC SURVEY IN THE HUBBLE ULTRA DEEP FIELD: THE INFRARED EXCESS OF UV-SELECTED z = 2–10 GALAXIES AS A FUNCTION OF UV-CONTINUUM SLOPE AND STELLAR MASS

“…The first results from these facilities indicated that there was no detectable dust-enshrouded star formation in normal or even extremely bright UV-selected z>5 galaxies (Walter et al 2012;Ouchi et al 2013;González-López et al 2014;Ota et al 2014;Riechers et al 2014;Maiolino et al 2015). Later, some bright z∼5-6 galaxies were weakly detected in the far-IR continuum (Capak et al 2015;Willott et al 2015), but at lower levels than seen in similar galaxies at lower redshifts using well known lower-redshift z∼0-2 IRX-β relations.…”

Dust formation in Milky Way-like galaxies

“…However, these dusty examples may not be representative of typical high-redshift galaxies. There is recent evidence of actively star-forming galaxies at z > 6.5 with little dust obscuration (Walter et al 2012;Kanekar et al 2013;Ouchi et al 2013), and observations suggest that dust extinction is generally decreased in low luminosity and high redshift systems (Bouwens et al 2012). Our current understanding of dust formation at high redshift is therefore still quite incomplete.…”

Analysis and design of efficient passive components for the millimeter-wave and THz bands

“…A great difference between young (E-galaxy is the youngest) and old galaxies (Arp220 is the oldest) is appreciated, and from the energy distribution it is clear that early universe formations have a dominant THz band content. Indeed, important research is carried out nowadays to clear up the hidden information inside the old universe using THz frequencies [26], [27]. Fig.…”

“…Fig. 2.4(c), from [27], compares the SED of several old galaxies, including many spiral and dwarf type galaxies. THz-band spectral information results decisive to observe star-forming environments in those galaxies.…”

“…In this graph it can be appreciated the importance of THz frequencies for thermal dust emission, and how abundant is the ionized carbon (see the spectral line at λ = 158µm, i.e., f = 1.9 THz). Several examples of employment of spectroscopy for remote space sensing and characterization: a) Spectral content of an interstellar cloud including includes dust continuum and molecular rotation line and atomic fine-structure line emissions assuming 30 K temperatures [25]; b) Spectral energy density (SED) of galaxies of different ages (E-galaxy the youngest, Arp220 the oldest) [1]; c) Spectral energy density of several known old galaxies and adjacent spiral and dwarf galaxies [27]; d) Spectrum of our galaxy, Milky Way. Half of the total emitted power falls in the THz region [29].…”