755 research outputs found
Keck Spectroscopy of Dwarf Elliptical Galaxies in the Virgo Cluster
Keck spectroscopy is presented for four dwarf elliptical galaxies in the
Virgo Cluster. At this distance, the mean velocity and velocity dispersion are
well resolved as a function of radius between 100 to 1000 pc, allowing a clear
separation between nuclear and surrounding galaxy light. We find a variety of
dispersion profiles for the inner regions of these objects, and show that none
of these galaxies is rotationally flattened.Comment: 4 pages, 2 figures, to appear in the proceedings of the Yale
Cosmology Workshop "The Shapes of Galaxies and their Halos", (ed. P.
Natarjan
Internal Kinematics of Distant Field Galaxies: I. Emission Line Widths for a Complete Sample of Faint Blue Galaxies at <z>=0.25
We present measurements of the OII(3727) emission line width for a complete
sample of 24 blue field galaxies (21.25=0.25, obtained
with the AUTOFIB fibre spectrograph on the Anglo-Australian Telescope. Most
emission lines are spectrally resolved, yet all have dispersions sigma<100km/s.
Five of the 24 sample members have OII doublet line flux ratios which imply gas
densities in excess of 100 cm^-3. The line emission in these galaxies may be
dominated by an active nucleus and the galaxies have been eliminated from the
subsequent analysis. The remaining 19 linewidths are too large by a factor of
two (7sigma significance) to be attributed to turbulent motions within an
individual star forming region, and therefore most likely reflect the orbital
motion of ionized gas in the galaxy. We use Fabry--Perot observations of nearby
galaxies to construct simulated datasets that mimic our observational setup at
z=0.25; these allow us to compute the expected distribution of (observable)
linewidths sigma_v for a galaxy of a given ``true'' (optical) rotation speed
v_c. These simulations include the effects of random viewing angles, clumpy
line emission, finite fibre aperture, and internal dust extinction on the
emission line profile. We assume a linewidth--luminosity--colour relation: ln[
v_c(M_B,B-R) ] = ln[v_c(-19,1)] - eta*(M_B+10) + zeta*[(B-R)-1] and determine
the range of parameters consistent with our data. We find a mean rotation speed
of v_c(-19,1)=66+-8km/s (68% confidence limits) for the distant galaxies with
M_B=-19 and B-R=1, with a magnitude dependence for v_c of eta=0.07+-0.08, and a
colour dependence of zeta =0.28+-0.25. Through comparison with several local
samples we show that this value of v_c(-19,1) is significantly lower than the
optical rotation speed of present-day galaxies with the same absolute magnitudeComment: TeX Text and Tables, no Figures. Compressed and uuencoded PS file of
the complete paper (43 pages including 9 figures) available at
http://zwicky.as.arizona.edu/~rix/; submitted to MNRA
The Dominance of Metal-Rich Streams in Stellar Halos: A Comparison Between Substructure in M31 and Lambda-CDM Models
Extensive photometric and spectroscopic surveys of the Andromeda galaxy (M31)
have discovered tidal debris features throughout M31's stellar halo. We present
stellar kinematics and metallicities in fields with identified substructure
from our on-going SPLASH survey of M31 red giant branch stars with the DEIMOS
spectrograph on the Keck II 10-m telescope. Radial velocity criteria are used
to isolate members of the kinematically-cold substructures. The substructures
are shown to be metal-rich relative to the rest of the dynamically hot stellar
population in the fields in which they are found. We calculate the mean
metallicity and average surface brightness of the various kinematical
components in each field, and show that, on average, higher surface brightness
features tend to be more metal-rich than lower surface brightness features.
Simulations of stellar halo formation via accretion in a cosmological context
are used to illustrate that the observed trend can be explained as a natural
consequence of the observed dwarf galaxy mass-metallicity relation. A
significant spread in metallicity at a given surface brightness is seen in the
data; we show that this is due to time effects, namely the variation in the
time since accretion of the tidal streams' progenitor onto the host halo. We
show that in this theoretical framework a relationship between the
alpha-enhancement and surface brightness of tidal streams is expected, which
arises from the varying times of accretion of the progenitor satellites onto
the host halo. Thus, measurements of the alpha-enrichment, metallicity, and
surface brightness of tidal debris can be used to reconstruct the luminosity
and time of accretion onto the host halo of the progenitors of tidal streams.Comment: 11 pages, 6 figures, published in Ap
Deep Photometry in a Remote M31 Major Axis Field Near G1
We present photometry from Hubble Space Telescope (HST)/Wide Field Planetary
Camera 2 parallel imagery of a remote M31 field at a projected distance of
about 34 kpc from the nucleus near the SW major axis. This field is near the
globular cluster G1, and near one of the candidate tidal plumes identified by
Ferguson et al. (2002). The F606W (V) and F814W (I) images were obtained in
parallel with Space Telescope Imaging Spectrograph spectroscopy of G1 (GO-9099)
and total 7.11 hours of integration time -- the deepest HST field in the outer
disk of M31 to date, reaching to V ~ 28. The color-magnitude diagram of the
field shows a clearly-defined red clump at V = 25.25 and a red giant branch
consistent with [Fe/H] ~ -0.7. The lack of a blue horizontal branch contrasts
with other M31 halo fields, the Andromeda dwarf spheroidals, and with the
nearby globular cluster G1. Comparing the observed luminosity function to the
Padova models, we find that at least some of the stellar population must be
younger than 6 - 8 Gyr. The outermost detected neutral hydrogen gas disk of M31
lies only 2 kpc in projection from our field. The finding that some giants in
the field have radial velocities close to that of the neutral hydrogen gas
(Reitzel, Guhathakurta, & Rich 2003) leads us to conclude that our field
samples the old, low-surface-brightness disk rather than the true Population II
spheroid.Comment: 15 pages, 3 figures. accepted for publication in the A
STEREO as a "Planetary Hazards" Mission
NASA's twin STEREO probes, launched in 2006, have advanced the art and science of space weather forecasting more than any other spacecraft or solar observatory. By surrounding the Sun, they provide previously-impossible early warnings of threats approaching Earth as they develop on the solar far side. They have also revealed the 3D shape and inner structure of CMEs-massive solar storms that can trigger geomagnetic storms when they collide with Earth. This improves the ability of forecasters to anticipate the timing and severity of such events. Moreover, the unique capability of STEREO to track CMEs in three dimensions allows forecasters to make predictions for other planets, giving rise to the possibility of interplanetary space weather forecasting too. STEREO is one of those rare missions for which "planetary hazards" refers to more than one world. The STEREO probes also hold promise for the study of comets and potentially hazardous asteroids
Elemental Abundances in M31: A Comparative Analysis of Alpha and Iron Element Abundances in the the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
We measured [Fe/H] and [α/Fe] using spectral synthesis of low-resolution stellar spectroscopy for 70 individual red-giant-branch stars across four fields spanning the outer disk, Giant Stellar Stream (GSS), and inner halo of M31. Fields at M31-centric projected distances of 23 kpc in the halo, 12 kpc in the halo, 22 kpc in the GSS, and 26 kpc in the outer disk are α-enhanced, with ⟨ [α/Fe]〉= 0.43, 0.50, 0.41, and 0.58, respectively. The 23 and 12 kpc halo fields are relatively metal-poor, with ⟨ [Fe/H]⟩ = −1.54 and −1.30, whereas the 22 kpc GSS and 26 kpc outer disk fields are relatively metal-rich with ⟨ [Fe/H]⟩ = −0.84 and −0.92, respectively. For fields with substructure, we separated the stellar populations into kinematically hot stellar halo components and kinematically cold components. We did not find any evidence of a radial [α/Fe] gradient along the high surface brightness core of the GSS between ~17 and 22 kpc. However, we found tentative suggestions of a negative radial [α/Fe] gradient in the stellar halo, which may indicate that different progenitor(s) or formation mechanisms contributed to the build up of the inner versus outer halo. Additionally, the [α/Fe] distribution of the metal-rich ([Fe/H] > −1.5), smooth inner stellar halo (r_(proj) ≾ 26 kpc) is inconsistent with having formed from the disruption of a progenitor(s) similar to present-day M31 satellite galaxies. The 26 kpc outer disk is most likely associated with the extended disk of M31, where its high α-enhancement provides support for an episode of rapid star formation in M31's disk possibly induced by a major merger
Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)
This paper investigates the discrepancy between recent measurements of the
density profile of the globular cluster 47 Tuc that have used HST data sets.
Guhathakurta et al. (1992) used pre-refurbishment WFPC1 V-band images to derive
r_c = 23" +/- 2". Calzetti et al. (1993) suggested that the density profile is
a superposition of two King profiles (r_c = 8" and r_c = 25") based on U-band
FOC images. De Marchi et al. (1996) used deep WFPC1 U-band images to derive r_c
= 12" +/- 2". Differences in the adopted cluster centers are not the cause of
the discrepancy. Our independent analysis of the data used by De Marchi et al.
reaches the following conclusions: (1) De Marchi et al.'s r_c ~ 12" value is
spuriously low, a result of radially-varying bias in the star counts in a
magnitude limited sample -- photometric errors and a steeply rising stellar
luminosity function cause more stars to scatter across the limiting magnitude
into the sample than out of it, especially near the cluster center where
crowding effects are most severe. (2) Changing the limiting magnitude to the
main sequence turnoff, away from the steep part of the luminosity function,
partially alleviates the problem and results in r_c = 18". (3) Combining such a
limiting magnitude with accurate photometry derived from PSF fitting, instead
of the less accurate aperture photometry employed by De Marchi et al., results
in a reliable measurement of the density profile which is well fit by r_c = 22"
+/- 2". Archival WFPC2 data are used to derive a star list with a higher degree
of completeness, greater photometric accuracy, and wider areal coverage than
the WFPC1 and FOC data sets; the WFPC2-based density profile supports the above
conclusions, yielding r_c = 24" +/- 1.9".Comment: 22 pages, 5 figures, 1 table; accepted for publication in PASP; see
http://www.ucolick.org/~raja/hgg.tar.gz for full-resolution figure
The DEEP2 Galaxy Redshift Survey: Redshift Identification of Single-Line Emission Galaxies
We present two methods for determining spectroscopic redshifts of galaxies in
the DEEP2 survey which display only one identifiable feature, an emission line,
in the observed spectrum ("single-line galaxies"). First, we assume each single
line is one of the four brightest lines accessible to DEEP2: Halpha, [OIII]
5007, Hbeta, or [OII] 3727. Then, we supplement spectral information with BRI
photometry. The first method, parameter space proximity (PSP), calculates the
distance of a single-line galaxy to galaxies of known redshift in (B-R), (R-I),
R, observed wavelength parameter space. The second method is an artificial
neural network (ANN). Prior information, such as allowable line widths and
ratios, rules out one or more of the four lines for some galaxies in both
methods. Based on analyses of evaluation sets, both methods are nearly perfect
at identifying blended [OII] doublets. Of the lines identified as Halpha in the
PSP and ANN methods, 91.4% and 94.2% respectively are accurate. Although the
methods are not this accurate at discriminating between [OIII] and Hbeta, they
can identify a single line as one of the two, and the ANN method in particular
unambiguously identifies many [OIII] lines. From a sample of 640 single-line
spectra, the methods determine the identities of 401 (62.7%) and 472 (73.8%)
single lines, respectively, at accuracies similar to those found in the
evaluation sets.Comment: 11 pages, 6 figures, accepted to Ap
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