arXiv subjects

Alan F. Heavens

Publications and source records attributed to Alan F. Heavens.

At least 55 records · Page 3Linked to original sources

arXiv2009

First lensing measurements of SZ-discovered clusters

We present the first lensing mass measurements of Sunyaev-Zel'dovich (SZ) selected clusters. Using optical imaging from the Southern Cosmology Survey (SCS), we present weak lensing masses for three clusters selected by their SZ emission in the South Pole Telescope survey (SPT). We confirm that the SZ selection procedure is successful in detecting mass concentrations. We also study the weak lensing signals from 38 optically-selected clusters in ~8 square degrees of the SCS survey. We fit Navarro, Frenk and White (NFW) profiles and find that the SZ clusters have amongst the largest masses, as high as 5x10^14 Msun. Using the best fit masses for all the clusters, we analytically calculate the expected SZ integrated Y parameter, which we find to be consistent with the SPT observations.

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arXiv2008

Physical Classification of Galaxies with MOPED/VESPA

The availability of high-quality spectra for a large number of galaxies in the SDSS survey allows for a more sophisticated extraction of information about their stellar populations than, e.g., the luminosity weighted age. Indeed, sophisticated and robust techniques to fully analyze galaxy spectra have now reached enough maturity as to trust their results and findings. By reconstructing the star formation and metallicity history of galaxies from the SDSS fossil record and analyzing how it relates to its environment, we have learned how to classify galaxies: to first order the evolution of a galaxy is determined by its present stellar mass, which in turn seems to be governed by the merger rate of dark halos.

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arXiv2008

Evidence for short-lived SN Ia progenitors

We use the VESPA algorithm and spectra from the Sloan Digital Sky Survey to investigate the star formation history of the host galaxies of 257 Type Ia supernovae. We find 5$σ$ evidence for a short-lived population of progenitors with lifetimes of less than 180 Myr, indicating a Type Ia supernova channel arising from stars in the mass range $\sim$3.5-8 $M_\odot$. As standardizeable candles, Type Ia supernovae play an important role in determining the expansion history of the Universe, but to be useful for future cosmological surveys, the peak luminosity needs to be free of uncorrected systematic effects at the level of 1-2%. If the different progenitor routes lead to supernovae with even moderately small differences in properties, then these need to be corrected for separately, or they could lead to a systematic bias in future supernovae surveys, as the prompt route is likely to increase in importance at high redshift. VESPA analysis of hosts could be a valuable tool in this, by identifying which progenitor route is most likely.

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arXiv2008

The role of spin in the formation and evolution of galaxies

Using the SDSS spectroscopic sample, we estimate the dark matter halo spin parameter lambda for ~53,000 disk galaxies for which MOPED star formation histories are available. We investigate the relationship between spin and total stellar mass, star formation history, and environment. First, we find a clear anti-correlation between stellar mass and spin, with low mass galaxies generally having high dark matter spins. Second, galaxies which have formed more than ~5% of their stars in the last 0.2 Gyr have more broadly distributed and typically higher spins (including a significant fraction with lambda > 0.1) than galaxies which formed a large fraction of their stars more than 10 Gyr ago. Finally, we find little or no correlation between the value of spin of the dark halo and environment as determined both by proximity to a new cluster catalog and a marked correlation study. This agrees well with the predictions from linear hierarchical torquing theory and numerical simulations.

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arXiv2008

The Cosmic Evolution of Metallicity from the SDSS Fossil Record

We present the time evolution of the stellar metallicity for SDSS galaxies, a sample that spans five orders of magnitude in stellar mass (10^7 - 10^{12} Msun). Assuming the BC03 stellar population models, we find that more massive galaxies are more metal-rich than less massive ones at all redshifts; the mass-metallicity relation is imprinted in galaxies from the epoch of formation. For galaxies with present stellar masses > 10^{10} Msun, the time evolution of stellar metallicity is very weak, with at most 0.2-0.3 dex over a Hubble time- for this reason the mass-metallicity relation evolves little with redshift. However, for galaxies with present stellar masses < 10^{10} Msun, the evolution is significant, with metallicity increasing by more than a decade from redshift 3 to the present. By being able to recover the metallicity history, we have managed to identify the origin of a recent discrepancy between the metallicity recovered from nebular lines and absorption lines. As expected, we show that the young population dominates the former while the old population the latter. We have investigated the dependence on the stellar models used and find that older stellar population synthesis codes do not produce a clear result. Finally, we have explored the relationship between cluster environment and metallicity, and find a strong correlation in the sense that galaxies in high density regions have high metallicity.

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arXiv2007

Recovering galaxy star formation and metallicity histories from spectra using VESPA

We introduce VErsatile SPectral Analysis (VESPA): a new method which aims to recover robust star formation and metallicity histories from galactic spectra. VESPA uses the full spectral range to construct a galaxy history from synthetic models. We investigate the use of an adaptative parametrization grid to recover reliable star formation histories on a galaxy-by-galaxy basis. Our goal is robustness as opposed to high resolution histories, and the method is designed to return high time resolution only where the data demand it. In this paper we detail the method and we present our findings when we apply VESPA to synthetic and real Sloan Digital Sky Survey (SDSS) spectroscopic data. We show that the number of parameters that can be recovered from a spectrum depends strongly on the signal-to-noise, wavelength coverage and presence or absence of a young population. For a typical SDSS sample of galaxies, we can normally recover between 2 to 5 stellar populations. We find very good agreement between VESPA and our previous analysis of the SDSS sample with MOPED.

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arXiv2007

The star formation histories of galaxies in the Sloan Digital Sky Survey

We present the results of a MOPED analysis of ~3 x 10^5 galaxy spectra from the Sloan Digital Sky Survey Data Release Three (SDSS DR3), with a number of improvements in data, modelling and analysis compared with our previous analysis of DR1. The improvements include: modelling the galaxies with theoretical models at a higher spectral resolution of 3Å; better calibrated data; an extended list of excluded emission lines, and a wider range of dust models. We present new estimates of the cosmic star formation rate, the evolution of stellar mass density and the stellar mass function from the fossil record. In contrast to our earlier work the results show no conclusive peak in the star formation rate out to a redshift around 2 but continue to show conclusive evidence for `downsizing' in the SDSS fossil record. The star formation history is now in good agreement with more traditional instantaneous measures. The galaxy stellar mass function is determined over five decades of mass, and an updated estimate of the current stellar mass density is presented. We also investigate the systematic effects of changes in the stellar population modelling, the spectral resolution, dust modelling, sky lines, spectral resolution and the change of data set. We find that the main changes in the results are due to the improvements in the calibration of the SDSS data, changes in the initial mass function and the theoretical models used.

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arXiv2006

The ages, metallicities and star formation histories of early-type galaxies in SDSS

We use the spectra of ~ 22,000 early-type galaxies, selected from the Sloan Digital Sky Survey, to infer the ages, metallicities and star formation histories of these galaxies. We find clear evidence of "downsizing", i.e. galaxies with larger velocity dispersion have older stellar populations. In particular, most early-type galaxies with velocity dispersion exceeding 200 km s-1 formed more than 90% of their current stellar mass at redshift z > 2.5. Therefore, star formation was suppressed around this redshift. We also show that chemical enrichment was rapid, lasting 1-2 Gyr and find evidence that [Fe/H] is sub-solar. We study the robustness of these results by comparing three different approaches: using (i) Lick absorption line indices; (ii) fitting a single-burst stellar population model to the whole spectrum (lines+continuum); and (iii) reconstructing the star formation and metallicity histories in multiple age-bins, providing a method to measure mass-weighted ages and metallicities. We find good agreement between the luminosity-weighted ages and metallicities computed with these three methods.

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arXiv2003

Star Formation and Metallicity History of the SDSS galaxy survey: unlocking the fossil record

Using MOPED we determine non-parametrically the star-formation and metallicity history of over 37,000 high-quality galaxy spectra from the Sloan Digital Sky Survey (SDSS) early data release. We use the entire spectral range, rather than concentrating on specific features, and we estimate the complete star formation history without prior assumptions about its form (by constructing so-called `population boxes'). The main results of this initial study are that the star formation rate in SDSS galaxies has been in decline for ~6 Gyr; a metallicity distribution for star-forming gas which is peaked ~3 Gyr ago at about solar metallicity, inconsistent with closed-box models, but consistent with infall models. We also determine the infall rate of gas in SDSS and show that it has been significant for the last 3 Gyr. We investigate errors using a Monte-Carlo Markov Chain algorithm. Further, we demonstrate that recovering star formation and metallicity histories for such a large sample becomes intractable without data compression methods, particularly the exploration of the likelihood surface. By exploring the whole likelihood surface we show that age-metallicity degeneracies are not as severe as by using only a few spectral features. We find that 65% of galaxies contain a significant old population (with an age of at least 8 Gyr), including recent starburst galaxies, and that over 97% have some stars older than 2 Gyr. It is the first time that the past star formation history has been determined from the fossil record of the present-day spectra of galaxies.

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arXiv2002

The bias of galaxies and the density of the universe from the 2dF galaxy redshift survey

By studying the bispectrum of the galaxy distribution in the 2dF galaxy redhsift survey (2dFGRS), we have shown that 2dFGRS galaxies are unbiased tracers of the mass distribution. This allows us the break the degeneracy intrinsic to power spectrum studies, between the matter density parameter Omega_m and the bias parameter b, and to obtain an accurate measurement of Omega_m: Omega_{m,z_{eff}}=0.27 \pm 0.06, a measurement obtained from the 2dFGRS alone, independently from other data sets. This result has to be interpreted as Omega_m at the effective redshift of the survey z_{eff}=0.17. Extrapolated at z=0 we obtain Omega_m=0.2 \pm 0.06 for the LCDM model. This constraint on the matter density parameter when combined with cosmic microwave background constraints on the flatness of the Universe, show firm evidence that the Universe is dominated by a vacuum energy component.

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arXiv2002

F stars, metallicity, and the ages of red galaxies at z > 1

We explore whether the rest-frame near-UV spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-UV spectra of F stars of known (sub-solar and super-solar) metallicity. F stars are of particular interest because the rest-frame near-UV spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turnoff. We find that, in the case of the F stars, where the HST ultraviolet spectra have high signal:noise, model-fitting with metallicity allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turnoff ages of these stars yielded by the model fitting are well constrained. Encouraged by this we have fitted these same variable- metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age-metallicity degeneracy is not so easily lifted for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED (Heavens et al., 2000) leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein-de Sitter universe, and favour a lambda-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3 - 5.

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arXiv2002

Fast Parameter Estimation from the CMB Power Spectrum

The statistical properties of a map of the primary fluctuations in the cosmic microwave background (CMB) may be specified to high accuracy by a few thousand power spectra measurements, provided the fluctuations are gaussian, yet the number of parameters relevant for the CMB is probably no more than about 10-20. There is consequently a large degree of redundancy in the power spectrum data. In this paper, we show that the MOPED data compression technique can reduce the CMB power spectrum measurements to about 10-20 numbers (one for each parameter), from which the cosmological parameters can be estimated virtually as accurately as from the complete power spectrum. This offers opportunities for very fast parameter estimation from real and simulated CMB skies, with accurate likelihood calculations at Planck resolution being speeded up by a factor of around five hundred million.

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arXiv2002

The 2dF Galaxy Redshift Survey: The bias of galaxies and the density of the Universe

We compute the bispectrum of the 2dF Galaxy Redshift Survey (2dFGRS) and use it to measure the bias parameter of the galaxies. This parameter quantifies the strength of clustering of the galaxies relative to the mass in the Universe. By analysing 80 million triangle configurations in the wavenumber range 0.1 < k < 0.5 h/Mpc (i.e. on scales roughly between 5 and 30 Mpc/h) we find that the linear bias parameter is consistent with unity: b_1=1.04 pm 0.11, and the quadratic (nonlinear) bias is consistent with zero: b_2=-0.054 pm 0.08. Thus, at least on large scales, optically-selected galaxies do indeed trace the underlying mass distribution. The bias parameter can be combined with the 2dFGRS measurement of the redshift distortion parameter beta = Omega_m^{0.6}/b_1, to yield Omega_m = 0.27 pm 0.06 for the matter density of the Universe, a result which is determined entirely from this survey, independently of other datasets. Our measurement of the matter density of the Universe should be interpreted as Omega_m at the effective redshift of the survey (z=0.17).

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arXiv2001

Testing dark matter with high-redshift supernovae

Dark matter in the Universe consisting of macroscopic objects such as primordial black holes may cause gravitational lensing of distant objects. The magnification associated with lensing will lead to additional scatter in the received flux from standard candles, and too small an observed scatter could rule out compact dark matter entirely. In this letter, we show how the scatter in fluxes of distant Type 1a supernovae could be used to distinguish between models with and without lensing by macroscopic dark matter. The proposed SNAP project, with $\sim 2400$ supernovae in the range $0.1\ls z\ls 1.7$, should be able to identify models at 99.9% confidence, if systematic errors are controlled. Note that this test is independent of any evolution of the mean supernova luminosity with redshift. The variances of the current Supernova Cosmology Project sample do not rule out compact lenses as dark matter: formally they favour such a population, but the significance is low, and removal of a single faint supernova from the sample reverses the conclusion.

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arXiv2001

On the trispectrum as a gaussian test for cosmology

In the standard model for structure formation, bound objects originate from the gravitational collapse of small perturbations arising from quantum fluctuations with random phases. In other scenarios, based on defects, structures are seeded by localized energy density. In principle, it is possible to differentiate between these models on the basis of their statistical properties; only in the former case is the initial density field an almost-perfect random gaussian field. In this paper, we investigate the use of the trispectrum of the galaxy density field, which is the connected four-point function in Fourier space, as a discriminant between gaussian and non-gaussian models. It has the advantage of having only weak non-linear growth. We define a related statistic $τ$ which, as a test of the gaussian hypothesis, is independent of cosmology, the power spectrum and biasing, in real space, and which is, in principle, a measure of the departure from gaussian statistics. For galaxy redshift surveys, the statistic depends on cosmology and bias only through the potentially observable parameter $β$. We compute the expected errors on the estimate of $τ$, and demonstrate with numerical simulations that it can be a useful discriminant of models, with the important proviso that any bias is linear on large scales. Whether it is the most effective method is uncertain and depends on the nature of the departure from gaussianity.

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arXiv2000

Detection Limits for Super-Hubble Suppression of Causal Fluctuations

We investigate to what extent future microwave background experiments might be able to detect a suppression of fluctuation power on large scales in flat and open universe models. Such suppression would arise if fluctuations are generated by causal processes, and a measurement of a small suppression scale would be problematic for inflation models, but consistent with many defect models. More speculatively, a measurement of a suppression scale of the order of the present Hubble radius could provide independent evidence for a fine-tuned inflation model leading to a low-density universe. We find that, depending on the primordial power spectrum, a suppression scale modestly larger than the visible Horizon can be detected, but that the detectability drops very rapidly with increasing scale. For models with two periods of inflation, there is essentially no possibility of detecting a causal suppression scale.

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arXiv2000

Full-sky correlations of peaks in the microwave background

We compute precise predictions for the two-point correlation function of local maxima (or minima) in the temperature of the microwave background, under the assumption that it is a random gaussian field. For a given power spectrum and peak threshold there are no adjustable parameters, and since this analysis does not make the small-angle approximation of Heavens & Sheth (1999), it is essentially complete. We find oscillatory features which are absent in the temperature autocorrelation function, and we also find that the small-angle approximation to the peak-peak correlation function is accurate to better than 0.01 on all scales. These high-precision predictions can form the basis of a sensitive test of the gaussian hypothesis with upcoming all-sky microwave background experiments MAP and Planck, affording a thorough test of the inflationary theory of the early Universe. To illustrate the effectiveness of the technique, we apply it to simulated maps of the microwave sky arising from the cosmic string model of structure formation, and compare with the bispectrum as a non-gaussian discriminant. We also show how peak statistics can be a valuable tool in assessing and statistically removing contamination of the map by foreground point sources.

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arXiv2000

Projected bispectrum in spherical harmonics and its application to angular galaxy catalogues

We present a theoretical and exact analysis of the bispectrum of projected galaxy catalogues. The result can be generalized to evaluate the projection in spherical harmonics of any 3D bispectrum and therefore has applications to cosmic microwave background and gravitational lensing studies. By expanding the 2D distribution of galaxies on the sky in spherical harmonics, we show how the 3-point function of the coefficients can be used in principle to determine the bias parameter of the galaxy sample. If this can be achieved, it would allow a lifting of the degeneracy between the bias and the matter density parameter of the Universe which occurs in linear analysis of 3D galaxy catalogues. In previous papers we have shown how a similar analysis can be done in three dimensions, and we show here, through an error analysis and by implementing the method on a simulated projected catalogue, that ongoing three-dimensional galaxy redshift surveys (even with all the additional uncertainties introduced by partial sky coverage, redshift-space distortions and smaller numbers) will do far better than all-sky projected catalogues with similar selection function.

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