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Register a new userThe Carnegie-Irvine Galaxy Survey. VI. Quantifying Spiral Structure
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The Carnegie-Irvine Galaxy Survey provides high-quality broad-band optical images of a large sample of nearby galaxies for detailed study of their structure. To probe the physical nature and possible cosmological evolution of spiral arms, a common feature of many disk galaxies, it is important to quantify their main characteristics. We describe robust methods to measure the number of arms, their mean strength, length, and pitch angle. The arm strength depends only weakly on the adopted radii over which it is measured, and it is stronger in bluer bands than redder bands. The vast majority of clearly two-armed (grand-design) spiral galaxies have systematically higher relative amplitude of the $m=2$ Fourier mode in the main spiral region. We use both one-dimensional and two-dimensional Fourier decomposition to measure the pitch angle, finding reasonable agreement between these two techniques with a scatter of $sim$2$deg$. To understand the applicability and limitations of our methodology to imaging surveys of local and distant galaxies, we create mock images with properties resembling observations of local ($z$ $lesssim$ 0.1) galaxies by the Sloan Digital Sky Survey and distant galaxies (0.1 $lesssim$ $z$ $lesssim$ 1.1) observed with the $Hubble$ $Space$ $Telescope$. These simulations lay the foundation for forthcoming quantitative statistical studies of spiral structure to understand its formation mechanism, dependence on galaxy properties, and cosmological evolution.
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The Carnegie-Irvine Galaxy Survey (CGS) is a comprehensive investigation of the physical properties of a complete, representative sample of 605 bright (B_T <= 12.9 mag) galaxies in the southern hemisphere. This contribution describes the isophotal analysis of the broadband (BVRI) optical imaging component of the project. We pay close attention to sky subtraction, which is particularly challenging for some of the large galaxies in our sample. Extensive crosschecks with internal and external data confirm that our calibration and sky subtraction techniques are robust with respect to the quoted measurement uncertainties. We present a uniform catalog of one-dimensional radial profiles of surface brightness and geometric parameters, as well as integrated colors and color gradients. Composite profiles highlight the tremendous diversity of brightness distributions found in disk galaxies and their dependence on Hubble type. A significant fraction of S0 and spiral galaxies exhibit non-exponential profiles in their outer regions. We perform Fourier decomposition of the isophotes to quantify non-axisymmetric deviations in the light distribution. We use the geometric parameters, in conjunction with the amplitude and phase of the m=2 Fourier mode, to identify bars and quantify their size and strength. Spiral arm strengths are characterized using the m=2 Fourier profiles and structure maps. Finally, we utilize the information encoded in the m=1 Fourier profiles to measure disk lopsidedness. The databases assembled here and in Paper I lay the foundation for forthcoming scientific applications of CGS.
Motivated by recent developments in our understanding of the formation and evolution of massive galaxies, we explore the detailed photometric structure of a representative sample of 94 bright, nearby elliptical galaxies, using high-quality optical images from the Carnegie-Irvine Galaxy Survey. The sample spans a range of environments and stellar masses, from M* = 10^{10.2} to 10^{12.0} solar mass. We exploit the unique capabilities of two-dimensional image decomposition to explore the possibility that local elliptical galaxies may contain photometrically distinct substructure that can shed light on their evolutionary history. Compared with the traditional one-dimensional approach, these two-dimensional models are capable of consistently recovering the surface brightness distribution and the systematic radial variation of geometric information at the same time. Contrary to conventional perception, we find that the global light distribution of the majority (>75%) of elliptical galaxies is not well described by a single Sersic function. Instead, we propose that local elliptical galaxies generically contain three subcomponents: a compact (R_e < 1 kpc) inner component with luminosity fraction f ~ 0.1-0.15; an intermediate-scale (R_e ~ 2.5 kpc) middle component with f ~ 0.2-0.25; and a dominant (f = 0.6), extended (R_e ~ 10 kpc) outer envelope. All subcomponents have average Sersic indices n ~ 1-2, significantly lower than the values typically obtained from single-component fits. The individual subcomponents follow well-defined photometric scaling relations and the stellar mass-size relation. We discuss the physical nature of the substructures and their implications for the formation of massive elliptical galaxies.
The Carnegie-Irvine Galaxy Survey (CGS) is a long-term program to investigate the photometric and spectroscopic properties of a statistically complete sample of 605 bright (B_T < 12.9 mag), southern (Dec. < 0) galaxies using the facilities at Las Campanas Observatory. This paper, the first in a series, outlines the scientific motivation of CGS, defines the sample, and describes the technical aspects of the optical broadband (BVRI) imaging component of the survey, including details of the observing program, data reduction procedures, and calibration strategy. The overall quality of the images is quite high, in terms of resolution (median seeing 1), field of view (8.9 X 8.9), and depth (median limiting surface brightness 27.5, 26.9, 26.4, and 25.3 mag/arcsec2 in the B, V, R, and I bands, respectively). We prepare a digital image atlas showing several different renditions of the data, including three-color composites, star-cleaned images, stacked images to enhance faint features, structure maps to highlight small-scale features, and color index maps suitable for studying the spatial variation of stellar content and dust. In anticipation of upcoming science analyses, we tabulate an extensive set of global properties for the galaxy sample. These include optical isophotal and photometric parameters derived from CGS itself, as well as published information on multiwavelength (ultraviolet, U-band, near-infrared, far-infrared) photometry, internal kinematics (central stellar velocity dispersions, disk rotational velocities), environment (distance to nearest neighbor, tidal parameter, group or cluster membership), and H I content. The digital images and science-level data products will be made publicly accessible to the community.
Many recent observations and numerical simulations suggest that nearby massive, early-type galaxies were formed through a two-phase process. In the proposed second phase, the extended stellar envelope was accumulated through many dry mergers. However, details of the past merger history of present-day ellipticals, such as the typical merger mass ratio, are difficult to constrain observationally. Within the context and assumptions of the two-phase formation scenario, we propose a straightforward method, using photometric data alone, to estimate the average mass ratio of mergers that contributed to the build-up of massive elliptical galaxies. We study a sample of nearby massive elliptical galaxies selected from the Carnegie-Irvine Galaxy Survey, using two-dimensional analysis to decompose their light distribution into an inner, denser component plus an extended, outer envelope, each having a different optical color. The combination of these two substructures accurately recovers the negative color gradient exhibited by the galaxy as whole. The color difference between the two components (<Delta(B-V)> ~ 0.10 mag; <Delta(B-R)> ~ 0.14 mag), based on the slope of the M_stellar-color relation for nearby early-type galaxies, can be translated into an estimate of the average mass ratio of the mergers. The rough estimate, 1:5 to 1:10, is consistent with the expectation of the two-phase formation scenario, suggesting that minor mergers were largely responsible for building up to the outer stellar envelope of present-day massive ellipticals. With the help of accurate photometry, large sample size, and more choices of colors promised by ongoing and future surveys, the approach proposed here can reveal more insights into the growth of massive galaxies during the last few Gyr.
The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993) with $R_0$=8.5 kpc, and $Theta_0$=220 km s$^{-1}$ or the newly fitted rotation curves with $R_0$=8.0 kpc and $Theta_0$=220 km s$^{-1}$ or $R_0$=8.4 kpc and $Theta_0$=254 km s$^{-1}$. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with $R_0$=8.0 kpc and $Theta_0$=220 km s$^{-1}$ and $R_0$=8.4 kpc and $Theta_0$=254 km s$^{-1}$ for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard $R_0$ and $Theta_0$.
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