874 research outputs found
A Search for Variable Stars and Planetary Occultations in NGC2301 I: Techniques
We observed the young open cluster NGC 2301 for 14 nights in Feb. 2004 using
the orthogonal transfer CCD camera (OPTIC). We used PSF shaping techniques
("square stars") during the observations allowing a larger dynamic range (4.5
magnitudes) of high photometric precision results (2 mmag) to be obtained.
These results are better than similar observing campaigns using standard CCD
imagers. This paper discusses our observational techniques and presents initial
results for the variability statistics found in NGC 2301. Details of the
variability statistics as functions of color, variability type, stellar type,
and cluster location will appear in paper II
The Dearth of Neutral Hydrogen in Galactic Dwarf Spheroidal Galaxies
We present new upper limits on the neutral hydrogen (H i) content within the stellar half-light ellipses of 15 Galactic dwarf spheroidal galaxies (dSphs), derived from pointed observations with the Green Bank Telescope (GBT) as well as Arecibo L-band Fast ALFA survey and Galactic All-Sky Survey data. All of the limits are more stringent than previously reported values, and those from the GBT improve upon constraints in the literature by a median factor of 23. Normalizing by V-band luminosity LV and dynamical mass Mdyn, we find and , irrespective of location in the Galactic halo. Comparing these relative H i contents to those of the Local Group and nearby neighbor dwarfs compiled by McConnachie, we find that the Galactic dSphs are extremely gas-poor. Our H i upper limits therefore provide the clearest picture yet of the environmental dependence of the H i content in Local Volume dwarfs. If ram pressure stripping explains the dearth of H i in these systems, then orbits in a relatively massive Milky Way are favored for the outer halo dSph Leo I, while Leo II and Canes Venatici I have had a pericentric passage in the past. For Draco and Ursa Minor, the interstellar medium mass that should accumulate through stellar mass loss in between pericentric passages exceeds by a factor of ~30. In Ursa Minor, this implies that either this material is not in the atomic phase, or that another mechanism clears the recycled gas on shorter timescales
Paper No 17.2: Head-Tracked Dynamic Exit Pupil Multiuser Autostereoscopic Display
In this paper, a glasses-free (autostereoscopic) display is described where the controlled illumination for a liquid crystal display (LCD) backlight is derived from a laser source where the laser is used not for its coherence properties but for its ability to have close control on its direction. This allows light exiting the LCD to be directed to the left and right eyes of several viewers who can move freely in both the X and Z directions. Light directions are determined by a spatial light modulator (SLM) that is controlled by a multiuser head tracker. This paper is principally concerned with the SLM and the Gabor superlens screen that is used to provide the angular magnification required to give a sufficiently wide viewing field
A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies
[abridged] We present the results of a spectroscopic survey of the recently
discovered faint Milky Way satellites Boo, UMaI, UMaII and Wil1. Using the
DEIMOS spectrograph on Keck, we have obtained samples that contain from 15 to
85 probable members of these satellites for which we derive radial velocities
precise to a few km/s down to i~21-22. About half of these stars are observed
with a high enough S/N to estimate their metallicity to within \pm0.2 dex. From
this dataset, we show that UMaII is the only object that does not show a clear
radial velocity peak. However, the measured systemic radial velocity
(v_r=115\pm5 km/s) is in good agreement with recent simulations in which this
object is the progenitor of the recently discovered Orphan Stream. The three
other satellites show velocity dispersions that make them highly dark-matter
dominated systems. In particular the Willman 1 object is not a globular cluster
given its metallicity scatter over -2.0<[Fe/H]<-1.0 and is therefore almost
certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity
dispersion of only 4.3_{-1.3}^{+2.3} km/s around a systemic velocity of
-12.3\pm2.3 km/s which implies a mass-to-light ratio of ~700 and a total mass
of ~5x10^5 Msun for this satellite, making it the least massive satellite
galaxy known to date. Such a low mass could mean that the 10^7 Msun limit that
had until now never been crossed for Milky Way and Andromeda satellite galaxies
may only be an observational limit and that fainter, less massive systems exist
within the Local Group. However, more modeling and an extended search for
potential extra-tidal stars are required to rule out the possibility that these
systems have not been significantly heated by tidal interaction.Comment: 24 pages, 11 figures, MNRAS accepte
Where are the Fossils of the First Galaxies? I. Local Volume Maps and Properties of the Undetected Dwarfs
We present a new method for generating initial conditions for LCDM N-body
simulations which provides the dynamical range necessary to follow the
evolution and distribution of the fossils of the first galaxies on Local
Volume, 5-10 Mpc, scales. The initial distribution of particles represents the
position, velocity and mass distribution of the dark and luminous halos
extracted from pre-reionization simulations. We confirm previous results that
ultra-faint dwarfs have properties compatible with being well preserved fossils
of the first galaxies. However, because the brightest pre-reionization dwarfs
form preferentially in biased regions, they most likely merge into non-fossil
halos with circular velocities >20-30 km/s. Hence, we find that the maximum
luminosity of true-fossils in the Milky Way is L_V<10^5 L_solar, casting doubts
on the interpretation that some classical dSphs are true-fossils. In addition,
we argue that most ultra-faints at small galactocentric distance, R<50 kpc, had
their stellar properties modified by tides, while a large population of fossils
is still undetected due to their extremely low surface brightness log(Sigma_V)
< -1.4. We estimate that the region outside R_50 (~ 400 kpc) up to 1 Mpc from
the Milky Way contains about a hundred true fossils of the first galaxies with
V-band luminosities 10^3 - 10^5 L_solar and half-light radii, r_hl ~ 100-1000
pc.Comment: published in ApJ October 2011 with minor revisions V. 741 article ID.
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Hundreds of Milky Way satellites? Luminosity bias in the satellite luminosity function
We correct the observed Milky Way satellite luminosity function for
luminosity bias using published completeness limits for the Sloan Digital Sky
Survey DR5. Assuming that the spatial distribution of Milky Way satellites
tracks the subhalos found in the Via Lactea LCDM N-body simulation, we show
that there should be between ~300 and ~600 satellites within 400 kpc of the Sun
that are brighter than the faintest known dwarf galaxies, and that there may be
as many as ~ 1000, depending on assumptions. By taking into account
completeness limits, we show that the radial distribution of known Milky Way
dwarfs is consistent with our assumption that the full satellite population
tracks that of subhalos. These results alleviate the primary worries associated
with the so-called missing satellites problem in CDM. We show that future, deep
wide-field surveys such as SkyMapper, the Dark Energy Survey (DES), PanSTARRS,
and the Large Synoptic Survey Telescope (LSST) will deliver a complete census
of ultra-faint dwarf satellites out to the Milky Way virial radius, offer new
limits on the free-streaming scale of dark matter, and provide unprecedented
constraints on the low-luminosity threshold of galaxy formation.Comment: 13 pages, 10 figures, ApJ In Pres
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