Alister W. Graham

Galaxy and Mass Assembly (GAMA): survey diagnostics and core data release

The Galaxy and Mass Assembly (GAMA) survey has been operating since 2008 February on the 3.9-m Anglo-Australian Telescope using the AAOmega fibre-fed spectrograph facility to acquire spectra with a resolution of R ≈ 1300 for 120 862 Sloan Digital Sky Survey selected galaxies. The target catalogue constitutes three contiguous equatorial regions centred at 9h (G09), 12h (G12) and 14.5h (G15) each of 12 × 4 deg2 to limiting fluxes of rpet < 19.4, rpet < 19.8 and rpet <19.4 mag, respectively (and additional limits at other wavelengths). Spectra and reliable redshifts have been acquired for over 98 per cent of the galaxies within these limits. Here we present the survey footprint, progression, data reduction, redshifting, re-redshifting, an assessment of data quality after 3 yr, additional image analysis products (including ugrizYJHK photometry, S´ersic profiles and photometric redshifts), observing mask and construction of our core survey catalogue (GamaCore). From this we create three science-ready catalogues: GamaCoreDR1 for public release, which includes data acquired during year 1 of operations within specified magnitude limits (2008 February to April); GamaCoreMainSurvey containing all data above our survey limits for use by the GAMA Team and collaborators; and GamaCore-AtlasSV containing year 1, 2 and 3 data matched to Herschel-ATLAS science demonstration data. These catalogues along with the associated spectra, stamps and profiles can be accessed via the GAMA website: http://www.gama-survey.org/

Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models

We use techniques from nonparametric function estimation theory to extract the density profiles, and their derivatives, from a set of N-body dark matter halos. We consider halos generated from ΛCDM simulations of gravitational clustering, as well as isolated spherical collapses. The logarithmic density slopes γ ≡ d log ρ/d log r of the ΛCDM halos are found to vary as power laws in radius, reaching values of γ ≈ -1 at the innermost resolved radii, ~10-2rvir. This behavior is significantly different from that of broken-power-law models like the Navarro-Frenk-White (NFW) profile but similar to that of models like de Vaucouleurss. Accordingly, we compare the N-body density profiles with various parametric models to find which provide the best fit. We consider an NFW-like model with arbitrary inner slope; Dehnen & McLaughlins anisotropic model; Einastos model (identical in functional form to Sersics model but fitted to the space density); and the density model of Prugniel & Simien that was designed to match the deprojected form of Sersics R1/n law. Overall, the best-fitting model to the ΛCDM halos is Einastos, although the Prugniel-Simien and Dehnen-McLaughlin models also perform well. With regard to the spherical-collapse halos, both the Prugniel-Simien and Einasto models describe the density profiles well, with an rms scatter some 4 times smaller than that obtained with either the NFW-like model or the three-parameter Dehnen-McLaughlin model. Finally, we confirm recent claims of a systematic variation in profile shape with halo mass.

A correlation between galaxy light concentration and supermassive black hole mass

We present evidence for a strong correlation between the concentration of bulges and the mass of their central supermassive black hole (Mbh)—more concentrated bulges have more massive black holes. Using C(1/3) from Trujillo, Graham, & Caon as a measure of bulge concentration, we find that log(Mbh/M☉) = 6.81(±0.95)C(1/3) + 5.03 ± 0.41. This correlation is shown to be marginally stronger (Spearmans rs = 0.91) than the relationship between the logarithm of the stellar velocity dispersion and log Mbh (Spearmans rs = 0.86) and has comparable or less scatter (0.31 dex in log Mbh, which decreases to 0.19 dex when we use only those galaxies whose supermassive black hole radii of influence are resolved and we remove one well-understood outlying data point).

Inclination-and dust-corrected galaxy parameters : bulge-to-disc ratios and size-luminosity relations

While galactic bulges may contain no significant dust of their own, the dust within galaxy discs can strongly attenuate the light from their embedded bulges. Furthermore, such dust inhibits the ability of observationally determined inclination corrections to recover intrinsic (i.e. dust-free) galaxy parameters. Using the sophisticated 3D radiative transfer model of Popescu et al. and Tuffs et al., together with the recent determination of the average face-on opacity by Driver et al. in nearby disc galaxies, we provide simple equations to correct (observed) disc central surface brightness and scalelengths for the effects of both inclination and dust in the B, V, I, J and K passbands. We then collate and homogenize various literature data sets and determine the typical intrinsic scalelengths, central surface brightness and magnitudes of galaxy discs as a function of morphological type. All galaxies have been carefully modelled in their respective papers with a Sersic R l/n bulge plus an exponential disc. Using the bulge magnitude corrections from Driver et al., we additionally derive the average, dust-corrected, bulge-to-disc flux ratio as a function of galaxy type. With values typically less than 1/3, this places somewhat uncomfortable constraints on some current semi-analytic simulations. Typical bulge sizes, profile shapes, surface brightness and deprojected densities are provided. Finally, given the two-component nature of disc galaxies, we present luminosity-size and (surface brightness)-size diagrams for discs and bulges. We also show that the distribution of elliptical galaxies in the luminosity-size diagram is not linear but strongly curved.

The Millennium Galaxy Catalogue: morphological classification and bimodality in the colour–concentration plane

Using 10095 galaxies (B < 20 mag) from the Millennium Galaxy Catalogue, we derive B-band luminosity distributions and selected bivariate brightness distributions for the galaxy population subdivided by eyeball morphology; Sersic index (n); two-degree Field Galaxy Redshift Survey (2dFGRS) η parameter; rest-(u - r) colour (global and core); MGC continuum shape; half-light radius; (extrapolated) central surface brightness; and inferred stellar mass-to-light ratio. All subdivisions extract highly correlated subsets of the galaxy population which consistently point towards two overlapping distributions: an old, red, inert, predominantly luminous, high central-surface brightness subset; and a young, blue, star forming, intermediate surface brightness subset. A clear bimodality in the observed distribution is seen in both the rest-(u - r) colour and log (n) distributions. Whilst the former bimodality was well established from Sloan Digital Sky Survey data, we show here that the rest-(u - r) colour bimodality becomes more pronounced when using the core colour as opposed to global colour. The two populations are extremely well separated in the colour-log(n) plane. Using our sample of 3314 (B < 19 mag) eyeball classified galaxies, we show that the bulge-dominated, early-type galaxies populate one peak and the bulge-less, late-type galaxies occupy the second. The early- and mid-type spirals sprawl across and between the peaks. This constitutes extremely strong evidence that the fundamental way to divide the luminous galaxy population (M BMGC -5 log h < -16 mag, i.e. dwarfs not included) is into bulges (old red, inert, high concentration) and discs (young, blue, star forming, low concentration) and that the galaxy bimodality reflects the two-component nature of galaxies and not two distinct galaxy classes. We argue that these two components require two independent formation mechanisms/processes and advocate early bulge formation through initial collapse and ongoing disc formation through splashback, infall and merging/accretion. We calculate the B-band luminosity densities and stellar mass densities within each subdivision and estimate that the z ≈ 0 stellar mass content in spheroids, bulges and discs is 35 ± 2, 18 ± 7 and 47 ± 7 per cent, respectively.

The Millennium Galaxy Catalogue: the B‐band attenuation of bulge and disc light and the implied cosmic dust and stellar mass densities

Based on our sample of 10 095 galaxies with bulge‐disc decompositions we derive the empirical BMGC-band internal attenuation‐inclination relation for galaxy discs and their associated central bulges. Our results agree well with the independently derived dust models of Tuffs et al., leading to a direct constraint on the mean opacity of spiral discs of τ f = 3.8 ± 0.7 (central face-on BMGC-band opacity). Depending on inclination, the BMGC-band attenuation correction varies from 0.2 to 1.1 mag for discs and from 0.8 to 2.6 mag for bulges. We find that, overall, 37 per cent of all BMGC-band photons produced in discs in the nearby Universe are absorbed by dust, a figure that rises to 71 per cent for bulge photons. The severity of internal dust extinction is such that one must incorporate internal dust corrections in all optical studies of large galaxy samples. This is particularly pertinent for optical Hubble Space Telescope comparative evolutionary studies as the dust properties will also be evolving. We use the new results to revise our recent estimates of the spheroid and disc luminosity functions. The implied stellar mass densities at redshift zero are somewhat higher than our earlier estimates: ρdiscs =

The Millennium Galaxy Catalogue: bulge–disc decomposition of 10 095 nearby galaxies

We have modelled the light distribution in 10095 galaxies from the Millennium Galaxy Catalogue (MGC), providing publically available structural catalogues for a large, rep- resentative sample of galaxies in the local Universe. Three different models were used: (1) a single Sersic function for the whole galaxy, (2) a bulge-disc decomposition model using a de Vaucouleurs (R 1/4 ) bulge plus exponential disc, (3) a bulge-disc decomposi- tion model using a Sersic (R 1/n ) bulge plus exponential disc. Repeat observations for � 700 galaxies demonstrate that stable measurements can be obtained for object com- ponents with a half-light radius comparable to, or larger than, the seeing half-width at half maximum. We show that with careful quality control, robust measurements can be obtained for large samples such as the MGC. We use the catalogues to show that the galaxy colour bimodality is due to the two-component nature of galaxies (i.e. bulges and discs) and not to two distinct galaxy populations. We conclude that under- standing galaxy evolution demands the routine bulge-disc decomposition of the giant galaxy population at all redshifts.

Galaxy And Mass Assembly (GAMA): Structural Investigation of Galaxies via Model Analysis

We present single-Sersic two-dimensional (2D) model fits to 167 600 galaxies modelled independently in the ugrizYJHK bandpasses using reprocessed Sloan Digital Sky Survey Data Release Seven (SDSS DR7) and UKIRT Infrared Deep Sky Survey Large Area Survey imaging data available from the Galaxy And Mass Assembly (GAMA) data base. In order to facilitate this study we developed Structural Investigation of Galaxies via Model Analysis (sigma), an r wrapper around several contemporary astronomy software packages including source extractor, psf extractor and galfit 3. sigma produces realistic 2D model fits to galaxies, employing automatic adaptive background subtraction and empirical point spread function measurements on the fly for each galaxy in GAMA. Using these results, we define a common coverage area across the three GAMA regions containing 138 269 galaxies. We provide Sersic magnitudes truncated at 10re which show good agreement with SDSS Petrosian and GAMA photometry for low Sersic index systems (n 4), recovering as much as Δm= 0.5 mag in the r band. We employ a K-band Sersic index/u−r colour relation to delineate the massive (n > ∼2) early-type galaxies (ETGs) from the late-type galaxies (LTGs). The mean Sersic index of these ETGs shows a smooth variation with wavelength, increasing by 30 per cent from g through K. LTGs exhibit a more extreme change in Sersic index, increasing by 52 per cent across the same range. In addition, ETGs and LTGs exhibit a 38 and 25 per cent decrease, respectively, in half-light radius from g through K. These trends are shown to arise due to the effects of dust attenuation and stellar population/metallicity gradients within galaxy populations.

An investigation into the prominence of spiral galaxy bulges

From a diameter-limited sample of 86 low-inclination (face-on) spiral galaxies, the bulge-to-disk size and luminosity ratios and other quantitative measurements for the prominence of the bulge are derived. The bulge and disk parameters have been estimated using a seeing-convolved Sersic r1/n bulge and a seeing-convolved exponential disk that were fitted to the optical (B, R, and I) and near-infrared (K) galaxy light profiles. In general, early-type spiral galaxy bulges have Sersic values of n > 1, and late-type spiral galaxy bulges have values of n 3 σ) for the early-type spirals than for the late-type spirals. Two new parameters are introduced to measure the prominence of the bulge. The first is the difference between the central surface brightness of the galaxy and the surface brightness level at which the bulge and disk contribute equally. The other test uses the radius at which the contribution from the disk and bulge light are equal, normalized for the effect of intrinsically different galaxy sizes. Both of these parameters reveal that the early-type spiral galaxies appear to have significantly (more than 2 σ in all passbands) bigger and brighter bulges than late-type spiral galaxies. This apparent contradiction with the re/h values can be explained with an iceberg-like scenario, in which the bulges in late-type spiral galaxies are relatively submerged in their disk. This can be achieved by varying the relative stellar density while maintaining the same effective bulge-to-disk ratio. The B/D luminosity ratio and the concentration index C31, in agreement with past studies, are positively correlated and decrease as one moves along the spiral Hubble sequence toward later spiral galaxy types, although for galaxies with large extended bulges the concentration index no longer traces the B/D luminosity ratio in a one-to-one fashion. A strong (Spearmans rank-order correlation coefficient, rs = 0.80) and highly significant positive correlation exists between the shape, n, of the bulge light profile and the bulge-to-disk luminosity ratio. The absolute bulge magnitude–log n diagram is used as a diagnostic tool for comparative studies with dwarf elliptical and ordinary elliptical galaxies. At least in the B band these objects occupy distinctly different regions of this parameter space. While the dwarf elliptical galaxies appear to be the faint extension to the brighter elliptical galaxies, the bulges of spiral galaxies do not; for a given luminosity they have a noticeably smaller shape parameter and hence a more dramatic decline of stellar density at large radii.

A New Empirical Model for the Structural Analysis of Early-Type Galaxies, and A Critical Review of the Nuker Model*

The Nuker law was designed to match the inner few (~3–10) arcseconds of predominantly nearby (30 Mpc) early-type galaxy light profiles; it was never intended to describe an entire profile. The Sersic model, on the other hand, was developed to fit the entire profile; however, because of the presence of partially depleted galaxy cores, the Sersic model cannot always describe the very inner region. We have therefore developed a new empirical model consisting of an inner power law, a transition region, and an outer Sersic model to connect the inner and outer structure of elliptical galaxies. We have additionally explored the stability of the Nuker model parameters. Surprisingly, none are found to be stable quantities; all are shown to vary systematically with a profiles fitted radial extent, and often by more than 100%. Considering elliptical galaxies spanning a range of 7.5 mag, we reveal that the central stellar densities of the underlying host galaxies increase with galaxy luminosity until the onset of core formation, detected only in the brightest elliptical galaxies. We suggest that the so-called power-law galaxies may actually be described by the Sersic model over their entire radial range.