C. Bridge

A dust-obscured massive maximum-starburst galaxy at a redshift of 6.34

Massive present-day early-type (elliptical and lenticular) galaxies probably gained the bulk of their stellar mass and heavy elements through intense, dust-enshrouded starbursts—that is, increased rates of star formation—in the most massive dark-matter haloes at early epochs. However, it remains unknown how soon after the Big Bang massive starburst progenitors exist. The measured redshift (z) distribution of dusty, massive starbursts has long been suspected to be biased low in z owing to selection effects, as confirmed by recent findings of systems with redshifts as high as ∼5 (refs 2–4). Here we report the identification of a massive starburst galaxy at z = 6.34 through a submillimetre colour-selection technique. We unambiguously determined the redshift from a suite of molecular and atomic fine-structure cooling lines. These measurements reveal a hundred billion solar masses of highly excited, chemically evolved interstellar medium in this galaxy, which constitutes at least 40 per cent of the baryonic mass. A ‘maximum starburst’ converts the gas into stars at a rate more than 2,000 times that of the Milky Way, a rate among the highest observed at any epoch. Despite the overall downturn in cosmic star formation towards the highest redshifts, it seems that environments mature enough to form the most massive, intense starbursts existed at least as early as 880 million years after the Big Bang.

The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE)

We present ground-based spectroscopic verification of 6 Y dwarfs (see also Cushing et al.), 89 T dwarfs, 8 L dwarfs, and 1 M dwarf identified by the Wide-field Infrared Survey Explorer (WISE). Eighty of these are cold brown dwarfs with spectral types ≥T6, six of which have been announced earlier by Mainzer et al. and Burgasser et al. We present color-color and color-type diagrams showing the locus of M, L, T, and Y dwarfs in WISE color space. Near-infrared and, in a few cases, optical spectra are presented for these discoveries. Near-infrared classifications as late as early Y are presented and objects with peculiar spectra are discussed. Using these new discoveries, we are also able to extend the optical T dwarf classification scheme from T8 to T9. After deriving an absolute WISE 4.6 μm (W2) magnitude versus spectral type relation, we estimate spectrophotometric distances to our discoveries. We also use available astrometric measurements to provide preliminary trigonometric parallaxes to four of our discoveries, which have types of L9 pec (red), T8, T9, and Y0; all of these lie within 10 pc of the Sun. The Y0 dwarf, WISE 1541–2250, is the closest at 2.8^(+1.3)_(–0.6) pc; if this 2.8 pc value persists after continued monitoring, WISE 1541–2250 will become the seventh closest stellar system to the Sun. Another 10 objects, with types between T6 and >Y0, have spectrophotometric distance estimates also placing them within 10 pc. The closest of these, the T6 dwarf WISE 1506+7027, is believed to fall at a distance of ~4.9 pc. WISE multi-epoch positions supplemented with positional info primarily from the Spitzer/Infrared Array Camera allow us to calculate proper motions and tangential velocities for roughly one-half of the new discoveries. This work represents the first step by WISE to complete a full-sky, volume-limited census of late-T and Y dwarfs. Using early results from this census, we present preliminary, lower limits to the space density of these objects and discuss constraints on both the functional form of the mass function and the low-mass limit of star formation.

VERY STRONG EMISSION-LINE GALAXIES IN THE WFC3 INFRARED SPECTROSCOPIC PARALLEL SURVEY AND IMPLICATIONS FOR HIGH-REDSHIFT GALAXIES* , **

The WFC3 Infrared Spectroscopic Parallel Survey uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths (EWs) higher than 200 A. A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin^2 area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). After spectroscopic follow-up of one of these galaxies with Keck/Low Resolution Imaging Spectrometer, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12 + log(O/H) =7.47 ± 0.11. After estimating the active galactic nucleus fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that the presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ~ 8 dropout surveys. In order to effectively remove low-redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow HST imaging surveys. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their spectral energy distribution (SED). Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore, the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

Explaining the [C II]157.7 μm Deficit in Luminous Infrared Galaxies : First Results from a Herschel/PACS Study of the GOALS Sample

We present the first results of a survey of the [C II] 157.7 μm emission line in 241 luminous infrared galaxies (LIRGs) comprising the Great Observatories All-sky Survey (GOALS) sample, obtained with the PACS instrument on board the Herschel Space Observatory. The [C II] luminosities, L_([C II]), of the LIRGs in GOALS range from ∼ 10^7 to 2×10^9 L_⊙. We find that LIRGs show a tight correlation of [C II]/FIR with far-IR flux density ratios, with a strong negative trend spanning from ∼ 10^(−2) to 10^(−4), as the average temperature of dust increases. We find correlations between the [C II]/FIR ratio and the strength of the 9.7 μm silicate absorption feature as well as with the luminosity surface density of the mid-IR emitting region (∑_(MIR)), suggesting that warmer, more compact starbursts have substantially smaller [C II]/FIR ratios. Pure star-forming LIRGs have a mean [C II]/FIR∼ 4 × 10^(−3), while galaxies with low 6.2 μm PAH equivalent widths (EWs), indicative of the presence of active galactic nuclei (AGN), span the full range in [C II]/FIR. However, we show that even when only pure star-forming galaxies are considered, the [C II]/FIR ratio still drops by an order of magnitude, from 10^(−2) to 10^(−3), with ∑_(MIR) and ∑_(IR), implying that the [C II] 157.7 μm luminosity is not a good indicator of the star formation rate (SFR) for most LIRGs, for it does not scale linearly with the warm dust emission most likely associated to the youngest stars. Moreover, even in LIRGs in which we detect an AGN in the mid-IR, the majority (2/3) of galaxies show [C II]/FIR≥ 10^(−3) typical of high 6.2 μm PAH EW sources, suggesting that most AGNs do not contribute significantly to the far-IR emission. We provide an empirical relation between the [C II]/FIR and the specific SFR (SSFR) for star-forming LIRGs. Finally, we present predictions for the starburst size based on the observed [C II] and far-IR luminosities which should be useful for comparing with results from future surveys of high-redshift galaxies with ALMA and CCAT.

Extending the Nearby Galaxy Heritage with WISE: First Results from the WISE Enhanced Resolution Galaxy Atlas

The Wide-field Infrared Survey Explorer (WISE) mapped the entire sky at mid-infrared wavelengths 3.4 μm, 4.6 μm, 12 μm, and 22 μm. The mission was primarily designed to extract point sources, leaving resolved and extended sources, for the most part, unexplored. Accordingly, we have begun a dedicated WISE Enhanced Resolution Galaxy Atlas (WERGA) project to fully characterize large, nearby galaxies and produce a legacy image atlas and source catalog. Here we demonstrate the first results of the WERGA project for a sample of 17 galaxies, chosen to be of large angular size, diverse morphology, and covering a range in color, stellar mass, and star formation. It includes many well-studied galaxies, such as M 51, M 81, M 87, M 83, M 101, and IC 342. Photometry and surface brightness decomposition is carried out after special super-resolution processing, achieving spatial resolutions similar to that of Spitzer Infrared Array Camera. The enhanced resolution method is summarized in the first paper of this two-part series. In this second work, we present WISE, Spitzer, and Galaxy Evolution Explorer (GALEX) photometric and characterization measurements for the sample galaxies, combining the measurements to study the global properties. We derive star formation rates using the polycyclic aromatic hydrocarbon sensitive 12 μm (W3) fluxes, warm-dust sensitive 22 μm (W4) fluxes, and young massive-star sensitive ultraviolet (UV) fluxes. Stellar masses are estimated using the 3.4 μm (W1) and 4.6 μm (W2) measurements that trace the dominant stellar mass content. We highlight and showcase the detailed results of M 83, comparing the WISE/Spitzer results with the Australia Telescope Compact Array H I gas distribution and GALEX UV emission, tracing the evolution from gas to stars. In addition to the enhanced images, WISEs all-sky coverage provides a tremendous advantage over Spitzer for building a complete nearby galaxy catalog, tracing both stellar mass and star formation histories. We discuss the construction of a complete mid-infrared catalog of galaxies and its complementary role of studying the assembly and evolution of galaxies in the local universe.

CHARACTERIZING THE MID-INFRARED EXTRAGALACTIC SKY WITH WISE AND SDSS

The Wide-field Infrared Survey Explorer (WISE) has completed its all-sky survey in four channels at 3.4-22 μm, detecting hundreds of millions of objects. We merge the WISE mid-infrared data with optical data from the Sloan Digital Sky Survey (SDSS) and provide a phenomenological characterization of WISE extragalactic sources. WISE is most sensitive at 3.4 μm (W1) and least sensitive at 22 μm (W4). The W1 band probes massive early-type galaxies out to z ≳ 1. This is more distant than SDSS identified early-type galaxies, consistent with the fact that 28% of 3.4 μm sources have faint or no r-band counterparts (r > 22.2). In contrast, 92%-95% of 12 μm and 22 μm sources have SDSS optical counterparts with r ≤ 22.2. WISE 3.4 μm detects 89.8% of the entire SDSS QSO catalog at S/N_(W1) >7σ, but only 18.9% at 22 μm with S/N_(W4) > 5σ. We show that WISE colors alone are effective in isolating stars (or local early-type galaxies), star-forming galaxies, and strong active galactic nuclei (AGNs)/QSOs at z ≾ 3. We highlight three major applications of WISE colors: (1) Selection of strong AGNs/QSOs at z ≤ 3 using W1 – W2 > 0.8 and W2 0.8, W2 6 (Vega) colors can be used to identify type-2 AGN candidates. The fraction of these type-2 AGN candidates is one-third of all WISE color-selected AGNs. (3) Selection of ultraluminous infrared galaxies (ULIRGs) at z ~ 2 with extremely red colors, r – W4 > 14 or well-detected 22 μm sources lacking detections in the 3.4 and 4.6 μm bands. The surface density of z ~ 2 ULIRG candidates selected with r – W4 > 14 is 0.9 ± 0.07 deg^(–2) at S/N_(W4) ≥ 5 (the corresponding, lowest flux density of 2.5 mJy), which is consistent with that inferred from smaller area Spitzer surveys. Optical spectroscopy of a small number of these high-redshift ULIRG candidates confirms our selection, and reveals a possible trend that optically fainter or r – W4 redder candidates are at higher redshifts.

A redshift survey of Herschel far-infrared selected starbursts and implications for obscured star formation

We present Keck spectroscopic observations and redshifts for a sample of 767 Herschel-SPIRE selected galaxies (HSGs) at 250, 350, and 500 μm, taken with the Keck I Low Resolution Imaging Spectrometer and the Keck II DEep Imaging Multi-Object Spectrograph. The redshift distribution of these SPIRE sources from the Herschel Multitiered Extragalactic Survey peaks at z = 0.85, with 731 sources at z < 2 and a tail of sources out to z ~ 5. We measure more significant disagreement between photometric and spectroscopic redshifts (〈Δz/(1 + z_(spec))〉 = 0.29) than is seen in non-infrared selected samples, likely due to enhanced star formation rates and dust obscuration in infrared-selected galaxies. The infrared data are used to directly measure integrated infrared luminosities and dust temperatures independent of radio or 24 μm flux densities. By probing the dust spectral energy distribution (SED) at its peak, we estimate that the vast majority (72%-83%) of z < 2 Herschel-selected galaxies would drop out of traditional submillimeter surveys at 0.85-1 mm. We find that dust temperature traces infrared luminosity, due in part to the SPIRE wavelength selection biases, and partially from physical effects. As a result, we measure no significant trend in SPIRE color with redshift; if dust temperature were independent of luminosity or redshift, a trend in SPIRE color would be expected. Composite infrared SEDs are constructed as a function of infrared luminosity, showing the increase in dust temperature with luminosity, and subtle change in near-infrared and mid-infrared spectral properties. Moderate evolution in the far-infrared (FIR)/radio correlation is measured for this partially radio-selected sample, with q_(IR) ∝(1 + z)^(–0.30±0.02) at z < 2. We estimate the luminosity function and implied star formation rate density contribution of HSGs at z < 1.6 and find overall agreement with work based on 24 μm extrapolations of the LIRG, ULIRG, and total infrared contributions. This work significantly increased the number of spectroscopically confirmed infrared-luminous galaxies at z » 0 and demonstrates the growing importance of dusty starbursts for galaxy evolution studies and the build-up of stellar mass throughout cosmic time.

The dominant role of mergers in the size evolution of massive early-type galaxies since z ~ 1

Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M_⋆ ≥ 10^(11) M_⊙) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h^(-1) kpc ≤ r_p ≤ 30 h^(-1) kpc and a relative velocity Δv ≤ 500 km s^(-1) in redshift space. We measured both major (stellar mass ratio μ ≡ M_(⋆,2)/M_(⋆,1) ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. The merger fraction and rate of massive galaxies evolves as a power-law (1 + z)^n, with major mergers increasing with redshift, n_(MM) = 1.4, and minor mergers showing little evolution, n_(mm) ~ 0. When split by their morphology, the minor merger fraction for early-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (n_(MM)^(LT) ~ 4 ) than for ETGs (n_(MM)^(ET)= 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find that μ ≥ 1/10 mergers can explain ~55% of the observed size evolution of these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra ~20%. The remaining ~5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, that merging is the main contributor to the size evolution of massive ETGs at z ≲ 1, accounting for ~50−75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events.

THE CFHTLS-DEEP CATALOG OF INTERACTING GALAXIES. I. MERGER RATE EVOLUTION TO z = 1.2

We present the rest-frame optical galaxy merger fraction between 0.2 10^(10.7) M_☉ are undergoing fewer mergers than less massive systems (M_* ~ 10^(10) M_☉), consistent with the expectations of galaxy assembly downsizing is observed. Our results also show that interacting galaxies have on average SFRs double that found in non-interacting field galaxies. We conclude that (1) the optical galaxy merger fraction does evolve with redshift, (2) the merger fraction depends mildly on stellar mass, with lower mass galaxies having higher merger fractions at z < 1, and (3) star formation is triggered at all phases of a merger, with larger enhancements at later stages, consistent with N-body simulations.

THE WFC3 INFRARED SPECTROSCOPIC PARALLEL (WISP) SURVEY

We present the WFC3 Infrared Spectroscopic Parallel (WISP) Survey. WISP is obtaining slitless, near-infrared grism spectroscopy of ~90 independent, high-latitude fields by observing in the pure-parallel mode with the Wide Field Camera Three on the Hubble Space Telescope for a total of ~250 orbits. Spectra are obtained with the G_(102) (λ = 0.8–1.17 μm, R ~ 210) and G_(141) grisms (λ = 1.11–1.67 μm, R ~ 130), together with direct imaging in the J and H bands (F110W and F140W, respectively). In the present paper, we present the first results from 19 WISP fields, covering approximately 63 arcmin^2. For typical exposure times (~6400 s in G_(102) and ~2700 s in G_(141)), we reach 5σ detection limits for emission lines of f ~ 5 × 10^(−17) erg s^(−1) cm^(−2) for compact objects. Typical direct imaging 5σ limits are 26.3 and 26.1 mag. (AB) in F110W and F140W, respectively. Restricting ourselves to the lines measured with the highest confidence, we present a list of 328 emission lines, in 229 objects, in a redshift range 0.3 < z < 3. The single-line emitters are likely to be a mix of Hα and [O_III]5007,4959 A, with Hα predominating. The overall surface density of high-confidence emission-line objects in our sample is approximately 4 per arcmin^2. These first fields show high equivalent width sources, active galactic nucleus, and post-starburst galaxies. The median observed star formation rate (SFR) of our Hα-selected sample is 4M_⊙ yr^(−1). At intermediate redshifts, we detect emission lines in galaxies as faint as H_(140) ~ 25, or M_R < −19, and are sensitive to SFRs down to less than 1M_⊙ yr^(−1). The slitless grisms on WFC3 provide a unique opportunity to study the spectral properties of galaxies much fainter than L^* at the peak of the galaxy assembly epoch.