The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3

Stellar archeology shows that massive elliptical galaxies today formed rapidly about ten billion years ago with star formation rates above several hundreds solar masses per year (M_sun/yr). Their progenitors are likely the sub-millimeter-bright galaxies (SMGs) at redshifts (z) greater than 2. While SMGs' mean molecular gas mass of 5x10^10 M_sun can explain the formation of typical elliptical galaxies, it is inadequate to form ellipticals that already have stellar masses above 2x10^11 M_sun at z ~ 2. Here we report multi-wavelength high-resolution observations of a rare merger of two massive SMGs at z = 2.3. The system is currently forming stars at a tremendous rate of 2,000 M_sun/yr. With a star formation efficiency an order-of-magnitude greater than that of normal galaxies, it will quench the star formation by exhausting the gas reservoir in only ~200 million years. At a projected separation of 19 kiloparsecs, the two massive starbursts are about to merge and form a passive elliptical galaxy with a stellar mass of ~4x10^11 M_sun. Our observations show that gas-rich major galaxy mergers, concurrent with intense star formation, can form the most massive elliptical galaxies by z ~ 1.5.Comment: Appearing in Nature online on May 22 and in print on May 30. Submitted here is the accepted version (including the Supplementary Information), see nature.com for the final versio

A dusty, normal galaxy in the epoch of reionization

Candidates for the modest galaxies that formed most of the stars in the early universe, at redshifts $z > 7$, have been found in large numbers with extremely deep restframe-UV imaging. But it has proved difficult for existing spectrographs to characterise them in the UV. The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths if the galaxies have become sufficiently enriched in dust and metals. So far, however, the most distant UV-selected galaxy detected in dust emission is only at $z = 3.25$, and recent results have cast doubt on whether dust and molecules can be found in typical galaxies at this early epoch. Here we report thermal dust emission from an archetypal early universe star-forming galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and determine its redshift to be $z = 7.5\pm0.2$ from a spectroscopic detection of the Ly{\alpha} break. A1689-zD1 is representative of the star-forming population during reionisation, with a total star-formation rate of about 12M$_\odot$ yr$^{-1}$. The galaxy is highly evolved: it has a large stellar mass, and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way. Dusty, evolved galaxies are thus present among the fainter star-forming population at $z > 7$, in spite of the very short time since they first appeared.Comment: Nature in press. 14 pages, 10 figures, including methods sectio

A multi-wavelength view of the star formation activity at z ∼ 3

We present a multi-wavelength, UV-to-radio analysis for a sample of massive (M * ∼ 1010 M ⊙) IRAC- and MIPS 24 μm detected Lyman break galaxies (LBGs) with spectroscopic redshifts z 3 in the GOODS-North field. For LBGs without individual 24 μm detections, we employ stacking techniques at 24 μm, 1.1mm, and 1.4GHz to construct the average UV-to-radio spectral energy distribution and find it to be consistent with that of a luminous infrared galaxy with L IR = 4.5 +1.1-2.3 × 1011 L ⊙ and a specific star formation rate of 4.3 Gyr-1 that corresponds to a mass doubling time 230 Myr. On the other hand, when considering the 24 μm detected LBGs we find among them galaxies with L IR>10 12 L ⊙, indicating that the space density of z 3 UV-selected ultra-luminous infrared galaxies (ULIRGs) is (1.5 0.5) × 10-5 Mpc-3. We compare measurements of star formation rates from data at different wavelengths and find that there is tight correlation (Kendall's τ>99.7%) and excellent agreement between the values derived from dust-corrected UV, mid-IR, millimeter, and radio data for the whole range of L IR up to L IR 1013 L ⊙. This range is greater than that for which the correlation is known to hold at z ∼ 2, possibly due to the lack of significant contribution from polycyclic aromatic hydrocarbons to the 24 μm flux at z ∼ 3. The fact that this agreement is observed for galaxies with L IR> 1012 L ⊙ suggests that star formation in UV-selected ULIRGs, as well as the bulk of star formation activity at this redshift, is not embedded in optically thick regions as seen in local ULIRGs and submillimeter-selected galaxies at z = 2. © 2010 The American Astronomical Society. All rights reserved

A two-parameter model for the infrared/submillimeter/radio spectral energy distributions of galaxies and active galactic nuclei

A two-parameter semi-empirical model is presented for the spectral energy distributions of galaxies with contributions to their infrared-submillimeter- radio emission from both star formation and accretion disk-powered activity. This model builds upon a previous one-parameter family of models for star-forming galaxies, and includes an update to the mid-infrared emission using an average template obtained from Spitzer Space Telescope observations of normal galaxies. Star-forming/active galactic nucleus (AGN) diagnostics based on polycyclic aromatic hydrocarbon equivalent widths and broadband infrared colors are presented, and example mid-infrared AGN fractional contributions are estimated from model fits to the Great Observatories All-Sky LIRG Survey sample of nearby U/LIRGS and the Five mJy Unbiased Spitzer Extragalactic Survey sample of 24 μm selected sources at redshifts 0 ≲ z ≲ 4. © 2014. The American Astronomical Society. All rights reserved.

KROSS: Mapping the Ha emission across the star-formation sequence at z~1

We present first results from the KMOS Redshift One Spectroscopic Survey (KROSS), an ongoing large kinematical survey of a thousand, z~1 star forming galaxies, with VLT KMOS. Out of the targeted galaxies (~500 so far), we detect and spatially resolve Ha emission in ~90% and 77% of the sample respectively. Based on the integrated Ha flux measurements and the spatially resolved maps we derive a median star formation rate (SFR) of ~7.0 Msun/yr and a median physical size of = 5.1kpc. We combine the inferred SFRs and effective radii measurements to derive the star formation surface densities ({\Sigma}SFR) and present a "resolved" version of the star formation main sequence (MS) that appears to hold at sub-galactic scales, with similar slope and scatter as the one inferred from galaxy integrated properties. Our data also yield a trend between {\Sigma}SFR and {\Delta}(sSFR) (distance from the MS) suggesting that galaxies with higher sSFR are characterised by denser star formation activity. Similarly, we find evidence for an anti-correlation between the gas phase metallicity (Z) and the {\Delta}(sSFR), suggesting a 0.2dex variation in the metal content of galaxies within the MS and significantly lower metallicities for galaxies above it. The origin of the observed trends between {\Sigma}SFR - {\Sigma}(sSFR) and Z - {\Delta}(sSFR) could be driven by an interplay between variations of the gas fraction or the star formation efficiency of the galaxies along and off the MS. To address this, follow-up observations of the our sample that will allow gas mass estimates are necessary.</r$_{\rm

The Molecular Gas Content of z = 3 Lyman Break Galaxies; Evidence of a non Evolving Gas Fraction in Main Sequence Galaxies at z &gt; 2

We present observations of the CO[3-2] emission towards two massive and infrared luminous Lyman Break Galaxies at z = 3.21 and z = 2.92, using the IRAM Plateau de Bure Interferometer, placing first constraints on the molecular gas masses (Mgas) of non-lensed LBGs. Their overall properties are consistent with those of typical (Main-Sequence) galaxies at their redshifts, with specific star formation rates ~1.6 and ~2.2 Gyr^(-1), despite their large infrared luminosities L_IR ~2-3 x 10^12 Lsun derived from Herschel. With one plausible CO detection (spurious detection probability of 10^(-3)) and one upper limit, we investigate the evolution of the molecular gas-to-stellar mass ratio (Mgas/M*) with redshift. Our data suggest that the steep evolution of Mgas/M* of normal galaxies up to z~2 is followed by a flattening at higher redshifts, providing supporting evidence for the existence of a plateau in the evolution of the specific star formation rate at z &gt; 2.5