11,508 research outputs found
A near-infrared and optical photometric study of the Sculptor dwarf spheroidal galaxy: implications for the metallicity spread
We present here a detailed study of the Sculptor dSph galaxy red giant branch
(RGB) and horizontal branch (HB) morphology, combining new near-infrared
photometry from CIRSI, with optical data from the ESO WFI. For a Sculptor-like
old and generally metal-poor system, the position of RGB stars on the
colour-magnitude diagram is mainly metallicity dependent. The advantage of
using optical-NIR colours is that the position of the RGB locus is much more
sensitive to metallicity than with optical colours alone. In contrast the
horizontal branch (HB) morphology is strongly dependent on both metallicity and
age. Therefore a detailed study of both the RGB in optical-NIR colours and the
HB can help break the age-metallicity degeneracy. Our measured photometric
width of the Sculptor giant branch corresponds to a range in metallicity of
0.75 dex. We detect the RGB and AGB bumps in both the NIR and optical
luminosity functions, and derive from them a mean metallicity of [M/H] = -1.3
+/- 0.1. From isochrone fitting we derive a mean metallicity of [Fe/H] = -1.42
with a dispersion of 0.2 dex. These photometric estimators are for the first
time consistent with individual metallicity measurements derived from
spectroscopic observations. No spatial gradient is detected in the RGB
morphology within a radius of 13 arcmin, twice the core radius. On the other
hand, a significant gradient is observed in the HB morphology index, confirming
the `second parameter problem' present in this galaxy. These observations are
consistent with an early extended period of star formation continuing in time
for a few Gyr. (Abridged)Comment: 9 pages, 10 figures. Accepted for publication in MNRA
Time-division SQUID multiplexers with reduced sensitivity to external magnetic fields
Time-division SQUID multiplexers are used in many applications that require
exquisite control of systematic error. One potential source of systematic error
is the pickup of external magnetic fields in the multiplexer. We present
measurements of the field sensitivity figure of merit, effective area, for both
the first stage and second stage SQUID amplifiers in three NIST SQUID
multiplexer designs. These designs include a new variety with improved
gradiometry that significantly reduces the effective area of both the first and
second stage SQUID amplifiers.Comment: 4 pages, 7 figures. Submitted for publication in the IEEE
Transactions on Applied Superconductivity, August 201
The Velocity Dispersion Profile of the Remote Dwarf Spheroidal Galaxy Leo I: A Tidal Hit and Run?
(abridged) We present kinematic results for a sample of 387 stars located
near Leo I based on spectra obtained with the MMT's Hectochelle spectrograph
near the MgI/Mgb lines. We estimate the mean velocity error of our sample to be
2.4 km/s, with a systematic error of < 1 km/s. We produce a final sample of 328
Leo I red giant members, from which we measure a mean heliocentric radial
velocity of 282.9 +/- 0.5 km/s, and a mean radial velocity dispersion of 9.2
+/- 0.4 km/s for Leo I. The dispersion profile of Leo I is flat out to beyond
its classical `tidal' radius. We fit the profile to a variety of equilibrium
dynamical models and can strongly rule out models where mass follows light.
Two-component Sersic+NFW models with tangentially anisotropic velocity
distributions fit the dispersion profile well, with isotropic models ruled out
at a 95% confidence level. The mass and V-band mass-to-light ratio of Leo I
estimated from equilibrium models are in the ranges 5-7 x 10^7 M_sun and 9-14
(solar units), respectively, out to 1 kpc from the galaxy center. Leo I members
located outside a `break radius' (about 400 arcsec = 500 pc) exhibit
significant velocity anisotropy, whereas stars interior appear to have
isotropic kinematics. We propose the break radius represents the location of
the tidal radius of Leo I at perigalacticon of a highly elliptical orbit. Our
scenario can account for the complex star formation history of Leo I, the
presence of population segregation within the galaxy, and Leo I's large outward
velocity from the Milky Way. The lack of extended tidal arms in Leo I suggests
the galaxy has experienced only one perigalactic passage with the Milky Way,
implying that Leo I may have been injected into its present orbit by a third
body a few Gyr before perigalacticon.Comment: ApJ accepted, 23 figures, access paper as a pdf file at
http://www.astro.lsa.umich.edu/~mmateo/research.htm
Variations Electrical Resistance of the Skin in Newborn Infants
Measurements were made of the large variations in apparent electrical resistance of the skin of fifteen newborn infants. Readings were taken every five minutes over two hour periods using a unidirectional current technique. The purpose of the experiment was to check Richter\u27s hypothesis that increases in palmar resistance afford an objective criterion of sleep
Advanced code-division multiplexers for superconducting detector arrays
Multiplexers based on the modulation of superconducting quantum interference
devices are now regularly used in multi-kilopixel arrays of superconducting
detectors for astrophysics, cosmology, and materials analysis. Over the next
decade, much larger arrays will be needed. These larger arrays require new
modulation techniques and compact multiplexer elements that fit within each
pixel. We present a new in-focal-plane code-division multiplexer that provides
multiplexing elements with the required scalability. This code-division
multiplexer uses compact lithographic modulation elements that simultaneously
multiplex both signal outputs and superconducting transition-edge sensor (TES)
detector bias voltages. It eliminates the shunt resistor used to voltage bias
TES detectors, greatly reduces power dissipation, allows different dc bias
voltages for each TES, and makes all elements sufficiently compact to fit
inside the detector pixel area. These in-focal-plane code-division multiplexers
can be combined with multi-gigahertz readout based on superconducting
microresonators to scale to even larger arrays.Comment: 8 pages, 3 figures, presented at the 14th International Workshop on
Low Temperature Detectors, Heidelberg University, August 1-5, 2011,
proceedings to be published in the Journal of Low Temperature Physic
Improved Searches for HI in Three Dwarf Spheroidal Galaxies
Previous searches for HI in our Galaxy's dwarf spheroidal companions have not
been complete enough to settle the question of whether or not these galaxies
have HI, especially in their outer parts. We present VLA observations of three
dwarf spheroidals: Fornax, Leo II, and Draco, all of which have known stellar
velocities. The new data show no HI emission or absorption. Column density
limits in emission are 4--7 x 10^18 atoms/cm^2 in the centers of the galaxies.
The importance of the new observations is that they cover larger areas than
previous searches and they are less plagued by confusion with foreground
(Galactic) HI. The apparent absence of neutral gas in the Fornax dwarf
spheroidal is especially puzzling because recent photometry shows evidence of
stars only 10^8 years old. We discuss whether the VLA observations could have
missed significant amounts of HI.Comment: 20 pages, 7 figures; to appear in the Astronomical Journa
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