arXiv subjects

Ivan Baldry

Publications and source records attributed to Ivan Baldry.

29 records · Page 2Linked to original sources

arXiv2012

Galaxy And Mass Assembly: Estimating galaxy group masses via caustic analysis

We have generated complementary halo mass estimates for all groups in the Galaxy And Mass Assembly Galaxy Group Catalogue (GAMA G3Cv1) using a modified caustic mass estimation algorithm, originally developed by Diaferio & Geller (1997). We calibrate the algorithm by applying it on a series of 9 GAMA mock galaxy light cones and investigate the effects of using different definitions for group centre and size. We select the set of parameters that provide median-unbiased mass estimates when tested on mocks, and generate mass estimates for the real group catalogue. We find that on average, the caustic mass estimates agree with dynamical mass estimates within a factor of 2 in 90.8 +/- 6.1% groups and compares equally well to velocity dispersion based mass estimates for both high and low multiplicity groups over the full range of masses probed by the G3Cv1.

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arXiv2011

A Magellan-IMACS-IFU Search for Dynamical Drivers of Nuclear Activity. I. Reduction Pipeline and Galaxy Catalog

Using the Inamori Magellan Areal Camera and Spectrograph (IMACS) integral-field unit (IFU) on the 6.5m Magellan telescope, we have designed the first statistically significant investigation of the two-dimensional distribution and kinematics of ionized gas and stars in the central kiloparsec regions of a well-matched sample of Seyfert and inactive control galaxies selected from the Sloan Digital Sky Survey. The goals of the project are to use the fine spatial sampling (0.2 arcsec/pixel) and large wavelength coverage (4000-7000A) of the IMACS-IFU to search for dynamical triggers of nuclear activity in the central region where active galactic nucleus (AGN) activity and dynamical timescales become comparable, to identify and assess the impact of AGN-driven outflows on the host galaxy and to provide a definitive sample of local galaxy kinematics for comparison with future three-dimensional kinematic studies of high-redshift systems. In this paper, we provide the first detailed description of the procedure to reduce and calibrate data from the IMACS-IFU in `long mode' to obtain two-dimensional maps of the distribution and kinematics of ionized gas and stars. The sample selection criteria are presented, observing strategy described and resulting maps of the sample galaxies presented along with a description of the observed properties of each galaxy and the overall observed properties of the sample.

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arXiv2010

Dependence of Star Formation Activity On Stellar Mass and Environment From the Redshift One LDSS-3 Emission Line Survey (ROLES)

Using the sample from the \it Redshift One LDSS3 Emission line Survey \rm (ROLES), we probe the dependence of star formation rate (SFR) and specific star formation rate (sSFR) as a function of stellar mass $M_*$ and environment as defined by local galaxy density, in the CDFS field. Our spectroscopic sample consists of 312 galaxies with $K_{AB}<24$, corresponding to stellar mass $\log(M_*/M_{\sun})>8.5$, and with [OII] derived star-formation rates SFR$>0.3M_{\sun}/$yr, at $0.889\leq z \leq 1.149$. The results have been compared directly with the Sloan Digital Sky Survey Stripe 82 sample at $0.032\leq z \leq 0.05$. For star-forming galaxies, we confirm that there is little correlation between SFR and density at $z\sim 0$. However, for the lowest mass galaxies in our $z\sim 1$ sample, those with $\log(M_*/M_{\sun})<10$, we find that both the median SFR and specific SFR {\it increase} significantly with increasing local density. The "downsizing" trend for low mass galaxies to be quenched progressively later in time appears to be more pronounced in moderately overdense environments. Overall we find that the evolution of star-formation in galaxies is most strongly driven by their stellar mass, with local galaxy density playing a role that becomes increasingly important for lower mass galaxies.

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arXiv2008

Galaxy Mass Growth in GDDS and SDSS

I present some new results related to our understanding of the masses of galaxies both in the local and high-redshift Universe. At high-redshift new Spitzer data on galaxies in the Gemini Deep Deep Survey allow us a more accurate measure of stellar mass to light ratios (using rest frame near-IR light) showing a refinement of the measurements but not great discrepancies. In the local universe a new method is explored to estimate the baryonic mass function of galaxies including contributions from unseen HI. This points to an interesting result: that the baryonic mass function of galaxies may in fact be quite steep, of comparable slope to the mass function of dark matter haloes.

astro-ph↗

arXiv2008

The energy output of the Universe from 0.1 micron to 1000 micron

The dominant source of electromagnetic energy in the Universe today (over ultraviolet, optical and near-infrared wavelengths) is starlight. However, quantifying the amount of starlight produced has proven difficult due to interstellar dust grains which attenuate some unknown fraction of the light. Combining a recently calibrated galactic dust model with observations of 10,000 nearby galaxies we find that (integrated over all galaxy types and orientations) only (11 +/- 2)% of the 0.1 micron photons escape their host galaxies; this value rises linearly (with log(lambda)) to (87 +/- 3)% at 2.1 micron. We deduce that the energy output from stars in the nearby Universe is (1.6+/-0.2) x 10^{35} W Mpc^{-3} of which (0.9+/-0.1) x 10^{35} W Mpc^{-3} escapes directly into the inter-galactic medium. Some further ramifications of dust attenuation are discussed, and equations that correct individual galaxy flux measurements for its effect are provided.

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arXiv2005

Monster Redshift Surveys through Dispersive Slitless Imaging: The Baryon Oscillation Probe

Wide-field imaging from space should not forget the dispersive dimension. We consider the capability of space-based imaging with a slitless grism: because of the low near-infrared background in space and the high sky-density of high redshift emission line galaxies this makes for a very powerful redshift machine with no moving parts. A small 1m space telescope with a 0.5 degree field of view could measure redshifts for 10^7 galaxies at 0.5<z<2 per year, this is a MIDEX class concept which we have dubbed `The Baryon Oscillation Probe' as the primary science case would be constraining dark energy evolution via measurement of the baryonic oscillations in the galaxy power spectrum. These ideas are generalizable to other missions such as SNAP and DESTINY.

astro-ph↗

arXiv2004

The 2dF Galaxy Redshift Survey: the local E+A galaxy population

We select a sample of low-redshift (z ~ 0.1) E+A galaxies from the 2dF Galaxy Redshift Survey (2dFGRS). The spectra of these objects are defined by strong hydrogen Balmer absorption lines (H-delta, H-gamma, H-beta) combined with a lack of [OII] 3727A emission, together implying a recently-truncated burst of star formation. The E+A spectrum is thus a signpost to galaxies in the process of evolution. We quantify the local environments, clustering properties and luminosity function of the E+A galaxies. We find that the environments are consistent with the ensemble of 2dFGRS galaxies: low-redshift E+A systems are located predominantly in the field, existing as isolated objects or in poor groups. However, the luminosity distribution of galaxies selected using three Balmer absorption lines H-delta-gamma-beta appears more typical of ellipticals. Indeed, morphologically these galaxies are preferentially spheroidal (E/S0) systems. In a small but significant number we find evidence for recent major mergers, such as tidal tails. We infer that major mergers are one important formation mechanism for E+A galaxies, as suggested by previous studies. At low redshift the merger probability is high in the field and low in clusters, thus these recently-formed spheroidal systems do not follow the usual morphology-density relation for ellipticals. Regarding the selection of E+A galaxies: we find that basing the Balmer-line criterion solely on H-delta absorption leads to a significant sub-population of disk systems with detectable H-alpha emission. In these objects the [OII] emission is presumably either obscured by dust or present with a low signal-to-noise ratio, whilst the (H-gamma, H-beta) absorption features are subject to emission-filling.

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arXiv2004

FORTIS: Pathfinder to the Lyman Continuum

Shull et al. (1999) have asserted that the contribution of stars, relative to quasars, to the metagalactic background radiation that ionizes most of the baryons in the universe remains almost completely unknown at all epochs. The potential to directly quantify this contribution at low redshift has recently become possible with the identification by GALEX of large numbers of sparsely distributed faint ultraviolet galaxies. Neither STIS nor FUSE nor GALEX have the ability to efficiently survey these sparse fields and directly measure the Lyman continuum radiation that may leak into the low redshift (z < 0.4) intergalactic medium. We present here a design for a new type of far ultraviolet spectrograph, one that is more sensitive, covers wider fields, and can provide spectra and images of a large number of objects simultaneously, called the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS). We intend to use a sounding rocket flight to validate the new instrument with a simple long-slit observation of the starburst populations in the galaxy M83. If however, the long-slit were replaced with microshutter array, this design could isolate the chains of blue galaxies found by GALEX over an ~30' diameter field-of-view and directly address the Lyman continuum problem in a long duration orbital mission. Thus, our development of the sounding rocket instrument is a pathfinder to a new wide field spectroscopic technology for enabling the potential discovery of the long hypothesized but elusive Lyman continuum radiation that is thought to leak from low redshift galaxies and contribute to the ionization of the universe.

astro-ph↗

arXiv2004

Substructure Analysis of Selected Low Richness 2dFGRS Clusters of Galaxies

Complementary one-, two-, and three-dimensional tests for detecting the presence of substructure in clusters of galaxies are applied to recently obtained data from the 2dF Galaxy Redshift Survey. The sample of 25 clusters used in this study includes 16 clusters not previously investigated for substructure. Substructure is detected at or greater than the 99% CL level in at least one test for 21 of the 25 clusters studied here. From the results, it appears that low richness clusters commonly contain subclusters participating in mergers. About half of the clusters have two or more components within 0.5 h^{-1} Mpc of the cluster centroid, and at least three clusters (Abell 1139, Abell 1663, and Abell S333) exhibit velocity-position characteristics consistent with the presence of possible cluster rotation, shear, or infall dynamics. The geometry of certain features is consistent with influence by the host supercluster environments. In general, our results support the hypothesis that low richness clusters relax to structureless equilibrium states on very long dynamical time scales (if at all).

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arXiv2004

The 2dF Galaxy Redshift Survey: The blue galaxy fraction and implications for the Butcher-Oemler effect

We derive the fraction of blue galaxies in a sample of clusters at z < 0.11 and the general field at the same redshift. The value of the blue fraction is observed to depend on the luminosity limit adopted, cluster-centric radius and, more generally, local galaxy density, but it does not depend on cluster properties. Changes in the blue fraction are due to variations in the relative proportions of red and blue galaxies but the star formation rate for these two galaxy groups remains unchanged. Our results are most consistent with a model where the star formation rate declines rapidly and the blue galaxies tend to be dwarfs and do not favour mechanisms where the Butcher-Oemler effect is caused by processes specific to the cluster environment.

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arXiv2002

The 2dF Galaxy Redshift Survey: The environmental dependence of galaxy star formation rates near clusters

We have measured the equivalent width of the H-alpha emission line for 11006 galaxies brighter than M_b=-19 (LCDM) at 0.05<z<0.1 in the 2dF Galaxy Redshift Survey (2dF), in the fields of seventeen known galaxy clusters. The limited redshift range ensures that our results are insensitive to aperture bias, and to residuals from night sky emission lines. We use these measurements to trace mustar, the star formation rate normalized to Lstar, as a function of distance from the cluster centre, and local projected galaxy density. We find that the distribution of mustar steadily skews toward larger values with increasing distance from the cluster centre, converging to the field distribution at distances greater than ~3 times the virial radius. A correlation between star formation rate and local projected density is also found, which is independent of cluster velocity dispersion and disappears at projected densities below ~1 galaxy (brighter than M_b=-19) per Mpc^2. This characteristic scale corresponds approximately to the mean density at the cluster virial radius. The same correlation holds for galaxies more than two virial radii from the cluster centre. We conclude that environmental influences on galaxy properties are not restricted to cluster cores, but are effective in all groups where the density exceeds this critical value. The present day abundance of such systems, and the strong evolution of this abundance, makes it likely that hierarchical growth of structure plays a significant role in decreasing the global average star formation rate. Finally, the low star formation rates well beyond the virialised cluster rule out severe physical processes, such as ram pressure stripping of disk gas, as being completely responsible for the variations in galaxy properties with environment.

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