182 research outputs found
Mitigation of H2RG persistence with image illumination
Residual charge generation, or image persistence, in infrared detectors is a problem that affects many low-light astronomical instruments. The HAWAII-2RG in the MMT and Magellan Infrared Spectrograph shows significant persistence when first powered up. We describe here how we reduce the persistence sensitivity of this detector by exposure to light
On the Rotation Period of (90377) Sedna
We present precise, ~1%, r-band relative photometry of the unusual solar
system object (90377) Sedna. Our data consist of 143 data points taken over
eight nights in October 2004 and January 2005. The RMS variability over the
longest contiguous stretch of five nights of data spanning nine days is only
1.3%. This subset of data alone constrain the amplitude of any long-period
variations with period P to be A<1% (P/20 days)^2. Over the course of any given
5-hour segment, the data exhibits significant linear trends not seen in a
comparison star of similar magnitude, and in a few cases these segments show
clear evidence for curvature at the level of a few millimagnitudes per hour^2.
These properties imply that the rotation period of Sedna is O(10 hours), cannot
be 10 days, unless the intrinsic light curve has
significant and comparable power on multiple timescales, which is unlikely. A
sinusoidal fit yields a period of P=(10.273 +/- 0.002) hours and semi-amplitude
of A=(1.1 +/- 0.1)%. There are additional acceptable fits with flanking periods
separated by ~3 minutes, as well as another class of fits with P ~ 18 hours,
although these later fits appear less viable based on visual inspection. Our
results indicate that the period of Sedna is likely consistent with typical
rotation periods of solar system objects, thus obviating the need for a massive
companion to slow its rotation.Comment: 7 pages, 4 figures, 2.5 tables. Final ApJL version, minor changes.
Full light curve data in tex
Mitigation of H2RG persistence with image illumination
Residual charge generation, or image persistence, in infrared detectors is a problem that affects many low-light astronomical instruments. The HAWAII-2RG in the MMT and Magellan Infrared Spectrograph shows significant persistence when first powered up. We describe here how we reduce the persistence sensitivity of this detector by exposure to light
Lensed Quasar Hosts
Gravitational lensing assists in the detection of quasar hosts by amplifying
and distorting the host light away from the unresolved quasar core images. We
present the results of HST observations of 30 quasar hosts at redshifts 1 < z <
4.5. The hosts are small in size (r_e <~ 6 kpc), and span a range of
morphologies consistent with early-types (though smaller in mass) to
disky/late-type. The ratio of the black hole mass (MBH, from the virial
technique) to the bulge mass (M_bulge, from the stellar luminosity) at 1<z<1.7
is broadly consistent with the local value; while MBH/M_bulge at z>1.7 is a
factor of 3--6 higher than the local value. But, depending on the stellar
content the ratio may decline at z>4 (if E/S0-like), flatten off to 6--10 times
the local value (if Sbc-like), or continue to rise (if Im-like). We infer that
galaxy bulge masses must have grown by a factor of 3--6 over the redshift range
3>z>1, and then changed little since z~1. This suggests that the peak epoch of
galaxy formation for massive galaxies is above z~1. We also estimate the duty
cycle of luminous AGNs at z>1 to be ~1%, or 10^7 yrs, with sizable scatter.Comment: 8 pages, 6 figures, review article with C. Impey at the conference on
"QSO Host Galaxies: Evolution and Environment", Aug. 29-Sep. 2, 2005, Lorentz
Center, Leiden, The Netherland
The Hypervelocity Star SDSS J090745.0+024507 is a Short-Period Variable
We present high-precision photometry of the hypervelocity star SDSS
J090745.0+024507 (HVS), which has a Galactic rest-frame radial velocity of
v=709 km/s, and so has likely been ejected from the supermassive black hole in
the Galactic center. Our data were obtained on two nights using the MMT 6.5m
telescope, and is supplemented by lower precision photometry obtained on four
nights using the FLWO 1.2m telescope. The high-precision photometry indicates
that the HVS is a short-period, low-amplitude variable, with period P=0.2-2
days and amplitude A = 2-10%. Together with the known effective temperature of
T_eff ~ 10,500 K (spectral type B9), this variability implies that the HVS is a
member of the class of slowly pulsating B-type main sequence stars, thus
resolving the previously-reported two-fold degeneracy in the luminosity and
distance of the star. The HVS has a heliocentric distance of 71 kpc, and an age
of ~0.35 Gyr. The time of ejection from the center of the Galaxy is < 100 Myr,
and thus the existence of the OS constitutes observational evidence of a
population of young stars in the proximity of the central supermassive black
hole ~0.1 Gyr ago. It is possible that the HVS was a member of a binary that
was tidally disrupted by the central black hole; we discuss constraints on the
properties of the companion's orbit.Comment: ApJL, submitted, 4 pages, 4 figure
PKS 1830-211: A Face-On Spiral Galaxy Lens
We present new Hubble Space Telescope images of the gravitational lens PKS
1830-211, which allow us to characterize the lens galaxy and update the
determination of the Hubble constant from this system. The I-band image shows
that the lens galaxy is a face-on spiral galaxy with clearly delineated spiral
arms. The southwestern image of the background quasar passes through one of the
spiral arms, explaining the previous detections of large quantities of
molecular gas and dust in front of this image. The lens galaxy photometry is
consistent with the Tully-Fisher relation, suggesting the lens galaxy is a
typical spiral galaxy for its redshift. The lens galaxy position, which was the
main source of uncertainty in previous attempts to determine H_0, is now known
precisely. Given the current time delay measurement and assuming the lens
galaxy has an isothermal mass distribution, we compute H_0 = 44 +/- 9 km/s/Mpc
for an Omega_m = 0.3 flat cosmological model. We describe some possible
systematic errors and how to reduce them. We also discuss the possibility
raised by Courbin et al. (2002), that what we have identified as a single lens
galaxy is actually a foreground star and two separate galaxies.Comment: 21 pp., 4 figs., accepted by ApJ, section added to discuss related
work by Courbin et al. (astro-ph/0202026
Chronic Ankle Instability and Neural Excitability of the Lower Extremity
Neuromuscular dysfunction of the leg and thigh musculature, including decreased strength and postural control, is common in patients with chronic ankle instability (CAI). Understanding how CAI affects specific neural pathways may provide valuable information for targeted therapies
The FIRST-Optical-VLA Survey for Lensed Radio Lobes
We present results from a survey for gravitationally lensed radio lobes.
Lensed lobes are a potentially richer source of information about galaxy mass
distributions than lensed point sources, which have been the exclusive focus of
other recent surveys. Our approach is to identify radio lobes in the FIRST
catalog and then search optical catalogs for coincident foreground galaxies,
which are candidate lensing galaxies. We then obtain higher-resolution images
of these targets at both optical and radio wavelengths, and obtain optical
spectra for the most promising candidates. We present maps of several radio
lobes that are nearly coincident with galaxies. We have not found any new and
unambiguous cases of gravitational lensing. One radio lobe in particular, FOV
J0743+1553, has two hot spots that could be multiple images produced by a
z=0.19 spiral galaxy, but the lensing interpretation is problematic.Comment: 38 pages, 18 figures, aastex, accepted to A
Halo Structures of Gravitational Lens Galaxies
We explore the halo structure of four gravitational lenses with
well-observed, thin Einstein rings. We find that the gravitational potentials
are well described by ellipsoidal density distributions in the sense that the
best-fit nonellipsoidal models have parameters consistent with their
ellipsoidal counterparts. We find upper limits on the standard parameters for
the deviation from an ellipse of |a_3/a_0|<0.023, 0.019, 0.037, and 0.035, and
|a_4/a_0|<0.034, 0.041, 0.051, and 0.064 for SDSS J0924+0219, HE0435-1223,
B1938+666, and PG1115+080, respectively. We find that the lens galaxies are at
the centers of their dark matter halos, and obtain upper limits for the offset
of each center of mass from the center of light of |Delta x|<0.004, 0.005,
0.009, and 0.005 arcsec, corresponding to 22, 29, 70, and 23 pc. These limits
also exclude the possibility of any significant lopsidednessof the dark matter
halos and set an upper limit of f_sat<sqrt(N)% on the mass fraction of massive
substructures inside the Einstein ring if they are divided over N satellites.
We also explore the properties of galaxies as substructures in groups for the
lens PG1115+080, finding evidence for dark matter halos associated with the
galaxies but no evidence for a clear distinction between satellite and central
galaxies.Comment: 10 pages, 5 figures, accepted for publication in The Astrophysical
Journa
- âŠ