Peter Erwin

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).

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.

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 R-band study of 65 S0‐Sb galaxies with the B-band 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 R25 or exponential disc scalelength h) and absolute sizes (kpc). S0‐Sab bars extend to ∼1‐10 kpc (mean ∼ 3.3 kpc), ∼0.2‐0.8R 25 (mean ∼ 0.38R 25) and ∼0.5‐2.5h (mean ∼ 1.4h). Late-type bars extend to only ∼0.5‐3.5 kpc, ∼0.05‐0.35R 25 and 0.2‐1.5h; their mean sizes are ∼1.5 kpc, ∼ 0.14R 25 and ∼0.6h .S bg alaxies resemble earlier-type galaxies in terms of bar size relative to h; their smaller R25-relative sizes may be a side effect of higher star formation, which increases R25 but not h. Sbc galaxies form a transition between the early- and late-type regimes. For S0‐Sbc galaxies, bar size correlates well with disc size (both R25 and h); these correlations are stronger than the known correlation with MB. 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 MB for Sc‐Sd galaxies. Because bar size scales with disc size and galaxy magnitude for most Hubble types, studies of bar evolution with redshift should select samples with similar distributions of disc size or magnitude (extrapolated to present-day values); otherwise, bar frequencies and sizes could be mis-estimated. Because early-type galaxies tend to have larger bars, resolution-limited studies will preferentially find bars in early-type galaxies (assuming no significant differential evolution in bar sizes). I show that the bars detected in Hubble Space Telescope (HST) nearinfrared(IR) images at z ∼ 1b ySheth et al. 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. Ke yw ords: galaxies: elliptical and lenticular, cD ‐ galaxies: evolution ‐ galaxies: spiral ‐ galaxies: structure.

Evolution of galactic discs: multiple patterns, radial migration, and disc outskirts

We investigate the evolution of galactic disks in N-body Tree-SPH simulations. We find that disks, initially truncated a t three scalelengths, can triple their radial extent, solely driven by se cular evolution. At the same time, the initial radial metall icity gradients are flattened and even reversed in the outer disks. Both Type I (si ngle exponential) and Type II (down-bending) observed disk surfacebrightness profiles can be explained by our findings. We show t hat profiles with breaks beyond the bar’s outer Lindblad reso nance, at present only explained as the effect of star-formation threshold, can occur even if no star fo rmation is considered. We explain these results with the strong angular momentum outward transfer, resulting from torques and radial migration associated with multiple patterns, such as central bars and spiral waves of different multiplicity. We find that even for stars ending up on co ld orbits, the changes in angular momentum exhibit complex structure as a function of radius, unlike the expected effect of transient spirals alone. We show that the bars in all of our simulations are the most effective drivers of radial migration through their corotatio n resonance, throughout the 3 Gyr of evolution studied. Focussing on one of our models, we find evidence for non-linear coupling among m = 1, 2, 3 and 4 density waves, where m is the pattern multiplicity. In this way the waves involved c onspire to carry the energy and angular momentum extracted by the first mode from the inner parts of the disk muc h farther out than a single mode could. We suggest that the naturally occurring larger resonance widths at galactic radii beyond four scale-lengths may have profound consequences on the formation and location of breaks in disk density profiles, provided spiral s are present at such large distances. We also consider the effect of gas inflow and show that when in-plane smooth gas accretion of∼ 5 M⊙/yr is included, the outer disks become more unstable, leading to a strong increase in the stellar velocity dispersion. This, i n turn, causes the formation of a Type III (up-bending) profil e in the old stellar population. We propose that observations of Type III surface brightness profiles, combined with an up-turn in the stella r velocity dispersions beyond the disk break, could be a signature of ongoing gas-accretion. The results of this study suggest that disk outskirts comprised of stars migrated from the inner disk would have relatively large radial velocity dispersions (> 30 km/s at 6 scale-lengths for Milky Way-size systems), and significant thickness when seen edge-on.

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 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 galaxys 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. A detailed statistical analysis of the general population of double-bar and inner-disk galaxies, as represented by this catalog, will be presented in a companion paper.

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

We present results from a survey of an unbiased sample of 38 early-type (S0–Sa), low-inclination, optically barred galaxies in the field, using images both from the ground and from space. Our goal was to find and characterize central stellar and gaseous structures: secondary bars, inner disks, and nuclear rings. We find that 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 extends to 240–750 pc in radius. Secondary bars are not systematically either parallel or perpendicular to the primary; we see cases where they lead the primary bar in rotation and others where they trail, which supports the hypothesis that the two bars of a double-bar system rotate independently. We see no significant effect of secondary bars on nuclear activity: our double-barred galaxies are no more likely to harbor a Seyfert or LINER nucleus than our single-barred galaxies. We find kiloparsec-scale inner disks in at least 20% of our sample; they occur 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 of these rings are dusty, sites of current or recent star formation, or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies (8% of the sample, but 15% of the S0s) 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. If we interpret nuclear rings, secondary bars, and (possibly) inner disks and nuclear spirals as signs of inner Lindblad resonances (ILRs), then between one and two-thirds of barred S0–Sa galaxies show evidence for ILRs.

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 � 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 � 100 pc, in a galaxy with an outer ring of radius � 14 kpc) and only � 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.

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.

THE HST/ACS COMA CLUSTER SURVEY. II. DATA DESCRIPTION AND SOURCE CATALOGS ∗

The Coma cluster, Abell 1656, was the target of an 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 early 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 or 1°) with a total coverage area of 274 arcmin2. The majority of the fields are located near the core region of Coma (19/25 pointings) with six additional fields in the southwest 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 its half-light radius. We have performed photometry for ~73,000 unique objects; approximately one-half of our detections are brighter than the 10σ 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 compact sources (primarily globular clusters but also ultra-compact dwarf galaxies) and a wide variety of extended galaxies from a cD galaxy to dwarf low surface brightness galaxies. The red sequence of Coma member galaxies has a color-magnitude relation with a constant slope and dispersion over 9 mag (–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.

Imfit: a fast, flexible new program for astronomical image fitting

I describe a new, open-source astronomical image-fitting program called IMFIT, specialized for galaxies but potentially useful for other sources, which is fast, flexible, and highly extensible. A key characteristic of the program is an object-oriented design which allows new types of image components (2D surface-brightness functions) to be easily written and added to the program. Image functions provided with IMFIT include the usual suspects for galaxy decompositions (Sérsic, exponential, Gaussian), along with Core-Sérsic and brokenexponential profiles, elliptical rings, and three components which perform line-of-sight integration through 3D luminosity-density models of disks and rings seen at arbitrary inclinations. Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead simplex, and Differential Evolution, allowing trade-offs between speed and decreased sensitivity to local minima in the fit landscape. Minimization can be done using the standard χ2 statistic (using either data or model values to estimate per-pixel Gaussian errors, or else user-supplied error images) or Poisson-based maximum-likelihood statistics; the latter approach is particularly appropriate for cases of Poisson data in the low-count regime. I show that fitting lowS/N galaxy images using χ2 minimization and individual-pixel Gaussian uncertainties can lead to significant biases in fitted parameter values, which are avoided if a Poisson-based statistic is used; this is true even when Gaussian read noise is present.I describe a new, open-source astronomical image-fitting program called Imfit, specialized for galaxies but potentially useful for other sources, which is fast, flexible, and highly extensible. A key characteristic of the program is an object-oriented design which allows new types of image components (2D surface-brightness functions) to be easily written and added to the program. Image functions provided with Imfit include the usual suspects for galaxy decompositions (Sersic, exponential, Gaussian), along with Core-Sersic and broken-exponential profiles, elliptical rings, and three components which perform line-of-sight integration through 3D luminosity-density models of disks and rings seen at arbitrary inclinations. Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead simplex, and Differential Evolution, allowing trade-offs between speed and decreased sensitivity to local minima in the fit landscape. Minimization can be done using the standard chi^2 statistic (using either data or model values to estimate per-pixel Gaussian errors, or else user-supplied error images) or Poisson-based maximum-likelihood statistics; the latter approach is particularly appropriate for cases of Poisson data in the low-count regime. I show that fitting low-S/N galaxy images using chi^2 minimization and individual-pixel Gaussian uncertainties can lead to significant biases in fitted parameter values, which are avoided if a Poisson-based statistic is used; this is true even when Gaussian read noise is present.