arXiv subjects

Peter Erwin

Publications and source records attributed to Peter Erwin.

71 records · Page 4Linked to original sources

arXiv2006

Double barred galaxies at intermediate redshifts: A feasibility study

Despite the increasing number of studies of barred galaxies at intermediate and high redshifts, double-barred (S2B) systems have only been identified in the nearby (z<0.04) universe thus far. In this feasibility study we demonstrate that the detection and analysis of S2Bs is possible at intermediate redshifts (0.1 < z < 0.5) with the exquisite resolution of the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a novel method. The radial profile of the Gini coefficient -- a model-independent structure parameter -- is able to detect bars in early-type galaxies that are large enough that they might host an inner bar of sufficient angular size. Using this method and subsequent examination with unsharp masks and ellipse fits we identified the two most distant S2Bs currently known (at redshifts z=0.103 and z=0.148). We investigate the underlying stellar populations of these two galaxies through a detailed colour analysis, in order to demonstrate the analysis that could be performed on a future sample of intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to push the limit for S2B detection and analysis out by a factor of ten in redshift and lookback time (z=0.5, t=5Gyr) compared to the previously known S2Bs. This in turn would provide new insight into the formation of these objects.

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arXiv2005

How Large Are the Bars in Barred Galaxies?

I present a study of the sizes (semimajor axes) of bars in disc galaxies, combining a detailed study of 65 S0-Sb galaxies with measurements of 70 Sb-Sd galaxies from Martin (1995). As has been noted before with smaller samples, bars in early-type (S0-Sb) galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies; this is true both for relative sizes (bar length as fraction of isophotal radius R_25 or exponential disc scale length h) and absolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~3.3 kpc), ~0.2-0.8 R_25 (mean \~0.38 R_25) and ~0.5-2.5 h (mean ~1.4 h). Late-type bars extend to only \~0.5-3.5 kpc, 0.05-0.35 R_25 and 0.2-1.5 h; mean sizes are ~1.5 kpc, 0.14 R_25 and 0.6 h. Sb galaxies resemble earlier-type galaxies in terms of bar size relative to h; their smaller R_25-relative sizes may be a side effect of higher star formation, which increases R_25 but not h. For S0-Sbc galaxies, bar size correlates well with disc size (both R_25 and h); these correlations are stronger than the known correlation with M_B. All correlations appear to be weaker or absent for late-type galaxies; in particular, there seems to be no correlation between bar size and either h or M_B for Sc-Sd galaxies. I show that the bars detected in HST near-IR images at z ~ 1 by Sheth et al. (2003) have absolute sizes consistent with those in bright, nearby S0-Sb galaxies. I also compare the sizes of real bars with those produced in simulations, and discuss some possible implications for scenarios of secular evolution along the Hubble sequence. Simulations often produce bars as large as -- or larger than -- those seen in S0-Sb galaxies, but rarely any as small as those in Sc-Sd galaxies. (Abridged.)

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arXiv2005

Anti-Truncation of Disks in Early-Type Barred Galaxies

The disks of spiral galaxies are commonly thought to be truncated: the radial surface brightness profile steepens sharply beyond a certain radius (3--5 inner-disk scale lengths). Here we present the radial brightness profiles of a number of barred S0--Sb galaxies with the opposite behavior: their outer profiles are distinctly shallower in slope than the main disk profile. We term these "anti-truncations"; they are found in at least 25% of a larger sample of barred S0--Sb galaxies. There are two distinct types of anti-truncations. About one-third show a fairly gradual transition and outer isophotes which are progressively rounder than the main disk isophotes, suggestive of a disk embedded within a more spheroidal outer zone -- either the outer extent of the bulge or a separate stellar halo. But the majority of the profiles have rather sharp surface-brightness transitions to the shallower, outer exponential profile and, crucially, outer isophotes which are not significantly rounder than the main disk; in the Sab--Sb galaxies, the outer isophotes include visible spiral arms. This suggests that the outer light is still part of the disk. A subset of these profiles are in galaxies with asymmetric outer isophotes (lopsided or one-armed spirals), suggesting that interactions may be responsible for at least some of the disklike anti-truncations.

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arXiv2004

Pseudobulges in Barred S0 Galaxies

We present preliminary results from an ongoing study of the bulges of S0 galaxies. We show that in a subsample of 14 barred S0 galaxies, fully half the photometrically defined bulges show kinematic signatures of "pseudobulges" -- that is, their kinematics are dominated by rotation. In four of these galaxies, we identify at least two subcomponents in the photometric bulge region: flatter, disk or bar components, assocated with disklike kinematics; and rounder "inner bulges,'' which appear to be hotter systems more like classical bulges.

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arXiv2004

Evidence for a New Elliptical-Galaxy Paradigm: Sersic and Core Galaxies

We fit the surface-brightness profiles of 21 elliptical galaxies using both the Sersic function and a new empirical model which combines an inner power law with an outer Sersic function. The profiles are combinations of deconvolved HST profiles from the literature and ellipse fits to the full WFPC2 mosaic images, and thus span a radial range from ~ 0.02 arcsec to ~ twice the half-light radius. We are able to accurately fit the entire profiles using either the Sersic function or our new model. In doing so, we demonstrate that most, if not all, so-called ``power-law'' galaxies are better described as ``Sersic galaxies'' -- they are well modeled by the three-parameter Sersic profile into the limits of HST resolution -- and that ``core'' galaxies are best understood as consisting of an outer Sersic profile with an inner power-law cusp, which is a downward deviation from the inward extrapolation of the Sersic profile. This definition of cores resolves ambiguities that result when the popular ``Nuker law'' is fitted to the profiles of ellipticals and bulges, particularly at lower luminosities. We also find that using the Nuker law to model core-galaxy nuclear profiles systematically overestimates the core radii by factors of 1.5--4.5 and underestimates the inner power-law slope by ~ 20--40% or more.

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arXiv2003

Double-Barred Galaxies: I. A Catalog of Barred Galaxies with Stellar Secondary Bars and Inner Disks

I present a catalog of 67 barred galaxies which contain distinct, elliptical stellar structures inside their bars. Fifty of these are double-barred galaxies: a small-scale, "inner" or "secondary" bar is embedded within a large-scale, "outer" or "primary" bar. I provide homogenized measurements of the sizes, ellipticities, and orientations of both inner and outer bars, along with with global parameters for the galaxies. The other 17 are classified as "inner-disk" galaxies, where a large-scale bar harbors an inner elliptical structure which is aligned with the galaxy's outer disk. Four of the double-barred galaxies also possess inner disks, located in between the inner and outer bars. While the inner-disk classification is ad-hoc -- and undoubtedly includes some inner bars with chance alignments (five such probable cases are identified) -- there is good evidence that inner disks form a statistically distinct population, and that at least some are indeed disks rather than bars. In addition, I list 36 galaxies which may be double-barred, but for which current observations are ambiguous or incomplete, and another 23 galaxies which have been previously suggested as potentially being double-barred, but which are probably *not*. False double-bar identifications are usually due to features such as nuclear rings and spirals being misclassified as bars; I provide some illustrated examples of how this can happen.

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arXiv2003

When Is a Bulge Not a Bulge? Inner Disks Masquerading as Bulges in NGC 2787 and NGC 3945

We present a detailed morphological, photometric, and kinematic analysis of two barred S0 galaxies with large, luminous inner disks inside their bars. We show that these structures, in addition to being geometrically disk-like, have exponential profiles (scale lengths $\sim$ 300--500 pc) distinct from the central, non-exponential bulges. We also find them to be kinematically disk-like. The inner disk in NGC 2787 has a luminosity roughly twice that of the bulge; but in NGC 3945, the inner disk is almost ten times more luminous than the bulge, which itself is extremely small (half-light radius $\approx$ 100 pc, in a galaxy with an outer ring of radius $\approx$ 14 kpc) and only $\sim$ 5% of the total luminosity -- a bulge/total ratio much more typical of an Sc galaxy. We estimate that at least 20% of (barred) S0 galaxies may have similar structures, which means that their bulge/disk ratios may be significantly overestimated. These inner disks dominate the central light of their galaxies; they are at least an order of magnitude larger than typical ``nuclear disks'' found in ellipticals and early-type spirals. Consequently, they must affect the dynamics of the bars in which they reside.

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arXiv2003

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, due to 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. Moreover, the stability of the Nuker model parameters are investigated. Surprisingly, none are found to be stable quantities; all are shown to vary systematically with a profile's fitted radial extent, and often by more than 100%. Considering elliptical galaxies spanning a range of 7.5 magnitudes, the central stellar density of the underlying host galaxy is observed to 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.

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arXiv2002

The Correlation Between Supermassive Black Hole Mass and the Structure of Ellipticals and Bulges

We demonstrate a strong correlation between supermassive black hole (SMBH) mass and the global structure of ellipticals and bulges: more centrally concentrated bulges and ellipticals (higher Sersic index $n$) host higher-mass black holes. This correlation is as good as that previously found between SMBH mass and central velocity dispersion, with comparable scatter. In addition, by carefully modeling the bulges of disk galaxies so that bars, inner disks, and the like do not accidentally contribute to bulge light, we find that the correlation between SMBH mass and bulge or elliptical luminosity is similarly close.

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arXiv2002

An Imaging Survey of Early-Type Barred Galaxies

This paper presents the results of a high-resolution imaging survey, using both ground-based and Hubble Space Telescope images, of a complete sample of nearby barred S0--Sa galaxies in the field, with a particular emphasis on identifying and measuring central structures within the bars: secondary bars, inner disks, nuclear rings and spirals, and off-plane dust. A discussion of the frequency and statistical properties of the various types of inner structures has already been published. Here, we present the data for the individual galaxies and measurements of their bars and inner structures. We set out the methods we use to find and measure these structures, and how we discriminate between them. In particular, we discuss some of the deficiencies of ellipse fitting of the isophotes, which by itself cannot always distinguish between bars, rings, spirals, and dust, and which can produce erroneous measurements of bar sizes and orientations.

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arXiv2002

Double Bars, Inner Disks, and Nuclear Rings in Early-Type Disk Galaxies

We present results from an imaging survey of an unbiased sample of thirty-eight early-type (S0--Sa), low-inclination, optically barred galaxies in the field. Our goal was to find and characterize central stellar and gaseous structures: secondary bars, inner disks, and nuclear rings. Bars inside bars are surprisingly common: at least one quarter of the sample galaxies (possibly as many as 40%) are double-barred, with no preference for Hubble type or the strength of the primary bar. A typical secondary bar is ~12% of the size of its primary bar and 240--750 pc in radius. We see no significant effect of secondary bars on nuclear activity. We also find kiloparsec-scale inner disks in at least 20% of our sample, almost exclusively in S0 galaxies. These disks are on average 20% the size of their host bar, and show a wider range of relative sizes than do secondary bars. Nuclear rings are present in about a third of our sample. Most are dusty, sites of current or recent star formation, or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies (15% of the S0's) appear to have purely stellar nuclear rings, with no evidence for dust or recent star formation. The fact that these central stellar structures are so common indicates that the inner regions of early-type barred galaxies typically contain dynamically cool and disklike structures. This is especially true for S0 galaxies, where secondary bars, inner disks, and/or stellar nuclear rings are present at least two thirds of the time. (abridged)

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arXiv2001

Stellar Nuclear Rings in Barred Galaxies: Fossils of Past Circumnuclear Starbursts?

We have found four barred S0 galaxies -- NGC 936, NGC 3945, NGC 4340, and NGC 4371 -- which contain smooth, luminous, purely stellar nuclear rings within their bars. These rings have little or no dust, no evidence for recent star formation, and are approximately the same color as surrounding bar and bulge. Thus, they are probably the aged remnants of bar-driven circumnuclear starburst episodes similar to those seen in barred galaxies today. Using kinematic data from long-slit spectroscopy, we construct rotation and resonance curves for two of the galaxies. In both cases, the nuclear rings appear to be located near or at the inner inner Lindblad resonances of the large-scale bars. We also discuss the difficulties inherent in detecting and identifying such rings, and show some of the surprising ways in which stellar rings can distort galaxy isophotes and ellipse fits.

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arXiv2001

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 (M_bh) -- more concentrated bulges have more massive black holes. Using C_{r_e}(1/3) from Trujillo, Graham & Caon (2001b) as a measure of bulge concentration, we find that log (M_bh/M_sun) = 6.81(+/-0.95)C_{r_e}(1/3) + 5.03(+/-0.41). This correlation is shown to be marginally stronger (Spearman's r_s=0.91) than the relationship between the logarithm of the stellar velocity dispersion and log M_bh (Spearman's r_s=0.86), and has comparable, or less, scatter (0.31 dex in log M_bh), which decreases to 0.19 dex when we use only those galaxies whose supermassive black hole's radius of influence is resolved and remove one well understood outlying data point).

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arXiv2001

Double Bars, Inner Disks, and Nuclear Rings in Barred Galaxies

We present results of a high-resolution imaging survey of barred S0--Sa galaxies which demonstrate that the central regions of these galaxies are surprisingly complex. We see many inner bars --- small, secondary bars (typically less than a kiloparsec in radius) located inside of, and probably rotating faster than, the large primary bars. These are present in about one quarter to one third of all our sample. In contrast to some theoretical expectations, they do not seem to enhance AGN activity significantly. A third of barred S0's appear to host kiloparsec-scale disks within their bars; but the frequency of such inner disks is much lower in our S0/a and Sa galaxies. In addition, we find one example of a triple barred galaxy, and two cases of purely stellar nuclear rings --- probably the fossil remnants of past circumnuclear starbursts. We comment briefly on results from an ongoing analysis of known double-barred systems, extending to Hubble types as late as Sbc, and discuss their characteristic sizes and orientations.

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arXiv2000

Kinematics and photometry as complementary tools in the study of barred galaxies

Recent advances in observational techniques and theoretical modelling of galaxy kinematics allow us to use more than just optical morphology to discern the structure and dynamics of galaxies. Here, we show for three barred galaxies (UGC 10205, NGC 6221 and NGC4340) that both kinematics and photometry are necessary to fully understand the properties of these galaxies. Kinematic and spectrophotometric data enable us to detect structures such as bars and gas shocks that are not directly visible in the images; conversely, imaging can be crucial for understanding the kinematic and dynamical behavior of a galaxy.

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arXiv1999

Triple Bars and Complex Central Structures in Disk Galaxies

We present an analysis of ground-based and HST images of three early-type barred galaxies. The first, NGC 2681, may be the clearest example yet of a galaxy with three concentric bars. The two other galaxies were previously suggested as triple-barred. Our analysis shows that while NGC 3945 is probably double-barred, NGC 4371 has only one bar; but both have intriguing central structures. NGC 3945 has a large, extremely bright disk inside its primary bar, with patchy dust lanes, a faint nuclear ring or pseudo-ring within the disk, and an apparent secondary bar crossing the ring. NGC 4371 has a bright nuclear ring only marginally bluer than the surrounding bulge and bar. There is no evidence for significant dust or star formation in either of these nuclear rings. The presence of stellar nuclear rings suggests that the centers of these galaxies are dynamically cool and disklike.

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arXiv1998

A WIYN Survey of Early-Type Barred Galaxies: Double Bars and Central Structures

We present results from a preliminary analysis of a recently-completed, multicolor imaging survey of nearby, early-type barred galaxies in the field, carried out with the WIYN telescope and supplemented with archival HST images. This forms a reasonably complete sample of nearby, bright, barred S0 and Sa galaxies in the field. The excellent seeing provided by WIYN allows us to examine the galaxies for central features such as circumnuclear rings and secondary bars; we find some evidence for dust lanes within secondary bars. The most striking results is the high frequency of double bars: our analysis suggests that at least $\sim 20%$ of barred S0-Sa galaxies possess secondary bars}. We also find an excellent candidate triply barred galaxy: NGC 2681.

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