192 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 Tully-Fisher Relation of Barred Galaxies
We present new data exploring the scaling relations, such as the Tully-Fisher
relation (TFR), of bright barred and unbarred galaxies. A primary motivation
for this study is to establish whether barredness correlates with, and is a
consequence of, virial properties of galaxies. Various lines of evidence
suggest that dark matter is dominant in disks of bright unbarred galaxies at
2.2 disk scale lengths, the point of peak rotation for a pure exponential disk.
We test the hypothesis that the TF plane of barred high surface brightness
galaxies is offset from the mean TFR of unbarred galaxies, as might be expected
if barred galaxies are ``maximal'' in their inner parts. We use existing and
new TF data to search for basic structural differences between barred and
unbarred galaxies. Our new data consist of 2-dimensional Halpha velocity fields
derived from SparsePak integral field spectroscopy (IFS) and V,I-band CCD
images collected at the WIYN Observatory for 14 strongly barred galaxies. We
use WIYN/SparsePak (2-D) velocity fields to show that long-slit (1-D) spectra
yield reliable circular speed measurements at or beyond 2.2 disk scale lengths,
far from any influence of the bar. This enables us to consider line width
measurements from extensive TF surveys which include barred and nonbarred disks
and derive detailed scaling relation comparisons. We find that for a given
luminosity, barred and unbarred galaxies have comparable structural and
dynamical parameters, such as peak velocities, scale lengths, or colors. In
particular, the location of a galaxy in the TF plane is independent of
barredness. In a global dynamical sense, barred and unbarred galaxies behave
similarly and are likely to have, on average, comparable fractions of luminous
and dark matter at a given radius. (abridged)Comment: Accepted for publication in the ApJ (September 1, 2003 issue, v594).
Appendix figures with I-band image and superimposed 2-D velocity field plus
rotation curves must be downloaded separately (due to size constraints) from
http://www.astro.ubc.ca/people/courteau/public/courteau03_TFbars.ps.g
Integral Field Unit Observations of NGC 891: Kinematics of the Diffuse Ionized Gas Halo
We present high and moderate spectral resolution spectroscopy of diffuse
ionized gas (DIG) emission in the halo of NGC 891. The data were obtained with
the SparsePak integral field unit at the WIYN Observatory. The wavelength
coverage includes the [NII]6548,6583, Halpha, and [SII]6716,6731 emission
lines. Position-velocity (PV) diagrams, constructed using spectra extracted
from four SparsePak pointings in the halo, are used to examine the kinematics
of the DIG. Using two independent methods, a vertical gradient in azimuthal
velocity is found to be present in the northeast quadrant of the halo, with
magnitude approximately 15-18 km/s/kpc, in agreement with results from HI
observations. The kinematics of the DIG suggest that this gradient begins at
approximately 1 kpc above the midplane. In another part of the halo, the
southeast quadrant, the kinematics are markedly different, and suggest rotation
at about 175 km/s, much slower than the disk but with no vertical gradient. We
utilize an entirely ballistic model of disk-halo flow in an attempt to
reproduce the kinematics observed in the northeast quadrant. Analysis shows
that the velocity gradient predicted by the ballistic model is far too shallow.
Based on intensity cuts made parallel to the major axis in the ballistic model
and an Halpha image of NGC 891 from the literature, we conclude that the DIG
halo is much more centrally concentrated than the model, suggesting that
hydrodynamics dominate over ballistic motion in shaping the density structure
of the halo. Velocity dispersion measurements along the minor axis of NGC 891
seem to indicate a lack of radial motions in the halo, but the uncertainties do
not allow us to set firm limits.Comment: 31 pages, 10 figures. Accepted for publication in the Astrophysical
Journa
The NIR Upgrade to the SALT Robert Stobie Spectrograph
The near infrared (NIR) upgrade to the Robert Stobie Spectrograph (RSS) on
the Southern African Large Telescope (SALT), RSS/NIR, extends the spectral
coverage of all modes of the visible arm. The RSS/NIR is a low to medium
resolution spectrograph with broadband imaging, spectropolarimetric, and
Fabry-Perot imaging capabilities. The visible and NIR arms can be used
simultaneously to extend spectral coverage from approximately 3200 A to 1.6 um.
Both arms utilize high efficiency volume phase holographic gratings via
articulating gratings and cameras. The NIR camera is designed around a
2048x2048 HAWAII-2RG detector housed in a cryogenic dewar. The Epps optical
design of the camera consists of 6 spherical elements, providing sub-pixel rms
image sizes of 7.5 +/- 1.0 um over all wavelengths and field angles. The exact
long wavelength cutoff is yet to be determined in a detailed thermal analysis
and will depend on the semi-warm instrument cooling scheme. Initial estimates
place instrument limiting magnitudes at J = 23.4 and H(1.4-1.6 um) = 21.6 for
S/N = 3 in a 1 hour exposure well below the sky noise.Comment: 12 pages, 10 figures, presented at SPIE, Astronomical Telescopes and
Instrumentation, 24 - 31 May 2006, Orlando, Florida US
SDSS-IV MaNGA: Variations in the N/O -- O/H relation bias metallicity gradient measurements
In this paper we use strong line calibrations of N/O and O/H in MaNGA spaxel
data to explore the systematics introduced by variations in N/O on various
strong-line metallicity diagnostics. We find radial variations in N/O at fixed
O/H which correlate with total galaxy stellar-mass; and which can induce systematic uncertainties in oxygen abundance gradients when
nitrogen-dependent abundance calibrations are used. Empirically, we find that
these differences are associated with variation in the local star formation
efficiency, as predicted by recent chemical evolution models for galaxies, but
we cannot rule out other processes such as radial migration and the accretion
of passive dwarf galaxies also playing a role.Comment: 8 pages, 4 figures. Submitted to ApJL. All comments are welcom
Isolated and non-isolated dwarfs in terms of modified Newtonian dynamics
Within the framework of modified Newtonian dynamics (MOND) we investigate the
kinematics of two dwarf spiral galaxies belonging to very different
environments, namely KK 246 in the Local Void and Holmberg II in the M81 group.
A mass model of the rotation curve of KK 246 is presented for the first time,
and we show that its observed kinematics are consistent with MOND. We re-derive
the outer rotation curve of Holmberg II, by modelling its HI data cube, and
find that its inclination should be closer to face-on than previously derived.
This implies that Holmberg II has a higher rotation velocity in its outer
parts, which, although not very precisely constrained, is consistent with the
MOND prediction.Comment: Accepted in A&A as a Research Note. 6 pages, 3 figure
The Unusual Infrared Object HDF-N J123656.3+621322
We describe an object in the Hubble Deep Field North with very unusual
near-infrared properties. It is readily visible in Hubble Space Telescope
NICMOS images at 1.6um and from the ground at 2.2um, but is undetected (with
signal-to-noise <~ 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1um. The
f_nu flux density drops by a factor >~ 8.3 (97.7% confidence) from 1.6 to
1.1um. The object is compact but may be slightly resolved in the NICMOS 1.6um
image. In a low-resolution, near-infrared spectrogram, we find a possible
emission line at 1.643um, but a reobservation at higher spectral resolution
failed to confirm the line, leaving its reality in doubt. We consider various
hypotheses for the nature of this object. Its colors are unlike those of known
galactic stars, except perhaps the most extreme carbon stars or Mira variables
with thick circumstellar dust shells. It does not appear to be possible to
explain its spectral energy distribution as that of a normal galaxy at any
redshift without additional opacity from either dust or intergalactic neutral
hydrogen. The colors can be matched by those of a dusty galaxy at z >~ 2, by a
maximally old elliptical galaxy at z >~ 3 (perhaps with some additional
reddening), or by an object at z >~ 10 whose optical and 1.1um light have been
suppressed by the intergalactic medium. Under the latter hypothesis, if the
luminosity results from stars and not an AGN, the object would resemble a
classical, unobscured protogalaxy, with a star formation rate >~ 100 M_sun/yr.
Such UV-bright objects are evidently rare at 2 < z < 12.5, however, with a
space density several hundred times lower than that of present-day L* galaxies.Comment: Accepted for publication in the Astrophysical Journal. 27 pages,
LaTeX, with 7 figures (8 files); citations & references updated + minor
format change
Integral Field Unit Observations of NGC 4302: Kinematics of the Diffuse Ionized Gas Halo
We present moderate resolution spectroscopy of extraplanar diffuse ionized
gas (EDIG) emission in the edge-on spiral galaxy NGC 4302. The spectra were
obtained with the SparsePak integral field unit (IFU) at the WIYN Observatory.
The spectra are used to construct position-velocity (PV) diagrams at several
ranges of heights above the midplane. Azimuthal velocities are directly
extracted from the PV diagrams using the envelope tracing method, and indicate
an extremely steep dropoff in rotational velocity with increasing height, with
magnitude ~30 km/s/kpc. We find evidence for a radial variation in the velocity
gradient on the receding side. We have also performed artificial observations
of galaxy models in an attempt to match the PV diagrams. The results of a
statistical analysis also favor a gradient of ~30 km/s/kpc. We compare these
results with an entirely ballistic model of disk-halo flow, and find a strong
dichotomy between the observed kinematics and those predicted by the model. The
disagreement is worse than we have found for other galaxies in previous
studies. The conclusions of this paper are compared to results from two other
galaxies, NGC 5775 and NGC 891. We find that the vertical gradient in rotation
speed, per unit EDIG scale height, for all three galaxies is consistent with a
constant magnitude (within the errors) of approximately 15-25 km/s/scaleheight,
independent of radius. This relationship is also true within the galaxy NGC
4302. We also discuss how the gradient depends on the distribution and
morphology of the EDIG and the star formation rates of the galaxies, and
consequences for the origin of the gas.Comment: 37 pages, 10 figures; accepted for publication in the Astrophysical
Journa
Galaxies in Southern Bright Star Fields I. Near-infrared imaging
As a prerequisite for cosmological studies using adaptive optics techniques,
we have begun to identify and characterize faint sources in the vicinity of
bright stars at high Galactic latitudes. The initial phase of this work has
been a program of K_s imaging conducted with SOFI at the ESO NTT. From
observations of 42 southern fields evenly divided between the spring and autumn
skies, we have identified 391 additional stars and 1589 galaxies lying at
separations 60" from candidate guide stars in the magnitude range 9.0 R 12.4.
When analyzed as a "discrete deep field" with 131 arcmin^2 area, our dataset
gives galaxy number counts that agree with those derived previously over the
range 16 K_s 20.5. This consistency indicates that in the aggregate, our fields
should be suitable for future statistical studies. We provide our source
catalogue as a resource for users of large telescopes in the southern
hemisphere.Comment: 10 pages, 7 figures, accepted by A&A; Table 3 is available at
http://www.rzg.mpg.de/~ajb/data.html pending upload to CD
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