42 research outputs found
The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun
We study the future orbital evolution and merging of the MW-M31-M33 system,
using a combination of collisionless N-body simulations and semi-analytic orbit
integrations. Monte-Carlo simulations are used to explore the consequences of
varying the initial phase-space and mass parameters within their observational
uncertainties. The observed M31 transverse velocity implies that the MW and M31
will merge t = 5.86 (+1.61-0.72) Gyr from now, after a first pericenter at t =
3.87 (+0.42-0.32) Gyr. M31 may (probability p=41%) make a direct hit with the
MW (defined here as a first-pericenter distance less than 25 kpc). Most likely,
the MW and M31 will merge first, with M33 settling onto an orbit around them.
Alternatively, M33 may make a direct hit with the MW first (p=9%), or M33 may
get ejected from the Local Group (p=7%). The MW-M31 merger remnant will
resemble an elliptical galaxy. The Sun will most likely (p=85%) end up at
larger radius from the center of the MW-M31 merger remnant than its current
distance from the MW center, possibly further than 50 kpc (p=10%). The Sun may
(p=20%) at some time in the next 10 Gyr find itself moving through M33 (within
10 kpc), but while dynamically still bound to the MW-M31 merger remnant. The
arrival and possible collision of M31 (and possibly M33) with the MW is the
next major cosmic event affecting the environment of our Sun and solar system
that can be predicted with some certainty. (Abridged)Comment: 58 pages, 16 figures, to be published in ApJ. Version with high
resolution figures and N-body movies available at
http://www.stsci.edu/~marel/M31 . Press materials, graphics, and
visualizations available at
http://hubblesite.org/newscenter/archive/releases/2012/2
UV Properties of Galactic Globular Clusters with GALEX II. Integrated colors
We present ultraviolet (UV) integrated colors of 44 Galactic globular
clusters (GGCs) observed with the Galaxy Evolution Explorer (GALEX) in both FUV
and NUV bands. This data-base is the largest homogeneous catalog of UV colors
ever published for stellar systems in our Galaxy. The proximity of GGCs makes
it possible to resolve many individual stars even with the somewhat low spatial
resolution of GALEX. This allows us to determine how the integrated UV colors
are driven by hot stellar populations, primarily horizontal branch stars and
their progeny. The UV colors are found to be correlated with various parameters
commonly used to define the horizontal branch morphology. We also investigate
how the UV colors vary with parameters like metallicity, age, helium abundance
and concentration. We find for the first time that GCs associated with the
Sagittarius dwarf galaxy have (FUV-V) colors systematically redder than GGCs
with the same metallicity. Finally, we speculate about the presence of an
interesting trend, suggesting that the UV color of GCs may be correlated with
the mass of the host galaxy, in the sense that more massive galaxies possess
bluer clusters.Comment: Accepted for publication by The Astronomical Journal. 36 pages, 9
figures, 1 tabl
Reddening, distance, and stellar content of the young open cluster Westerlund 2
We present deep UBVIC photometric data of the young open cluster Westerlund 2. An abnormal reddening law of RV, cl = 4.14 ± 0.08 was found for the highly reddened early-type members (E(B−V)≧1.45E(B−V)≧1.45), whereas a fairly normal reddening law of RV, fg = 3.33 ± 0.03 was confirmed for the foreground early-type stars (E(B − V)fg < 1.05). The distance modulus was determined from zero-age main-sequence fitting to the reddening-corrected colour–magnitude diagram of the early-type members to be V0 − MV = 13.9 ± 0.14 (random error) +0.4−0.1−0.1+0.4 (the upper limit of systematic error) mag (d=6.0±0.4+1.2−0.3d=6.0±0.4−0.3+1.2 kpc). To obtain the initial mass function, pre-main-sequence (PMS) stars were selected by identifying the optical counterparts of Chandra X-ray sources and mid-infrared emission stars from the Spitzer GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) source catalogue. The initial mass function shows a shallow slope of Γ = −1.1 ± 0.1 down to log m = 0.7. The total mass of Westerlund 2 is estimated to be at least 7 400 M⊙. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be ≲ 1.5 Myr. We confirmed the existence of a clump of PMS stars located ∼1 arcmin north of the core of Westerlund 2, but we could not find any clear evidence for an age difference between the core and the northern clump
The starburst cluster westerlund 1: The initial mass function and mass segregation
Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVIC and JKS photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral-type-intrinsic-color relation, we obtain the mean interstellar reddening of 〈E(B-V)〉 = 4.19 ± 0.23 and 〈E(J-KS )〉 = 1.70 ± 0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster - 3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0 ± 1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of Γ = -0.8 ± 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 × 104 M⊙. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low luminosity yellow hypergiants
The Proper Motion Field Along the Magellanic Bridge: a New Probe of the LMC-SMC Interaction
We present the first detailed kinematic analysis of the proper motions (PMs)
of stars in the Magellanic Bridge, from both the \textit{Gaia} Data Release 2
catalog and from \textit{Hubble Space Telescope} Advanced Camera for Surveys
data. For the \textit{Gaia} data, we identify and select two populations of
stars in the Bridge region, young main sequence (MS) and red giant stars. The
spatial locations of the stars are compared against the known H {\small I} gas
structure, finding a correlation between the MS stars and the H {\small I} gas.
In the \textit{Hubble Space Telescope} fields our signal comes mainly from an
older MS and turn-off population, and the proper motion baselines range between
and 13 years. The PMs of these different populations are found to be
consistent with each other, as well as across the two telescopes. When the
absolute motion of the Small Magellanic Cloud is subtracted out, the residual
Bridge motions display a general pattern of pointing away from the Small
Magellanic Cloud towards the Large Magellanic Cloud. We compare in detail the
kinematics of the stellar samples against numerical simulations of the
interactions between the Small and Large Magellanic Clouds, and find general
agreement between the kinematics of the observed populations and a simulation
in which the Clouds have undergone a recent direct collision.Comment: 13 pages, 10 figures, 2 tables, submitted to ApJ, accepted February
8th, 201
Nonlinear Color-Metallicity Relations of Globular Clusters. III. On the Discrepancy in Metallicity between Globular Cluster Systems and their Parent Elliptical Galaxies
One of the conundrums in extragalactic astronomy is the discrepancy in
observed metallicity distribution functions (MDFs) between the two prime
stellar components of early-type galaxies-globular clusters (GCs) and halo
field stars. This is generally taken as evidence of highly decoupled
evolutionary histories between GC systems and their parent galaxies. Here we
show, however, that new developments in linking the observed GC colors to their
intrinsic metallicities suggest nonlinear color-to-metallicity conversions,
which translate observed color distributions into strongly-peaked, unimodal
MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar
to the MDFs of resolved field stars in nearby elliptical galaxies and those
produced by chemical evolution models of galaxies. The GC MDF shape,
characterized by a sharp peak with a metal-poor tail, indicates a virtually
continuous chemical enrichment with a relatively short timescale. The
characteristic shape emerges across three orders of magnitude in the host
galaxy mass, suggesting a universal process of chemical enrichment among
various GC systems. Given that GCs are bluer than field stars within the same
galaxy, it is plausible that the chemical enrichment processes of GCs ceased
somewhat earlier than that of field stellar population, and if so, GCs
preferentially trace the major, vigorous mode of star formation events in
galactic formation. We further suggest a possible systematic age difference
among GC systems, in that the GC systems in more luminous galaxies are older.
This is consistent with the downsizing paradigm of galaxies and supports
additionally the similar nature shared by GCs and field stars. Our findings
suggest that GC systems and their parent galaxies have shared a more common
origin than previously thought, and hence greatly simplify theories of galaxy
formation.Comment: 55 pages, 7 figures, 5 tables; Accepted for publication in Ap
UV Properties of Galactic Globular Clusters with GALEX I. The Color-Magnitude Diagrams
We present GALEX data for 44 Galactic globular clusters obtained during 3
GALEX observing cycles between 2004 and 2008. This is the largest homogeneous
data set on the UV photometric properties of Galactic globular clusters ever
collected. The sample selection and photometric analysis are discussed, and
color-magnitude diagrams are presented. The blue and intermediate-blue
horizontal branch is the dominant feature of the UV color-magnitude diagrams of
old Galactic globular clusters. Our sample is large enough to display the
remarkable variety of horizontal branch shapes found in old stellar
populations. Other stellar types that are obviously detected are blue
stragglers and post core-He burning stars. The main features of UV
color-magnitude diagrams of Galactic globular clusters are briefly discussed.
We establish the locus of post-core He burning stars in the UV color-magnitude
diagram and present a catalog of candidate AGB-manqu \'e, post early-AGB, and
post-AGB stars within our cluster sample.Comment: Accepted for publication by The Astronomical Journal. 46 pages,
including 21 Figures and 3 tables. All data will be made publicly available
by the time the article is published. In the meantime, please contact the
authors for data requests. Revised version fixed error with figure numbers
and caption