Raymond G. Carlberg

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.

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.

Discovery of the Low-Redshift Optical Afterglow of GRB 011121 and Its Progenitor Supernova SN 2001ke*

We present the discovery and follow-up observations of the afterglow of the gamma-ray burst GRB 011121 and its associated supernova SN 2001ke. Images were obtained with the Optical Gravitational Lensing Experiment 1.3 m telescope in BVRI passbands, starting 10.3 hr after the burst. The temporal analysis of our early data indicates a steep decay, independent of wavelength, with Fν ∝ t-1.72±0.05. There is no evidence for a break in the light curve earlier than 2.5 days after the burst. The spectral energy distribution determined from the early broadband photometry is a power law with Fν ∝ ν-0.66±0.13 after correcting for a large reddening. Spectra obtained with the Magellan 6.5 m Baade telescope reveal narrow emission lines from the host galaxy that provide a redshift of z = 0.362 ± 0.001 to the GRB. We also present late R- and J-band observations of the afterglow ~7-17 days after the burst. The late-time photometry shows a large deviation from the initial decline, and our data combined with Hubble Space Telescope photometry provide strong evidence for a supernova peaking about 12 rest-frame days after the GRB. The first spectrum ever obtained of a GRB supernova at cosmological distance revealed a blue continuum. SN 2001ke was more blue near maximum than SN 1998bw and faded more quickly, which demonstrates that a range of properties are possible in supernovae that generate GRBs. The blue color is consistent with a supernova interacting with circumstellar gas, and this progenitor wind is also evident in the optical afterglow. This is the best evidence to date that classical, long GRBs are generated by core-collapse supernovae.

A proto-galaxy candidate at z = 2.7 discovered by its young stellar population

A protogalaxy candidate at z=2.72 has been discovered serendipitously by the CNOC cluster redshift survey. The candidate is an extremely luminous (V=20.5 mag, absolute mag -26) and well resolved disk-like galaxy. The redshift is identified from a dozen strong UV absorption lines. No emission lines are found between 1000 and 2000A (rest), including Ly alpha. The photometric data fit the spectral energy distributions of a stellar population from 400 million years to an arbitrarily young age, dependent on the amount of dust extinction. However, the presence of a strong P-Cygni profile in CIV~indicates that a very substantial component of the stellar population must be younger than ~ 10 Myr. We interpret this object as an early-type galaxy observed within about 100 million years of the initial burst of star formation which created most of its stellar mass. Because of the resolved, regular, and smooth nature of the object, it is unlikely that the high luminosity is due to gravitational lensing.

Toward a cosmological Hubble diagram for Type II-P supernovae

We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based on five events at redshift up to z ~ 0.3. This diagram was constructed using photometry from the Canada-France-Hawaii Telescope Supernova Legacy Survey and absorption-line spectroscopy from the Keck Observatory. The method used to measure distances to these supernovae is based on recent work by Hamuy & Pinto and exploits a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe II λ 5169 P Cygni feature observed during the plateau phases. We present three refinements to this method that significantly improve the practicality of measuring the distances of SNe II-P at cosmologically interesting redshifts. These are an extinction correction measurement based on the V-I colors at day 50, a cross-correlation measurement for the expansion velocity, and the ability to extrapolate such velocities accurately over almost the entire plateau phase. We apply this revised method to our data set of high-redshift SNe II-P and find that the resulting Hubble diagram has a scatter of only 0.26 mag, thus demonstrating the feasibility of measuring the expansion history, with present facilities, using a method independent of that based on supernovae of Type Ia.

Red nuggets at high redshift: structural evolution of quiescent galaxies over 10 Gyr of cosmic history

We present an analysis of the size growth seen in early-type galaxies over 10 Gyr of cosmic time. Our analysis is based on a homogeneous synthesis of published data from 16 spectroscopic surveys observed at similar spatial resolution, augmented by new measurements for galaxies in the Gemini Deep Deep Survey. In total, our sample contains structural data for 465 galaxies (mainly early-type) in the redshift range 0.2 < z < 2.7. The size evolution of passively evolving galaxies over this redshift range is gradual and continuous, with no evidence for an end or change to the process around z ~ 1, as has been hinted at by some surveys which analyze subsets of the data in isolation. The size growth appears to be independent of stellar mass, with the mass-normalized half-light radius scaling with redshift as Re ∝(1 + z)–1.62 ± 0.34. Surprisingly, this power law seems to be in good agreement with the recently reported continuous size evolution of UV-bright galaxies in the redshift range z ~ 0.5-3.5. It is also in accordance with the predictions from recent theoretical models.

Superluminous supernovae at redshifts of 2.05 and 3.90

A rare class of ‘superluminous’ supernovae that are about ten or more times more luminous at their peaks than other types of luminous supernova has recently been found at low to intermediate redshifts. A small subset of these events have luminosities that evolve slowly and result in radiated energies of up to about 1051 ergs. Therefore, they are probably examples of ‘pair-instability’ or ‘pulsational pair-instability’ supernovae with estimated progenitor masses of 100 to 250 times that of the Sun. These events are exceedingly rare at low redshift, but are expected to be more common at high redshift because the mass distribution of the earliest stars was probably skewed to high values. Here we report the detection of two superluminous supernovae, at redshifts of 2.05 and 3.90, that have slowly evolving light curves. We estimate the rate of events at redshifts of 2 and 4 to be approximately ten times higher than the rate at low redshift. The extreme luminosities of superluminous supernovae extend the redshift limit for supernova detection using present technology, previously 2.36 (ref. 8), and provide a way of investigating the deaths of the first generation of stars to form after the Big Bang.

The Las Campanas Infrared Survey – II. Photometric redshifts, comparison with models and clustering evolution

The Las Campanas Infrared (LCIR) Survey, using the Cambridge Infra-Red Survey Instrument (CIRSI), reaches H∼21 over nearly 1 deg^2. In this paper we present results from 744 arcmin^2 centred on the Hubble Deep Field South for which UBVRI optical data are publicly available. Making conservative magnitude cuts to ensure spatial uniformity, we detect 3177 galaxies to H=20.0 in 744 arcmin^2 and a further 842 to H=20.5 in a deeper subregion of 407 arcmin^2. We compare the observed optical–infrared (IR) colour distributions with the predictions of semi-analytic hierarchical models and find reasonable agreement. We also determine photometric redshifts, finding a median redshift of ∼0.55. We compare the redshift distributions N(z) of E, Sbc, Scd and Im spectral types with models, showing that the observations are inconsistent with simple passive-evolution models while semi-analytic models provide a reasonable fit to the total N(z) but underestimate the number of z∼1 red spectral types relative to bluer spectral types. We also present N(z) for samples of extremely red objects (EROs) defined by optical–IR colours. We find that EROs with R-H>4 and H 4 comprise ∼18 per cent of the observed galaxy population, while in semi-analytic models they contribute only ∼4 per cent. We also determine the angular correlation function w(θ) for magnitude, colour, spectral type and photometric redshift-selected subsamples of the data and use the photometric redshift distributions to derive the spatial clustering statistic ξ(r) as a function of spectral type and redshift out to z∼1.2. Parametrizing ξ(r) by ξ(rc,z)=[rc/r∗(z)]^(-1.8), where r_c is in comoving coordinates, we find that r∗(z) increases by a factor of 1.5–2 from z=0 to z∼1.2. We interpret this as a selection effect – the galaxies selected at z∼1.2 are intrinsically very luminous, about 1–1.5 mag brighter than L∗. When galaxies are selected by absolute magnitude, we find no evidence for evolution in r∗ over this redshift range. Extrapolated to z=0, we find r∗(z=0)∼6.5 h^(-1) Mpc for red galaxies and r∗(z=0)∼2–4 h^(-1) Mpc for blue galaxies. We also find that, while the angular clustering amplitude of EROs with R-H>4 or I-H>3 is up to four times that of the whole galaxy population, the spatial clustering length r∗(z=1) is ∼7.5–10.5 h^(-1) Mpc, which is only a factor of ∼1.7 times r∗(z=1) for R-H<4 and I-H<3 galaxies lying in a similar redshift and luminosity range. This difference is similar to that observed between red and blue galaxies at low redshifts.

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.

Galaxy groups at 0.3 ≤z≤ 0.55 – I. Group properties

The evolution of galaxies in groups may have important implications for the evolution of the star formation history of the universe, since many processes which operate in groups may suppress star formation and the fraction of galaxies in bound groups grows rapidly between z=1 and the present day. In this paper, we present an investigation of the properties of galaxies in galaxy groups at intermediate redshift (z ~ 0.4). The groups were selected from the CNOC2 redshift survey as described in Carlberg et al., 2001, with further spectroscopic follow-up undertaken at the Magellan telescope in order to improve the completeness and depth of the sample. We present the data for the individual groups, and find no clear trend in the fraction of passive galaxies with group velocity dispersion and group concentration. We stack the galaxy groups in order to compare the properties of group galaxies with those of field galaxies at the same redshift. The groups contain a larger fraction of passive galaxies than the field, this trend being particularly clear for galaxies brighter than M_{B_J}<-20 in the higher velocity dispersion groups. In addition, we see evidence for an excess of bright passive galaxies in the groups relative to the field. In contrast, the luminosity functions of the star forming galaxies in the groups and the field are consistent. These trends are qualitatively consitent with the differences between group and field galaxies seen in the local universe.