30 research outputs found
Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm
NGC 205, a close satellite of the M31 galaxy, is our nearest example of a
dwarf elliptical galaxy. Photometric and kinematic observations suggest that
NGC 205 is undergoing tidal distortion from its interaction with M31. Despite
earlier attempts, the orbit and progenitor properties of NGC 205 are not well
known. We perform an optimized search for these unknowns by combining a genetic
algorithm with restricted N-body simulations of the interaction. This approach,
coupled with photometric and kinematic observations as constraints, allows for
an effective exploration of the parameter space. We represent NGC 205 as a
static Hernquist potential with embedded massless test particles that serve as
tracers of surface brightness. We explore 3 distinct, initially stable
configurations of test particles: cold rotating disk, warm rotating disk, and
hot, pressure-supported spheroid. Each model reproduces some, but not all, of
the observed features of NGC 205, leading us to speculate that a rotating
progenitor with substantial pressure support could match all of the
observables. Furthermore, plausible combinations of mass and scale length for
the pressure-supported spheroid progenitor model reproduce the observed
velocity dispersion profile. For all 3 models, orbits that best match the
observables place the satellite 11+/-9 kpc behind M31 moving at very large
velocities: 300-500 km/s on primarily radial orbits. Given that the observed
radial component is only 54 km/s, this implies a large tangential motion for
NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not
associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC
205's velocity appears to be near or greater than its escape velocity,
signifying that the satellite is likely on its first M31 passage.Comment: 34 pages, 20 figures, accepted for publication in the Astrophysical
Journal, A pdf version with high-resolution figures may be obtained from
http://www.ucolick.org/~kirsten/ms.pd
Strangers in the night: Discovery of a dwarf spheroidal galaxy on its first Local Group infall
We present spectroscopic observations of the AndXII dwarf spheroidal galaxy
using DEIMOS/Keck-II, showing it to be moving rapidly through the Local Group
(-556 km/s heliocentric velocity, -281 km/s relative to Andromeda from the MW),
falling into the Local Group from ~115 kpc beyond Andromeda's nucleus. AndXII
therefore represents a dwarf galaxy plausibly falling into the Local Group for
the first time, and never having experienced a dense galactic environment. From
Green Bank Telescope observations, a limit on the H{I} gas mass of <3000 Msun
suggests that AndXII's gas could have been removed prior to experiencing the
tides of the Local Group galaxies. Orbit models suggest the dwarf is close to
the escape velocity of M31 for published mass models. AndXII is our best direct
evidence for the late infall of satellite galaxies, a prediction of
cosmological simulations.Comment: 4 pages 5 figures 1 table, accepted in ApJ, july issu
A kinematically selected, metal-poor stellar halo in the outskirts of M31
We present evidence for a metal-poor, [Fe/H] =0.2 dex,
stellar halo component detectable at radii from 10 kpc to 70 kpc, in our
nearest giant spiral neighbor, the Andromeda galaxy. This metal-poor sample
underlies the recently-discovered extended rotating component, and has no
detected metallicity gradient. This discovery uses a large sample of 9861
radial velocities of Red Giant Branch (RGB) stars obtained with the Keck-II
telescope and DEIMOS spectrograph, with 827 stars with robust radial velocity
measurements isolated kinematically to lie in the halo component primarily by
windowing out the extended rotating component which dominates the photometric
profile of Andromeda out to 50 kpc (de-projected). The stars lie in 54
spectroscopic fields spread over an 8 square degree region, and are expected to
fairly sample the halo to a radius of 70 kpc. The halo sample shows no
significant evidence for rotation. Fitting a simple model in which the velocity
dispersion of the component decreases with radius, we find a central velocity
dispersion of 152\kms decreasing by -0.90\kms/\kpc. By fitting a
cosmologically-motivated NFW halo model to the halo stars we constrain the
virial mass of M31 to be greater than 9.0 \times 10^{11} \msun with 99%
confidence. The properties of this halo component are very similar to that
found in our Milky Way, revealing that these roughly equal mass galaxies may
have led similar accretion and evolutionary paths in the early Universe.Comment: 13 pages, 12 figures, accepted in ApJ. substantially revised versio
The Flattened Dark Matter Halo of M31 as Deduced from the Observed HI Scale Heights
In this paper, we use the outer-galactic HI scale height data as well as the
observed rotation curve as constraints to determine the halo density
distribution of the Andromeda galaxy (M31). We model the galaxy as a
gravitationally-coupled system of stars and gas, responding to the external
force-field of a known Hernquist bulge and the dark matter halo, the density
profile of the latter being characterized by four free parameters. The
parameter space of the halo is optimized so as to match the observed HI
thickness distribution as well as the rotation curve on an equal footing,
unlike the previous studies of M31 which were based on rotation curves alone.
We show that an oblate halo, with an isothermal density profile, provides the
best fit to the observed data. This gives a central density of 0.011 M_sun
/pc^3, a core radius of 21 kpc, and an axis ratio of 0.4. The main result from
this work is the flattened dark matter halo for M31, which is required to match
the outer galactic HI scale height data. Interestingly, such flattened halos
lie at the most oblate end of the distribution of halo shapes found in recent
cosmological simulations.Comment: 21 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure Along the Southeastern Minor Axis and its Relationship to the Giant Southern Stream
We present the discovery of a kinematically-cold stellar population along the
SE minor axis of the Andromeda galaxy (M31) that is likely the forward
continuation of M31's giant southern stream. This discovery was made in the
course of an on-going spectroscopic survey of red giant branch (RGB) stars in
M31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellar
kinematics are investigated in eight fields located 9-30 kpc from M31's center
(in projection). A likelihood method based on photometric and spectroscopic
diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky
Way dwarf stars: for the first time, this is done without using radial velocity
as a selection criterion, allowing an unbiased study of M31's stellar
kinematics. The radial velocity distribution of the 1013 M31 RGB stars shows
evidence for the presence of two components. The broad (hot) component has a
velocity dispersion of 129 km/s and presumably represents M31's virialized
spheroid. A significant fraction (19%) of the population is in a narrow (cold)
component centered near M31's systemic velocity with a velocity dispersion that
decreases with increasing radial distance, from 55.5 km/s at R_proj=12 kpc to
10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the cold
component matches that of the "Southeast shelf" predicted by the Fardal et al.
(2007) orbital model of the progenitor of the giant southern stream. The
metallicity distribution of the cold component matches that of the giant
southern stream, but is about 0.2 dex more metal rich on average than that of
the hot spheroidal component. We discuss the implications of our discovery on
the interpretation of the intermediate-age spheroid population found in this
region in recent ultra-deep HST imaging studies.Comment: 23 pages, 16 figures, 2 tables, accepted for publication in the
Astrophysical Journal. Changes from previous version: expanded discussion in
sections 4.2 and 7.2, removal of section 7.1.4 and associated figure
(discussion moved to section 7.1.2
The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies
[Abridged]We compare both the Milky Way and M31 galaxies to local external
disk galaxies within the same mass range, using their relative locations in the
planes formed by V_flat versus M_K, j_disk, and the average Fe abundance of
stars in the galaxy outskirts. We find, for all relationships, that the MW is
systematically offset by ~ 1 sigma, showing a significant deficiency in stellar
mass, in angular momentum, in disk radius and [Fe/H] in the stars in its
outskirts at a given V_flat. On the basis of their location in the M_K, V_flat,
and R_d volume, the fraction of spirals like the MW is 7+/-1%, while M31
appears to be a "typical'' spiral. Our Galaxy appears to have escaped any
significant merger over the last ~10 Gyrs which may explain why it is deficient
by a factor 2 to 3 in stellar mass, angular momentum and outskirts metallicity
and then, unrepresentative of the typical spiral. As with M31, most local
spirals show evidence for a history shaped mainly by relatively recent merging.
We conclude that the standard scenario of secular evolution is generally unable
to reproduce the properties of most (if not all) spiral galaxies. However, the
so-called "spiral rebuilding'' scenario proposed by Hammer et al. 2005 is
consistent with the properties of both distant galaxies and of their
descendants - the local spirals.Comment: 14 pages, 6 figures, to appear in Ap
The Hubble flow around the Local Group
We use updated data on distances and velocities of galaxies in the proximity
of the Local Group (LG) in order to establish properties of the local Hubble
flow. For 30 neighbouring galaxies with distances 0.7 < D_LG < 3.0 Mpc, the
Local flow is characterized by the Hubble parameter H_loc = (78+/-2)
km/(s*Mpc), the mean-square peculiar velocity sigma_v = 25 km/s, corrected for
errors of radial velocity measurements (~4 km/s) and distance measurements (~10
km/s), as well as the radius of the zero-velocity surface R_0 = (0.96+/-0.03)
Mpc. The minimum value for sigma_v is achieved when the barycenter of the LG is
located at the distance D_c = (0.55+/-0.05) D_M31 towards M31 corresponding to
the Milky Way-to-M31 mass ratio M_MW / M_M31 ~ 4/5. In the reference frame of
the 30 galaxies at 0.7 - 3.0 Mpc, the LG barycenter has a small peculiar
velocity ~(24+/-4) km/s towards the Sculptor constellation. The derived value
of R_0 corresponds to the total mass M_T(LG) = (1.9+/-0.2) 10^12 M_sun with
Omega_m = 0.24 and a topologically flat universe, a value in good agreement
with the sum of virial mass estimates for the Milky Way and M31.Comment: 14 pages, 6 figures, 1 table. Accepted for publication in MNRA
Searching for dark matter in X-rays: how to check the dark matter origin of a spectral feature
A signal from decaying dark matter (DM) can be unambiguously distinguished
from spectral features of astrophysical or instrumental origin by studying its
spatial distribution. We demonstrate this approach by examining the recent
claim of 0912.0552 regarding the possible DM origin of the 2.5 keV line in
Chandra observations of the Milky Way satellite known as Willman 1. Our
conservative strategy is to adopt a relatively large dark mass for Willman 1
and relatively small dark masses for the comparison objects. We analyze
archival observations by XMM-Newton of M31 and Fornax dwarf spheroidal galaxy
(dSph) and Chandra observations of Sculptor dSph. By performing a conservative
analysis of X-ray spectra, we show the absence of a DM decay line with
parameters consistent with those of 0912.0552. For M31, the observations of the
regions between 10 and 20 kpc from the center, where the uncertainties in the
DM distribution are minimal, make a strong exclusion at the level above
10sigma. The minimal estimate for the amount of DM in the central 40 kpc of M31
is provided by the model of 0912.4133, assuming the stellar disk's mass to
light ratio ~8 and almost constant DM density within a core of 28 kpc. Even in
this case one gets an exclusion at 5.7sigma from central region of M31 whereas
modeling all processed data from M31 and Fornax produces more than 14sigma
exclusion. Therefore, despite possible systematic uncertainties, we exclude the
possibility that the spectral feature at ~2.5 keV found in 0912.0552 is a DM
decay line. We conclude, however, that the search for DM decay line, although
demanding prolonged observations of well-studied dSphs, M31 outskirts and other
similar objects, is rather promising, as the nature of a possible signal can be
checked. An (expected) non-observation of a DM decay signal in the planned
observations of Willman 1 should not discourage further dedicated observations.Comment: 16 pages, 8 figures; journal version; analysis of additional data
from M31 outskirts and comments on arXiv:1001.4055 are adde
The kinematic identification of a thick stellar disc in M31
We present the first characterization of a thick disc component in the
Andromeda galaxy (M31) using kinematic data from the DEIMOS multi-object
spectrograph instrument on Keck II. Using 21 fields in the South West of the
galaxy, we measure the lag of this component with respect to the thin disc, as
well as the dispersion, metallicity and scale length of the component. We find
an average lag between the two components of =46.0+/-3.9km/s. The velocity
dispersion of the thick disc is sigma_{thick}=50.8+/-1.9km/s, greater than the
value of dispersion we determine for the thin disc,
sigma_{thin}=35.7+/-1.0km/s. The thick disc is more metal poor than the thin
disc, with [Fe/H]_{spec}=-1.0+/-0.1 compared to [Fe/H]_{spec}=-0.7+/-0.05 for
the thin disc. We measure a radial scale length of the thin and thick discs of
h_r=7.3+/-1.0 kpc and h_r=8.0+/-1.2 kpc. From this, we infer scale heights for
both discs of 1.1+/-0.2 kpc and 2.8+/-0.6 kpc, both of which are ~2--3 times
larger than those observed in the Milky Way. We estimate a mass range for the
thick disc component of 2.4x10^{10}Msun< M_{*,thick} <4.1x10^{10}Msun. This
value provides a useful constraint on possible formation mechanisms, as any
proposed method for forming a thick disc must be able to heat (or deposit) at
least this amount of material.Comment: 22 pages, 17 figures. Minor revisions made to text following referee
report. Accepted for publication in MNRA
Geological controls on the geometry of incised-valley fills: Insights from a global dataset of late-Quaternary examples
Incised valleys that develop due to relative sea-level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised-valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database-driven statistical analysis of 151 late-Quaternary incised-valley fills has been performed, the aim being to investigate the geological controls on their geometry.
Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised-valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised-valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised-valley fills, likely through effects on relative sea-level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley-fill geometry is positively correlated with the associated drainage-basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley-fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley-margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised-valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley-fill thickness and present-day coastal-prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves.
These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of source-to-sink sediment route-ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs