158 research outputs found
A Minor Axis Surface Brightness Profile for M31
We use data from the Isaac Newton Telescope Wide Field Camera survey of M31
to determine the surface brightness profile of M31 along the south-east minor
axis. We combine surface photometry and faint red giant branch star counts to
trace the profile from the innermost regions out to a projected radius of 4
degrees (~55 kpc) where the V-band surface brightness is 32 mag per square
arcsec; this is the first time the M31 minor axis profile has been mapped over
such a large radial distance using a single dataset. We confirm the finding by
Pritchet & van den Bergh (1994) that the minor axis profile can be described by
a single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20
kpc. Beyond this, the surface brightness profile flattens considerably and is
consistent with either a power-law of index -2.3 or an exponential of
scalelength 14 kpc. The fraction of the total M31 luminosity contained in this
component is ~2.5%. While it is tempting to associate this outer component with
a true Population II halo in M31, we find that the mean colour of the stellar
population remains approximately constant at V-i~1.6 from 0.5-3.5 degrees along
the minor axis. This result suggests that the same metal-rich stellar
population dominates both structural components.Comment: 11 pages, 3 figures, ApJ Letters in press, extremely minor
modification
Inferring the Andromeda Galaxy's mass from its giant southern stream with Bayesian simulation sampling
M31 has a giant stream of stars extending far to the south and a great deal
of other tidal debris in its halo, much of which is thought to be directly
associated with the southern stream. We model this structure by means of
Bayesian sampling of parameter space, where each sample uses an N-body
simulation of a satellite disrupting in M31's potential. We combine constraints
on stellar surface densities from the Isaac Newton Telescope survey of M31 with
kinematic data and photometric distances. This combination of data tightly
constrains the model, indicating a stellar mass at last pericentric passage of
log(M_s / Msun) = 9.5+-0.1, comparable to the LMC. Any existing remnant of the
satellite is expected to lie in the NE Shelf region beside M31's disk, at
velocities more negative than M31's disk in this region. This rules out the
prominent satellites M32 or NGC 205 as the progenitor, but an overdensity
recently discovered in M31's NE disk sits at the edge of the progenitor
locations found in the model. M31's virial mass is constrained in this model to
be log(M200) = 12.3+-0.1, alleviating the previous tension between
observational virial mass estimates and expectations from the general galactic
population and the timing argument. The techniques used in this paper, which
should be more generally applicable, are a powerful method of extracting
physical inferences from observational data on tidal debris structures.Comment: 27 pages, 10 figures. Accepted by MNRA
The masses of Local Group dwarf spheroidal galaxies: The death of the universal mass profile
We investigate the claim that all dwarf spheroidal galaxies (dSphs) reside
within halos that share a common, universal mass profile as has been derived
for dSphs of the Galaxy. By folding in kinematic information for 25 Andromeda
dSphs, more than doubling the previous sample size, we find that a singular
mass profile can not be found to fit all the observations well. Further, the
best-fit dark matter density profile measured for solely the Milky Way dSphs is
marginally discrepant (at just beyond the 1 sigma level) with that of the
Andromeda dSphs, where a profile with lower maximum circular velocity, and
hence mass, is preferred. The agreement is significantly better when three
extreme Andromeda outliers, And XIX, XXI and XXV, all of which have large
half-light radii (>600pc) and low velocity dispersions (sigma_v < 5km/s) are
omitted from the sample. We argue that the unusual properties of these outliers
are likely caused by tidal interactions with the host galaxy.Comment: ApJ in press, 16 pages, 7 figures. Updated to address referee
comment
A trio of new Local Group galaxies with extreme properties
We report on the discovery of three new dwarf galaxies in the Local Group.
These galaxies are found in new CFHT/MegaPrime g,i imaging of the south-western
quadrant of M31, extending our extant survey area to include the majority of
the southern hemisphere of M31's halo out to 150 kpc. All these galaxies have
stellar populations which appear typical of dwarf spheroidal (dSph) systems.
The first of these galaxies, Andromeda XVIII, is the most distant Local Group
dwarf discovered in recent years, at ~1.4 Mpc from the Milky Way (~ 600 kpc
from M31). The second galaxy, Andromeda XIX, a satellite of M31, is the most
extended dwarf galaxy known in the Local Group, with a half-light radius of r_h
~ 1.7 kpc. This is approximately an order of magnitude larger than the typical
half-light radius of many Milky Way dSphs, and reinforces the difference in
scale sizes seen between the Milky Way and M31 dSphs (such that the M31 dwarfs
are generally more extended than their Milky Way counterparts). The third
galaxy, Andromeda XX, is one of the faintest galaxies so far discovered in the
vicinity of M31, with an absolute magnitude of order M_V ~ -6.3. Andromeda
XVIII, XIX and XX highlight different aspects of, and raise important questions
regarding, the formation and evolution of galaxies at the extreme faint-end of
the luminosity function. These findings indicate that we have not yet sampled
the full parameter space occupied by dwarf galaxies, although this is an
essential pre-requisite for successfully and consistently linking these systems
to the predicted cosmological dark matter sub-structure.Comment: 32 pages, 7 figures (ApJ preprint format). Accepted for publication
in Ap
A kinematic study of the Andromeda dwarf spheroidal system
We present a homogeneous kinematic analysis of red giant branch stars within
18 of the 28 Andromeda dwarf spheroidal (dSph) galaxies, obtained using the
Keck I LRIS and Keck II DEIMOS spectrographs. Based on their g-i colors (taken
with the CFHT MegaCam imager), physical positions on the sky, and radial
velocities, we assign probabilities of dSph membership to each observed star.
Using this information, the velocity dispersions, central masses and central
densities of the dark matter halos are calculated for these objects, and
compared with the properties of the Milky Way dSph population. We also measure
the average metallicity ([Fe/H]) from the co-added spectra of member stars for
each M31 dSph and find that they are consistent with the trend of decreasing
[Fe/H] with luminosity observed in the Milky Way population. We find that three
of our studied M31 dSphs appear as significant outliers in terms of their
central velocity dispersion, And XIX, XXI and XXV, all of which have large
half-light radii (>700 pc) and low velocity dispersions (sigma_v<5 km/s). In
addition, And XXV has a mass-to-light ratio within its half-light radius of
just [M/L]_{half}=10.3^{+7.0}_{-6.7}, making it consistent with a simple
stellar system with no appreciable dark matter component within its 1 sigma
uncertainties. We suggest that the structure of the dark matter halos of these
outliers have been significantly altered by tides.Comment: 41 pages, 23 figures. Accepted for publication in Ap
PAndAS in the mist: The stellar and gaseous mass within the halos of M31 and M33
Large scale surveys of the prominent members of the Local Group have provided
compelling evidence for the hierarchical formation of massive galaxies,
revealing a wealth of substructure that is thought to be the debris from
ancient and on-going accretion events. In this paper, we compare two extant
surveys of the M31-M33 subgroup of galaxies; the Pan-Andromeda Archaeological
Survey (PAndAS) of the stellar structure, and a combination of observations of
the HI gaseous content, detected at 21cm. Our key finding is a marked lack of
spatial correlation between these two components on all scales, with only a few
potential overlaps between stars and gas.The paucity of spatial correlation
significantly restricts the analysis of kinematic correlations, although there
does appear to the HI kinematically associated with the Giant Stellar Stream
where it passes the disk of M31. These results demonstrate that that different
processes must significantly influence the dynamical evolution of the stellar
and HI components of substructures, such as ram pressure driving gas away from
a purely gravitational path. Detailed modelling of the offset between the
stellar and gaseous substructure will provide a determination of the properties
of the gaseous halo of M31 and M33.Comment: 11 pages, 6 figures. Accepted for publication in the Astrophysical
Journal. Figure quality reduced. High quality version available at
http://www.physics.usyd.edu.au/~gfl/Arxiv_Papers/PAndAS_Mist
The host-galaxy response to the afterglow of GRB 100901A
For Gamma-Ray Burst 100901A, we have obtained Gemini-North and Very Large
Telescope optical afterglow spectra at four epochs: one hour, one day, three
days and one week after the burst, thanks to the afterglow remaining unusually
bright at late times. Apart from a wealth of metal resonance lines, we also
detect lines arising from fine-structure levels of the ground state of Fe II,
and from metastable levels of Fe II and Ni II at the host redshift (z =
1.4084). These lines are found to vary significantly in time. The combination
of the data and modelling results shows that we detect the fall of the Ni II 4
F9/2 metastable level population, which to date has not been observed. Assuming
that the population of the excited states is due to the UV-radiation of the
afterglow, we estimate an absorber distance of a few hundred pc. This appears
to be a typical value when compared to similar studies. We detect two
intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of
the data, we do not see significant variation in the absorption lines of these
two intervening systems.Comment: 17 pages, 9 figures. Accepted by Monthly Notices of the Royal
Astronomical Society on Jan 11th 201
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