Optical Surface Photometry Research Articles

Unlocking the secrets of stellar haloes using combined star counts and surface photometry

The stellar haloes of galaxies can currently be studied either through observations of resolved halo stars or through surface photometry. Curiously, the two methods appear to give conflicting results, as a number of surface photometry measurements have revealed integrated colours that are too red to be reconciled with the halo properties inferred from the study of resolved stars. Several explanations for this anomaly have been proposed - including dust photoluminescence, extinction of extragalactic background light and a bottom-heavy stellar initial mass function. A decisive test is, however, still lacking. Here, we explain how observations of the halo of a nearby galaxy, involving a combination of both surface photometry and bright star counts, can be used to distinguish between the proposed explanations. We derive the observational requirements for this endeavour and find that star counts in filters VI and surface photometry in filters VIJ appears to be the optimal strategy. Since the required halo star counts are already available for many nearby galaxies, the most challenging part of this test is likely to be the optical surface photometry, which requires several nights of exposure time on a 4-8 m telescope, and the near-IR surface photometry, which is most readily carried out using the upcoming James Webb Space Telescope.

Optical multiband surface photometry of a sample of Seyfert galaxies

Context. Parallel analysis of the large-scale morphology and local environment of matched active and control galaxy samples plays an important role in studies of the fueling of active galactic nuclei.

On the Structural Differences between Disk and Dwarf Galaxies

Gas-rich dwarf and disk galaxies overlap in numerous physical quantities, which makes their classification subjective. We report the discovery of a separation of dwarfs and disks into two unique sequences in the mass (luminosity) versus scale length plane. This provides an objective classification scheme for late-type galaxies that only requires optical or near-IR surface photometry of a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a continuous relation between baryonic mass and rotational velocity, we conclude that the difference between dwarfs and disks must be due to their distribution of stellar light such that dwarfs are more diffuse than disk galaxies. This structural separation may be due to a primordial difference between low- and high-mass galaxies or produced by hierarchical mergers in which disks are built up from dwarfs. Structural differences between dwarf and disk galaxies may also be driven by the underlying kinematics, where the strong rotation in disks produces an axially symmetric object that undergoes highly efficient star formation in contrast to the more weakly rotating, more disordered motion of dwarfs that produces a diffuse, triaxial object with a history of inefficient star formation.

Optical and Near‐Infrared Color Profiles in Nearby Early‐Type Galaxies and the Implied Age and Metallicity Gradients

We present results of an age and metallicity gradient analysis inferred from both optical and near-infrared surface photometry. The analysis is based on a sample of 36 nearby early-type galaxies, obtained from the Early Data Release of the Sloan Digital Sky Survey and the Two Micron All Sky Survey. Surface brightness profiles were derived in each band and used to study the color gradients of the galaxies. Using simple stellar population models with both optical and near-infrared colors, we may interpret the color gradients in terms of age and metallicity gradients of galaxies. Using g(Z) equivalent to d log Z(met)/d log R and g(A) = d log (age)/d log R to represent the metallicity and age gradients, we found a median value of g(Z) = -0.25 +/- 0.03 for the metallicity gradient, with a dispersion sigma(gZ) = 0.19 +/- 0.02. The corresponding values for the age gradients were g(A) = 0.02 +/- 0.04 and sigma(gA) = 0.25 +/- 0.03. These results are in good agreement with recent observational results, as well as with recent simulations that suggest that both monolithic collapse and major mergers have played important roles in the formation of early-type galaxies. Our results demonstrate the potential of using multi-wave band colors obtained from current and future optical and infrared surveys in constraining the age and metallicity gradients of early-type galaxies.

Sample of minor merger of galaxies: Optical CCD surface photometry and HII region properties

We present the results of the B, V and I photometry of eleven southern minor mergers. The total apparent B magni- tude, integrated B−V and V − I colours were measured. We built B, V ,a ndI equivalent profiles for each galaxy and decomposed them into bulge and disk components when possible. From Hα+N(II) images we have estimated the basic photometric param- eters of the HII regions, such as position, size, B − V and V − I colours, Hα+(NII) luminosity and EW(Hα + (NII)) equivalent width. Primary components have blue absolute magnitudes in the range −22 < MB < −18, with a peak at MB = −22. The magnitudes of the secondary components are in the range −22 < MB < −16 with a maximum at MB = −19. Most pairs have ∆MB ∼ 2, which means that in luminosity the primary galaxy is on average about 6 times brighter than the secondary. We found a linear correlation between the luminosity ratios of the components and their ratio of major diameters, leading to mass ratios between 0.04 < Msecondary/M primary < 0.2, suggesting indeed that our sample is formed by minor mergers. On average the galaxies have colours bluer than those of isolated galaxies with the same morphological type. Most of the HII regions and evolved star-forming regions of the sample were formed between 3.6 to 13.7 Myr ago with an average of (6.3 ± 0.7) Myr. The HII region properties, luminosity, sizes and ages are similar in both components. The HII regions have log (Hα+(NII)) lumi- nosity between 38.6 and 41.7. The HII region luminosity function for the whole sample fits a power law of index α = -1.33. The linear correlation between the luminosity L(Hα+(NII)) and the size of the HII regions has slope of 2.12 ± 0.06. We found that the disk of the primary component is more luminous than those of Lu's sample, while the disk of the secondary is smaller and fainter. A plot of the disk parameters does not change with colour. This indicates that the different stellar populations in the disks were affected in the same way.

A study of H I-selected galaxies in the Hercules cluster

The present study focuses on a sample of 22 galaxies detected in the blind VLA H i survey of the Hercules cluster by Dickey ([CITE]), 18 of which were selected on an H i line width smaller than 270 km s-1 and 4 others with only tentative optical counterparts in the Palomar Sky Survey. Sensitive single-dish H i line spectra were obtained for 20 of them, and for one the VLA detection was not confirmed. Optical surface photometry has been carried out for 10 objects, for 8 of which optical spectroscopy was obtained as well. Based on various criteria, we classify two sample galaxies (ce-143 and ne-204) as genuine dwarfs whereas the remaining sample objects are found to be intrinsically luminous galaxies. For those objects investigated in optical wavelengths we determine properties similar to those of actively star-forming galaxies, and find that approximately one half of them has properties intermediate between those of dwarf galaxies and low-luminosity disc galaxies. No optical redshifts could be determined for two of the galaxies (sw-103 and sw-194) and their physical association with the H i clouds detected at their positions therefore remains uncertain. A particularly interesting object in our sample is the Tidal Dwarf Galaxy candidate ce-061 in the galaxy merger IC 1182.

Optical surface photometry of a sample of disk galaxies

This work presents the structural decomposition of a sample of 11 disk galaxies, which span a range of different morphological types. The U, B, V, R, and I photometric information given in Paper I (color and color-index images and luminosity, ellipticity, and position-angle profiles) has been used to decide what types of components form the galaxies before carrying out the decomposition. We find and model such components as bulges, disks, bars, lenses and rings.

Dynamics of Ringed Barred Spiral Galaxies. I. Surface Photometry and Kinematics of NGC 1433 and NGC 6300

This paper presents new near-infrared images and surface photometry and Hα Fabry-Perot radial velocities for NGC 1433 and NGC 6300, two large and previously well-studied nearby ringed barred spiral galaxies. Stellar absorption-line radial velocities and new optical surface photometry are also presented for NGC 1433. The data are intended to set the stage for dynamical modeling of the two galaxies, with the principal goals being to derive mass distributions, bar mass-to-light ratios, bar pattern speeds, and resonance locations, parameters which have been derived for very few ringed disk galaxies. The presence of strong rings and pseudorings in the two galaxies allows the possibility to link specific orbital resonances with the observed rings. The new data allow us to derive a fairly complete composite rotation curve of NGC 1433, leading us to make some interesting preliminary judgments about the structure of the galaxy. The new data also verify the previous finding that the radial velocity of the Seyfert nucleus in NGC 6300 differs from the actual systemic velocity of the galaxy by nearly 100 km s-1. We demonstrate in this paper that the offset is not an artifact of significant extinction in the inner regions of the galaxy.

Age constraints for blue LSBGs

Optical and near-infrared surface photometry of the bluest low surface brightness galaxies (Ronnback and Bergvall, 1994, and Bergvall et al., 1999) has produced a sample that yields information on what is probably the youngest and/or metal-poorest tail of the LSBG population. Using the newest version of a spectral evolutionary code (described in Zackrisson et al., 2000), we compared model predictions to observations for thirty galaxies in an attempt to constrain their ages.

Modelling the Dust Content of Spiral Galaxies

We compare optical and near-infrared surface photometry of seven edge-on spiral galaxies with corresponding surface photometry calculated from a realistic model of spiral galaxies which takes into account both absorption and scattering by the interstellar dust. For the stars and the dust in the disc we use exponential distributions in both directions, radially and perpendicular to the plane of the disc, while the de Vaucouleurs (R1/4) profile is used for the description of the bulge. The effect of the spiral structure in the galactic discs is also examined and it is found that the simple exponential disc model is able to describe quite accurately the real galaxy. From this analysis we were able to obtain some general conclusions, the most significant of which are:1) The face-on central optical depth is less than unity in all optical bands, indicating that typical spiral galaxies like those we have modelled would be completely transparent if they were to be seen face on.2) The dust scale height is about half that of the stars, which means that dust is more concentrated near the plane of the disc.3) The dust scale length is about 1.4 times larger than that of the stars and dust is more radially extended than the stars.4) The gas-to-dust mass ratio calculated, with the dust mass derived from the model, is close to the value derived for our Galaxy.5) The derived extinction law matches the Galactic extinction law quite well, indicating a universal dust behaviour.

Photometrical Constraints on the Age of IZw18

Two studies of the star formation history of IZw18 are presented: HST/NICMOS imaging has been used to contruct a near-IR CMD, which give a lower limit of 1 Gyr to the age of IZw18 (Ostlin, 2000), in agreement with the optical CMD (Aloisi et al., 1999). Optical and near-IR surface photometry was extracted from deep images obtained with the Nordic Optical Telescope and the UKIRT. The colours become redder with increasing radius. The halo colours have been compared to predictions from the PEGASE.2 (Fioc and Rocca-Volmerange, 1999) spectral synthesis model and, on the assumption of being of stellar origin, give a lower limit of 1 Gyr to the age of the underlying population.

Optical surface photometry of radio galaxies

Optical imaging observations for 50 radio galaxies are presented. For each object isophotal contours, photometric profiles, structural parameters (position angle, ellipticity, Fourier coefficients), and total magnitudes are given. These observations, obtained in the Cousins R band, complement the data presented in a previous paper and are part of a larger project aimed at studying the optical properties of low redshift () radio galaxies (Govoni et al. 1999). Comments for each individual source are reported.

Optical Surface Photometry of a Sample of Disk Galaxies. I. Observations and Data Reduction

We present accurate optical surface photometry in the U, B, V, R, and I passbands for 11 disk galaxies. The sample has been selected in order to study the different morphological structures present in disk galaxies and includes all morphological types. For each galaxy, we present surface brightness, ellipticity, and position angle radial profiles from ellipse fits to the isophotes. Color index images and color index profiles in U-B, B-V, and B-I are also shown. The photometric information obtained is crucial to understanding the different morphological structures presented in all these galaxies, and to obtain their mass distributions. The latter topic will be the subject of a forthcoming paper.

The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. II. Models

The first paper in this series presented optical (UBVI) and near-infrared (H) surface photometry and Hα Fabry-Perot interferometry of IC 4214, an excellent example of a multiringed galaxy with a weak bar. In the present paper, we analyze the nonaxisymmetric gravitational potential of IC 4214, as defined by the near-infrared data. The gas behavior is modeled with two-dimensional sticky particle simulations that employ a rigidly rotating nonaxisymmetric potential. Our goals are to interpret the morphology and velocity field of IC 4214 and to obtain a model from which the bar pattern speed, halo contribution, and relative bulge and disk masses can be deduced. We also study the orbital families in the derived IC 4214 model. A fairly good match, in both morphology and kinematics, is obtained for Ωbar ≈ 40 ± 5 km s-1 kpc-1, placing the bar corotation slightly beyond the end of the bar (rbar/rcr ≈ 0.72 ± 0.05). The deduced bar amplitude implies that the visible mass of the disk dominates over the unseen halo (Mdisk/Mhalo > 3–4). Moreover, our model accounts quantitatively for the large difference in the orientation of IC 4214 as derived in our first paper with photometric and kinematic methods.

An Optical, Near-Infrared, and Kinematic Study of Four Early-Type Resonance Ring Galaxies

?????We present optical and near-infrared surface photometry of four early-type galaxies considered to have resonance ring phenomena on the basis of morphology. We also present kinematics and rotation curves for three of the galaxies. The four galaxies are exceptional examples of resonance ring galaxies, and from these observations we derive the characteristic light distributions, the colors of the rings, and the near-infrared disk mass-to-light ratios. We also deproject the galaxies to examine intrinsic ring shapes and bar/ring alignments. From models of the rotation curves, we compute the Lindblad precession frequencies to examine resonance locations in the disks of the galaxies. Under the assumption that the outer rings and pseudorings seen in these galaxies are linked to the outer Lindblad resonance, we find that the inner rings of the sample galaxies lie near the inner 4:1 resonance and that the sharp ends of the bars lie on average at only 0.65 times the predicted radius of corotation.

Disk Galaxies in the Outer Local Supercluster: Optical CCD Surface Photometry and Distribution of Galaxy Disk Parameters

We report new B-band CCD surface photometry on a sample of 76 disk galaxies brighter than BT = 14.5 mag in the Uppsala General Catalogue of Galaxies that are confined within a volume located in the outer part of the Local Supercluster. With our earlier published I-band CCD and high signal-to-noise ratio 21 cm H I data, this paper completes our optical surface photometry campaign on this galaxy sample. As an application of this data set, the B-band photometry is used here to illustrate two selection effects that have been somewhat overlooked in the literature but that may be important in deriving the distribution function of disk central surface brightness (CSB) of disk galaxies from a diameter- and/or flux-limited sample: a Malmquist-type bias against disk galaxies with small disk scale lengths (DSLs) at a given CSB and a disk inclination-dependent selection effect that may, for example, bias toward inclined disks near the threshold of a diameter-limited selection if disks are not completely opaque in the optical. Taking into consideration these selection effects, we present a method of constructing a volume-sampling function and a way to interpret the derived distribution function of CSB and DSL. Application of this method to our galaxy sample implies that if galaxy disks are optically thin, CSB and DSL may well be correlated in the sense that, up to an inclination-corrected limiting CSB of about 24.5 mag arcsec-2 that is adequately probed by our galaxy sample, the DSL distribution of galaxies with a lower CSB may have a longer tail toward large values unless the distribution of disk galaxies as a function of CSB rises rapidly toward faint values.

An Optical and H [CSC]i[/CSC] Study of NGC 5850: Victim of a High-Speed Encounter?

We present optical CCD surface photometry and VLA H I observations of NGC 5850, one of the largest and brightest barred spirals of the inner ring variety in the sky. The broadband images reveal numerous morphological peculiarities, particularly in the spiral arms. Structural asymmetries are more obvious in H I, the most pronounced being a large-scale displacement of gas to the west and northwest of the nucleus. Most of the (3.3 ± 0.1) × 109 M☉ of atomic hydrogen is concentrated in the prominent optical ring and faint spiral arms, with very low H I surface densities (ΣH I) in the bulge and interarm regions. We detect approximately 2 × 107 M☉ of H I in the southwestern half of the nuclear ring. The H I surface density drops rapidly outside the arms, and we find no evidence for either large-scale tidal features or an extended gas disk above 0.05 M☉ pc-2 (3 σ). Overall, the intensity-weighted H I velocity field appears fairly regular, yet still shows clear deviations from circular rotation that we attribute to a warped oval disk and streaming motions across the arms. Radio continuum emission at 20 cm is dominated by a faint bulge component that peaks at the optical nucleus. The spiral arms are not detected. The absence of extended X-ray emission throughout the NGC 5846 group and the close similarity between NGC 5850's optical and H I morphologies argue against ram pressure stripping through a dense intergalactic medium as the cause of the galaxy's peculiar morphology. We attribute it instead to a high-speed encounter with the massive elliptical galaxy NGC 5846. We identify the peculiar spiral arms as a disrupted outer pseudoring. Star formation in NGC 5850 has not been enhanced relative to other intermediate field spirals, nor have significant gas masses been transported to the nucleus. This may be attributed to the recent (200 Myr) nature of the interaction.

The peculiar rotation curve of NGC 157

We present the results of a new H i, optical, and Hα interferometric study of the nearby spiral galaxy NGC 157. Our combined C- and D-array observations with the VLA show a large-scale, ring-like structure in the neutral hydrogen underlying the optical disc, together with an extended, low surface density component going out to nearly twice the Holmberg radius. Beginning just inside the edge of the star-forming disc, the line of nodes in the gas disc commences a 60° warp, while at the same time, the rotation velocity drops by almost half its peak value of 200 km s−1, before levelling off again in the outer parts. While a flat rotation curve in NGC 157 cannot be ruled out, supportive evidence for an abrupt decline comes from the ionized gas kinematics, the optical surface photometry, and the global H i profile. A standard ‘maximum-disc’ mass model predicts comparable amounts of dark and luminous matter within NGC 157. Alternatively, a model employing a disc truncated at 2 disc scalelengths could equally well account for the unusual form of the rotation curve in NGC 157.

Near-infrared and optical broadband surface photometry of 86 face-on disk dominated galaxies. II. A two-dimensional method to determine bulge and disk parameters

In this Paper I present a new two-dimensional decomposition technique, which models the surface photometry of a galaxy with an exponential light profile for both bulge and disk and, when necessary, with a Freeman bar. The new technique was tested for systematic errors on both artificial and real data and compared with widely used one-dimensional decomposition techniques, where the luminosity profile of the galaxy is used. The comparisons indicate that a decomposition of the two-dimensional image of the galaxy with an exponential light profile for both bulge and disk yields the most reproducible and representative bulge and disk parameters. An extensive error analysis was made to determine the reliability of the model parameters. If the model with an exponential bulge profile is a reasonable description of a galaxy, the maximum errors in the derived model parameters are of order 20%. The uncertainties in the model parameters will increase, if the exponential bulge function is replaced by other often used bulge functions as the de Vaucouleurs law. All decomposition methods were applied to the optical and near-infrared data set presented by de Jong & van der Kruit (1994), which comprises 86 galaxies in six passbands.

The universal rotation curve of spiral galaxies — I. The dark matter connection

Abstract We use a homogeneous sample of about 1100 optical and radio rotation curves (RCs) and relative surface photometry to investigate the main mass structure properties of spirals, over a range of 6 mag and out to ≾ 1.5 and 2 optical radii (for the optical and radio data, respectively). We confirm the strong dependence on luminosity for both the profile and the amplitude of RCs claimed by Persic &amp; Salucci. Spiral RCs show the striking feature that a single global parameter, e.g. luminosity, dictates the rotational velocity at any radius for any object, so revealing the existence of a universal RC. At high luminosities, there is a slight discrepancy between the profiles of RCs and those predicted from the luminous matter (LM) distributions: this implies a small, yet detectable, amount of dark matter (DM). At low luminosities, the failure of the LM prediction is much more severe, and the DM is the only relevant mass component. We show that the universal RC implies a number of scaling properties between dark and luminous galactic structure parameters: (i) the DM/LM mass ratio scales inversely with luminosity; (ii) the central halo density scales as L−0.7; (iii) the halo core radius is comparable to the optical radius, but shrinks for low luminosities; (iv) the total halo mass scales as L0.5. Such scaling properties can be represented as a curve in the (luminosity)-(DM/LM mass ratio)-(DM core radius)-(DM central density) space, which provides a geometrical description of the tight coupling between the dark and the luminous matter in spiral galaxies.