434 research outputs found
Bulgeless Giant Galaxies Challenge our Picture of Galaxy Formation by Hierarchical Clustering
We dissect giant Sc-Scd galaxies with Hubble Space Telescope photometry and
Hobby-Eberly Telescope spectroscopy. We use HET's High Resolution Spectrograph
(resolution = 15,000) to measure stellar velocity dispersions in the nuclear
star clusters and pseudobulges of the pure-disk galaxies M33, M101, NGC 3338,
NGC 3810, NGC 6503, and NGC 6946. We conclude: (1) Upper limits on the masses
of any supermassive black holes are MBH <= (2.6+-0.5) * 10**6 M_Sun in M101 and
MBH <= (2.0+-0.6) * 10**6 M_Sun in NGC 6503. (2) HST photometry shows that the
above galaxies contain tiny pseudobulges that make up <~ 3 % of the stellar
mass but no classical bulges. We inventory a sphere of radius 8 Mpc centered on
our Galaxy to see whether giant, pure-disk galaxies are common or rare. In this
volume, 11 of 19 galaxies with rotation velocity > 150 km/s show no evidence
for a classical bulge. Four may contain small classical bulges that contribute
5-12% of the galaxy light. Only 4 of the 19 giant galaxies are ellipticals or
have classical bulges that contribute 1/3 of the galaxy light. So pure-disk
galaxies are far from rare. It is hard to understand how they could form as the
quiescent tail of a distribution of merger histories. Recognition of
pseudobulges makes the biggest problem with cold dark matter galaxy formation
more acute: How can hierarchical clustering make so many giant, pure-disk
galaxies with no evidence for merger-built bulges? This problem depends
strongly on environment: the Virgo cluster is not a puzzle, because >2/3 of its
stellar mass is in merger remnants.Comment: 28 pages, 16 Postscript figures, 2 tables; requires emulateapj.sty
and apjfonts.sty; accepted for publication in ApJ; for a version with full
resolution figures, see http://chandra.as.utexas.edu/~kormendy/kdbc.pd
STRUCTURE AND FORMATION OF cD GALAXIES: NGC 6166 IN ABELL 2199
Hobby–Eberly Telescope (HET) spectroscopy is used to measure the velocity dispersion profile of the nearest prototypical cD galaxy, NGC 6166 in the cluster Abell 2199. We also present composite surface photometry from many telescopes. We confirm the defining feature of a cD galaxy; i.e., (we suggest), a halo of stars that fills the cluster center and that is controlled dynamically by cluster gravity, not by the central galaxy. Our HET spectroscopy shows that the velocity dispersion of NGC 6166 rises from σ ≃ 300 km s[superscript −1] in the inner r ~ 10" to σ = 865 ± 58 km s[superscript −1] at r ~ 100'' in the cD halo. This extends published observations of an outward σ increase and shows for the first time that σ rises all the way to the cluster velocity dispersion of 819 ± 32 km s[superscript −1]. We also observe that the main body of NGC 6166 moves at +206 ± 39 km s[superscript −1] with respect to the cluster mean velocity, but the velocity of the inner cD halo is ~70 km s[superscript −1] closer to the cluster velocity. These results support our picture that cD halos consist of stars that were stripped from individual cluster galaxies by fast tidal encounters.
However, our photometry does not confirm the widespread view that cD halos are identifiable as an extra, low-surface-brightness component that is photometrically distinct from the inner, steep-Sérsic-function main body of an otherwise-normal giant elliptical galaxy. Instead, all of the brightness profile of NGC 6166 outside its core is described to ±0.037 V mag arcsec[superscript −2] by a single Sérsic function with index n ≃ 8.3. The cD halo is not recognizable from photometry alone. This blurs the distinction between cluster-dominated cD halos and the similarly-large-Sérsic-index halos of giant, core-boxy-nonrotating ellipticals. These halos are believed to be accreted onto compact, high-redshift progenitors ("red nuggets") by large numbers of minor mergers. They belong dynamically to their central galaxies. Still, cDs and core-boxy-nonrotating Es may be more similar than we think: both may have outer halos made largely via minor mergers and the accumulation of tidal debris.
We construct a main-body+cD-halo decomposition that fits both the brightness and dispersion profiles. To fit σ(r), we need to force the component Sérsic indices to be smaller than a minimum-x[superscript 2] photometric decomposition would suggest. The main body has M[subscript V] ≃ -22.8 ≃ 30% of the total galaxy light. The cD halo has M[subscript V] ≃ -23.7, ~1/2 mag brighter than the brightest galaxy in the Virgo cluster. A mass model based on published cluster dynamics and X-ray observations fits our observations if the tangential dispersion is larger than the radial dispersion at r ≃ 20"–60". The cD halo is as enhanced in α element abundances as the main body of NGC 6166. Quenching of star formation in [< over ~]1 Gyr suggests that the center of Abell 2199 has been special for a long time during which dynamical evolution has liberated a large mass of now-intracluster stars
Design and construction progress of LRS2-B: a new low resolution integral field spectrograph for the Hobby-Eberly Telescope
The upcoming Wide-Field Upgrade (WFU) has ushered in a new era of
instrumentation for the Hobby-Eberly Telescope (HET). Here, we present the
design, construction progress, and lab tests completed to date of the
blue-optimized second generation Low Resolution Spectrograph (LRS2-B). LRS2-B
is a dual-channel, fiber fed instrument that is based on the design of the
Visible Integral Field Replicable Unit Spectrograph (VIRUS), which is the new
flagship instrument for carrying out the HET Dark Energy eXperiment (HETDEX).
LRS2-B utilizes a microlens-coupled integral field unit (IFU) that covers a
7"x12" area on the sky having unity fill-factor with ~300 spatial elements that
subsample the median HET image quality. The fiber feed assembly includes an
optimized dichroic beam splitter that allows LRS2-B to simultaneously observe
370 nm to 470 nm and 460 nm to 700 nm at fixed resolving powers of R \approx
1900 and 1200, respectively. We discuss the departures from the nominal VIRUS
design, which includes the IFU, fiber feed, camera correcting optics, and
volume phase holographic grisms. Additionally, the motivation for the selection
of the wavelength coverage and spectral resolution of the two channels is
briefly discussed. One such motivation is the follow-up study of spectrally and
(or) spatially resolved Lyman-alpha emission from z ~ 2.5 star-forming galaxies
in the HETDEX survey. LRS2-B is planned to be a commissioning instrument for
the HET WFU and should be on-sky during quarter 4 of 2013. Finally, we mention
the current state of LRS2-R, the red optimized sister instrument of LRS2-B.Comment: 22 pages, 12 figures, 2 tables. To be published in Proc. SPIE, 2012,
"Ground-Based and Airborne Instrumentation for Astronomy IV", 8446-103. In
v2, a note has been added indicating that this paper has been superseded by
arXiv:1407:601
The Influence of Motion and Stress on Optical Fibers
We report on extensive testing carried out on the optical fibers for the
VIRUS instrument. The primary result of this work explores how 10+ years of
simulated wear on a VIRUS fiber bundle affects both transmission and focal
ratio degradation (FRD) of the optical fibers. During the accelerated lifetime
tests we continuously monitored the fibers for signs of FRD. We find that
transient FRD events were common during the portions of the tests when motion
was at telescope slew rates, but dropped to negligible levels during rates of
motion typical for science observation. Tests of fiber transmission and FRD
conducted both before and after the lifetime tests reveal that while
transmission values do not change over the 10+ years of simulated wear, a clear
increase in FRD is seen in all 18 fibers tested. This increase in FRD is likely
due to microfractures that develop over time from repeated flexure of the fiber
bundle, and stands in contrast to the transient FRD events that stem from
localized stress and subsequent modal diffusion of light within the fibers.
There was no measurable wavelength dependence on the increase in FRD over 350
nm to 600 nm. We also report on bend radius tests conducted on individual
fibers and find the 266 microns VIRUS fibers to be immune to bending-induced
FRD at bend radii of R > 10cm. Below this bend radius FRD increases slightly
with decreasing radius. Lastly, we give details of a degradation seen in the
fiber bundle currently deployed on the Mitchell Spectrograph (formally VIRUS-P)
at McDonald Observatory. The degradation is shown to be caused by a localized
shear in a select number of optical fibers that leads to an explosive form of
FRD. In a few fibers, the overall transmission loss through the instrument can
exceed 80%.Comment: 19 pages, 22 figure
Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593
We present Ks-band surface photometry of NGC 7690 (Hubble type Sab) and NGC
4593 (SBb). We find that, in both galaxies, a major part of the "bulge" is as
flat as the disk and has approximately the same color as the inner disk. In
other words, the "bulges" of these galaxies have disk-like properties. We
conclude that these are examples of "pseudobulges" -- that is, products of
secular dynamical evolution. Nonaxisymmetries such as bars and oval disks
transport disk gas toward the center. There, star formation builds dense
stellar components that look like -- and often are mistaken for -- merger-built
bulges but that were constructed slowly out of disk material. These
pseudobulges can most easily be recognized when, as in the present galaxies,
they retain disk-like properties. NGC 7690 and NGC 4593 therefore contribute to
the growing evidence that secular processes help to shape galaxies.
NGC 4593 contains a nuclear ring of dust that is morphologically similar to
nuclear rings of star formation that are seen in many barred and oval galaxies.
The nuclear dust ring is connected to nearly radial dust lanes in the galaxy's
bar. Such dust lanes are a signature of gas inflow. We suggest that gas is
currently accumulating in the dust ring and hypothesize that the gas ring will
starburst in the future. The observations of NGC 4593 therefore suggest that
major starburst events that contribute to pseudobulge growth can be episodic.Comment: 10 pages, 3 Postscript figures; requires emulateapj.cls,
apjfonts.sty, and psfig.sty; accepted for publication in ApJ; for a version
with full resolution figures, see
http://chandra.as.utexas.edu/~kormendy/n7690.pd
Optical and UV Light Curves of the Accretion Disk Corona Source 4U 1822-371
The eclipsing low-mass X-ray binary 4U is the prototypical accretion disk
corona (ADC) system. We have obtained new time-resolved UV spectrograms of 4U
with the Hubble Space Telescope and new V- and J-band light curves with the
1.3-m SMARTS telescope at CTIO. We present an updated ephemeris for the times
of the optical/UV eclipses. Model light curves do not give acceptable fits to
the UV eclipses unless the models include an optically-thick ADC.Comment: 3 pages, 2 figures, from A Population Explosion: The Nature and
Evolution of X-ray Binaries in Diverse Environment
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