Pathways to quiescence: SHARDS view on the star formation histories of massive quiescent galaxies at 1.0 < z < 1.5

We present star formation histories (SFHs) for a sample of 104 massive (stellar mass M > 10^10 M_⊙) quiescent galaxies (MQGs) at z = 1.0–1.5 from the analysis of spectrophotometric data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) and HST/WFC3 G102 and G141 surveys of the GOODS-North field, jointly with broad-band observations from ultraviolet (UV) to far-infrared (far-IR). The sample is constructed on the basis of rest-frame UVJ colours and specific star formation rates (sSFRs = SFR/Mass). The spectral energy distributions (SEDs) of each galaxy are compared to models assuming a delayed exponentially declining SFH. A Monte Carlo algorithm characterizes the degeneracies, which we are able to break taking advantage of the SHARDS data resolution, by measuring indices such as MgUV and D4000. The population of MQGs shows a duality in their properties. The sample is dominated (85 per cent) by galaxies with young mass-weighted ages, t_M t_M < 2 Gyr, short star formation time-scales, 〈τ〉 ∼ 60–200 Myr, and masses log(M/M_⊙) ∼ 10.5. There is an older population (15 per cent) with t_M t_M = 2–4 Gyr, longer star formation time-scales, 〈τ〉∼ 400 Myr, and larger masses, log(M/M_⊙) ∼ 10.7. The SFHs of our MQGs are consistent with the slope and the location of the main sequence of star-forming galaxies at z > 1.0, when our galaxies were 0.5–1.0 Gyr old. According to these SFHs, all the MQGs experienced a luminous infrared galaxy phase that lasts for ∼500 Myr, and half of them an ultraluminous infrared galaxy phase for ∼100 Myr. We find that the MQG population is almost assembled at z ∼ 1, and continues evolving passively with few additions to the population.

No surviving evolved companions of the progenitor of SN 1006

Type Ia supernovae are thought to occur when a white dwarf made of carbon and oxygen accretes sufficient mass to trigger a thermonuclear explosion(1). The accretion could be slow, from an unevolved (main-sequence) or evolved (subgiant or giant) star(2,3) (the single-degenerate channel), or rapid, as the primary star breaks up a smaller orbiting white dwarf(3,4) (the double-degenerate channel). A companion star will survive the explosion only in the single-degenerate channel(5). Both channels might contribute to the production of type Ia supernovae(6,7), but the relative proportions of their contributions remain a fundamental puzzle in astronomy. Previous searches for remnant companions have revealed one possible case for SN 1572 (refs 8, 9), although that has been questioned(10). More recently, observations have restricted surviving companions to be small, main-sequence stars(11-13), ruling out giant companions but still allowing the single-degenerate channel. Here we report the results of a search for surviving companions of the progenitor of SN 1006 (ref. 14). None of the stars within 4 arc minutes of the apparent site of the explosion is associated with the supernova remnant, and we can firmly exclude all giant and subgiant stars from being companions of the progenitor. In combination with previous results, our findings indicate that fewer than 20 per cent of type Ia supernovae occur through the single-degenerate channel.

A bright z=5.2 lensed submillimeter galaxy in the field of Abell 773 HLSJ091828.6+514223

During our Herschel Lensing Survey (HLS) of massive galaxy clusters, we have discovered an exceptionally bright source behind the z = 0.22 cluster Abell 773, which appears to be a strongly lensed submillimeter galaxy (SMG) at z = 5.2429. This source is unusual compared to most other lensed sources discovered by Herschel so far, because of its higher submm flux (∼200 mJy at 500 μm) and its high redshift. The dominant lens is a foreground z = 0.63 galaxy, not the cluster itself. The source has a far-infrared (FIR) luminosity of L_FIR = 1.1 × 10^14/μ L_⨀, where μ is the magnification factor, likely ∼11. We report here the redshift identification through CO lines with the IRAM-30 m, and the analysis of the gas excitation, based on CO(7–6), CO(6–5), CO(5–4) detected at IRAM and the CO(2–1) at the EVLA. All lines decompose into a wide and strong red component, and a narrower and weaker blue component, 540 km s^−1 apart. Assuming the ultraluminous galaxy (ULIRG) CO-to-H_2 conversion ratio, the H_2 mass is 5.8×10^11/μ M_⨀, of which one third is in a cool component. From the CI(^3P_2−^3 P_1) line we derive a C_I/H_2 number abundance of 6 × 10^−5 similar to that in other ULIRGs. The H_2O_p(2, 0, 2−1, 1, 1) line is strong only in the red velocity component, with an intensity ratio I(H_2O)/I(CO) ∼ 0.5, suggesting a strong local FIR radiation field, possibly from an active nucleus (AGN) component. We detect the [NII]205 μm line for the first time at high-z. It shows comparable blue and red components, with a strikingly broad blue one, suggesting strong ionized gas flows.

The AGN, star-forming, and morphological properties of luminous ir-bright/optically-faint galaxies

We present the active galactic nucleus (AGN), star-forming, and morphological properties of a sample of 13 MIR-luminous (∫_24 700 μJy) IR-bright/optically-faint galaxies (IRBGs, ∫_24/f_R≲ 1000). While these z ∼ 2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only seven are formally detected in the X-ray and four lack X-ray emission at even the 2σ level. Spitzer InfraRed Spectrograph (IRS) spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable polycyclic aromatic hydrocarbon (PAH) emission responsible for ∼25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30%–40% of luminous IRBGs have star formation rates in the ultraluminous infrared galaxy (ULIRG) range (∼100–2000 M_⨀ yr^−1). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGNs, 80% of which are candidates to be Compton thick. Furthermore, the mean X-ray luminosity of the sample, log L_2–10 keV(erg s^−1) ∼44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ∼70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.

MID-IR luminosities and UV/optical star formation rates at z < 1.4

Ultraviolet (UV) nonionizing continuum and mid-infrared (IR) emission constitute the basis of two widely used star formation (SF) indicators at intermediate and high redshifts. We study 2430 galaxies with z < 1.4 in the Extended Groth Strip with deep MIPS 24 μm observations from FIDEL, spectroscopy from DEEP2, and UV, optical, and near-IR photometry from the AEGIS. The data are coupled with dust-reddened stellar population models and Bayesian spectral energy distribution (SED) fitting to estimate dust-corrected star formation rates (SFRs). In order to probe the dust heating from stellar populations of various ages, the derived SFRs were averaged over various timescales—from 100 Myr for "current" SFR (corresponding to young stars) to 1-3 Gyr for long-timescale SFRs (corresponding to the light-weighted age of the dominant stellar populations). These SED-based UV/optical SFRs are compared to total IR luminosities extrapolated from 24 μm observations, corresponding to 10-18 μm rest frame. The total IR luminosities are in the range of normal star-forming galaxies and luminous IR galaxies (10^10-10^12 L_☉). We show that the IR luminosity can be estimated from the UV and optical photometry to within a factor of 2, implying that most z < 1.4 galaxies are not optically thick. We find that for the blue, actively star-forming galaxies the correlation between the IR luminosity and the UV/optical SFR shows a decrease in scatter when going from shorter to longer SFR-averaging timescales. We interpret this as the greater role of intermediate age stellar populations in heating the dust than what is typically assumed. Equivalently, we observe that the IR luminosity is better correlated with dust-corrected optical luminosity than with dust-corrected UV light. We find that this holds over the entire redshift range. Many so-called green valley galaxies are simply dust-obscured actively star-forming galaxies. However, there exist 24 μm detected galaxies, some with L_IR>10^11 L_☉, yet with little current SF. For them a reasonable amount of dust absorption of stellar light (but presumably higher than in nearby early-type galaxies) is sufficient to produce the observed levels of IR, which includes a large contribution from intermediate and old stellar populations. In our sample, which contains very few ultraluminous IR galaxies, optical and X-ray active galactic nuclei do not contribute on average more than ~50% to the mid-IR luminosity, and we see no evidence for a large population of "IR excess" galaxies.

Determining star formation rates for infrared galaxies

We show that measures of star formation rates (SFRs) for infrared galaxies using either single-band 24 μm or extinction-corrected Paα luminosities are consistent in the total infrared luminosity = L(TIR) ~ 10^10 L_☉ range. MIPS 24 μm photometry can yield SFRs accurately from this luminosity upward: SFR(M_☉ yr^–1) = 7.8 × 10^–10 L(24 μm, L_☉) from L(TIR) = 5× 10^9 L_☉ to 10^11 L_☉ and SFR = 7.8 × 10^–10 L(24 μm, L_☉)(7.76 × 10^–11 L(24))^0.048 for higher L(TIR). For galaxies with L(TIR) ≥ 10^10 L_☉, these new expressions should provide SFRs to within 0.2 dex. For L(TIR) ≥ 10^11 L_☉, we find that the SFR of infrared galaxies is significantly underestimated using extinction-corrected Paα (and presumably using any other optical or near-infrared recombination lines). As a part of this work, we constructed spectral energy distribution templates for eleven luminous and ultraluminous purely star forming infrared galaxies and over the spectral range 0.4 μm to 30 cm. We use these templates and the SINGS data to construct average templates from 5 μm to 30 cm for infrared galaxies with L(TIR) = 5× 10^9 to 10^13 L_☉. All of these templates are made available online.