94 research outputs found
Recombination Ghosts in Littrow Configuration: Implications for Spectrographs Using Volume Phase Holographic Gratings
We report the discovery of optical ghosts generated when using Volume Phase
Holographic (VPH) gratings in spectrographs employing the Littrow
configuration. The ghost is caused by light reflected off the detector surface,
recollimated by the camera, recombined by, and reflected from, the grating and
reimaged by the camera onto the detector. This recombination can occur in two
different ways. We observe this ghost in two spectrographs being developed by
the University of Wisconsin - Madison: the Robert Stobie Spectrograph for the
Southern African Large Telescope and the Bench Spectrograph for the WIYN 3.5m
telescope. The typical ratio of the brightness of the ghost relative to the
integrated flux of the spectrum is of order 10^-4, implying a recombination
efficiency of the VPH gratings of order 10^-3 or higher, consistent with the
output of rigorous coupled wave analysis. Any spectrograph employing VPH
gratings, including grisms, in Littrow configuration will suffer from this
ghost, though the general effect is not intrinsic to VPH gratings themselves
and has been observed in systems with conventional gratings in non-Littrow
configurations. We explain the geometric configurations that can result in the
ghost as well as a more general prescription for predicting its position and
brightness on the detector. We make recommendations for mitigating the ghost
effects for spectrographs and gratings currently built. We further suggest
design modifications for future VPH gratings to eliminate the problem entirely,
including tilted fringes and/or prismatic substrates. We discuss the resultant
implications on the spectrograph performance metrics.Comment: 13 pages, 8 figures, emulateapj style, accepted for publication in
PAS
The DiskMass Survey. X. Radio synthesis imaging of spiral galaxies
We present results from 21 cm radio synthesis imaging of 28 spiral galaxies
from the DiskMass Survey obtained with the VLA, WSRT, and GMRT facilities. We
detail the observations and data reduction procedures and present a brief
analysis of the radio data. We construct 21 cm continuum images, global HI
emission-line profiles, column-density maps, velocity fields, and
position-velocity diagrams. From these we determine star formation rates
(SFRs), HI line widths, total HI masses, rotation curves, and
azimuthally-averaged radial HI column-density profiles. All galaxies have an HI
disk that extends beyond the readily observable stellar disk, with an average
ratio and scatter of R_{HI}/R_{25}=1.35+/-0.22, and a majority of the galaxies
appear to have a warped HI disk. A tight correlation exists between total HI
mass and HI diameter, with the largest disks having a slightly lower average
column density. Galaxies with relatively large HI disks tend to exhibit an
enhanced stellar velocity dispersion at larger radii, suggesting the influence
of the gas disk on the stellar dynamics in the outer regions of disk galaxies.
We find a striking similarity among the radial HI surface density profiles,
where the average, normalized radial profile of the late-type spirals is
described surprisingly well with a Gaussian profile. These results can be used
to estimate HI surface density profiles in galaxies that only have a total HI
flux measurement. We compare our 21 cm radio continuum luminosities with 60
micron luminosities from IRAS observations for a subsample of 15 galaxies and
find that these follow a tight radio-infrared relation, with a hint of a
deviation from this relation at low luminosities. We also find a strong
correlation between the average SFR surface density and the K-band surface
brightness of the stellar disk.Comment: 22 pages + Appendix, 16 figures + Atlas, 5 tables. Accepted for
publication in Astronomy & Astrophysic
The DiskMass Survey. VIII. On the Relationship Between Disk Stability and Star Formation
We study the relationship between the stability level of late-type galaxy
disks and their star-formation activity using integral-field gaseous and
stellar kinematic data. Specifically, we compare the two-component (gas+stars)
stability parameter from Romeo & Wiegert (Q_RW), incorporating stellar
kinematic data for the first time, and the star-formation rate estimated from
21cm continuum emission. We determine the stability level of each disk
probabilistically using a Bayesian analysis of our data and a simple dynamical
model. Our method incorporates the shape of the stellar velocity ellipsoid
(SVE) and yields robust SVE measurements for over 90% of our sample. Averaging
over this subsample, we find a meridional shape of sigma_z/sigma_R =
0.51^{+0.36}_{-0.25} for the SVE and, at 1.5 disk scale lengths, a stability
parameter of Q_RW = 2.0 +/- 0.9. We also find that the disk-averaged
star-formation-rate surface density (Sigma-dot_e,*) is correlated with the
disk-averaged gas and stellar mass surface densities (Sigma_e,g and Sigma_e,*)
and anti-correlated with Q_RW. We show that an anti-correlation between
Sigma-dot_e,* and Q_RW can be predicted using empirical scaling relations, such
that this outcome is consistent with well-established statistical properties of
star-forming galaxies. Interestingly, Sigma-dot_e,* is not correlated with the
gas-only or star-only Toomre parameters, demonstrating the merit of calculating
a multi-component stability parameter when comparing to star-formation
activity. Finally, our results are consistent with the Ostriker et al. model of
self-regulated star-formation, which predicts
Sigma-dot_e,*/Sigma_e,g/sqrt(Sigma_e,*). Based on this and other theoretical
expectations, we discuss the possibility of a physical link between disk
stability level and star-formation rate in light of our empirical results.Comment: Accepted for publication in ApJ. 15 pages, 6 figures, 2 tables. An
electronic version of Table 1 is available by request, or at
http://www.astro.rug.nl/~westfall/research/dmVIII_table1.tx
SparsePak: A Formatted Fiber Field-Unit for The WIYN Telescope Bench Spectrograph. II. On-Sky Performance
We present a performance analysis of SparsePak and the WIYN Bench
Spectrograph for precision studies of stellar and ionized gas kinematics of
external galaxies. We focus on spectrograph configurations with echelle and
low-order gratings yielding spectral resolutions of ~10000 between 500-900nm.
These configurations are of general relevance to the spectrograph performance.
Benchmarks include spectral resolution, sampling, vignetting, scattered light,
and an estimate of the system absolute throughput. Comparisons are made to
other, existing, fiber feeds on the WIYN Bench Spectrograph. Vignetting and
relative throughput are found to agree with a geometric model of the optical
system. An aperture-correction protocol for spectrophotometric standard-star
calibrations has been established using independent WIYN imaging data and the
unique capabilities of the SparsePak fiber array. The WIYN
point-spread-function is well-fit by a Moffat profile with a constant power-law
outer slope of index -4.4. We use SparsePak commissioning data to debunk a
long-standing myth concerning sky-subtraction with fibers: By properly treating
the multi-fiber data as a ``long-slit'' it is possible to achieve precision sky
subtraction with a signal-to-noise performance as good or better than
conventional long-slit spectroscopy. No beam-switching is required, and hence
the method is efficient. Finally, we give several examples of science
measurements which SparsePak now makes routine. These include H
velocity fields of low surface-brightness disks, gas and stellar
velocity-fields of nearly face-on disks, and stellar absorption-line profiles
of galaxy disks at spectral resolutions of ~24,000.Comment: To appear in ApJSupp (Feb 2005); 19 pages text; 7 tables; 27 figures
(embedded); high-resolution version at
http://www.astro.wisc.edu/~mab/publications/spkII_pre.pd
SDSS-IV MaNGA: Radial Gradients in Stellar Population Properties of Early-Type and Late-Type Galaxies
We derive ages, metallicities, and individual element abundances of early-
and late-type galaxies (ETGs and LTGs) out to 1.5 R. We study a large
sample of 1900 galaxies spanning in stellar mass,
through key absorption features in stacked spectra from the SDSS-IV/MaNGA
survey. We use mock galaxy spectra with extended star formation histories to
validate our method for LTGs and use corrections to convert the derived ages
into luminosity- and mass-weighted quantities. We find flat age and negative
metallicity gradients for ETGs and negative age and negative metallicity
gradients for LTGs. Age gradients in LTGs steepen with increasing galaxy mass,
from Gyr/R for the lowest mass galaxies to
Gyr/R for the highest mass ones. This strong
gradient-mass relation has a slope of . Comparing local age and
metallicity gradients with the velocity dispersion within galaxies
against the global relation with shows that internal processes
regulate metallicity in ETGs but not age, and vice versa for LTGs. We further
find that metallicity gradients with respect to local show a much
stronger dependence on galaxy mass than radial metallicity gradients. Both
galaxy types display flat [C/Fe] and [Mg/Fe], and negative [Na/Fe] gradients,
whereas only LTGs display gradients in [Ca/Fe] and [Ti/Fe]. ETGs have
increasingly steep [Na/Fe] gradients with local reaching
dex/ km/s for the highest masses. [Na/Fe] ratios are correlated with
metallicity for both galaxy types across the entire mass range in our sample,
providing support for metallicity dependent supernova yields.Comment: 21 pages, 21 figures, 4 tables + Appendi
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