Browsing by Subject "galaxies :"

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Bars In Disk-Dominated And Bulge-Dominated Galaxies At Z Similar To 0: New Insights From Similar To 3600 SDSS Galaxies
(2008-03) Barazza, Fabio D.; Jogee, Shardha; Marinova, Irina; Barazza, Fabio D.; Jogee, Shardha; Marinova, Irina
We present a study of large-scale bars in the local universe, based on a large sample of 3692 galaxies, with 18.5 <= M(g) < -22.0 mag and redshift 0.01 <= z < 0.03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and Se e rsic cuts yield a similar sample of similar to 2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined (60 degrees) systems, we find the following results. (1) The optical r-band fraction (f(opt-r)) of barred galaxies, when averaged over the whole sample, is similar to 48%-52%. (2) When galaxies are separated according to half light radius (r(e)), or normalized r(e)/R(24), which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: f(opt-r) rises sharply, from similar to 40% in galaxies that have small r(e)/R(24) and visually appear to host prominent bulges, to similar to 70% for galaxies that have large r(e)/R(24) and appear disk-dominated. (3) For galaxies with bluer colors, f(opt-r) rises significantly (by similar to 30%). A weaker rise (by similar to 15%-20%) is seen for lower luminosities or lower masses. (4) While hierarchical Lambda CDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that similar to 20% of disk galaxies appear to be "quasi-bulgeless.'' (5) We outline how the effect of a decreasing resolution and a rising obscuration of bars by gas and dust over z = 0.2-1.0 can cause a significant artificial loss of bars, and an artificial reduction in the optical bar fraction over z = 0.2-1.0.
An Explanation For The Observed Weak Size Evolution Of Disk Galaxies
(2008-01) Somerville, Rachel S.; Barden, Marco; Rix, Hans-Walter; Bell, Eric F.; Beckwith, Steven V. W.; Borch, Andrea; Caldwell, John A. R.; Haussler, Boris; Heymans, Catherine; Jahnke, Knud; Jogee, Shardha; McIntosh, Daniel H.; Meisenheimer, Klaus; Peng, Chen Y.; Sanchez, Sebastian F.; Wisotzki, Lutz; Wolf, Christian; Caldwell, John A. R.
Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z similar to 1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS) and high-redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the Lambda CDM model evolves by about a factor of 2 over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z similar to 1 is about 15%-20%, in good agreement with the observational constraints from GEMS. At redshift z similar to 2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero point of the Tully-Fisher relation) is only about 10% larger at z similar to 1 (20% at z similar to 2) than at the present day.
GEMS : Galaxy Fitting Catalogs and Testing Parametric Galaxy Fitting Codes : GALFIT and GIM2D
(2007-10) Haeussler, Boris; McIntosh, Daniel H.; Barden, Marco; Bell, Eric F.; Rix, Hans-Walter; Borch, Andrea; Beckwith, Steven V. W.; Caldwell, John A. R.; Heymans, Catherine; Jahnke, Knud; Jogee, Shardha; Koposov, Sergey E.; Meisenheimer, Klaus; Sanchez, Sebastian F.; Somerville, Rachel S.; Wisotzki, Lutz; Wolf, Christian; Caldwell, John A. R.
In the context of measuring the structures of intermediate-redshift galaxies with HST ACS surveys, we tune, test, and compare two widely used fitting codes (GALFIT and GIM2D) for fitting single-component Sersic models to both simulated and real galaxy data. Our study focuses on the GEMS survey with the sensitivity of typical HST survey data, and we include our final catalog of fit results for all 41,495 objects detected in GEMS. Using simulations, we find that fitting accuracy depends sensitively on galaxy profile shape. Exponential disks are well fit and have small measurement errors, whereas fits to de Vaucouleurs profiles show larger uncertainties owing to the large amount of light at large radii. Both codes provide reliable fits with little systematic error for galaxies with effective surface brightnesses brighter than that of the sky; the formal uncertainties returned by these codes significantly underestimate the true uncertainties (as estimated using the simulations). We find that GIM2D suffers significant systematic errors for spheroids with close companions owing to the difficulty of effectively masking out neighboring galaxy light; there appears to be no work-around to this important systematic in GIM2D's current implementation. While this crowding error affects only a small fraction of galaxies in GEMS, it must be accounted for in the analysis of deeper cosmological images or of more crowded fields with GIM2D. In contrast, GALFIT results are robust to the presence of neighbors because it can simultaneously fit the profiles of multiple companions as well as the galaxy of interest. We find GALFIT's robustness to nearby companions and factor of greater than or similar to 20 faster runtime speed are important advantages over GIM2D for analyzing large HST ACS data sets.
The Hubble Space Telescope Advanced Camera for Surveys Coma Cluster Survey. I. Survey Objectives and Design
(2008-06) Carter, David; Goudfrooij, Paul; Mobasher, Bahram; Ferguson, Henry C.; Puzia, Thomas H.; Aguerri, Alfonso L.; Balcells, Marc; Batcheldor, Dan; Bridges, Terry J.; Davies, Jonathan I.; Erwin, Peter; Graham, Alister W.; Guzman, Rafael; Hammer, Derek; Hornschemeier, Ann; Hoyos, Carlos; Hudson, Michael J.; Huxor, Avon; Jogee, Shardha; Komiyama, Yutaka; Lotz, Jennifer; Lucey, John R.; Marzke, Ronald O.; Merritt, David; Miller, Bryan W.; Miller, Neal A.; Mouhcine, Mustapha; Okamura, Sadanori; Peletier, Reynier F.; Phillipps, Steven; Poggianti, Bianca M.; Sharples, Ray M.; Smith, Russell J.; Trentham, Neil; Tully, R. Brent; Valentijn, Edwin; Kleijn, Gijs Verdoes; Jogee, Shardha
We describe the HST ACS Coma Cluster Treasury survey, a deep two-passband imaging survey of one of the nearest rich clusters of galaxies, the Coma Cluster (Abell 1656). The survey was designed to cover an area of 740 arcmin(2) in regions of different density of both galaxies and intergalactic medium within the cluster. The ACS failure of 2007 January 27 leaves the survey 28% complete, with 21 ACS pointings (230 arcmin(2)) complete, and partial data for a further four pointings (44 arcmin(2)). The predicted survey depth for 10 sigma detections for optimal photometry of point sources is g' = 27.6 in the F475W filter and I-C = 26.8 mag in F814 (AB magnitudes). Initial simulations with artificially injected point sources show 90% recovered at magnitude limits of g' = 27.55 and I-C = 26.65. For extended sources, the predicted 10 sigma limits for a 1 arcsec(2) region are g' = 25.8 mag arcsec(-2) and I-C = 25.0 mag arcsec(-2). We highlight several motivating science goals of the survey, including study of the faint end of the cluster galaxy luminosity function, structural parameters of dwarf galaxies, stellar populations and their effect on colors and color gradients, evolution of morphological components in a dense environment, the nature of ultracompact dwarf galaxies, and globular cluster populations of cluster galaxies of a range of luminosities and types. This survey will also provide a local rich cluster benchmark for various well-known global scaling relations and explore new relations pertaining to the nuclear properties of galaxies.
The Kinematics Of Thick Disks In Nine External Galaxies
(2008-08) Yoachim, Peter; Dalcanton, Julianne J.; Yoachim, Peter
We present kinematic measurements of thin- and thick-disk components in a sample of nine edge-on galaxies. We extract stellar and ionized gas rotation curves at and above the galaxies' midplanes using the Ca II triplet absorption features and H alpha emission lines measured with the GMOS spectrographs on Gemini-North and Gemini-South. For the higher mass galaxies in the sample, we fail to detect differences between the thin- and thick-disk kinematics. In the lower mass galaxies, there is a wide range of thick-disk behavior, including thick disks with substantial lag and one counterrotating thick disk. We compare our rotation curves with expectations from thick-disk formation models and conclude that the wide variety of thick-disk kinematics favors a formation scenario in which thick-disk stars are accreted or formed during merger events as opposed to models that form thick disks through gradual thin- disk heating.
The Multiwavelength Survey By Yale-Chile (MUSYC) Wide K-Band Imaging, Photometric Catalogs, Clustering, And Physical Properties Of Galaxies At Z Similar To 2
(2008-07) Blanc, Guillermo A.; Lira, Paulina; Barrientos, L. Felipe; Aguirre, Paula; Francke, Harod; Taylor, Edward N.; Quadri, Ryan; Marchesini, Danilo; Infante, Leopoldo; Gawiser, Eric; Hall, Patrick B.; Willis, Jon P.; Herrera, David; Maza, Jose; Blanc, Guillermo A.
We present K-band imaging of two similar to 30' x 30' fields covered by the Multiwavelength Survey by Yale-Chile (MUSYC) Wide NIR Survey. The SDSS 1030+05 and Cast 1255 fields were imaged with the Infrared Side Port Imager (ISPI) on the 4 m Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO) to a 5 sigma point-source limiting depth of K similar to 20 (Vega). Combining these data with the MUSYC optical UBVRIz imaging, we created multiband K-selected source catalogs for both fields. These catalogs, together with the MUSYC K-band catalog of the Extended Chandra Deep Field South (ECDF-S) field, were used to select K 20 BzK galaxies over an area of 0.71 deg(2). This is the largest area ever surveyed for BzK galaxies. We present number counts, redshift distributions, and stellar masses for our sample of 3261 BzK galaxies (2502 star-forming [sBzK] and 759 passively evolving [pBzK]), as well as reddening and star formation rate estimates for the star-forming BzK systems. We also present two-point angular correlation functions and spatial correlation lengths for both sBzK and pBzK galaxies and show that previous estimates of the correlation function of these galaxies were affected by cosmic variance due to the small areas surveyed. We have measured correlation lengths r(0) of 8.89 +/- 2.03 and 10.82 +/- 1.72 Mpc for sBzK and pBzK galaxies, respectively. This is the first reported measurement of the spatial correlation function of passive BzK galaxies. In the Lambda CDM scenario of galaxy formation, these correlation lengths at z similar to 2 translate into minimum masses of similar to 4 x 10(12) and similar to 9 x 10(12) M(circle dot) for the dark matter halos hosting sBzK and pBzK galaxies, respectively. The clustering properties of the galaxies in our sample are consistent with their being the descendants of bright Lyman break galaxies at z similar to 3, and the progenitors of present-day > 1L* galaxies.
Tests Of The Radial Tremaine-Weinberg Method
(2008-04) Meidt, S. E.; Rand, Richard J.; Merrifield, M. R.; Debattista, Victor P.; Shen, Juntai T.; Shen, Juntai T.
At the intersection of galactic dynamics, evolution, and global structure, issues such as the relation between bars and spirals and the persistence of spiral patterns can be addressed through the characterization of the angular speeds of the patterns and their possible radial variation. The radial Tremaine-Weinberg (TWR) method, a generalized version of the Tremaine-Weinberg method for observationally determining a single, constant pattern speed, allows the pattern speed to vary arbitrarily with radius. Here we perform tests of the TWR method with regularization on several simulated galaxy data sets. The regularization is employed as a means of smoothing intrinsically noisy solutions, as well as for testing model solutions of different radial dependence (e. g., constant, linear, or quadratic). We test these facilities in studies of individual simulations and demonstrate successful measurement of both bar and spiral pattern speeds in a single disk, secondary bar pattern speeds, and spiral winding (in the first application of a TW calculation to a spiral simulation). We also explore the major sources of error in the calculation and find uncertainty in the major-axis position angle most dominant. In all cases, the method is able to extract pattern speed solutions where discernible patterns exist to within 20% of the known values, suggesting that the TWR method should be a valuable tool in the area of galactic dynamics. For utility, we also discuss the caveats in, and compile a prescription for, applications to real galaxies.
Trimming Down The Willman 1 dSph
(2008-06) Siegel, Michael H.; Shetrone, Matthew D.; Irwin, Michael; Siegel, Michael H.; Shetrone, Matthew D.
Willman 1 is a small low-surface-brightness object identified in the Sloan Digital Sky Survey and tentatively classified as a very low luminosity dSph galaxy. Further study has supported this classification while hinting that it may be undergoing disruption by the Milky Way potential. In an effort to better constrain the nature of Willman 1, we present a comprehensive analysis of the brightest stars in a 0.6 deg(2) field centered on the overdensity. High-resolution Hobby-Eberly Terlescope (HET) spectra of two previously identified Willman 1 red giant branch (RGB) stars show that one is a metal-rich foreground dwarf while the other is a metal-poor giant. The one RGB star that we confirm as a member of Willman 1 has a low metallicity ([Fe/H] = -2.2) and a surprisingly low alpha-element abundance ([alpha/Fe]= -0.11). Washington+DDO51 photometry indicates that 2-5 of the seven brightest Willman 1 stars identified in previous studies are actually dwarf stars, including some of the more metal-rich stars that have been used to argue both for an abundance spread and a more metal-rich stellar population than galaxies of similar luminosity. The remaining stars are too blue or too faint for photometric classification. The Washington+DDO51 photometry identifies three potential RGB stars in the field but HET spectra show that they are background halo stars. Time series photometry identifies one apparent variable star in the field, but it is unlikely to be associated with Willman 1. Our wide-field survey indicates that over 0.6 deg(2), Willman 1 does not have a single RR Lyrae star, a single blue horizontal branch (BHB) star, or a single RGB star beyond its tidal radius. While our results confirm that Willman 1 is most likely a low-luminosity metal-poor dSph galaxy, the possibility remains that it is a tidally disrupted metal-poor globular cluster.