Finding Young Stellar Populations in Elliptical Galaxies from Independent Components of Optical Spectra

Elliptical galaxies are believed to consist of a single population of old stars formed together at an early epoch in the Universe, yet recent analyses of galaxy spectra seem to indicate the presence of significant younger populations of stars in them. The detailed physical modelling of such populations is computationally expensive, inhibiting the detailed analysis of the several million galaxy spectra becoming available over the next few years. Here we present a data mining application aimed at decomposing the spectra of elliptical galaxies into several coeval stellar populations, without the use of detailed physical models. This is achieved by performing a linear independent basis transformation that essentially decouples the initial problem of joint processing of a set of correlated spectral measurements into that of the independent processing of a small set of prototypical spectra. Two methods are investigated: (1) A fast projection approach is derived by exploiting the correlation structure of neighboring wavelength bins within the spectral data. (2) A factorisation method that takes advantage of the positivity of the spectra is also investigated. The preliminary results show that typical features observed in stellar population spectra of different evolutionary histories can be convincingly disentangled by these methods, despite the absence of input physics. The success of this basis transformation analysis in recovering physically interpretable representations indicates that this technique is a potentially powerful tool for astronomical data mining.Comment: 12 Pages, 7 figures; accepted in SIAM 2005 International Conference on Data Mining, Newport Beach, CA, April 200

The star formation history of elliptical galaxies

The results of an investigation into the star formation history of elliptical galaxies, and the subsequent implications for the still controversial issue of their formation and evolution are presented. New, high signal-to-noise data and new, non-solar metallicity models motivate the re-investigation of elliptical galaxy formation with synthetic stellar population models, which has been difficult in the past because of uncertainties in the model spectra and the effects of age-metallicity degeneracy.Three different groups of elliptical galaxies, covering a range of redshift and nuclear activity are investigated. The rest-frame optical spectra of a population of 24 low-redshift (z ≃ 0.2) AGN host galaxies, together with the high-quality rest-frame ultraviolet spectra of two high-redshift (z ≃ 1.5) mJy radio galaxies and ultraviolet-to-optical spectra of two lowredshift (z < 0.1) inactive galaxies are compared with a range of simple, near-instantaneous starburst stellar population model spectra from various authors. With the benefit of nonsolar metallicity models, the attempt is made to reliably determine the ages and metallicities of the stellar populations of the observed galaxies via continuum fitting.A simultaneous test of the ability of these models to accurately reproduce the spectra of real stellar populations is also presented. This is carried out both by testing the models’ ability to recover the correct ages and metallicities of two well-studied F stars and the Sun, in the regimes where these stars dominate the integrated model flux, and by investigating the quality of the continuum fits to the observed galaxy spectra.With high-quality data, and reliable model spectra, it is possible to lift age-metallicity degeneracy, and robustly constrain the ages and metallicities of the galaxies studied. Models with stellar populations of more than one age and / or metallicity have been constructed, and this has allowed composite galaxy populations to be disentangled.The results are consistent with the existence of two classes of elliptical galaxies, one formed at high redshift, with predominantly passive evolution, and the other formed at lower redshift, from galaxy-galaxy merging, with associated star formation. The spectroscopic determination of the stellar content of the galaxies studied has enabled their epochs of star formation to be determined, and hence their formation routes.The constraints imposed by the reliable identification of old stellar populations at known redshift favour a A-dominated cosmological model, and strongly reject an Einstein-de Sitter universe

The Sun, stellar-population models, and the age estimation of high-redshift galaxies

Given sufficiently deep optical spectroscopy, the age estimation of high-redshif t ($z > 1$) galaxies has been claimed to be a relatively robust process (e.g. Dunlop et al. 1996) due to the fact that, for ages $< 5$Gyr, the near-ultraviolet light of a stellar population is expected to be dominated by `well-understood' main-sequence (MS) stars. Recently, however, the reliability of this process has been called into question by Yi et al (2000), who claim to have developed models in which the spectrum produced by the main sequence reddens much more rapidly than in the models of Jimenez et al (2000a), leading to much younger age estimates for the reddest known high-redshift ellipticals. In support of their revised age estimates, Yi et al cite the fact that their models can reproduce the spectrum of the Sun at an age of 5 Gyr, whereas the solar spectrum is not reproduced by the Jimenez et al models until $\simeq 10$ Gyr. Here we confirm this discrepancy, but point out that this is in fact a {\it strength} of the Jimenez et al models and indicative of some flaw in the models of Yi et al (which, in effect, imply that the Sun will turn into a red giant any minute now). We have also explored the models of Worthey (1994) (which are known to differ greatly from those of Jimenez et al in the treatment of post-MS evolution) and find that the main-sequence component of Worthey's models also cannot reproduce the solar spectrum until an age of 9-10 Gyr. We conclude that either the models of Yi et al are not as main-sequence dominated at 4-5 Gyr as claimed, or that the stellar evolutionary timescale in these models is in error by a factor possibly as high as two. (abridged)Comment: Submitted to MNRAS, final versio

Premature dismissal of high-redshift elliptical galaxies

It has recently been argued that single-collapse high-redshift models for elliptical galaxy formation can be rejected because they predict large numbers of very red galaxies at intermediate redshifts which are not seen in deep optical-infrared surveys. We argue, however, that this conclusion is premature since, while much effort has been invested in refining the predictions of hierarchical CDM models, only very simplistic models have been used to study the evolution of galaxies in other cosmogonies (e.g. isocurvature models). We demonstrate that the use of a more realistic multi-zone chemo-dynamical single-collapse model, yields colours at intermediate redshifts which are much bluer than inferred from the one-zone model, and indeed are comparable to those predicted by hierarchical merging despite still allowing $> 90$ of the final stellar mass of elliptical galaxies to be formed in the first Gyr of their evolution. We, therefore, conclude that the one-zone model should be avoided to predict the colours of high-redshift galaxies and that the use of realistic multi-zone models allows the existence of ellipticals at high redshift, being their dismissal premature.Comment: Submitted to MNRA

Young stellar populations in early-type galaxies in the Sloan Digital Sky Survey

We use a purely data-driven rectified factor analysis to identify early-type galaxies with recent star formation in DR4 of the SDSS Spectroscopic Catalogue. We compare the spectra and environment of these galaxies with `normal' early-types, and a sample of independently selected E+A galaxies. We calculate the projected local galaxy surface density (Sigma_5 and Sigma_10) for each galaxy in our sample, and find that the dependence, on projected local density, of the properties of E+As is not significantly different from that of early-types with young stellar populations, dropping off rapidly towards denser environments, and flattening off at densities < 0.1-0.3 Mpc^-2. The dearth of E+A galaxies in dense environments confirms that E+As are most likely the products of galaxy-galaxy merging or interactions, rather than star-forming galaxies whose star formation has been quenched by processes unique to dense environments. We see a tentative peak in the number of E+A galaxies at Sigma_10 \~ 0.1-0.3 Mpc^-2, which may represent the local galaxy density at which the rate of galaxy-galaxy merging or interaction rate peaks. Analysis of the spectra of our early-types with young stellar populations suggests that they have a stellar component dominated by F stars, ~ 1-4 Gyr old, together with a mature, metal-rich population characteristic of `typical' early-types. The young stars represent > 10% of the stellar mass in these galaxies. This, together with the similarity of the environments in which this `E+F' population and the E+A galaxy sample are found, suggests that E+F galaxies used to be E+A galaxies, but have evolved by a further ~ one to a few Gyr. Our factor analysis is sensitive enough to identify this hidden population. (Abridged)Comment: 7 pages, 5 figures, submitted to MNRAS, minor revisio

The star-formation histories of elliptical galaxies across the fundamental plane

We present the first results from a study designed to test whether, given high-quality spectrophotometry spanning the mid-UV--optical wavelength regime, it is possible to distinguish the metal content (Z) and star-formation history (sfh) of individual elliptical galaxies with sufficient accuracy to establish whether their formation history is linked to their detailed morphology and position on the Fundamental Plane. From a detailed analysis of UV-optical spectrophotometry of the `cuspy' elliptical galaxy NGC 3605 and the giant elliptical NGC 5018 we find that: 1) optical spectra with l > 3500 A may not contain sufficient data to robustly uncover all the stellar populations present in individual galaxies, even in such relatively passive objects as ellipticals, 2) the addition of the UV data approaching l = 2500 A holds the key to establishing well-constrained sfhs, from which we can infer a formation and evolution history which is consistent with their photometric properties, 3) despite the superficial similarity of their spectra, the two galaxies have very different `recent' sfhs -- the smaller, cuspy elliptical NGC 3605 contains a high-Z population of age ~= 1 Gyr, and has a position on the fundamental plane typical of the product of a low-z gas-rich merger (most likely at z ~ 0.08), while the giant elliptical NGC 5018, with a sub-solar secondary population, appears to have gained its more recent stars via mass transfer / accretion of gas from its spiral companion, 4) despite these differences in detailed history, more than 85% of the stellar mass of both galaxies is associated with an old (9-12 Gyr) stellar population of near-solar Z. This pilot study provides strong motivation for the construction and analysis of high-quality UV-optical spectra for a substantial sample of ellipticals spanning the Fundamental Plane.Comment: 11 pages, 10 figures, submitted to MNRAS, revised versio

The ages of quasar host galaxies

We present the results of fitting deep off-nuclear optical spectroscopy of radio-quiet quasars, radio-loud quasars and radio galaxies at z ~ 0.2 with evolutionary synthesis models of galaxy evolution. Our aim was to determine the age of the dynamically dominant stellar populations in the hos t galaxies of these three classes of powerful AGN. Some of our spectra display residual nuclear contamination at the shortest wavelengths, but the detailed quality of the fits longward of the 4000A break provide unequivocal proof, if further proof were needed, that quasars lie in massive galaxies with (at least at z ~ 0.2) evolved stellar populations. By fitting a two-component model we have separated the very blue (starburst and/or AGN contamination) from the redder underlying spectral energy distribution, and find that the hosts of all three classes of AGN are dominated by old stars of age 8 - 14 Gyr. If the blue component is attributed to young stars, we find that, at most, 1% of the baryonic mass of these galaxies is involved in star-formation activity at the epoch of observation. These results strongly support the conclusion reached by McLure et al. (1999) that the host galaxies of luminous quasars are massive ellipticals which formed prior to the peak epoch of quasar activity at z ~ 2.5.Comment: 24 pages, LaTeX, uses MNRAS style file, incorporates 19 postscript figures, final version, to be published in MNRA

Repeat-Driven Generation of Antigenic Diversity in a Major Human Pathogen, Trypanosoma cruzi.

Trypanosoma cruzi, a zoonotic kinetoplastid protozoan parasite, is the causative agent of American trypanosomiasis (Chagas disease). Having a very plastic, repetitive and complex genome, the parasite displays a highly diverse repertoire of surface molecules, with pivotal roles in cell invasion, immune evasion and pathogenesis. Before 2016, the complexity of the genomic regions containing these genes impaired the assembly of a genome at chromosomal level, making it impossible to study the structure and function of the several thousand repetitive genes encoding the surface molecules of the parasite. We here describe the genome assembly of the Sylvio X10/1 genome sequence, which since 2016 has been used as a reference genome sequence for T. cruzi clade I (TcI), produced using high coverage PacBio single-molecule sequencing. It was used to analyze deep Illumina sequence data from 34 T. cruzi TcI isolates and clones from different geographic locations, sample sources and clinical outcomes. Resolution of the surface molecule gene distribution showed the unusual duality in the organization of the parasite genome, a synteny of the core genomic region with related protozoa flanked by unique and highly plastic multigene family clusters encoding surface antigens. The presence of abundant interspersed retrotransposons in these multigene family clusters suggests that these elements are involved in a recombination mechanism for the generation of antigenic variation and evasion of the host immune response on these TcI strains. The comparative genomic analysis of the cohort of TcI strains revealed multiple cases of such recombination events involving surface molecule genes and has provided new insights into T. cruzi population structure

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Effect of angiotensin-converting enzyme inhibitor and angiotensin receptor blocker initiation on organ support-free days in patients hospitalized with COVID-19

IMPORTANCE Overactivation of the renin-angiotensin system (RAS) may contribute to poor clinical outcomes in patients with COVID-19. Objective To determine whether angiotensin-converting enzyme (ACE) inhibitor or angiotensin receptor blocker (ARB) initiation improves outcomes in patients hospitalized for COVID-19. DESIGN, SETTING, AND PARTICIPANTS In an ongoing, adaptive platform randomized clinical trial, 721 critically ill and 58 non–critically ill hospitalized adults were randomized to receive an RAS inhibitor or control between March 16, 2021, and February 25, 2022, at 69 sites in 7 countries (final follow-up on June 1, 2022). INTERVENTIONS Patients were randomized to receive open-label initiation of an ACE inhibitor (n = 257), ARB (n = 248), ARB in combination with DMX-200 (a chemokine receptor-2 inhibitor; n = 10), or no RAS inhibitor (control; n = 264) for up to 10 days. MAIN OUTCOMES AND MEASURES The primary outcome was organ support–free days, a composite of hospital survival and days alive without cardiovascular or respiratory organ support through 21 days. The primary analysis was a bayesian cumulative logistic model. Odds ratios (ORs) greater than 1 represent improved outcomes. RESULTS On February 25, 2022, enrollment was discontinued due to safety concerns. Among 679 critically ill patients with available primary outcome data, the median age was 56 years and 239 participants (35.2%) were women. Median (IQR) organ support–free days among critically ill patients was 10 (–1 to 16) in the ACE inhibitor group (n = 231), 8 (–1 to 17) in the ARB group (n = 217), and 12 (0 to 17) in the control group (n = 231) (median adjusted odds ratios of 0.77 [95% bayesian credible interval, 0.58-1.06] for improvement for ACE inhibitor and 0.76 [95% credible interval, 0.56-1.05] for ARB compared with control). The posterior probabilities that ACE inhibitors and ARBs worsened organ support–free days compared with control were 94.9% and 95.4%, respectively. Hospital survival occurred in 166 of 231 critically ill participants (71.9%) in the ACE inhibitor group, 152 of 217 (70.0%) in the ARB group, and 182 of 231 (78.8%) in the control group (posterior probabilities that ACE inhibitor and ARB worsened hospital survival compared with control were 95.3% and 98.1%, respectively). CONCLUSIONS AND RELEVANCE In this trial, among critically ill adults with COVID-19, initiation of an ACE inhibitor or ARB did not improve, and likely worsened, clinical outcomes. TRIAL REGISTRATION ClinicalTrials.gov Identifier: NCT0273570