arXiv subjects

Peter Erwin

Publications and source records attributed to Peter Erwin.

At least 55 records · Page 3Linked to original sources

arXiv2012

The HST/ACS Coma Cluster Survey. VIII. Barred Disk Galaxies in the Core of the Coma Cluster

(ABRIDGED) We use high resolution (~0.1") F814W ACS images from the HST ACS Treasury survey of the Coma cluster at z~0.02 to study bars in massive disk galaxies (S0s), and in dwarf galaxies in the Coma core. Our study helps constrain the evolution of bars and disks in dense environments and provides a comparison point for studies in lower density environments and at higher redshifts. (1) We characterize the fraction and properties of bars in a sample of 32 bright (M_V <= -18, M_* > 10^9.5 M_sun) S0 galaxies, which dominate the population of massive disk galaxies in the Coma core. Measuring the S0 bar fraction must be handled carefully, as the results depend on the method used: the bar fraction for bright S0s in the Coma core is 50%+/-11%, 65%+/-11%, and 60%+/-11% for three methods of bar detection: strict ellipse fit criteria, relaxed ellipse fit criteria, and visual classification. (2) We compare the S0 bar fraction across different environments (Coma core, A901/902, Virgo). We find that the bar fraction among bright S0 galaxies does not show a statistically significant variation across environments spanning two orders of magnitude in galaxy number density (n~300-10,000 gal/Mpc^3). We speculate that the S0 bar fraction is not significantly enhanced in rich clusters because S0s in rich clusters are less prone to bar instabilities as they are dynamically hot and gas poor due to ram pressure stripping and accelerated star formation. In addition, high-speed encounters in rich clusters may be less effective than slow, strong encounters in inducing bars. (3) We analyze a sample of 333 faint (M_V > -18) dwarf galaxies in the Coma core. Using unsharp-masking, we find only 13 galaxies with bar and/or spiral structure. The paucity of disk structures in Coma dwarfs suggests that either disks are not common in these galaxies, or that any disks present are too hot to develop instabilities.

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arXiv2011

The Relation Between Galaxy Morphology and Environment in the Local Universe: An RC3-SDSS Picture

We present an analysis of the z ~ 0 morphology-environment relation for 911 bright (M_B < -19) galaxies, matching classical RC3 morphologies to the SDSS-based group catalog of Yang et al. We study how the relative fractions of spirals, lenticulars, and ellipticals depend on halo mass over a range of 10^11.7-10^14.8 h^-1 Msol. We pay particular attention to how morphology relates to central (most massive) vs satellite galaxy status. The fraction of galaxies which are elliptical is a strong function of stellar mass; it is also a strong function of halo mass, but only for central galaxies. We interpret this in a scenario where elliptical galaxies are formed, probably via mergers, as central galaxies within their halos; satellite ellipticals are previously central galaxies accreted onto larger halos. The overall fraction of S0 galaxies increases strongly with halo mass, from ~10% to ~70%. We find striking differences between the central and satellites: 20+/-2% of central M_* > 10^10.5 Msol galaxies are S0 regardless of halo mass, but satellite S0 galaxies are only found in massive (> 10^13 h^-1 Msol) halos, where they are 69+/-4% of the M_* > 10^10.5 Msol satellite population. This suggests two channels for S0 formation: one for central galaxies, and another which transforms lower mass (M_* <~ 10^11 Msol) accreted spirals into satellite S0 galaxies in massive halos. Analysis of finer morphological structure (bars and rings in disk galaxies) shows some trends with stellar mass, but none with halo mass; this is consistent with other recent studies which indicate that bars are not strongly influenced by galaxy environment. Radio sources in high-mass central galaxies are common, similarly so for elliptical and S0 galaxies, with a frequency that increases with halo mass. Emission-line AGN (mostly LINERs) are more common in S0s, but show no strong environmental trends (abridged).

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arXiv2011

Do Nuclear Star Clusters and Black Holes Follow the Same Host-Galaxy Correlations?

Studies have suggested that there is a strong correlation between the masses of nuclear star clusters (NSCs) and their host galaxies, a correlation which said to be an extension of the well-known correlations between supermassive black holes (SMBHs) and their host galaxies. But careful analysis of disk galaxies -- including 2D bulge/disk/bar decompositions -- shows that while SMBHs correlate with the stellar mass of the bulge component of galaxies, the masses of NSCs correlate much better with the total galaxy stellar mass. In addition, the mass ratio M_nsc/M_star,tot for NSCs in spirals (at least those with Hubble types Sc and later) is typically an order of magnitude smaller than the mass ratio M_bh/M_star, bulge of SMBHs. The absence of a universal "central massive object" correlation argues against common formation and growth mechanisms for both SMBHs and NSCs. We also discuss evidence for a break in the NSC-host galaxy correlation: galaxies with Hubble types earlier than Sbc appear to host systematically more massive NSCs than do types Sc and later.

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arXiv2011

A Strong Dichotomy in S0 Disk Profiles Between the Virgo Cluster and the Field

We report evidence for a striking difference between S0 galaxies in the local field and in the Virgo Cluster. While field S0 galaxies have disks whose surface-brightness profiles are roughly equally divided between the three main types (Types I, II, and III: single-exponential, truncated, and antitruncated), Virgo S0s appear to be entirely lacking in disk truncations. More specifically, the fraction of truncations in S0 galaxies with M_B < -17 is 28% +7/-6% for the field, versus 0% +4/-0% for the Virgo Cluster galaxies; the difference is significant at the 99.7% level. The discrepancy is made up almost entirely by Type I profiles, which are almost twice as frequent in the Virgo Cluster as they are in the field. This suggests that S0 formation may be driven by different processes in cluster and field environments, and that outer-disk effects can be useful tests of S0 formation models.

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arXiv2011

The Outer Disks of Early-Type Galaxies. II. Surface-Brightness Profiles of Unbarred Galaxies and Trends with Hubble Type

We present azimuthally averaged radial profiles of R-band surface brightness for a complete sample of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred galaxies. Following very careful sky subtraction, the profiles can typically be determined down to brightness levels well below 27 mag arcsec^{-2} and in the best cases below 28 mag arcsec^{-2}. We classified the profiles according to the scheme used previously for the barred sample: Type I profiles are single unbroken exponential radial declines in brightness; Type II profiles ("truncations") have an inner shallow slope (usually exponential) which changes at a well defined break radius to a steeper exponential; and Type III profiles ("antitruncations") have an inner exponential that is steeper, giving way to a shallower outer (usually exponential) decline. By combining these profiles with previous studies, we can make the first clear statements about the trends of outer-disk profile types along the Hubble sequence (including both barred and unbarred galaxies), and their global frequencies. We find that Type I profiles are most frequent in early-type disks, decreasing from one-third of all S0--Sa disks to barely 10% of the latest type spirals. Conversely, Type II profiles (truncations) increase in frequency with Hubble type, from only ~25% of S0 galaxies to ~80% of Sd--Sm spirals. Overall, the fractions of Type I, II, and III profiles for all disk galaxies (Hubble types S0--Sm) are: 21%, 50%, and 38%, respectively; this includes galaxies (~8% of the total) with composite Type II+III profiles (counted twice). Finally, we note the presence of bars in ten galaxies previously classified (optically) as "unbarred". This suggests that ~20% of optically unbarred galaxies are actually barred; the bars in such cases can be weak, obscured by dust, or so large as to be mistaken for the main disk

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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

The central black hole mass of the high-sigma but low-bulge-luminosity lenticular galaxy NGC 1332

The masses of the most massive supermassive black holes (SMBHs) predicted by the M_BH-sigma and M_BH-luminosity relations appear to be in conflict. Which of the two relations is the more fundamental one remains an open question. NGC 1332 is an excellent example that represents the regime of conflict. It is a massive lenticular galaxy which has a bulge with a high velocity dispersion sigma of ~320 km/s; bulge--disc decomposition suggests that only 44% of the total light comes from the bulge. The M_BH-sigma and the M_BH-luminosity predictions for the central black hole mass of NGC 1332 differ by almost an order of magnitude. We present a stellar dynamical measurement of the SMBH mass using an axisymmetric orbit superposition method. Our SINFONI integral-field unit (IFU) observations of NGC 1332 resolve the SMBH's sphere of influence which has a diameter of ~0.76 arcsec. The sigma inside 0.2 arcsec reaches ~400 km/s. The IFU data allow us to increase the statistical significance of our results by modelling each of the four quadrants separately. We measure a SMBH mass of (1.45 \pm 0.20) x 10^9 M_sun with a bulge mass-to-light ratio of 7.08 \pm 0.39 in the R-band. With this mass, the SMBH of NGC 1332 is offset from the M_BH-luminosity relation by a full order of magnitude but is consistent with the M_BH-sigma relation.

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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.

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arXiv2010

The Coexistence of Classical Bulges, Pseudobulges, and Supermassive Black Holes

Some S0 and early-type spiral galaxies possess "composite bulges"; in these galaxies, the photometric bulge -- the central stellar light in excess of the disk light -- is composed of both a "(disky) pseudobulge", with a flattened, disklike morphology and relatively cool stellar kinematics, and a rounder, kinematically hot "classical bulge" embedded within. I speculate that supermassive black holes (SMBH) in such galaxies may correlate with the classical-bulge component only, and not with the pseudobulge component; preliminary comparisons with SMBH masses appear to support this hypothesis.

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arXiv2010

Do Nuclear Star Clusters and Supermassive Black Holes Follow the Same Host-Galaxy Correlations?

Recent studies have suggested a strong correlation between the masses of nuclear star clusters and their host galaxies, an extension of the known correlations between supermassive black holes (SMBHs) and their host galaxies. By focusing on disk galaxies with well-determined black hole and nuclear cluster masses, we argue that there is not a universal "central massive object" correlation after all: careful analysis shows that while SMBHs correlate better with the stellar masses of the bulge components, nuclear star clusters clearly correlate better with total galaxy stellar mass.

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arXiv2009

Do black hole masses scale with classical bulge luminosities only? The case of the two composite pseudobulge galaxies NGC3368 and NGC3489

It is now well established that all galaxies with a massive bulge component harbour a central supermassive black hole (SMBH). The mass of the SMBH correlates with bulge properties such as the bulge mass and the velocity dispersion, which implies that the bulge and the SMBH of a galaxy have grown together during the formation process. The spiral galaxy NGC3368 and the S0 galaxy NGC3489 both host a pseudobulge and a much smaller classical bulge component at the centre. We present high resolution, near-infrared IFU data of these two galaxies, taken with SINFONI at the VLT, and use axisymmetric orbit models to determine the masses of the SMBHs. The SMBH mass of NGC3368 is M_BH=7.5x10^6 M_sun with an error of 1.5x10^6 M_sun, which mostly comes from the non-axisymmetry in the data. For NGC3489, a solution without black hole cannot be excluded when modelling the SINFONI data alone, but can be clearly ruled out when modelling a combination of SINFONI, OASIS and SAURON data, for which we obtain M_BH=6.00^{+0.56}_{-0.54} (stat) +/- 0.64 (sys) x 10^6 M_sun. Although both galaxies seem to be consistent with the M_BH-sigma relation, at face value they do not agree with the relation between bulge magnitude and black hole mass when the total bulge magnitude (i.e., including both classical bulge and pseudobulge) is considered; the agreement is better when only the small classical bulge components are considered. However, taking into account the ageing of the stellar population could change this conclusion.

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arXiv2009

Double-Barred Galaxies

I present a brief review of what is known about double-barred galaxies, where a small ("inner") bar is nested inside a larger ("outer") bar; the review is focused primarily on their demographics and photometric properties. Roughly 20% of S0--Sb galaxies are double-barred; they may be rarer in later Hubble types. Inner bars are typically ~ 500 pc in radius (~ 12% the size of outer bars), but sizes range from ~ 100 pc to > 1 kpc. The structure of at least some inner bars appears very similar to that of outer bars (and single large-scale bars). Direct and indirect evidence all support the hypothesis that inner bars rotate independently of outer bars, although actual pattern speeds for inner bars are poorly constrained. Finally, I note that inner bars do not appear to promote nuclear activity.

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arXiv2008

Where have all the bulges gone?

Several recent studies indicate that bulges are more complex than merely structureless relaxed stellar systems. We study the HST images of a sample of 130 nearby early type (S0-Sab) disc galaxies and detect pure structureless bulges with the Sersic index n > 2 for only 12% of the galaxies. Other galaxies show varied substructure in their inner regions (inner bars, inner spiral arms, inner rings) and sometimes contain no bulge at all. Inner substructure is more common for these galaxies, which also display structure at larger scales.

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arXiv2008

The HST/ACS Coma Cluster Survey: I - Survey Objectives and Design

We describe the HST ACS Coma cluster Treasury survey, a deep two-passband imaging survey of one of the nearest rich clusters of galaxies, the Coma cluster (Abell 1656). The survey was designed to cover an area of 740 square arcmin in regions of different density of both galaxies and intergalactic medium within the cluster. The ACS failure of January 27th 2007 leaves the survey 28% complete, with 21 ACS pointings (230 square arcmin) complete, and partial data for a further 4 pointings (44 square arcmin). Predicted survey depth for 10 sigma detections for optimal photometry of point sources is g' = 27.6 in the F475W filter, and IC=26.8 mag in F814 (AB magnitudes). Initial simulations with artificially injected point sources show 90% recovered at magnitude limits of g' = 27.55 and IC = 26.65. For extended sources, the predicted 10 sigma limits for a 1 square arcsecond region are g' = 25.8 mag/sq. arcsec and IC = 25.0 mag/sq. arcsec. We highlight several motivating science goals of the survey, including study of the faint end of the cluster galaxy luminosity function, structural parameters of dwarf galaxies, stellar populations and their effect on colors and color gradients, evolution of morphological components in a dense environment, the nature of ultra compact dwarf galaxies, and globular cluster populations of cluster galaxies of a range of luminosities and types. This survey will also provide a local rich cluster benchmark for various well known global scaling relations and explore new relations pertaining to the nuclear properties of galaxies.

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arXiv2007

Trends for Outer Disk Profiles

The surface-brightness profiles of galaxy disks fall into three main classes, based on whether they are simple exponentials (Type I), bend down at large radii (Type II, "truncations") or bend up at large radii (Type III, "antitruncations"). Here, we discuss how the frequency of these different profiles depends on Hubble type, environment, and the presence or absence of bars; these trends may herald important new tests for disk formation models.

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arXiv2007

The Coexistence of Classical Bulges and Disky Pseudobulges in Early-Type Disk Galaxies

Close examination of so-called "pseudobulges" in several early-type disk galaxies indicates that they are actually composite structures consisting of both a flattened, kinematically cool disklike structure ("disky pseudobulge") and a rounder, kinematically hot spheroidal structure ("classical bulge"). This indicates that pseudobulges, thought to form from internal secular evolution, and classical bulges, thought to form from rapid mergers, are not exclusive phenomena: some galaxies can have both.

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arXiv2007

The Outer Disks of Early-Type Galaxies. I. Surface-Brightness Profiles of Barred Galaxies

We present a study of 66 barred, early-type (S0-Sb) disk galaxies, focused on the disk surface brightness profile outside the bar region and the nature of Freeman Type I and II profiles, their origins, and their possible relation to disk truncations. This paper discusses the data and their reduction, outlines our classification system, and presents $R$-band profiles and classifications for all galaxies in the sample. The profiles are derived from a variety of different sources, including the Sloan Digital Sky Survey (Data Release 5). For about half of the galaxies, we have profiles derived from more than one telescope; this allows us to check the stability and repeatability of our profile extraction and classification. The vast majority of the profiles are reliable down to levels of mu_R ~ 27 mag arcsec^-2; in exceptional cases, we can trace profiles down to mu_R > 28. We can typically follow disk profiles out to at least 1.5 times the traditional optical radius R_25; for some galaxies, we find light extending to ~ 3 R_25. We classify the profiles into three main groups: Type I (single-exponential), Type II (down-bending), and Type III (up-bending). The frequencies of these types are approximately 27%, 42%, and 24%, respectively, plus another 6% which are combinations of Types II and III. We further classify Type II profiles by where the break falls in relation to the bar length, and in terms of the postulated mechanisms for breaks at large radii ("classical trunction" of star formation versus the influence of the Outer Lindblad Resonance of the bar). We also classify the Type III profiles by the probable morphology of the outer light (disk or spheroid). Illustrations are given for all cases. (Abridged)

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arXiv2007

The Outer Structure of Galactic Disks: Connections Between Bars, Disks, and Environments

Surface-brightness profiles for early-type (S0-Sb) disks exhibit three main classes (Type I, II, and III). Type II profiles are more common in barred galaxies, and most of the time appear to be related to the bar's Outer Lindblad Resonance. Roughly half of barred galaxies in the field have Type II profiles, but almost none in the Virgo Cluster do; this might be related to ram-pressure stripping in clusters. A strong \textit{anti}correlation is found between Type III profiles ("antitruncations") and bars: Type III profiles are most common when there is no bar, and least common when there is a strong bar.

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