arXiv subjects

Alister W. Graham

Publications and source records attributed to Alister W. Graham.

At least 127 records · Page 7Linked to original sources

arXiv2011

The HST/ACS Coma Cluster Survey IV. Intergalactic Globular Clusters and the Massive Globular Cluster System at the Core of the Coma Galaxy Cluster

Intracluster stellar populations are a natural result of tidal interactions in galaxy clusters. Measuring these populations is difficult, but important for understanding the assembly of the most massive galaxies. The Coma cluster is one of the nearest truly massive galaxy clusters, and is host to a correspondingly large system of globular clusters (GCs). We use imaging from the HST/ACS Coma Cluster Survey to present the first definitive detection of a large population of intracluster GCs (IGCs) that fills the Coma cluster core and is not associated with individual galaxies. The GC surface density profile around the central massive elliptical galaxy, NGC 4874, is dominated at large radii by a population of IGCs that extend to the limit of our data (R<520 kpc). We estimate that there are 47000+/-1600 (random) +4000/-5000 (systematic) IGCs out to this radius, and that they make up ~70% of the central GC system, making this the largest GC system in the nearby Universe. Even including the GC systems of other cluster galaxies, IGCs still make up ~30-45% of the GCs in the cluster core. Observational limits from previous studies of the intracluster light (ICL) suggest that the IGC population has a high specific frequency. If the IGC population has a specific frequency similar to high-S_N dwarf galaxies, then the ICL has a total stellar mass of ~10^12 M_sun within the cluster core. The ICL makes up approximately half of the stellar luminosity and one-third of the stellar mass of the central (NGC4874+ICL) system. The color distribution of the IGC population is bimodal, with blue, metal-poor GCs outnumbering red, metal-rich GCs by a ratio of 4:1. The fraction of red IGCs (20%), and the red color of those GCs, implies that IGCs can originate from the halos of relatively massive, L* galaxies, and not solely from the disruption of dwarf galaxies. (Abridged)

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arXiv2010

An expanded M_bh-sigma diagram, and a new calibration of active galactic nuclei masses

[Abridged] We present an updated and improved M_bh-sigma diagram containing 64 galaxies for which M_bh measurements (not just upper limits) are available. Due to new and increased black hole masses at the high-mass end, and a better representation of barred galaxies at the low-mass end, the "classical" (all morphological type) M_bh-sigma relation for predicting black hole masses is log(M_bh/M_Sun) = (5.13+/-0.34)log[sigma/200] + (8.13+/-0.05), with an rms scatter of 0.43 dex. Modifying the regression analysis to correct for a hitherto over-looked sample bias in which black holes with masses <10^6 M_Sun are not (yet) detectable, the relation steepens further to give log(M_bh/M_Sun) = (5.95+/-0.44)log[sigma/200] + (8.15+/-0.06). We have also updated the "barless" and "elliptical-only" M_bh-sigma relations introduced by Graham and Hu in 2008 due to the offset nature of barred/disc galaxies. These relations have a total scatter as low as 0.34 dex and currently define the upper envelope of points in the M_bh-sigma diagram. These relations also have a slope consistent with the value 5, in agreement with the prediction by Silk & Rees based on feedback from massive black holes in bulges built by monolithic-collapse. Using updated virial products and velocity dispersions from 28 active galactic nuclei, we determine that the optimal scaling factor f - which brings their virial products in line with the 64 directly measured black hole masses - is 2.8^{+0.7}_{-0.5}. This is roughly half the value reported by Onken et al. and Woo et al., and consequently halves the mass estimates of most high-redshift quasars. We have explored the results after separating the samples into barred and non-barred galaxies, and we have also developed a preliminary corrective term to the velocity dispersion based on bar dynamics.

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arXiv2010

The HST/ACS Coma Cluster Survey III. Structural Parameters of Galaxies using single-Sérsic Fits

We present a catalogue of structural parameters for 8814 galaxies in the 25 fields of the HST/ACS Coma Treasury Survey. Parameters from Sérsic fits to the two-dimensional surface brightness distributions are given for all galaxies from our published Coma photometric catalogue with mean effective surface brightness brighter than 26.0 mag/sq. arcsec and brighter than 24.5 mag (equivalent to absolute magnitude - 10.5), as given by the fits, all in F814W(AB). The sample comprises a mixture of Coma members and background objects; 424 galaxies have redshifts and of these 163 are confirmed members. The fits were carried out using both the Gim2D and Galfit codes. We provide the following parameters: Galaxy ID, RA, DEC, the total corrected automatic magnitude from the photometric catalogue, the total magnitude of the model (F814W_AB), the geometric mean effective radius Re, the mean surface brightness within the effective radius <μ>_e, the Sérsic index n, the ellipticity and the source position angle. The selection limits of the catalogue and the errors listed for the Sérsic parameters come from extensive simulations of the fitting process using synthetic galaxy models. The agreement between Gim2D and Galfit parameters is sensitive to details of the fitting procedure; for the settings employed here the agreement is excellent over the range of parameters covered in the catalogue. We define and present two goodness-of-fit indices which quantify the degree to which the image can be approximated by a Sérsic model with concentric, coaxial elliptical isophotes; such indices may be used to objectively select galaxies with more complex structures such as bulge-disk, bars or nuclear components. We make the catalog available in electronic format at Astro-WISE and MAST.

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arXiv2010

The HST/ACS Coma Cluster Survey. II. Data Description and Source Catalogs

The Coma cluster was the target of a HST-ACS Treasury program designed for deep imaging in the F475W and F814W passbands. Although our survey was interrupted by the ACS instrument failure in 2007, the partially completed survey still covers ~50% of the core high-density region in Coma. Observations were performed for 25 fields that extend over a wide range of cluster-centric radii (~1.75 Mpc) with a total coverage area of 274 arcmin^2. The majority of the fields are located near the core region of Coma (19/25 pointings) with six additional fields in the south-west region of the cluster. In this paper we present reprocessed images and SExtractor source catalogs for our survey fields, including a detailed description of the methodology used for object detection and photometry, the subtraction of bright galaxies to measure faint underlying objects, and the use of simulations to assess the photometric accuracy and completeness of our catalogs. We also use simulations to perform aperture corrections for the SExtractor Kron magnitudes based only on the measured source flux and half-light radius. We have performed photometry for ~73,000 unique objects; one-half of our detections are brighter than the 10-sigma point-source detection limit at F814W=25.8 mag (AB). The slight majority of objects (60%) are unresolved or only marginally resolved by ACS. We estimate that Coma members are 5-10% of all source detections, which consist of a large population of unresolved objects (primarily GCs but also UCDs) and a wide variety of extended galaxies from a cD galaxy to dwarf LSB galaxies. The red sequence of Coma member galaxies has a constant slope and dispersion across 9 magnitudes (-21<M_F814W<-13). The initial data release for the HST-ACS Coma Treasury program was made available to the public in 2008 August. The images and catalogs described in this study relate to our second data release.

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arXiv2010

A dozen new galaxies caught in the act: Gas stripping and extended emission line regions in the Coma cluster

We present images of extended H-alpha clouds associated with 14 member galaxies in the Coma cluster obtained from deep narrow band imaging observations with Suprime-Cam at the Subaru Telescope. The parent galaxies of the extended H-alpha clouds are distributed farther than 0.2 Mpc from the peak of X-ray emission of the cluster. Most of the galaxies have colors bluer than g-r approx 0.5 and they account for 57% of the blue (g-r<0.5) bright (r<17.8 mag) galaxies in the central region of the Coma cluster. They reside near the red- and blue-shifted edges of the Coma cluster's radial velocity distribution. These findings suggest that the most of the parent galaxies were recently captured by the Coma cluster potential and are now infalling toward the cluster center with their disk gas being stripped off and producing the observed H-alpha clouds.

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arXiv2010

How non-linear scaling relations unify dwarf and giant elliptical galaxies

Dwarf elliptical galaxies are frequently excluded from bright galaxy samples because they do not follow the same linear relations in diagrams involving effective half light radii R_e or mean effective surface brightnesses _e. However, using two linear relations which unite dwarf and bright elliptical galaxies we explain how these lead to curved relations when one introduces either the half light radius or the associated surface brightness. In particular, the curved _e - R_e relation is derived here. This and other previously misunderstood curved relations, once heralded as evidence for a discontinuity between faint and bright elliptical galaxies at M_B ~ -18 mag, actually support the unification of such galaxies as a single population whose structure (i.e. stellar concentration) varies continuously with stellar luminosity and mass.

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arXiv2010

Ultra-Compact Dwarfs in the Core of the Coma Cluster

We have discovered both a red and a blue subpopulation of Ultra-Compact Dwarf (UCD) galaxy candidates in the Coma galaxy cluster. We analyzed deep F475W (Sloan g) and F814W (I) Hubble Space Telescope images obtained with the Advanced Camera for Surveys Wide Field Channel as part of the Coma Cluster Treasury Survey and have fitted the light profiles of ~5000 point-like sources in the vicinity of NGC 4874, one of the two central dominant galaxies of the Coma cluster. Although almost all of these sources are globular clusters that remain unresolved, we found that 52 objects have effective radii between ~10 and 66 pc, in the range spanned by Dwarf Globular Transition Objects (DGTO) and UCDs. Of these 52 compact objects, 25 are brighter than M_V ~-11 mag, a magnitude conventionally thought to separate UCDs and globular clusters. The UCD/DGTO candidates have the same color and luminosity distribution as the most luminous globular clusters within the red and blue subpopulations of the immensely rich NGC 4874 globular cluster system. Unlike standard globular clusters, blue and red UCD/DGTO subpopulations have the same median effective radius. The spatial distribution of UCD/DGTO candidates reveal that they congregate towards NGC 4874, and are not uniformly distributed. We find a relative deficit of UCD/DGTOs compared with globular clusters in the inner 15 kpc around NGC 4874, however at larger radii UCD/DGTO and globular clusters follow the same spatial distribution.

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arXiv2010

Galaxy and Mass Assembly (GAMA): FUV, NUV, ugrizYJHK Petrosian, Kron and Sèrsic photometry

In order to generate credible 0.1-2 μm SEDs, the GAMA project requires many Gigabytes of imaging data from a number of instruments to be re-processed into a standard format. In this paper we discuss the software infrastructure we use, and create self-consistent ugrizYJHK photometry for all sources within the GAMA sample. Using UKIDSS and SDSS archive data, we outline the pre-processing necessary to standardise all images to a common zeropoint, the steps taken to correct for seeing bias across the dataset, and the creation of Gigapixel-scale mosaics of the three 4x12 deg GAMA regions in each filter. From these mosaics, we extract source catalogues for the GAMA regions using elliptical Kron and Petrosian matched apertures. We also calculate Sérsic magnitudes for all galaxies within the GAMA sample using SIGMA, a galaxy component modelling wrapper for GALFIT 3. We compare the resultant photometry directly, and also calculate the r band galaxy LF for all photometric datasets to highlight the uncertainty introduced by the photometric method. We find that (1) Changing the object detection threshold has a minor effect on the best-fitting Schechter parameters of the overall population (M* +/- 0.055mag, α +/- 0.014, Φ* +/- 0.0005 h^3 Mpc^{-3}). (2) An offset between datasets that use Kron or Petrosian photometry regardless of the filter. (3) The decision to use circular or elliptical apertures causes an offset in M* of 0.20mag. (4) The best-fitting Schechter parameters from total-magnitude photometric systems (such as SDSS modelmag or Sérsic magnitudes) have a steeper faint-end slope than photometry dependent on Kron or Petrosian magnitudes. (5) Our Universe's total luminosity density, when calculated using Kron or Petrosian r-band photometry, is underestimated by at least 15%.

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arXiv2010

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

The Galaxy And Mass Assembly (GAMA) survey has been operating since February 2008 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 SDSS selected galaxies. The target catalogue constitutes three contiguous equatorial regions centred at 9h (G09), 12h (G12) and 14.5h (G15) each of 12 x 4 sq.deg to limiting fluxes of r < 19.4, r < 19.8, and r < 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 years, additional image analysis products (including ugrizYJHK photometry, Sersic 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 (February 2008 to April 2008); GamaCoreMainSurvey containing all data above our survey limits for use by the GAMA team and collaborators; and GamaCoreAtlasSv 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/

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arXiv2010

Comment on "A Non-Parametric Estimate of Mass 'Scoured' in Galaxy Cores" (arXiv:1006.0488)

This comment is in response to the article titled "A Non-Parametric Estimate of Mass Scoured in Galaxy Cores" (arXiv:1006.0488) written by Hopkins and Hernquist. This comment politely mentions two relevant papers in which the main conclusion from Hopkins & Hernquist had already been published six years ago using the core-Sersic model. It then explains why Hopkins & Hernquist's concern about the core-Sersic model is not valid.

astro-ph.CO↗

arXiv2010

On the transition from nuclear-cluster to black-hole dominated galaxy cores

Giant elliptical galaxies, believed to be built from the merger of lesser galaxies, are known to house a massive black hole at their center rather than a compact star cluster. If low- and intermediate-mass galaxies do indeed partake in the hierarchical merger scenario, then one needs to explain why their dense nuclear star clusters are not preserved in merger events. A valuable clue may the recent revelation that nuclear star clusters and massive black holes frequently co-exist in intermediate mass bulges and elliptical galaxies. In an effort to understand the physical mechanism responsible for the disappearance of nuclear star clusters, we have numerically investigated the evolution of merging star clusters with seed black holes. Using black holes that are 1-5% of their host nuclear cluster mass, we reveal how their binary coalescence during a merger dynamically heats the newly wed star cluster, expanding it, significantly lowering its central stellar density, and thus making it susceptible to tidal destruction during galaxy merging. Moreover, this mechanism provides a pathway to explain the observed reduction in the nucleus-to-galaxy stellar mass ratio as one proceeds from dwarf to giant elliptical galaxies.

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arXiv2010

Barred Galaxies in the Coma Cluster

We use ACS data from the HST Treasury survey of the Coma cluster (z~0.02) to study the properties of barred galaxies in the Coma core, the densest environment in the nearby Universe. This study provides a complementary data point for studies of barred galaxies as a function of redshift and environment. From ~470 cluster members brighter than M_I = -11 mag, we select a sample of 46 disk galaxies (S0--Im) based on visual classification. The sample is dominated by S0s for which we find an optical bar fraction of 47+/-11% through ellipse fitting and visual inspection. Among the bars in the core of the Coma cluster, we do not find any very large (a_bar > 2 kpc) bars. Comparison to other studies reveals that while the optical bar fraction for S0s shows only a modest variation across low-to-intermediate density environments (field to intermediate-density clusters), it can be higher by up to a factor of ~2 in the very high-density environment of the rich Coma cluster core.

astro-ph.CO↗

arXiv2010

Discovery of an optical counterpart to the hyperluminous X-ray source in ESO 243-49

The existence of black holes of masses ~ 10^2-10^5 Msun has important implications for the formation and evolution of star clusters and supermassive black holes. One of the strongest candidates to date is the hyperluminous X-ray source HLX1, possibly located in the S0-a galaxy ESO243-49, but the lack of an identifiable optical counterpart had hampered its interpretation. Using the Magellan telescope, we have discovered an unresolved optical source with R = (23.80 +/- 0.25) mag and V = (24.5 +/- 0.3) mag within HLX1's positional error circle. This implies an average X-ray/optical flux ratio ~ 500. Taking the same distance as ESO243-49, we obtain an intrinsic brightness M_R = (-11.0 +/- 0.3) mag, comparable to that of a massive globular cluster. Alternatively, the optical source is consistent with a main-sequence M star in the Galactic halo (for example an M4.4 star at ~ 2.5 kpc). We also examined the properties of ESO243-49 by combining Swift/UVOT observations with stellar population modelling. We found that the overall emission is dominated by a ~5 Gyr old stellar population, but the UV emission at ~2000 Ang is mostly due to ongoing star-formation at a rate of ~ 0.03 Msun/yr. The UV emission is more intense (at least a 9-sigma enhancement above the mean) North East of the nucleus, in the same quadrant as HLX1. With the combined optical and X-ray measurements, we put constraints on the nature of HLX1. We rule out a foreground star and a background AGN. Two alternative scenarios are still viable. HLX1 could be an accreting intermediate-mass black hole in a star cluster, which may itself be the stripped nucleus of a dwarf galaxy that passed through ESO243-49, an event which might have caused the current episode of star formation. Or, it could be a neutron star in the Galactic halo, accreting from an M4-M5 donor star.

astro-ph.HE↗

arXiv2009

GAMA: towards a physical understanding of galaxy formation

The Galaxy And Mass Assembly (GAMA) project is the latest in a tradition of large galaxy redshift surveys, and is now underway on the 3.9m Anglo-Australian Telescope at Siding Spring Observatory. GAMA is designed to map extragalactic structures on scales of 1kpc - 1Mpc in complete detail to a redshift of z~0.2, and to trace the distribution of luminous galaxies out to z~0.5. The principal science aim is to test the standard hierarchical structure formation paradigm of Cold Dark Matter (CDM) on scales of galaxy groups, pairs, discs, bulges and bars. We will measure (1) the Dark Matter Halo Mass Function (as inferred from galaxy group velocity dispersions); (2) baryonic processes, such as star formation and galaxy formation efficiency (as derived from Galaxy Stellar Mass Functions); and (3) the evolution of galaxy merger rates (via galaxy close pairs and galaxy asymmetries). Additionally, GAMA will form the central part of a new galaxy database, which aims to contain 275,000 galaxies with multi-wavelength coverage from coordinated observations with the latest international ground- and space-based facilities: GALEX, VST, VISTA, WISE, HERSCHEL, GMRT and ASKAP. Together, these data will provide increased depth (over 2 magnitudes), doubled spatial resolution (0.7"), and significantly extended wavelength coverage (UV through Far-IR to radio) over the main SDSS spectroscopic survey for five regions, each of around 50 deg^2. This database will permit detailed investigations of the structural, chemical, and dynamical properties of all galaxy types, across all environments, and over a 5 billion year timeline.

astro-ph.CO↗

arXiv2009

The Millennium Galaxy Catalogue: The $M_{bh}$--$L_{spheroid}$ derived supermassive black hole mass function

Supermassive black hole mass estimates are derived for 1743 galaxies from the Millennium Galaxy Catalogue using the recently revised empirical relation between supermassive black hole mass and the luminosity of the host spheroid. The MGC spheroid luminosities are based on $R^{1/n}$-bulge plus exponential-disc decompositions. The majority of black hole masses reside between $10^6 M_{\odot}$ and an upper limit of $2\times10^9 M_{\odot}$. Using previously determined space density weights, we derive the SMBH mass function which we fit with a Schechter-like function. Integrating the black hole mass function over $10^6< M_{bh}/ M_{\odot} < 10^{10}$ gives a supermassive black hole mass density of ($3.8 \pm 0.6) \times 10^5 h^{3}_{70} M_{\odot}$ Mpc$^{-3}$ for early-type galaxies and ($0.96 \pm 0.2) \times10^5 h^{3}_{70} M_{\odot}$ Mpc$^{-3}$ for late-type galaxies. The errors are estimated from Monte Carlo simulations which include the uncertainties in the $M_{bh}$--$L$ relation, the luminosity of the host spheroid and the intrinsic scatter of the $M_{bh}$--$L$ relation. Assuming supermassive black holes form via baryonic accretion we find that ($0.008\pm0.002) h_{70}^{3}$ per cent of the Universe's baryons are currently locked up in supermassive black holes. This result is consistent with our previous estimate based on the $M_{bh}$--$n$ (S{é}rsic index) relation.

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arXiv2009

Quantifying the Coexistence of Massive Black Holes and Dense Nuclear Star Clusters

In large spheroidal stellar systems, such as elliptical galaxies, one invariably finds a 10^6-10^9 M_Sun supermassive black hole at their centre. In contrast, within dwarf elliptical galaxies one predominantly observes a 10^5-10^7 M_Sun nuclear star cluster. To date, few galaxies have been found with both type of nuclei coexisting and even less have had the masses determined for both central components. Here we identify one dozen galaxies housing nuclear star clusters and supermassive black holes whose masses have been measured. This doubles the known number of such hermaphrodite nuclei - which are expected to be fruitful sources of gravitational radiation. Over the host spheroid (stellar) mass range from 10^8 to 10^11 M_Sun, we find that a galaxy's nucleus-to-spheroid (baryon) mass ratio is not a constant value but decreases from a few percent to ~0.3 percent such that log[(M_BH+M_NC)/M_sph] = -(0.39+/-0.07)log[M_sph/10^10 M_Sun] -(2.18+/-0.07). Once dry merging has commenced by M_sph ~ 10^11 M_Sun and the nuclear star clusters have disappeared, this ratio is expected to become a constant value. As a byproduct of our investigation, we have found that the projected flux from resolved nuclear star clusters can be well approximated with Sersic functions having a range of indices from ~0.5 to ~3, the latter index describing the Milky Way's nuclear star cluster.

astro-ph.CO↗

arXiv2009

The M_bh-sigma diagram, and the offset nature of barred active galaxies

From a sample of 50 predominantly inactive galaxies with direct supermassive black hole mass measurements, it has recently been established that barred galaxies tend to reside rightward of the M_bh-sigma relation defined by non-barred galaxies. Either black holes in barred galaxies tend to be anaemic or the central velocity dispersions in these galaxies have a tendency to be elevated by the presence of the bar. The latter option is in accord with studies connecting larger velocity dispersions in galaxies with old bars, while the former scenario is at odds with the observation that barred galaxies do not deviate from the M_bh-luminosity relation. Using a sample of 88 galaxies with active galactic nuclei, whose supermassive black hole masses have been estimated from their associated emission lines, we reveal for the first time that they also display this same general behavior in the M_bh-sigma diagram depending on the presence of a bar or not. A new symmetrical and non-symmetrical "barless" M_bh-sigma relation is derived using 82 non-barred galaxies. The barred galaxies are shown to reside on or up to ~1 dex below this relation. This may explain why narrow-line Seyfert 1 galaxies appear offset from the "barless" M_bh-sigma relation, and has far reaching implications given that over half of the disk galaxy population are barred.

astro-ph.CO↗

arXiv2009

The Millennium Galaxy Catalogue: Exploring the Color-Concentration Bimodality via Bulge-Disc Decomposition

We investigate the origin of the galaxy color-concentration bimodality at the bright-end of the luminosity function (M(B) - 5 log h < -18 mag) with regard to the bulge-disc nature of galaxies. Via (2D) surface brightness profile modeling with GIM2D, we subdivide the local galaxy population in the Millennium Galaxy Catalogue into one-component and two-component systems. We reveal that one-component (elliptical and disc-only) systems define the two peaks of the galaxy color-concentration distribution (with total stellar mass densities of 0.7 +/- 0.1 and 1.3 +/- 0.1 x 10^8 h Msol Mpc^-3 respectively), while two-component systems contribute to both a bridging population and the red, concentrated peak (with total stellar mass densities of 1.1 +/- 0.1 and 1.8 +/- 0.2 x 10^8 h Msun Mpc^-3 respectively). Moreover, luminous, `bulge-less, red discs' and `disc-less, blue bulges' (blue ellipticals) are exceptionally rare (with volume-densities of 1.7 +/- 0.3 and 1.1 \pm 0.1 x 10^-4 h^3 Mpc^-3 respectively). Finally, within the two-component population we confirm a previously-reported correlation between bulge and disc color (with a mean offset of only <(u-r)_bulge-(u-r)_disc> = 0.22 +/- 0.02 mag).

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