Ronald J. Buta

Third Reference Catalogue of Bright Galaxies

This new, enlarged reference catalogue of bright galaxies in three volumes reflects the explosive growth of extragalactic astronomy over the last 15 years. With data on more than 23,000 galaxies, it includes all galaxies with apparent diameters larger than one arc minute, magnitudes brighter than about magnitude 15.5, and redshifts not larger than 15,000 km/sec, as well as many other objects of interest. Volume 1 contains the explanations and references; volumes 2 and 3 contain the catalogue proper. The catalogue gives for each galaxy, the position, names, type and luminosity class, optical diameters, optical and infrared magnitudes, various colour indices and radial velocities. The work also makes reference to papers on bright galaxies published between 1913 and 1988. This dictionary/encyclopaedia on stellar systems is intended for researchers in astronomy.

Multicomponent decompositions for a sample of S0 galaxies

We have estimated the bulge-to-total (B/T) light ratios in the Ks band for a sample of 24 SO, S0/a and Sa galaxies by applying a two-dimensional multicomponent decomposition method. For the disc an exponential function is used, the bulges are fitted by a Sersic R 1/n function and the bars and ovals are described either by a Sersic or a Ferrers function. In order to avoid non-physical solutions, preliminary characterization of the structural components is made by inspecting the radial profiles of the orientation parameters and the low azimuthal wavenumber Fourier amplitudes and phases. In order to identify also the inner structures, unsharp masks were created: previously undetected inner spiral arms were found in NGC 1415 and marginally in NGC 3941. Most importantly, we found that S0s have a mean (B/T) K ratio of 0.24 ± 0.11, which is significantly smaller than the mean (B/T) R = 0.6 generally reported in the literature. Also, the surface brightness profiles of the bulges in S0s were found to be more exponential-like than generally assumed, the mean shape parameter of the bulge being = 2.1 ± 0.7. We did not find examples of barred S0s lacking the disc component, but we found some galaxies (NGC 718, 1452 and 4608) having a non-exponential disc in the bar region. To our knowledge, our study is the first attempt to apply a multicomponent decomposition method for a moderately sized sample of early-type disc galaxies.

Photometric scaling relations of lenticular and spiral galaxies

Photometric scaling relations are studied for S0 galaxies and compared with those obtained for spirals. New two-dimensional multi-component decompositions are presented for 122 early-type disc galaxies, using deep K s -band images. Combining them with our previous decompositions, the final sample consists of 175 galaxies (Near-Infrared Survey of S0s, NIRSOS: 117 SOs + 22 S0/a and 36 Sa galaxies). As a comparison sample we use the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS) of nearly 200 spirals, for which similar multi-component decompositions have previously been made by us. The improved statistics, deep images and the homogeneous decomposition method used allow us to re-evaluate the parameters of the bulges and discs. For spirals we largely confirm previous results, which are compared with those obtained for S0s. Our main results are as follows. (1) Important scaling relations are present, indicating that the formative processes of bulges and discs in S0s are coupled [e.g. M 0 K (disc) = 0.63 M 0 K (bulge) -9.3], as has been found previously for spirals [for OSUBSGS spirals M 0 K (disc) = 0.38 M 0 K (bulge) -15.5; the rms deviation from these relations is 0.5 mag for S0s and spirals]. (2) We obtain median r eff /h 0 r ~ 0.20, 0.15 and 0.10 for S0, S0/a-Sa and Sab-Sc galaxies, respectively: these values are smaller than predicted by simulation models in which bulges are formed by galaxy mergers. (3) The properties of bulges of S0s are different from the elliptical galaxies, which are manifested in the M 0 K (bulge) versus r eff relation, in the photometric plane (μ 0 , n, reff ), and to some extent also in the Kormendy relation (〈μ〉 eff versus r eff ). The bulges of S0s are similar to bulges of spirals with M 0 K (bulge) < -20 mag. Some S0s have small bulges, but their properties are not compatible with the idea that they could evolve to dwarfs by galaxy harassment. (4) The relative bulge flux (B/T) for S0s covers the full range found in the Hubble sequence, even with 13 per cent having B/T < 0.15, typical for late-type spirals. (5) The values and relations of the parameters of the discs [h 0 r , M 0 K (disc), μ 0 (0)] of the S0 galaxies in NIRS0S are similar to those obtained for spirals in the OSUBSGS. Overall, our results support the view that spiral galaxies with bulges brighter than -20 mag in the K band can evolve directly into S0s, due to stripping of gas followed by truncated star formation.

Properties of Bars and Bulges in the Hubble Sequence

Properties of bars and bulges in the Hubble sequence are discussed, based on the analysis of 216 disk galaxies (S0s and spirals from NIRS0S and OSUBGS surveys, respectively). For that purpose we have collected together, and completed when necessary, the various analysis we have previously made separately for early and late types. We find strong photometric and kinematic evidence of pseudobulges in the S0-S0/a galaxies: their bulges are on average fairly exponential, inner disks are common (in 56%), and in many of the galaxies the bulges are rotationally supported. This would be difficult to understand in such gas poor galaxies as in S0s, i f these pseudobulge candidates were formed by star formation in the disk in a a similar manner as in spirals. A more likely explanation is that pseudobulges in the early-type galaxies are bar-related structures, connected to the evolution of bars, which interpretation is supported by our Fourier analysis and structural decompositions. Bars in the early-type galaxies are found to have many characteristics of evolved systems: (1) they have flat-top/double peaked Fourier amplitude profiles, (2) bars have typically sharp outer cut-offs, (3) the higher Fourier modes appear in the amplitude profiles, and (4) many bars have also ansae-type morphologies. We show the distributions of bar strength in different Hubble type bins using four bar strength indicators, Qg, A2, fbar and the bar length, which are expected to give important clues for understanding the mechanism of how bars evolve.

RECONSTRUCTING THE STELLAR MASS DISTRIBUTIONS OF GALAXIES USING S4G IRAC 3.6 AND 4.5 μm IMAGES. I. CORRECTING FOR CONTAMINATION BY POLYCYCLIC AROMATIC HYDROCARBONS, HOT DUST, AND INTERMEDIATE-AGE STARS

With the aim of constructing accurate two-dimensional maps of the stellar mass distribution in nearby galaxies from Spitzer Survey of Stellar Structure in Galaxies 3.6 and 4.5 μm images, we report on the separation of the light from old stars from the emission contributed by contaminants. Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust content, and star formation histories are presented to demonstrate our approach. To isolate the old stellar light from contaminant emission (e.g., hot dust and the 3.3 μm polycyclic aromatic hydrocarbon (PAH) feature) in the IRAC 3.6 and 4.5 μm bands we use an independent component analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. The contaminants are studied by comparison with the non-stellar emission imaged at 8 μm, which is dominated by the broad PAH feature. Using the measured 3.6 μm/8 μm ratio to select individual contaminants, we find that hot dust and PAHs together contribute between ~5% and 15% to the integrated light at 3.6 μm, while light from regions dominated by intermediate-age (AGB and RSG) stars accounts for only 1%-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate-age stars contribute upward of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing stellar mass, as will be the focus in a companion paper.

Reconstructing the Stellar Mass Distributions of Galaxies Using S4G IRAC 3.6 and 4.5 μm Images. II.: The Conversion from Light to Mass

We present a new approach for estimating the 3.6 μm stellar mass-to-light (M/L) ratio Υ_3.6 in terms of the [3.6]-[4.5] colors of old stellar populations. Our approach avoids several of the largest sources of uncertainty in existing techniques using population synthesis models. By focusing on mid-IR wavelengths, we gain a virtually dust extinction-free tracer of the old stars, avoiding the need to adopt a dust model to correctly interpret optical or optical/near-IR colors normally leveraged to assign the mass-to-light ratio Upsilon. By calibrating a new relation between near-IR and mid-IR colors of giant stars observed in GLIMPSE we also avoid the discrepancies in model predictions for the [3.6]-[4.5] colors of old stellar populations due to uncertainties in the molecular line opacities assumed in template spectra. We find that the [3.6]-[4.5] color, which is driven primarily by metallicity, provides a tight constraint on Upsilon3.6, which varies intrinsically less than at optical wavelengths. The uncertainty on Υ3.6 of ~0.07 dex due to unconstrained age variations marks a significant improvement on existing techniques for estimating the stellar M/L with shorter wavelength data. A single Υ3.6 = 0.6 (assuming a Chabrier initial mass function (IMF)), independent of [3.6]-[4.5] color, is also feasible because it can be applied simultaneously to old, metal-rich and young, metal-poor populations, and still with comparable (or better) accuracy (~0.1 dex) than alternatives. We expect our Υ3.6 to be optimal for mapping the stellar mass distributions in S4G galaxies, for which we have developed an independent component analysis technique to first isolate the old stellar light at 3.6 μm from nonstellar emission (e.g., hot dust and the 3.3 polycyclic aromatic hydrocarbon feature). Our estimate can also be used to determine the fractional contribution of nonstellar emission to global (rest-frame) 3.6 μm fluxes, e.g., in WISE imaging, and establishes a reliable basis for exploring variations in the stellar IMF.

COMPARISON OF BAR STRENGTHS AND FRACTIONS OF BARS IN ACTIVE AND NONACTIVE GALAXIES

Gravitational perturbation strengths and bar fractions in active and nonactive galaxies are compared using the Ohio State University Bright Galaxy Survey, which forms a statistically well defined sample of 180 disk galaxies. Bar fractions are studied using (1) the optical and near-IR classification of bars made by Eskridge and coworkers in 2002 and (2) our own bar classification based on Fourier decomposition of near-IR images (Fourier bars). The gravitational perturbation strengths are calculated using the bar torque method, taking the maximum ratio Qg of the tangential force to the mean background radial force as a measure of the nonaxisymmetric perturbation. In addition, two-dimensional bulge-disk-bar decomposition is used to study the properties of bulges of the sample galaxies. In the near-IR, Seyfert galaxies, LINERs, and H II/starburst galaxies were found to have a similar fraction, 72%, of Fourier bars (or SB-type bars), compared to 55% in the nonactive galaxies. However, if SAB-type bars are also included, practically all (95%) H II/starburst galaxies have bars. In addition, a large fraction (34%) of bars in LINERs are obscured by dust in the optical region. We find that bars in early-type galaxies are at the same time long and massive and have weak perturbation strengths. Weak perturbation strengths can be explained by dilution of the nonaxisymmetric forces by the massive bulges: for a bulge-to-disk mass ratio B/D ranging from 0 to 1, the dilution may reduce Qg from as high as 0.6 to as low as 0.1. On the other hand, bar length (relative to disk scale length) is not correlated with B/D, contrary to expectation. Seyfert- or LINER-type nuclear activity is present in most galaxies that have thin and thick planar bar components, whereas nuclear activity does not appear in those late-type galaxies that have extremely massive bars and strong perturbation strengths.

Bar‐induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey – I

Bar-induced perturbation strengths are calculated for a well-defined magnitude-limited sample of 180 spiral galaxies, based on the Ohio State University Bright Galaxy Survey. We use a gravitational torque method, the ratio of the maximal tangential force to the mean axisymmetric radial force, as a quantitative measure of the bar strength. The gravitational potential is inferred from an H-band light distribution by assuming that the M/L ratio is constant throughout the disc. Galaxies are deprojected using orientation parameters based on B-band images. In order to eliminate artificial stretching of the bulge, two-dimensional bar‐bulge‐disc decomposition has been used to derive a reliable bulge model. This bulge model is subtracted from an image, the disc is deprojected assuming it is thin, and then the bulge is added back by assuming that its mass distribution is spherically symmetric. We find that removing the artificial bulge stretch is important especially for galaxies having bars inside large bulges. We also find that the masses of the bulges can be significantly overestimated if bars are not taken into account in the decomposition. Bars are identified using Fourier methods by requiring that the phases of the main modes (m = 2, m = 4) are maintained nearly constant in the bar region. With such methods, bars are found in 65 per cent of the galaxies in our sample, most of them being classified as SB-type systems in the near-infrared by Eskridge and co-workers. We also suggest that as much as ≈70 per cent of the galaxies classified as SAB-types in the near-infrared might actually be non-barred systems, many of them having central ovals. It is also possible that a small fraction of the SAB-type galaxies have weak non-classical bars with spiral-like morphologies. Ke yw ords: galaxies: spiral ‐ galaxies: statistics.

Galaxies, Human Eyes, and Artificial Neural Networks

The quantitative morphological classification of galaxies is important for understanding the origin of type frequency and correlations with environment. However, galaxy morphological classification is still mainly done visually by dedicated individuals, in the spirit of Hubbles original scheme and its modifications. The rapid increase in data on galaxy images at low and high redshift calls for a re-examination of the classification schemes and for automatic methods. Here are shown results from a systematic comparison of the dispersion among human experts classifying a uniformly selected sample of more than 800 digitized galaxy images. These galaxy images were then classified by six of the authors independently. The human classifications are compared with each other and with an automatic classification by an artificial neural network, which replicates the classification by a human expert to the same degree of agreement as that between two human experts.

A Comparative study of morphological classifications of APM galaxies

We investigate the consistency of visual morphological classifications of galaxies by comparing classifications for 831 galaxies from six independent observers. The galaxies were classified on laser print copy images or on computer screen produced from scans with the Automated Plate Measuring (APM) machine. Classifications are compared using the Revised Hubble numerical type index T. We find that individual observers agree with one another with rms combined dispersions of between 1.3 and 2.3 type units, typically about 1.8 units. The dispersions tend to decrease slightly with increasing angular diameter and, in some cases, with increasing axial ratio