arXiv subjects

A. M. Hopkins

Publications and source records attributed to A. M. Hopkins.

At least 127 records · Page 7Linked to original sources

arXiv2015

Galaxy And Mass Assembly (GAMA) Blended Spectra Catalog: Strong Galaxy-Galaxy Lens and Occulting Galaxy Pair Candidates

We present the catalogue of blended galaxy spectra from the Galaxy And Mass Assembly (GAMA) survey. These are cases where light from two galaxies are significantly detected in a single GAMA fibre. Galaxy pairs identified from their blended spectrum fall into two principal classes: they are either strong lenses, a passive galaxy lensing an emission-line galaxy; or occulting galaxies, serendipitous overlaps of two galaxies, of any type. Blended spectra can thus be used to reliably identify strong lenses for follow-up observations (high resolution imaging) and occulting pairs, especially those that are a late-type partly obscuring an early-type galaxy which are of interest for the study of dust content of spiral and irregular galaxies. The GAMA survey setup and its autoz automated redshift determination were used to identify candidate blended galaxy spectra from the cross-correlation peaks. We identify 280 blended spectra with a minimum velocity separation of 600 km/s, of which 104 are lens pair candidates, 71 emission-line-passive pairs, 78 are pairs of emission-line galaxies and and 27 are pairs of galaxies with passive spectra. We have visually inspected the candidates in the Sloan Digital Sky Survey (SDSS) and Kilo Degree Survey (KiDS) images. Many blended objects are ellipticals with blue fuzz (Ef in our classification). These latter "Ef" classifications are candidates for possible strong lenses, massive ellipticals with an emission-line galaxy in one or more lensed images. The GAMA lens and occulting galaxy candidate samples are similar in size to those identified in the entire SDSS. This blended spectrum sample stands as a testament of the power of this highly complete, second-largest spectroscopic survey in existence and offers the possibility to expand e.g., strong gravitational lens surveys.

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arXiv2015

The SAMI Galaxy Survey: instrument specification and target selection

The SAMI Galaxy Survey will observe 3400 galaxies with the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) on the Anglo-Australian Telescope (AAT) in a 3-year survey which began in 2013. We present the throughput of the SAMI system, the science basis and specifications for the target selection, the survey observation plan and the combined properties of the selected galaxies. The survey includes four volume limited galaxy samples based on cuts in a proxy for stellar mass, along with low-stellar mass dwarf galaxies all selected from the Galaxy And Mass Assembly (GAMA) survey. The GAMA regions were selected because of the vast array of ancillary data available, including ultraviolet through to radio bands. These fields are on the celestial equator at 9, 12, and 14.5 hours, and cover a total of 144 square degrees (in GAMA-I). Higher density environments are also included with the addition of eight clusters. The clusters have spectroscopy from 2dFGRS and SDSS and photometry in regions covered by the Sloan Digital Sky Survey (SDSS) and/or VLT Survey Telescope/ATLAS. The aim is to cover a broad range in stellar mass and environment, and therefore the primary survey targets cover redshifts 0.004 < z < 0.095, magnitudes r$_{pet}$ < 19.4, stellar masses $10^{7} - 10^{12}$ M$_{sol}$, and environments from isolated field galaxies through groups to clusters of $10^{15}$ M$_{sol}$.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Bivariate functions of H$α$ star forming galaxies

We present bivariate luminosity and stellar mass functions of H$α$ star forming galaxies drawn from the Galaxy And Mass Assembly (GAMA) survey. While optically deep spectroscopic observations of GAMA over a wide sky area enable the detection of a large number of $0.001<{SFR}_{Hα}$ (M$_{\odot}$ yr$^{-1}$)$<100$ galaxies, the requirement for an H$α$ detection in targets selected from an $r$-band magnitude limited survey leads to an incompleteness due to missing optically faint star forming galaxies. Using $z<0.1$ bivariate distributions as a reference we model the higher-$z$ distributions, thereby approximating a correction for the missing optically faint star forming galaxies to the local SFR and stellar mass densities. Furthermore, we obtain the $r$-band LFs and stellar mass functions of H$α$ star forming galaxies from the bivariate LFs. As our sample is selected on the basis of detected H$α$ emission, a direct tracer of on-going star formation, this sample represents a true star forming galaxy sample, and is drawn from both photometrically classified blue and red sub-populations, though mostly from the blue population. On average 20-30% of red galaxies at all stellar masses are star forming, implying that these galaxies may be dusty star forming systems.

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arXiv2014

The SAMI Galaxy Survey: Cubism and covariance, putting round pegs into square holes

We present a methodology for the regularisation and combination of sparse sampled and irregularly gridded observations from fibre-optic multi-object integral-field spectroscopy. The approach minimises interpolation and retains image resolution on combining sub-pixel dithered data. We discuss the methodology in the context of the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) Galaxy Survey underway at the Anglo-Australian Telescope. The SAMI instrument uses 13 fibre bundles to perform high-multiplex integral-field spectroscopy across a one degree diameter field of view. The SAMI Galaxy Survey is targeting 3000 galaxies drawn from the full range of galaxy environments. We demonstrate the subcritical sampling of the seeing and incomplete fill factor for the integral-field bundles results in only a 10% degradation in the final image resolution recovered. We also implement a new methodology for tracking covariance between elements of the resulting datacubes which retains 90% of the covariance information while incurring only a modest increase in the survey data volume.

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arXiv2014

The SAMI Galaxy Survey: Early Data Release

We present the Early Data Release of the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. The SAMI Galaxy Survey is an ongoing integral field spectroscopic survey of ~3400 low-redshift (z<0.12) galaxies, covering galaxies in the field and in groups within the Galaxy And Mass Assembly (GAMA) survey regions, and a sample of galaxies in clusters. In the Early Data Release, we publicly release the fully calibrated datacubes for a representative selection of 107 galaxies drawn from the GAMA regions, along with information about these galaxies from the GAMA catalogues. All datacubes for the Early Data Release galaxies can be downloaded individually or as a set from the SAMI Galaxy Survey website. In this paper we also assess the quality of the pipeline used to reduce the SAMI data, giving metrics that quantify its performance at all stages in processing the raw data into calibrated datacubes. The pipeline gives excellent results throughout, with typical sky subtraction residuals in the continuum of 0.9-1.2 per cent, a relative flux calibration uncertainty of 4.1 per cent (systematic) plus 4.3 per cent (statistical), and atmospheric dispersion removed with an accuracy of 0."09, less than a fifth of a spaxel.

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arXiv2014

Galaxy evolution across the optical emission-line diagnostic diagrams?

The discovery of the M-sigma relation, the local galaxy bimodality, and the link between black-hole and host-galaxy properties, have raised the question whether AGN play a role in galaxy evolution. Several theoretical models implement AGN feedback to explain the observed galaxy luminosity function, and possibly the color and morphological transformation of spiral galaxies into passive ellipticals. To understand the importance of AGN feedback, a study of the AGN populations in the radio-optical domain is crucial. A mass sequence linking star-forming galaxies and AGN has been already noted in previous works, and it is now investigated as possible evolutionary sequence. We observed a sample of 119 intermediate-redshift (0.04<z<0.4) SDSS-FIRST radio emitters with the Effelsberg 100-m telescope at 4.85 and 10.45 GHz and obtained spectral indices. We find indications of spectral index flattening in high-metallicity star-forming galaxies, composite galaxies, and Seyferts. This "flattening sequence" along the [NII]-based emission-line diagnostic diagram is consistent with the hardening of galaxy ionizing field, due to nuclear activity. After combining our data with FIRST measurements at 1.4 GHz, we find that the three-point radio spectra of Seyferts and LINERs show substantial differences, attributable to small radio core components and larger (arcsecond sized) jet/lobe components, respectively. A visual inspection of FIRST images seems to confirm this hypothesis. Galaxies along this sequence are hypothesized to be transitioning from the active star-forming galaxies (blue cloud) to the passive elliptical galaxies (red sequence). This supports the suggestion that AGN play a role in shutting down star-formation, and allow the transition from one galaxy class to the other.

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arXiv2014

Galaxy And Mass Assembly (GAMA): The unimodal nature of the dwarf galaxy population

In this paper we aim to (i) test the number of statistically distinct classes required to classify the local galaxy population, and, (ii) identify the differences in the physical and star formation properties of visually-distinct galaxies. To accomplish this, we analyse the structural parameters (effective radius r_e, effective surface brightness within r_e (mu_e), central surface brightness (mu_0), and S'ersic index (n)), obtained by fitting the light profile of 432 galaxies (0.002<z<=0.02 Viking Z-band), and their spectral energy distribution using multi-band photometry in 18 broadbands to obtain the stellar mass (M*), the star formation rate (SFR), the specific SFR (sSFR) and the dust mass (M_{dust}), respectively. We show that visually distinct, star-forming dwarf galaxies (irregulars, blue spheroids and low surface brightness galaxies) form a unimodal population in a parameter space mapped by mu_e, mu_0, n, r_e, SFR, sSFR, M*, M_{dust} and (g-i). The SFR and sSFR distribution of passively evolving (dwarf) ellipticals on the other hand, statistically distinguish them from other galaxies with similar luminosity, while the giant galaxies clearly segregate into star-forming spirals and passive lenticulars. We therefore suggest that the morphology classification scheme(s) used in literature for dwarf galaxies only reflect the observational differences based on luminosity and surface brightness among the apparent distinct classes, rather than any physical differences between them.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Galaxy close-pairs, mergers, and the future fate of stellar mass

We use a highly complete subset of the GAMA-II redshift sample to fully describe the stellar mass dependence of close-pairs and mergers between 10^8 Msun and 10^12 Msun. Using the analytic form of this fit we investigate the total stellar mass accreting onto more massive galaxies across all mass ratios. Depending on how conservatively we select our robust merging systems, the fraction of mass merging onto more massive companions is 2.0%-5.6%. Using the GAMA-II data we see no significant evidence for a change in the close-pair fraction between redshift $z = 0.05-0.2$. However, we find a systematically higher fraction of galaxies in similar mass close-pairs compared to published results over a similar redshift baseline. Using a compendium of data and the function $γ_M =A(1+z)m$ to predict the major close-pair fraction, we find fitting parameters of $A = 0.021 \pm 0.001$ and $m = 1.53 \pm 0.08$, which represents a higher low-redshift normalisation and shallower power-law slope than recent literature values. We find that the relative importance of in-situ star-formation versus galaxy merging is inversely correlated, with star-formation dominating the addition of stellar material below Mstar and merger accretion events dominating beyond Mstar. We find mergers have a measurable impact on the whole extent of the GSMF, manifest as a deepening of the dip in the GSMF over the next Gyr and an increase in Mstar by as much as 0.01-0.05 dex.

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arXiv2014

Galaxy And Mass Assembly (GAMA): autoz spectral redshift measurements, confidence and errors

The Galaxy And Mass Assembly (GAMA) survey has obtained spectra of over 230000 targets using the Anglo-Australian Telescope. To homogenise the redshift measurements and improve the reliability, a fully automatic redshift code was developed (autoz). The measurements were made using a cross-correlation method for both absorption-line and emission-line spectra. Large deviations in the high-pass filtered spectra are partially clipped in order to be robust against uncorrected artefacts and to reduce the weight given to single-line matches. A single figure of merit (FOM) was developed that puts all template matches onto a similar confidence scale. The redshift confidence as a function of the FOM was fitted with a tanh function using a maximum likelihood method applied to repeat observations of targets. The method could be adapted to provide robust automatic redshifts for other large galaxy redshift surveys. For the GAMA survey, there was a substantial improvement in the reliability of assigned redshifts and in the lowering of redshift uncertainties with a median velocity uncertainty of 33 km/s.

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arXiv2014

ATLAS 1.4 GHz Data Release 2 -- I. Observations of the CDF-S and ELAIS-S1 fields and methods for constructing differential number counts

This is the first of two papers describing the second data release (DR2) of the Australia Telescope Large Area Survey (ATLAS) at 1.4 GHz, which comprises deep wide-field observations in total intensity, linear polarization, and circular polarization over the Chandra Deep Field-South and European Large Area Infrared Space Observatory Survey-South 1 regions. DR2 improves upon the first data release by maintaining consistent data reductions across the two regions, including polarization analysis, and including differential number counts in total intensity and linear polarization. Typical DR2 sensitivities across the mosaicked multi-pointing images are 30 uJy/beam at approximately 12"x6" resolution over a combined area of 6.4 square degrees. In this paper we present detailed descriptions of our data reduction and analysis procedures, including corrections for instrumental effects such as positional variations in image sensitivity, bandwidth smearing with a non-circular beam, and polarization leakage, and application of the BLOBCAT source extractor. We present the DR2 images and catalogues of components (discrete regions of radio emission) and sources (groups of physically associated radio components). We describe new analytic methods to account for resolution bias and Eddington bias when constructing differential number counts of radio components.

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arXiv2014

Galaxy and Mass Assembly (GAMA): The evolution of bias in the radio source population to z ~ 1.5

We present a large-scale clustering analysis of radio galaxies in the Very Large Array (VLA) Faint Images of the Radio Sky at Twenty-cm (FIRST) survey over the Galaxy And Mass Assembly (GAMA) survey area, limited to S1.4 GHz >1 mJy with spectroscopic and photometric redshift limits up to r < 19.8 and r < 22 mag, respectively. For the GAMA spectroscopic matches, we present the redshift-space and projected correlation functions, the latter of which yielding a correlation length r0 ~ 8.2 Mpc/h and linear bias of ~1.9 at z ~ 0.34. Furthermore, we use the angular two-point correlation function w(θ) to determine spatial clustering properties at higher redshifts. We find r0 to increase from ~6 to ~14 Mpc/h between z = 0.3 and z = 1.55, with the corresponding bias increasing from ~2 to ~10 over the same range. Our results are consistent with the bias prescription implemented in the SKADS simulations at low redshift, but exceed these predictions at z > 1. This is indicative of an increasing (rather than fixed) halo mass and/or AGN fraction at higher redshifts or a larger typical halo mass for the more abundant FRI sources.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Testing galaxy formation models through the most massive galaxies in the Universe

We have analysed the growth of Brightest Group Galaxies and Brightest Cluster Galaxies (BGGs/BCGs) over the last 3 billion years using a large sample of 883 galaxies from the Galaxy And Mass Assembly Survey. By comparing the stellar mass of BGGs and BCGs in groups and clusters of similar dynamical masses, we find no significant growth between redshift $z=0.27$ and $z=0.09$. We also examine the number of BGGs/BCGs that have line emission, finding that approximately 65 per cent of BGGs/BCGs show H$α$ in emission. From the galaxies where the necessary spectroscopic lines were accurately recovered (54 per cent of the sample), we find that half of this (i.e. 27 per cent of the sample) harbour on-going star formation with rates up to $10\,$M$_{\odot}$yr$^{-1}$, and the other half (i.e. 27 per cent of the sample) have an active nucleus (AGN) at the centre. BGGs are more likely to have ongoing star formation, while BCGs show a higher fraction of AGN activity. By examining the position of the BGGs/BCGs with respect to their host dark matter halo we find that around 13 per cent of them do not lie at the centre of the dark matter halo. This could be an indicator of recent cluster-cluster mergers. We conclude that BGGs and BCGs acquired their stellar mass rapidly at higher redshifts as predicted by semi-analytic models, mildly slowing down at low redshifts.

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arXiv2014

Galaxy And Mass Assembly (GAMA): Mid-Infrared Properties and Empirical Relations from $WISE$

The Galaxy And Mass Assembly (GAMA) survey furnishes a deep redshift catalog that, when combined with the Wide-field Infrared Explorer ($WISE$), allows us to explore for the first time the mid-infrared properties of $> 110, 000$ galaxies over 120 deg$^2$ to $z\simeq 0.5$. In this paper we detail the procedure for producing the matched GAMA-$WISE$ catalog for the G12 and G15 fields, in particular characterising and measuring resolved sources; the complete catalogs for all three GAMA equatorial fields will be made available through the GAMA public releases. The wealth of multiwavelength photometry and optical spectroscopy allows us to explore empirical relations between optically determined stellar mass (derived from synthetic stellar population models) and 3.4micron and 4.6micron WISE measurements. Similarly dust-corrected Halpha-derived star formation rates can be compared to 12micron and 22micron luminosities to quantify correlations that can be applied to large samples to $z<0.5$. To illustrate the applications of these relations, we use the 12micron star formation prescription to investigate the behavior of specific star formation within the GAMA-WISE sample and underscore the ability of WISE to detect star-forming systems at $z\sim0.5$. Within galaxy groups (determined by a sophisticated friends-of-friends scheme), results suggest that galaxies with a neighbor within 100$\,h^{-1} $kpc have, on average, lower specific star formation rates than typical GAMA galaxies with the same stellar mass.

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arXiv2013

Galaxy And Mass Assembly (GAMA): Resolving the role of environment in galaxy evolution

We present observations of 18 galaxies from the Galaxy And Mass Assembly (GAMA) survey made with the SPIRAL optical integral field unit (IFU) on the Anglo-Australian Telescope. The galaxies are selected to have a narrow range in stellar mass (6x10^9Msolar < M* <2x10^10 Msolar) in order to focus on the effects of environment. Local galaxy environments are measured quantitatively using 5th nearest neighbour surface densities. We find that the total star formation rates (SFR) measured from the IFU data are consistent with total SFRs measured from aperture correcting either GAMA or Sloan Digital Sky Survey single-fibre observations. The mean differences are SFR_GAMA/SFR_IFU = 1.26+/-0.23, sigma=0.90 and for the Sloan Digital Sky Survey we similarly find SFR_Brinchmann/SFR_IFU = 1.34+/-0.17, sigma=0.67. Examining the relationships with environment, we find off-centre and clumpy Halpha emission is not significantly dependent on environment, being present in 2/7 (29^+20_-11 per cent) galaxies in high-density environments (>0.77 Mpc^-2), and 5/11 (45^+15_-13 per cent) galaxies in low-density environments (<0.77 Mpc^-2). We find a weak but not significant relationship of the total star formation rates of star-forming galaxies with environment. Due to the size of our sample and the scatter observed we do not draw a definitive conclusion about a possible SFR dependence on environment. Examining the spatial distribution of the Halpha emission, we find no evidence for a change in shape or amplitude of the radial profile of star-forming galaxies with environment. If these observations are borne out in larger samples this would infer that any environment-driven star-formation suppression must either act very rapidly (the `infall-and-quench' model) or that galaxies must evolve in a density-dependent manner (an `in-situ evolution' model).

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arXiv2013

Galaxy and Mass Assembly (GAMA): Witnessing the assembly of the cluster Abell 1882

We present a combined optical and X-ray analysis of the rich cluster Abell 1882 with the aim of identifying merging substructure and understanding the recent assembly history of this system. Our optical data consist of spectra drawn from the Galaxy and Mass Assembly (GAMA) survey, which lends itself to this kind of detailed study thanks to its depth and high spectroscopic completeness. We use 283 spectroscopically confirmed cluster members to detect and characterize substructure. We complement the optical data with X-ray data taken with both Chandra and XMM. Our analysis reveals that A1882 harbors two main components, A1882A and A1882B, which have a projected separation of 2Mpc and a line of sight velocity difference of v_{los}=-428km/s. The primary system, A1882A, has velocity dispersion sigma_v=500km/s and Chandra (XMM) temperature kT=3.57keV (3.31keV) while the secondary, A1882B, has sigma_v=457km/s and Chandra (XMM) temperature kT=2.39keV (2.12keV). The optical and X-ray estimates for the masses of the two systems are consistent within the uncertainties and indicate that there is twice as much mass in A1882A (M_{500}=1.5-1.9x10^{14}Msolar) when compared with A1882B (M_{500}=0.8-1.0x10^{14}Msolar). We interpret the A1882A/A1882B system as being observed prior to a core passage. Supporting this interpretation is the large projected separation of A1882A and A1882B and the dearth of evidence for a recent (<2Gyr) major interaction in the X-ray data. Two-body analyses indicate that A1882A and A1882B form a bound system with bound incoming solutions strongly favored. We compute blue fractions of f_b=0.28 and 0.18 for the spectroscopically confirmed member galaxies within r_{500} of the centers of A1882A and A1882B, respectively. These blue fractions do not differ significantly from the blue fraction measured from an ensemble of 20 clusters with similar mass and redshift.

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arXiv2013

Galaxy And Mass Assembly (GAMA): Linking Star Formation Histories and Stellar Mass Growth

We present evidence for stochastic star formation histories in low-mass (M* < 10^10 Msun) galaxies from observations within the Galaxy And Mass Assembly (GAMA) survey. For ~73,000 galaxies between 0.05<z<0.32, we calculate star formation rates (SFR) and specific star formation rates (SSFR = SFR/M*) from spectroscopic Halpha measurements and apply dust corrections derived from Balmer decrements. We find a dependence of SSFR on stellar mass, such that SSFRs decrease with increasing stellar mass for star-forming galaxies, and for the full sample, SSFRs decrease as a stronger function of stellar mass. We use simple parametrizations of exponentially declining star formation histories to investigate the dependence on stellar mass of the star formation timescale and the formation redshift. We find that parametrizations previously fit to samples of z~1 galaxies cannot recover the distributions of SSFRs and stellar masses observed in the GAMA sample between 0.05<z<0.32. In particular, a large number of low-mass (M* < 10^10 Msun) galaxies are observed to have much higher SSFRs than can be explained by these simple models over the redshift range of 0.05<z<0.32, even when invoking mass-dependent staged evolution. For such a large number of galaxies to maintain low stellar masses, yet harbour such high SSFRs, requires the late onset of a weak underlying exponentially declining SFH with stochastic bursts of star formation superimposed.

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arXiv2013

Galaxy And Mass Assembly (GAMA): A deeper view of the mass, metallicity, and SFR relationships

A full appreciation of the role played by gas metallicity (Z), star-formation rate (SFR), and stellar mass is fundamental to understanding how galaxies form and evolve. The connections between these three parameters at different redshifts significantly affect galaxy evolution, and thus provide important constraints for galaxy evolution models. Using data from the Sloan Digital Sky Survey-Data Release 7 (SDSS-DR7) and the Galaxy and Mass Assembly (GAMA) surveys we study the relationships and dependencies between SFR, Z, and stellar mass, as well as the Fundamental Plane for star-forming galaxies. We combine both surveys using volume-limited samples up to a redshift of z ~ 0.36. The GAMA and SDSS surveys complement each other when analyzing the relationships between SFR, Mass and Z. We present evidence for SFR and metallicity evolution to z ~ 0.2. We study the dependencies between SFR, Mass, Z, and specific star-formation rate (SSFR) on the M-Z, M-SFR, M-SSFR, Z-SFR, and Z-SSFR relations, finding strong correlations between all. Based on those dependencies, we propose a simple model that allows us to explain the different behaviour observed between low and high mass galaxies. Finally, our analysis allows us to confirm the existence of a Fundamental Plane, for which Mass=f(Z, SFR) in star-forming galaxies.

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arXiv2013

Galaxy And Mass Assembly: Evolution of the Halpha luminosity function and star formation rate density up to z<0.35

Measurements of the low-z Halpha luminosity function have a large dispersion in the local number density of sources, and correspondingly in the SFR density. The possible causes for these discrepancies include limited volume sampling, biases arising from survey sample selection, different methods of correcting for dust obscuration and AGN contamination. The Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey (SDSS) provide deep spectroscopic observations over a wide sky area enabling detection of a large sample of star-forming galaxies spanning 0.001<SFR(Halpha)<100 with which to robustly measure the evolution of the SFR density in the low-z universe. The large number of high SFR galaxies present in our sample allow an improved measurement of the bright end of the luminosity function, indicating that the decrease in number density of sources at bright luminosities is best described by a Saunders functional form rather than the traditional Schechter function. This result is consistent with other published luminosity functions in the FIR and radio. For GAMA and SDSS we find the r-band apparent magnitude limit, combined with the subsequent requirement for Halpha detection leads to an incompleteness due to missing bright Halpha sources with faint r-band magnitudes.

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