324 research outputs found
Proper motions for HST observations in three off-axis bulge fields
Aims. This is the second in a series of papers that attempt to unveil the
kinematic structure of the Galactic bulge through studying radial velocities
and proper motions. We report here ~15000 new proper motions for three low
foreground-extinction off-axis fields of the Galactic bulge. Methods. Proper
motions were derived from a combination of Hubble Space Telescope Wide Field
Planetary Camera 2 (WFPC2) and Advanced Camera for Surveys (ACS) images taken 8
and 9 years apart, and ACS observations taken 9 and 10 years apart, and they
reach accuracies better than 0.9 mas/yr for more than ~10000 objects with
magnitudes F814W < 24. Results. The proper motion distributions in these fields
are similar to those of Galactic minor axis bulge fields. We observe the
rotation of main sequence stars below the turn-off within the Galactic bulge,
as in the minor axis fields. Conclusions. Our stellar proper motions
measurements show a significant bulge rotation for fields as far from the
galactic plane as b=-8.Comment: 14 pages, 14 figures, published in Astronomy & Astrophysic
Metallicity Distribution Functions, Radial Velocities, and Alpha Element Abundances in Three Off-Axis Bulge Fields
We present radial velocities and chemical abundance ratios of [Fe/H], [O/Fe],
[Si/Fe], and [Ca/Fe] for 264 red giant branch (RGB) stars in three Galactic
bulge off-axis fields located near (l,b)=(-5.5,-7), (-4,-9), and (+8.5,+9). The
results are based on equivalent width and spectrum synthesis analyses of
moderate resolution (R~18,000), high signal-to-noise ratio (S/N~75-300) spectra
obtained with the Hydra spectrographs on the Blanco 4m and WIYN 3.5m
telescopes. The targets were selected from the blue side of the giant branch to
avoid cool stars that would be strongly affected by CN and TiO; however, a
comparison of the color-metallicity distribution in literature samples suggests
our selection of bluer targets should not present a significant bias against
metal-rich stars. We find a full range in metallicity that spans
[Fe/H]\approx-1.5 to +0.5, and that, in accordance with the previously observed
minor-axis vertical metallicity gradient, the median [Fe/H] also declines with
increasing Galactic latitude in off-axis fields. The off-axis vertical [Fe/H]
gradient in the southern bulge is estimated to be ~0.4 dex/kpc. The (+8.5,+9)
field exhibits a higher than expected metallicity, with a median [Fe/H]=-0.23,
that might be related to a stronger presence of the X--shaped bulge structure
along that line-of-sight. All fields exhibit an identical, strong decrease in
velocity dispersion with increasing metallicity that is consistent with
observations in similar minor-axis outer bulge fields. Additionally, the
[O/Fe], [Si/Fe], and [Ca/Fe] versus [Fe/H] trends are identical among our three
fields, and are in good agreement with past bulge studies. [abridged]Comment: Accepted for Publication in the Astrophysical Journal; 120 pages
(main text ends on page 24); 22 figures (figures end on page 46); 6 tables;
electronic versions of the tables can be made available upon request to
author C. Johnso
Kinematics of the X-shaped Milky Way Bulge: Expectations from a Self-consistent N-body Model
We explore the kinematics (both the radial velocity and the proper motion) of
the vertical X-shaped feature in the Milky Way with an N-body bar/bulge model.
From the solar perspective, the distance distribution of particles is
double-peaked in fields passing through the X-shape. The separation and
amplitude ratio between the two peaks qualitatively match the observed trends
towards the Galactic bulge. We confirm clear signatures of cylindrical rotation
in the pattern of mean radial velocity across the bar/bulge region. We also
find possible imprints of coherent orbital motion inside the bar structure in
the radial velocity distribution along l=0 degree, where the near and far sides
of the bar/bulge show excesses of approaching and receding particles. The
coherent orbital motion is also reflected in the slight displacement of the
zero-velocity-line in the mean radial velocity, and the displacement of the
maximum/minimum in the mean longitudinal proper motion across the bulge region.
We find some degree of anisotropy in the stellar velocity within the X-shape,
but the underlying orbital family of the X-shape cannot be clearly
distinguished. Two potential applications of the X-shape in previous literature
are tested, i.e., bulge rotation and Galactic center measurements. We find that
the proper motion difference between the two sides of the X-shape can be used
to estimate the mean azimuthal streaming motion of the bulge, but not the
pattern speed of the bar. We also demonstrate that the Galactic center can be
located with the X-shape, but the accuracy depends on the fitting scheme, the
number of fields, and their latitudinal coverage.Comment: Minor changes to match the ApJ accepted version; 17 pages; emulateapj
format. The electronic tables of our model result are available upon reques
The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
The central kiloparsecs of the Milky Way are known to host an old, spheroidal
stellar population, whose spatial and kinematical properties set it apart from
the boxy/peanut structure that constitutes most of the central stellar mass.
The nature of this spheroidal population, whether a small classical bulge, the
innermost stellar halo or a population of disk stars with large initial
velocity dispersion, remains unclear. This structure is also a promising
candidate to host some of the oldest stars in the Galaxy. Here we address the
topic of the inner stellar spheroid age, using spectroscopic and photometric
metallicities for a sample of 935 RR Lyrae stars that are constituents of this
component. By means of stellar population synthesis, we derive an
age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae
stars in the bulge spheroid, an extremely ancient age of Gyr
and conclude they were among the first stars to form in what is now the Milky
Way galaxy. Our age estimate for the central spheroid shows remarkable
agreement with the age profile that has been inferred for the Milky Way stellar
halo, suggesting a connection between the two structures. However, we find mild
evidence for a transition in the halo properties at ~kpc. We
also investigate formation scenarios for metal-rich RR Lyrae stars, such as
binarity and helium variations, and whether they can provide alternative
explanations for the properties of our sample. We conclude that, within our
framework, the only viable alternative is to have younger, slightly
helium-rich, RR Lyrae stars, a hypothesis that would open intriguing questions
for the formation of the inner stellar spheroid.Comment: Accepted for publication in Astronomy & Astrophysics. 17 pages, 15
figure
Light, Alpha, and Fe-Peak Element Abundances in the Galactic Bulge
C. I. Johnson, et al., “Light, Alpha, and Fe-Peak Element Abundances in the Galactic Bulge”, The Astronomical Journal, Vol. 148(4), September 2014. This version of record is available online at: http://iopscience.iop.org/article/10.1088/0004-6256/148/4/67/meta © 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.AWe present radial velocities and chemical abundances of O, Na, Mg, Al, Si, Ca, Cr, Fe, Co, Ni, and Cu for a sample of 156 red giant branch stars in two Galactic bulge fields centered near (l,b)=(+5.25,-3.02) and (0,-12). The (+5.25,-3.02) field also includes observations of the bulge globular cluster NGC 6553. The results are based on high resolution (R~20,000), high signal-to-noise (S/N>70) FLAMES-GIRAFFE spectra obtained through the ESO archive. However, we only selected a subset of the original observations that included spectra with both high S/N and that did not show strong TiO absorption bands. The present work extends previous analyses of this data set beyond Fe and the alpha-elements Mg, Si, Ca, and Ti. While we find reasonable agreement with past work, the data presented here indicate that the bulge may exhibit a different chemical composition than the local thick disk, especially at [Fe/H]>-0.5. In particular, the bulge [alpha/Fe] ratios may remain enhanced to a slightly higher [Fe/H] than the thick disk and the Fe-peak elements Co, Ni, and Cu appear enhanced compared to the disk. There is also some evidence that the [Na/Fe] (but not [Al/Fe]) trends between the bulge and local disk may be different at low and high metallicity. We also find that the velocity dispersion decreases as a function of increasing [Fe/H] for both fields, and do not detect any significant cold, high velocity population. A comparison with chemical enrichment models indicates that a significant fraction of hypernovae are required to explain the bulge abundance trends, and that initial mass functions that are steep, top-heavy (and do not include strong outflow), or truncated to avoid including contributions from stars >40 solar masses are ruled out, in particular because of disagreement with the Fe-peak abundance data. [abridged]Peer reviewedFinal Published versio
Constraints on the Formation of the Globular Cluster IC 4499 from Multi-Wavelength Photometry
We present new multiband photometry for the Galactic globular cluster IC 4499
extending well past the main sequence turn-off in the U, B, V, R, I, and DDO51
bands. This photometry is used to determine that IC4499 has an age of 12 pm 1
Gyr and a cluster reddening of E(B-V) = 0.22 pm 0.02. Hence, IC 4499 is coeval
with the majority of Galactic GCs, in contrast to suggestions of a younger age.
The density profile of the cluster is observed to not flatten out to at least
r~800 arcsec, implying that either the tidal radius of this cluster is larger
than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the
situation in some other, more massive, globular clusters, no anomalous color
spreads in the UV are detected among the red giant branch stars. The small
uncertainties in our photometry should allow the detection of such signatures
apparently associated with variations of light elements within the cluster,
suggesting that IC 4499 consists of a single stellar population.Comment: accepted to MNRA
A Photometric Study of the Outer Halo Globular Cluster NGC 5824
Multi-wavelength CCD photometry over 21 years has been used to produce deep
color-magnitude diagrams together with light curves for the variables in the
Galactic globular cluster NGC 5824. Twenty-one new cluster RR Lyrae stars are
identified, bringing the total to 47, of which 42 have reliable periods
determined for the first time. The color-magnitude diagram is matched using
BaSTI isochrones with age of ~Gyr. and reddening is found to be ; using the period-Wesenheit relation in two colors the distance
modulus is corresponding to a distance of 30.9 Kpc.
The observations show no signs of populations that are significantly younger
than the ~Gyr stars. The width of the red giant branch does not allow for a
spread in [Fe/H] greater than dex, and there is no photometric
evidence for widened or parallel sequences. The pseudo-color
magnitude diagram shows a bifurcation of the red giant branch that by analogy
with other clusters is interpreted as being due to differing spectral
signatures of the first (75\%) and second (25\%) generations of stars whose age
difference is close enough that main sequence turnoffs in the color-magnitude
diagram are unresolved. The cluster main sequence is visible against the
background out to a radial distance of arcmin. We conclude that NGC
5824 appears to be a classical Oosterhoff Type II globular cluster, without
overt signs of being a remnant of a now-disrupted dwarf galaxy.Comment: 26 pages, 15 figures, 4 tables, accepted for publication in
Astronomical Journa
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