Engineering topics

Rix, Hans-Walter

Publications and source records attributed to Rix, Hans-Walter.

28 records · Page 2

NASA NTRS2013

The Radial Distribution of Star Formation in Galaxies at z~1 From The 3D-HST Survey

The assembly of galaxies can be described by the distribution of their star formation as a function of cosmic time.Thanks to the WFC3 grism on the Hubble Space Telescope (HST) it is now possible to measure this beyond thelocal Universe. Here we present the spatial distribution of H emission for a sample of 54 strongly star-forming-galaxies at z~1 in the 3D-HST Treasury survey. By stacking the Halpha emission, we find that star formation occurredin approximately exponential distributions at z~1, with a median Sersic index of n=1.0 plus or minus 0.2. The stacks areelongated with median axis ratios of b/a 0.58 plus or minus 0.09 in Halpha consistent with (possibly thick) disks at randomorientation angles. Keck spectra obtained for a subset of eight of the galaxies show clear evidence for rotation, withinclination corrected velocities of 90-330 km per second. The most straightforward interpretation of our results is that starformation in strongly star-forming galaxies at z~1 generally occurred in disks. The disks appear to be scaled-upversions of nearby spiral galaxies: they have EW(Halpha)~100 Angstroms out to the solar orbit and they have star formation surface densities above the threshold for driving galactic scale winds.

Radial↗

NASA NTRS2011

What Turns Galaxies Off? the Different Morphologies of Star-Forming and Quiescent Galaxies Since z Approximates 2 from CANDELS

We use HST/WFC3 imaging from the CANDELS multicyc1e treasury survey, in conjunction with the Sloan Digital Sky Survey, to explore the evolution of galactic structure for galaxies with stellar masses > 3 x 10(exp 10) Solar Mass from Z= 2.2 to the present epoch, a time span of 10 Gyr. We explore the relationship between rest-frame optical color, stellar mass, star formation activity and the structural parameters of galaxies as determined from parametric fits to the surface brightness profiles of galaxies. We confirm the dramatic evolution from z= 2.2 to the present day in the number density of non-star-forming galaxies above 3 x 10(exp 10) Solar Mass reported by other authors. We find that the vast majority of these quiescent systems have concentrated light profiles, as parameterized by the Sersic index, and the population of concentrated galaxies grows similarly rapidly. We examine the joint distribution of star formation activity, Sersic index, stellar mass, mass divided by radius (a proxy for velocity dispersion), and stellar surface density. Quiescence correlates poorly with stellar mass at all z < 2.2 (given the approx < 0.2 dex scatter between halo mass and stellar mass at z approximates 0 inferred by More et al, this argues against halo mass being the only factor determining quiescence). Quiescence correlates better with Sersic index, 'velocity dispersion' and stellar surface density, where Sersic index correlates the best (increasingly so at lower redshift). Yet, there is significant scatter between quiescence and galaxy structure: while the vast majority of quiescent galaxies have prominent bulges, many of them have significant disks, and a number of bulge-dominated galaxies have significant star formation. Noting the rarity of quiescent galaxies without prominent bulges, we argue that a prominent bulge (and, perhaps by association, a supermassive black hole) is a necessary but not sufficient condition for quenching star formation on galactic scales over the last 10 Gyr; such a result is qualitatively consistent with the expectations of the AGN feedback paradigm.

Bell, Eric F.↗

NASA NTRS2001

A Young Stellar Cluster in the Nucleus of NGC 4449

We have obtained 1-2 A resolution optical Echellette spectra of the nuclear star cluster in the nearby starburst galaxy NGC4449. The light is clearly dominated by a very young (6 - 10 Myr) population of stars. For our age dating, we have used recent population synthesis models to interpret the observed equivalent width of stellar absorption features such as the H I Balmer series and the Ca II triplet around 8500 A. We also compare the observed spectrum of the nuclear cluster to synthesized spectra for stellar populations of varying ages. All these approaches yield a consistent cluster age. Metallicity estimates based on the relative intensities of various ionization lines yield no evidence for significant enrichment in the center of this low mass galaxy: the metallicity of the nuclear cluster is about one fourth of the solar value, in agreement with independent estimates for the disk material of NGC4449.

Boeker, Torsten↗

NASA NTRS1995

Detection of compact ultraviolet nuclear emission in liner galaxies

Low-ionization nuclear emission-line regions (LINERs), which exist in a large fraction of galaxies, may be the least luminous manifestation of quasar activity. As such, they may make possible the study of the active galactic nucleus (AGN) phenomenon in the nearest galaxies. The nature of LINERs has, however, remained controversial because an AGN-like nonstellar continuum source has not been directly observed in them. We report the detection of bright (greater than or approximately = 2 x 10(exp -16) ergs/s/sq cm/A), unresolved (full width at half maximum (FWHM) less than or approximately = 0.1 sec) point sources of UV (approximately 2300 A) emission in the nuclei of nine nearby galaxies. The galaxies were imaged using the Faint Object Camera (FOC) on the Hubble Space Telescope (HST), and seven of them are from a complete sample of 110 nearby galaxies that was observed with HST. Ground-based optical spectroscopy reveals that five of the nuclei are LINERs, three are starburst nuclei, and one is a Seyfert nucleus. The observed UV flux in each of the five LINERs implies an ionizing flux that is sufficient to account to the observed emission lines through photoionization. The detection of a strong UV continuum in the LINERs argues against shock excitation as the source of the observed emission lines, and supports the idea that photoionization excites the lines in at least some objects of this class. We have analyzed ground-based spectra for most of the northern-hemisphere galaxies in the HST sample and find that 26 of them are LINERs, among which only the above five LINERs have a detected nuclear UV source.

Maoz, Dan↗

NASA NTRS1994

Galaxy mergers and gravitational lens statistics

We investigate the impact of hierarchical galaxy merging on the statistics of gravitational lensing of distant sources. Since no definite theoretical predictions for the merging history of luminous galaxies exist, we adopt a parameterized prescription, which allows us to adjust the expected number of pieces comprising a typical present galaxy at z approximately 0.65. The existence of global parameter relations for elliptical galaxies and constraints on the evolution of the phase space density in dissipationless mergers, allow us to limit the possible evolution of galaxy lens properties under merging. We draw two lessons from implementing this lens evolution into statistical lens calculations: (1) The total optical depth to multiple imaging (e.g., of quasars) is quite insensitive to merging. (2) Merging leads to a smaller mean separation of observed multiple images. Because merging does not reduce drastically the expected lensing frequency, it cannot make lambda-dominated cosmologies compatible with the existing lensing observations. A comparison with the data from the Hubble Space Telescope (HST) Snapshot Survey shows that models with little or no evolution of the lens population are statistically favored over strong merging scenarios. A specific merging scenario proposed to Toomre can be rejected (95% level) by such a comparison. Some versions of the scenario proposed by Broadhurst, Ellis, & Glazebrook are statistically acceptable.

Rix, Hans-Walter↗

NASA NTRS1993

Mapping the stellar backbones of spiral galaxies

We present optical and IR surface photometry of M51 (NGC 5194) at B, V, R, I, J, K, and CO. These data are used to establish whether K-band images of spiral galaxies provide reliable maps of stellar surface mass density features such as massive spiral arms or bars. The main distorting agents in the mapping at shorter wavelengths are dust extinction and luminous young stars. From modeling the color changes across the main dust lanes in M51, we find the optical depths to be about 0.5 in the K band. For these optical depths the K-band flux is attenuated by only less than 10 percent even in the dust lanes. From monitoring the gravity-sensitive CO index across the spiral arms we find that young red supergiants do not significantly distort the K-band image. On this basis, we conclude that K-band images do trace the massive disk star population and allow to map azimuthal variation in the surface mass density of the stellar disk. In M51 we find the surface mass density contrast to range from 1.8 to 3, as expected from N-body simulations of the galaxy's tidal encounter with NGC 5195. The spiral arm amplitude in M51 clearly shows smooth, strong radial variations, with a maximum at about 130 arcsec and minima at 45 and 170 arcsec.

Rix, Hans-Walter↗

NASA NTRS1992

NGC 4550 - A laboratory for testing galaxy formation

The kinematic and photometric structure of the two components of the E7/S0 galaxy NGC 4550 are evaluated quantitatively to determine the system's morphology. Attention is given to whether the data indicate a rotating bulge with a counterrotating disk or a pair of counterstreaming disks, and formation scenarios are discussed in the light of the findings. The data examined include long-slit spectroscopy with evidence of bimodality in the line-of-sight velocity distribution. Two photometrically inseparable counterstreaming disk components are identified and their velocities given. The structure and the formation scenarios given point to the feasibility of absorption of external material by galaxy disks without extreme heating. The possibility of detecting stellar counterrotation in S0 disks is discussed based on the study of NGC 4550.

Rix, Hans-Walter↗

NASA NTRS1992

Hubble Space Telescope Wide Field Camera imaging of the gravitational lens 2237 + 0305

Images of the gravitational lens system 2237 + 0305, taken with the HST Wide Field Camera, are analyzed. Positions for the four quasar images, accurate to +/-0.015 arcsec, and relative magnitudes in U and R, accurate to +/-0.06 and 0.04 mag, respectively, are determined. The upper limits on the observed brightness of the fifth image are found to be less than or approximately equal to 7 percent of the brightest quasar image. The mass of the lens inside 0.9 arcsec is found to be 1.08 +/-0.02 x 10 exp 10 solar masses/h100 corresponding to a mass-to-light ratio in B of 12.3h100. This solar mass/solar luminosity estimate agrees with values obtained from stellar dynamics for other elliptical galaxies. A comparison of predictions from this mass model with the measured central velocity dispersion yields a distance-independent agreement to within 10 percent, assuming isotropic velocity dispersions.

Rix, Hans-Walter↗

NASA NTRS1991

An infrared/optical investigation of 100 micron 'cirrus'

IRAS Skyflux images, in conjunction with ESO/SRC photographic plates, are presently used to study a sample of 151 compact cyrrus sources. IR colors entail at least two dust components; a simple two-temperature dust model yields a mean physical temperature of 22.5 + or - 0.3 K for the warm dust, and 173 + or - 8 K for the hot dust, which constitutes 15-42 percent of total IR luminosity. A variable proportion of cold dust may contribute significantly to the optical brightness in two isolated cloud complexes.

Paley, Elizabeth S.↗

NASA NTRS1988

Quasar microlensing and dark matter

The amplification of quasar brightness due to gravitational lensing by foreground objects is discussed. It is shown that a recently published sample of X-ray-selected quasars behind foreground galaxies shows a statistically significant brightening compared to a control sample. Correlations with galaxy redshift and impact parameter predicted by microlensing are also demonstrated. A technique is described to measure the mean density of the lenses from a small number of identified cases of microlensing. It is shown that, in this sample, amplification bias is important in determining the mean intensity enhancement and must be included in the density estimate. Assuming that at least two of the four intrinsically brightest quasars behind galaxies are indeed microlensed, the present data yield a formal lower limit on the mean density parameter of lenses Omega(l) greater than 0.25 at 95 percent confidence. These data also imply that a considerable quantity of dark matter exists in macroscopic objects outside the visible parts of galaxies but is still highly correlated with them.

Rix, Hans-Walter↗