Damien Le Borgne

The type Ia supernova SNLS-03D3bb from a super-Chandrasekhar-mass white dwarf star

The accelerating expansion of the Universe, and the need for dark energy, were inferred from observations of type Ia supernovae. There is a consensus that type Ia supernovae are thermonuclear explosions that destroy carbon–oxygen white dwarf stars that have accreted matter from a companion star, although the nature of this companion remains uncertain. These supernovae are thought to be reliable distance indicators because they have a standard amount of fuel and a uniform trigger: they are predicted to explode when the mass of the white dwarf nears the Chandrasekhar mass of 1.4 solar masses (M[circdot]). Here we show that the high-redshift supernova SNLS-03D3bb has an exceptionally high luminosity and low kinetic energy that both imply a super-Chandrasekhar-mass progenitor. Super-Chandrasekhar-mass supernovae should occur preferentially in a young stellar population, so this may provide an explanation for the observed trend that overluminous type Ia supernovae occur only in ‘young’ environments. As this supernova does not obey the relations that allow type Ia supernovae to be calibrated as standard candles, and as no counterparts have been found at low redshift, future cosmology studies will have to consider possible contamination from such events.

Cosmic Star Formation History and Its Dependence on Galaxy Stellar Mass

We examine the cosmic star formation rate (SFR) and its dependence on galaxy stellar mass over the redshift range 0.8 1010.8 M☉) was 6 times higher at z = 2 than it is today. It drops steeply from z = 2, reaching the present-day value at z ~ 1. In contrast, the SFR density of intermediate-mass galaxies (1010.2 M☉ ≤ M* < 1010.8 M☉) declines more slowly and may peak or plateau at z ~ 1.5. We use the characteristic growth time tSFR ≡ ρ/ρSFR to provide evidence of an associated transition in massive galaxies from a burst to a quiescent star formation mode at z ~ 2. Intermediate-mass systems transit from burst to quiescent mode at z ~ 1, while the lowest mass objects undergo bursts throughout our redshift range. Our results show unambiguously that the formation era for galaxies was extended and proceeded from high- to low-mass systems. The most massive galaxies formed most of their stars in the first ~3 Gyr of cosmic history. Intermediate-mass objects continued to form their dominant stellar mass for an additional ~2 Gyr, while the lowest mass systems have been forming over the whole cosmic epoch spanned by the GDDS. This view of galaxy formation clearly supports downsizing in the SFR where the most massive galaxies form first and galaxy formation proceeds from larger to smaller mass scales.

Evolved Galaxies at z > 1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems

We present spectroscopic evidence from the Gemini Deep Deep Survey for a significant population of color-selected red galaxies at 1.3 1.5 old galaxies have a sky density greater than 0.1 arcmin-2. Conservative age estimates for 20 galaxies with z > 1.3, z = 1.49, give a median age of 1.2 Gyr and zf = 2.4. One-quarter of the galaxies have inferred zf > 4. Models restricted to [Fe/H] ? 0 give median ages and zf of 2.3 Gyr and 3.3, respectively. These galaxies are among the most massive and contribute ~50% of the stellar mass density at 1 < z < 2. The derived ages and most probable star formation histories suggest a high star formation rate (~300-500 M? yr-1) phase in the progenitor population. We argue that most of the red galaxies are not descendants of the typical z ~ 3 Lyman break galaxies. Galaxies associated with luminous submillimeter sources have the requisite star formation rates to be the progenitor population. Our results point toward early and rapid formation for a significant fraction of present-day massive galaxies.

The Gemini Deep Deep Survey. VIII. When Did Early-Type Galaxies Form?

We have used the Hubble Space Telescopes Advanced Camera for Surveys (Ford et al. 2003) to measure the cumulative mass density in morphologically selected early-type galaxies over the redshift range 0.8 < z < 1.7. Our imaging data set covers four well-separated sight lines and is roughly intermediate (in terms of both depth and area) between the GOODS/GEMS imaging data and the images obtained in the Hubble Deep Field campaigns. Our images contain 144 galaxies with ultradeep spectroscopy obtained as part of the Gemini Deep Deep Survey. These images have been analyzed using a new purpose-written morphological analysis code, which improves the reliability of morphological classifications by adopting a quasi-Petrosian image thresholding technique. We find that at z ~ 1 about 80% of the stars living in the most massive galaxies reside in early-type systems. This fraction is similar to that seen in the local universe. However, we detect very rapid evolution in this fraction over the range 0.8 < z < 1.7, suggesting that over this redshift range the strong morphology-mass relationship seen in the nearby universe is beginning to fall into place. By comparing our images to published spectroscopic classifications, we show that little ambiguity exists in connecting spectral classes to morphological classes for spectroscopically quiescent systems. However, the mass density function of early-type galaxies is evolving more rapidly than that of spectroscopically quiescent systems, which we take as further evidence that we are witnessing the formation of massive early-type galaxies over the 0.8 < z < 1.7 redshift range.

Gemini Deep Deep Survey. VI. Massive Hδ-strong Galaxies at z ≃ 1

We show that there has been a dramatic decline in the abundance of massive galaxies with strong Hstellar absorption lines from z ∼ 1.2 to the present. These H�-strong, or HDS, galaxies have undergone a recent and rapid break in their star-formation activity. Combining data from the Gemini Deep Deep and the Sloan Digital Sky Surveys to make mass-matched samples (M⋆ > 10 10.2 M⊙, with 25 and 50,255 galaxies, respectively), we find that the fraction of galaxies in an HDS phase has decreased from about 50% at z = 1.2 to a few percent today. This decrease in fraction is due to an actual decrease in the number density of massive HDS systems by a factor of 2-4, coupled with an increase in the number density of massive galaxies by ∼ 30 percent. We show that this result depends only weakly on the threshold chosen for the Hequivalent width to define HDS systems (if greater than 4u and corresponds to a (1 + z) 2.5±0.7 evolution. Spectral synthesis studies of the high-redshift population using the Pcode, treating HA, EW(O II), Dn4000, and rest-frame colors, favor models in which the Balmer absorption features in massive H�-strong systems are the echoes of intense episodes of star-formation that faded ≃ 1 Gyr prior to the epoch of observation. The z = 1.4 − 2 epoch appears to correspond to a time at which massive galaxies are in transition from a mode of sustained star formation to a relatively quiescent mode with weak and rare star-formation episodes. We argue that the most likely local descendants of the distant massive HDS galaxies are passively evolving massive galaxies in the field and small groups. Subject headings: Galaxies: formation — galaxies: evolution — galaxies: high-redshift

When do early-type galaxies form?

We have used the Hubble Space Telescopes Advanced Camera for Surveys to measure the mass density function of morphologically selected early-type galaxies in the Gemini Deep Deep Survey fields, over the redshift range 0.9 < z < 1.6. Our imaging data set covers four well-separated sight-lines, and is roughly intermediate (in terms of both depth and area) between the GOODS/GEMS imaging data, and the images obtained in the Hubble Deep Field campaigns. Our images contain 144 galaxies with ultra-deep spectroscopy, and they have been analyzed using a new purpose-written morphological analysis code which improves the reliability of morphological classifications by adopting a quasi-petrosian image thresholding technique. We find that at z = 1 approximately 70% of the stars in massive galaxies reside in early-type systems. This fraction is remarkably similar to that seen in the local Universe. However, we detect very rapid evolution in this fraction over the range 1.0 < z < 1.6, suggesting that in this epoch the strong color-morphology relationship seen in the nearby Universe is beginning to fall into place.

Star-Forming, Recently Star-Forming, and “Red and Dead” Galaxies at 1 < Z < 2

We summarize some of the key results on red galaxies presented in the first five papers from the Gemini Deep Deep Survey (GDDS). The GDDS is an ultra-deep (K < 20.6 mag, I < 24.5 mag) spectroscopic redshift survey, which preferentially targeted galaxies in the redshift range 0.8 < z < 2. The survey was completed in 2004, and the data is publicly available. The primary goal of the GDDS was to make the first direct measurement of the evolving stellar mass function over 0.8 < z < 2. We find that ∼ 15% of the local stellar mass density is already in place by z = 1.8, rising to 40-50% by z = 1. Nearly half the stellar mass density at z = 1.8 is in massive “red-and-dead” galaxies. Almost all of these red-and-dead galaxies exhibit early-type morphologies, although around 20% of these show tidal distortions consistent with recent (dry?) merger activity. A decomposition of the star-formation history of the Universe into the sum of individual histories for mass-segregated galaxy populations reveals strong evidence for the “down-sizing” paradigm espoused by Cowie et al. (1997). We find that the most massive galaxies form early in the history of the Universe and galaxy formation proceeds from larger to smaller mass scales.