Teh Effect of Stellar Contamination on Different Measures of Galaxy Photometry
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Fundamental parameters of brightest cluster galaxies
Photometry and spectroscopy of a sample of the brightest cluster galaxies (BCGs) are presented, and the relationships between scale length, surface brightness, velocity dispersion, and line strength are determined and compared with the relations determined for lower luminosity ellipticals (LLEs). It is found that the relation between log r(e), log sigma, and mean surface brightness reported by Dressler et al. for LLEs also describes the relation for BCGs fairly well. Fundamental plane solutions for BCGs are derived which can be used as distance indicators with a predicted error of about 21 percent per galaxy. Only a weak correlation is found between the Na D 5893 line strength and velocity dispersion for BCGs, in contrast to the significant correlation between these quantities reported for LLEs. It is found that the mean mass-to-light ratio for BCGs is a factor of about two greater than for the LLEs in the Virgo and Coma sample of Dressler et al.
Oegerle, William R.↗
Surface photometry of galaxies. I - The SB0 NGC 2950
Surface photometry of NGC 2950, a classic example of the SB0 type galaxy, is presented. The analysis shows that except for the rudimentary bar the galaxy closely resembles an S0 galaxy. The disk component has an exponential light curve with a central surface brightness of B(0) = 21.65 per square second of arc and a scale size of 4.3 kpc. The nucleus, bar, and disk components contain 29, 22, and 48 percent, respectively, of the light. Integral and differential colors are presented and discussed in terms of the three components mentioned above.
Crane, P.↗
CCD surface photometry of galaxies
We present here the results based on analysis of broad band optical images of radio loud and radio quiet galaxies selected from a set of x-ray galaxies. Data reduction techniques, and surface photometry using the IRAF data reduction package are described. The radial surface brightness profile of each galaxy is obtained. The disc, bulge, and nucleus of the galaxy are modeled based on the estimated brightness profile. The model of the galaxy is constructed, which is subtracted from the observed image to enhance the small scale features in the galaxy. Color maps of the galaxy are obtained and compared with those of normal galaxies. The optical properties of the galaxy are compared with its x-ray and radio properties.
Anupama, G. C.↗
The effect of spiral arms on the Sérsic photometry of galaxies
The Sérsic profile is a widely used model for describing the surface brightness distribution of galaxies. Spiral galaxies, however, are qualitatively different from a Sérsic model. Aims. The goal of this study is to assess how accurately the total flux and half-light radius of a galaxy with spiral arms can be recovered when fitted with a Sérsic profile. Methods. I selected a sample of bulge-dominated galaxies with spiral arms. Using photometric data from the Hyper Suprime-Cam survey, I estimated the contribution of the spiral arms to their total flux. Then I generated simulated images of galaxies with similar characteristics, fitted them with a Sérsic model, and quantified the error on the determination of the total flux and half-light radius. Results. Spiral arms can introduce biases on the photometry of galaxies in a way that depends on the underlying smooth surface brightness profile, the location of the arms, and the depth of the photometric data. A set of spiral arms accounting for 10% of the flux of a bulge-dominated galaxy typically causes the total flux and the half-light radius to be overestimated by 15% and 30%, respectively. This bias, however, is much smaller if the galaxy is disk-dominated. Conclusions. Galaxies with a prominent bulge and a non-zero contribution from spiral arms are the most susceptible to biases in the total flux and half-light radius when fitted with a Sérsic profile. If photometric measurements with high accuracy are required, then measurements over finite apertures are to be preferred over global estimates of the flux.
79 ASTRONOMY AND ASTROPHYSICS↗
Middle-ultraviolet photometry of Virgo cluster galaxies
Photographic photometry of Virgo cluster galaxies has been performed in a wavelength band extending from 1620-3200 A using sounding rocket techniques. The observational results are middle ultraviolet magnitudes, U2421, or faint limits for U2421, for 201 galaxies within 5.5 arcmin of the cluster center. A strong negative correlation is found between U2421 - V and V for all observed S0 galaxies and a similar but weaker correlation for the observed ellipticals. No such correlation is found for spiral galaxies regardless of their projection angles on the sky. The measured colors are generally compatible with colors computed from previously generated composite spectra.
Smith, A. M.↗
Intermediate bandpass photometry of galaxies and globular clusters
Galaxies and globular clusters, obtaining data by measurements with intermediate bandpass photometry
Neff, J. S.↗
Preliminary results of OAO ultraviolet photoelectric photometry of galaxies
General results are presented based on nearly completed reductions of data for approximately 35 galaxies of all Hubble types. In general the visual and ultraviolet energy distributions are well correlated. The energy distribution in late-type galaxies appears to turn up sharply in the region around 2000 A, and this is tentatively interpreted as due to the presence of early-type stars whose energy distribution is modified by interstellar dust within these objects. Similar turnups may be present in several elliptical galaxies but the evidence at this time is not definitive.
Code, A. D.↗
Surface colour photometry of galaxies with Schmidt telescopes.
A method is described which owes its practicality to the capability of Schmidt telescopes to record a number of galaxy images on a single plate and to the existence of high speed computer controlled area-scanning precision microdensitometers such as the Photometric Data Systems model 1010. The method of analysis results in quantitative color-index information which is displayed in a manner that allows any user to effectively study the morphological properties of the distribution of color-index in galaxies.
Wray, J. D.↗
Photometry of resolved galaxies. III - GR 8
Photometry of 84 resolved stellar images in GR 8 is presented. The numbers are derived by applying point spread function fitting techniques on digital images obtained with a three-phase Texas Instruments CCD. The resulting color-magnitude diagrams are discussed. These data are compared with theoretical stellar evolutionary tracks and similar results for other resolved galaxies.
Hoessel, J. G.↗
High-resolution surface photometry of elliptical galaxies
High-resolution CCD surface photometry profiles have been obtained for a sample of 42 nearby elliptical and SO galaxies as a first step in a program to investigate their central structure and core properties. A comparison of the present profiles to those of other observers shows that the accuracy of the central surface brightnesses is to better than 0.02 mag rms. Central resolution is limited by the atmospheric seeing PSF which is measured for each profile. A hybrid Fourier deconvolution procedure has been developed to correct the surface photometry for seeing in a model-independent way. Tests of the procedure on simulated galaxy images show that intrinsic core radii equal to the seeing FWHM or larger can be recovered from the observations. Application of the deconvolution procedure to the observed galaxies yields surface photometry profiles of slightly sub-arc second resolution.
Lauer, T. R.↗
Surface photometry of Virgo cluster galaxies - Barred galaxies
Photographic surface photometry in B and V is presented for three barred galaxies in the Virgo cluster: N4548, N4596, and N4608. Intercomparisons of luminosity and color profiles and standard photometric parameters indicate that for these galaxies: (1) the nuclear component follows the fourth-root-of-radius luminosity law for both B and V, (2) the luminosity profiles along the bar show a characteristic shoulder with a slight fall in B-V color profile at the brightest point in the bar, the strength of the effect declining from N4548 to N4608, (3) the integrated bar component is slightly bluer than the nucleus, and (4) as the disk, arm, and ring components contribute less to the total luminosity of the system, the contribution of the bar increases as does the equivalent gradient.
Benedict, G. F.↗
DSPS: Differentiable stellar population synthesis
ABSTRACT Models of stellar population synthesis (SPS) are the fundamental tool that relates the physical properties of a galaxy to its spectral energy distribution (SED). In this paper, we present DSPS: a python package for SPS. All of the functionality in DSPS is implemented natively in the JAX library for automatic differentiation, and so our predictions for galaxy photometry are fully differentiable, and directly inherit the performance benefits of JAX, including portability onto GPUs. DSPS also implements several novel features, such as i) a flexible empirical model for stellar metallicity that incorporates correlations with stellar age, ii) support for the Diffstar model that provides a physically-motivated connection between the star formation history of a galaxy (SFH) and the mass assembly of its underlying dark matter halo. We detail a set of theoretical techniques for using autodiff to calculate gradients of predictions for galaxy SEDs with respect to SPS parameters that control a range of physical effects, including SFH, stellar metallicity, nebular emission, and dust attenuation. When forward modelling the colours of a synthetic galaxy population, we find that DSPS can provide a factor of 5 speed-up over standard SPS codes on a CPU, and a factor of 300-400 on a modern GPU. When coupled with gradient-based techniques for optimization and inference, DSPS makes it practical to conduct expansive likelihood analyses of simulation-based models of the galaxy–halo connection that fully forward model galaxy spectra and photometry.
79 ASTRONOMY AND ASTROPHYSICS↗
Dark Energy Survey Year 3 results: redshift calibration of the weak lensing source galaxies
ABSTRACT Determining the distribution of redshifts of galaxies observed by wide-field photometric experiments like the Dark Energy Survey (DES) is an essential component to mapping the matter density field with gravitational lensing. In this work we describe the methods used to assign individual weak lensing source galaxies from the DES Year 3 Weak Lensing Source Catalogue to four tomographic bins and to estimate the redshift distributions in these bins. As the first application of these methods to data, we validate that the assumptions made apply to the DES Y3 weak lensing source galaxies and develop a full treatment of systematic uncertainties. Our method consists of combining information from three independent likelihood functions: self-organizing map p(z) (sompz), a method for constraining redshifts from galaxy photometry; clustering redshifts (WZ), constraints on redshifts from cross-correlations of galaxy density functions; and shear ratios (SRs), which provide constraints on redshifts from the ratios of the galaxy-shear correlation functions at small scales. Finally, we describe how these independent probes are combined to yield an ensemble of redshift distributions encapsulating our full uncertainty. We calibrate redshifts with combined effective uncertainties of σ〈z〉 ∼ 0.01 on the mean redshift in each tomographic bin.
79 ASTRONOMY AND ASTROPHYSICS↗
New Cepheid distances to nearby galaxies based on BVRI CCD photometry. II - The local group galaxy M33
A true distance modulus to the nearby spiral galaxy M33 has been determined based on CCD photometry obtained at BVRI wavelengths. M33 is presently one of five nearby galaxies used in the calibration of the IR Tully-Fisher relation, and thereby in the determination of the Hubble constant. Using period-luminosity relations at several wavelengths offers the advantage that the distance moduli derived can be corrected for the effects of interstellar extinction. These data indicate that there is internal reddening affecting the Cepheid photometry in M33 which must be accounted for if a true distance modulus is to be obtained for this galaxy. Adopting a true distance modulus to the LMC of 18.5 mag, the new CCD data yield a true distance to M33 of 24.64 + or - 0.09 mag, corresponding to a linear distance of 840 kpc. A mean value of the total color excess (foreground and internal) for the Cepheids in M33 is estimated to be E(B - V) = 0.10 + or - 0.09 mag, assuming a value for the total mean LMC Cepheid color excess of 0.10 mag.
Freedman, Wendy L.↗
Self-supervised Representation Learning for Astronomical Images
Sky surveys are the largest data generators in astronomy, making automated tools for extracting meaningful scientific information an absolute necessity. We show that, without the need for labels, self-supervised learning recovers representations of sky survey images that are semantically useful for a variety of scientific tasks. These representations can be directly used as features, or fine-tuned, to outperform supervised methods trained only on labeled data. We apply a contrastive learning framework on multiband galaxy photometry from the Sloan Digital Sky Survey (SDSS), to learn image representations. We then use them for galaxy morphology classification and fine-tune them for photometric redshift estimation, using labels from the Galaxy Zoo 2 data set and SDSS spectroscopy. In both downstream tasks, using the same learned representations, we outperform the supervised state-of-the-art results, and we show that our approach can achieve the accuracy of supervised models while using 2-4 times fewer labels for training. The codes, trained models, and data can be found at https://portal.nersc.gov/project/dasrepo/self-supervised-learning-sdss.
79 ASTRONOMY AND ASTROPHYSICS↗
DIGS: deep inference of galaxy spectra with neural posterior estimation
Abstract With the advent of billion-galaxy surveys with complex data, the need of the hour is to efficiently model galaxy spectral energy distributions (SEDs) with robust uncertainty quantification. The combination of simulation-based inference (SBI) and amortized neural posterior estimation (NPE) has been successfully used to analyse simulated and real galaxy photometry both precisely and efficiently. In this work, we utilise this combination and build on existing literature to analyse simulated noisy galaxy spectra. Here, we demonstrate a proof-of-concept study of spectra that is (a) an efficient analysis of galaxy SEDs and inference of galaxy parameters with physically interpretable uncertainties; and (b) amortized calculations of posterior distributions of said galaxy parameters at the modest cost of a few galaxy fits with Markov chain Monte Carlo (MCMC) methods. We utilise the SED generator and inference framework Prospector to generate simulated spectra, and train a dataset of 2 × 10 6 spectra (corresponding to a five-parameter SED model) with NPE. We show that SBI—with its combination of fast and amortized posterior estimations—is capable of inferring accurate galaxy stellar masses and metallicities. Our uncertainty constraints are comparable to or moderately weaker than traditional inverse-modelling with Bayesian MCMC methods (e.g. 0.17 and 0.26 dex in stellar mass and metallicity for a given galaxy, respectively). We also find that our inference framework conducts rapid SED inference (0.9–1.2 × 10 5 galaxy spectra via SBI/NPE at the cost of 1 MCMC-based fit). With this work, we set the stage for further work that focuses of SED fitting of galaxy spectra with SBI, in the era of JWST galaxy survey programs and the wide-field Roman Space Telescope spectroscopic surveys.
spectroscopy↗
An H II region beyond the optical disk of the galaxy
The detection of a faint H II region at a kinematic distance of 28 kpc from the Galactic center is reported. The velocity and morphology of H-alpha emission obtained using a Fabry-Perot indicate that the H II region is associated with a molecular cloud detected by Digel et al. (1993). The ionizing source must be close to the cloud and is constrained to be an early B star; a distant blue star, probably an early B supergiant, is identified as the ionizing star. Although its precise luminosity is uncertain, the likely range implies a distance modulus placing the H II region well beyond the optical disk of the Galaxy. Photometry of other stars associated with the GMC/H II region will allow a reliable distance determination and extend the rotation curve of the Galaxy by 60 percent. The implications of current star formation beyond the optical disk are discussed.
De Geus, E. J.↗