702 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
High-resolution spectroscopy of extremely metal-poor stars in the least evolved galaxies: Bootes II
We present high-resolution Magellan/MIKE spectra of the four brightest
confirmed red giant stars in the ultra-faint dwarf galaxy Bootes II (Boo II).
These stars all inhabit the metal-poor tail of the Boo II metallicity
distribution function. The chemical abundance pattern of all detectable
elements in these stars is consistent with that of the Galactic halo. However,
all four stars have undetectable amounts of neutron-capture elements Sr and Ba,
with upper limits comparable to the lowest ever detected in the halo or in
other dwarf galaxies. One star exhibits significant radial velocity variations
over time, suggesting it to be in a binary system. Its variable velocity has
likely increased past determinations of the Boo II velocity dispersion. Our
four stars span a limited metallicity range, but their enhanced
{\alpha}-abundances and low neutron-capture abundances are consistent with the
interpretation that Boo II has been enriched by very few generations of stars.
The chemical abundance pattern in Boo II confirms the emerging trend that the
faintest dwarf galaxies have neutron-capture abundances distinct from the halo,
suggesting the dominant source of neutron-capture elements in halo stars may be
different than in ultra-faint dwarfs.Comment: 10 pages, 5 figures, 4 tables. Updated to match ApJ accepted versio
Evidence for the disky origin of luminous Virgo dwarf ellipticals from the kinematics of their globular cluster systems
We report evidence for dynamically significant rotation in the globular
cluster systems of two luminous Virgo dwarf ellipticals, VCC1261 and VCC1528.
Including previous results for VCC1087, the globular cluster systems of all
three Virgo dwarf ellipticals studied in detail to date exhibit v_rot/sigma >
1. Taking the rotation seen in the globular clusters as maximal disk rotation,
we find all three dEs lie on the r-band Tully-Fisher relation. We argue that
these data support the hypothesis that luminous dEs are the remnants of
transformed disk galaxies. We also obtained deep, longslit data for the stars
in VCC1261 and VCC1528. Both these galaxies show rapid rotation in their inner
regions, with spatial scales of ~0.5 kpc. These rotation velocities are similar
to those seen in the GC systems. Since our longslit data for Virgo dEs extend
out to 1-2 effective radii (typical of deep observations), whereas the globular
clusters extend out to 4--7 effective radii, we conclude that non-detections of
rotation in many luminous dEs may simply be due to a lack of radial coverage in
the stellar data, and that globular clusters represent singularly sensitive
probes of the dynamics of dEs. Based on these data, we suggest that gas disks
are significant sites of globular cluster formation in the early universe.Comment: To appear in the AJ, corrected typographical errors in Table 1, added
a referenc
Local Group Dwarf Elliptical Galaxies: II. Stellar Kinematics to Large Radii in NGC 147 and NGC 185
We present kinematic and metallicity profiles for the M31 dwarf elliptical
(dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most
extensive spectroscopic radial coverage for any dE galaxy, extending to a
projected distance of eight half-light radii (8 r_eff = 14'). We achieve this
coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442
member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast
to previous studies, we find that both dEs have significant internal rotation.
We measure a maximum rotational velocity of 17+/-2 km/s for NGC 147 and 15+/-5
km/s for NGC 185. The velocity dispersions decrease gently with radius with an
average dispersion of 16+/-1 km/s for NGC 147 and 24+/-1 km/s for NGC 185. Both
dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for
a radial metallicity gradient. We construct two-integral axisymmetric dynamical
models and find that the observed kinematical profiles cannot be explained
without modest amounts of non-baryonic dark matter. We measure central
mass-to-light ratios of ML_V = 4.2+/-0.6 and ML_V = 4.6+/-0.6 for NGC 147 and
NGC 185, respectively. Both dE galaxies are consistent with being primarily
flattened by their rotational motions, although some anisotropic velocity
dispersion is needed to fully explain their observed shapes. The velocity
profiles of all three Local Group dEs (NGC 147, NGC 185 and NGC 205) suggest
that rotation is more prevalent in the dE galaxy class than previously assumed,
but is often manifest only at several times the effective radius. Since all dEs
outside the Local Group have been probed to only inside the effective radius,
this opens the door for formation mechanisms in which dEs are transformed or
stripped versions of gas-rich rotating progenitor galaxies.Comment: 16 pages, 7 figures. accepted to A
Stellar Kinematics of the Andromeda II Dwarf Spheroidal Galaxy
We present kinematical profiles and metallicity for the M31 dwarf spheroidal
(dSph) satellite galaxy Andromeda II (And II) based on Keck DEIMOS spectroscopy
of 531 red giant branch stars. Our kinematical sample is among the largest for
any M31 satellite and extends out to two effective radii (r_eff = 5.3' = 1.1
kpc). We find a mean systemic velocity of -192.4+-0.5 km/s and an average
velocity dispersion of sigma_v = 7.8+-1.1 km/s. While the rotation velocity
along the major axis of And II is nearly zero (<1 km/s), the rotation along the
minor axis is significant with a maximum rotational velocity of v_max=8.6+-1.8
km/s. We find a kinematical major axis, with a maximum rotational velocity of
v_max=10.9+-2.4 km/s, misaligned by 67 degrees to the isophotal major axis. And
II is thus the first dwarf galaxy with evidence for nearly prolate rotation
with a v_max/sigma_v = 1.1, although given its ellipticity of epsilon = 0.10,
this object may be triaxial. We measured metallicities for a subsample of our
data, finding a mean metallicity of [Fe/H] = -1.39+- 0.03 dex and an internal
metallicity dispersion of 0.72+-0.03 dex. We find a radial metallicity gradient
with metal-rich stars more centrally concentrated, but do not observe a
significant difference in the dynamics of two metallicity populations. And II
is the only known dwarf galaxy to show minor axis rotation making it a unique
system whose existence offers important clues on the processes responsible for
the formation of dSphs.Comment: 14 pages, 10 figures, 4 tables, accepted for publication in Ap
Structure and Dynamics of the Globular Cluster Palomar 13
We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of σ = 2.2 ± 0.4 km s^(–1). We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is σ = 0.7^(+0.6)_(–0.5) km s^(–1). Combining our DEIMOS data with literature values, our final velocity dispersion is σ = 0.4^(+0.4)_( –0.3) km s^(–1). We determine a spectroscopic metallicity of [Fe/H] = –1.6 ± 0.1 dex, placing a 1σ upper limit of σ_([Fe/H]) ~ 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M_V = –2.8 ± 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (Σ α r^η, η = –2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M_(1/2) = 1.3^(+2:7)_(–1.3) × 10^3 M_☉ and a mass-to-light ratio of M/L_V = 2.4^(+5.0)_(–2.4) M_☉/L_☉. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics
NGC 770: A Counter-Rotating Core in a Low-Luminosity Elliptical Galaxy
We present evidence for a counter-rotating core in the low-luminosity (M_B =
-18.2) elliptical galaxy NGC 770 based on internal stellar kinematic data. This
counter-rotating core is unusual as NGC 770 is not the primary galaxy in the
region and it lies in an environment with evidence of on-going tidal
interactions. We discovered the counter-rotating core via single-slit Keck/ESI
echelle spectroscopy; subsequent integral field spectroscopy was obtained with
the Gemini/GMOS IFU. The counter-rotating region has a peak rotation velocity
of 21 km/s as compared to the main galaxy's rotation speed of greater than 45
km/s in the opposite direction. The counter-rotating region extends to a radius
of 4'' (0.6 kpc), slightly smaller than the half-light radius of the galaxy
which is 5.3'' (0.8 kpc) and is confined to a disk whose scale height is less
than 0.8'' (0.1 kpc). We compute an age and metallicity of the inner
counter-rotating region of 3 +/- 0.5 Gyr and [Fe/H] = 0.2 +/- 0.2 dex, based on
Lick absorption-line indices. The lack of other large galaxies in this region
limits possible scenarios for the formation of the counter-rotating core. We
discuss several scenarios and favor one in which NGC 770 accreted a small
gas-rich dwarf galaxy during a very minor merging event. If this scenario is
correct, it represents one of the few known examples of merging between two
dwarf-sized galaxies.Comment: 26 pages, 9 figures. Accepted to AJ. See this
http://www.ociw.edu/~mgeha/geha.ps.gz for version with high resolution
figure
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