22 research outputs found
Major Substructure in the M31 Outer Halo: the South-West Cloud
We undertake the first detailed analysis of the stellar population and
spatial properties of a diffuse substructure in the outer halo of M31. The
South-West Cloud lies at a projected distance of ~100 kpc from the centre of
M31, and extends for at least ~50 kpc in projection. We use Pan-Andromeda
Archaeological Survey photometry of red giant branch stars to determine a
distance to the South-West Cloud of 793 +/- 45 kpc. The metallicity of the
cloud is found to be [Fe/H] = -1.3 +/- 0.1. This is consistent with the
coincident globular clusters PAndAS-7 and PAndAS-8, which have metallicities
determined using an independent technique of [Fe/H] = -1.35 +/- 0.15. We
measure a brightness for the Cloud of M_V = -12.1 mag; this is ~75 per cent of
the luminosity implied by the luminosity-metallicity relation. Under the
assumption that the South-West Cloud is the visible remnant of an accreted
dwarf satellite, this suggests that the progenitor object was amongst M31's
brightest dwarf galaxies prior to disruption.Comment: 13 pages, 9 figures, accepted for publication in MNRA
The elusive stellar halo of the Triangulum galaxy
The stellar haloes of large galaxies represent a vital probe of the processes of galaxy evolution. They are the remnants of the initial bouts of star formation during the collapse of the protogalactic cloud, coupled with imprint of ancient and ongoing accretion events. Previously, we have reported the tentative detection of a possible, faint, extended stellar halo in the Local Group spiral, the Triangulum galaxy (M33). However, the presence of substructure surrounding M33 made interpretation of this feature difficult. Here, we employ the final data set from the Pan-Andromeda Archaeological Survey, combined with an improved calibration and a newly derived contamination model for the region to revisit this claim. With an array of new fitting algorithms, fully accounting for contamination and the substantial substructure beyond the prominent stellar disc in M33, we reanalyse the surrounds to separate the signal of the stellar halo and the outer halo substructure. Using more robust search algorithms, we do not detect a large-scale smooth stellar halo and place a limit on the maximum surface brightness of such a feature of ÎŒV = 35.5 mag arcsecâ2, or a total halo luminosity of L < 106âLâ
The elusive stellar halo of the Triangulum galaxy
The stellar halos of large galaxies represent a vital probe of the processes
of galaxy evolution. They are the remnants of the initial bouts of star
formation during the collapse of the proto-galactic cloud, coupled with imprint
of ancient and on-going accretion events. Previously, we have reported the
tentative detection of a possible, faint, extended stellar halo in the Local
Group spiral, the Triangulum Galaxy (M33). However, the presence of
substructure surrounding M33 made interpretation of this feature difficult.
Here, we employ the final data set from the Pan-Andromeda Archaeological Survey
(PAndAS), combined with an improved calibration and a newly derived
contamination model for the region to revisit this claim. With an array of new
fitting algorithms, fully accounting for contamination and the substantial
substructure beyond the prominent stellar disk in M33, we reanalyse the
surrounds to separate the signal of the stellar halo and the outer halo
substructure. Using more robust search algorithms, we do not detect a large
scale smooth stellar halo and place a limit on the maximum surface brightness
of such a feature of = 35.5 mags per square arcsec, or a total halo
luminosity of .Comment: 17 pages, 23 figures, accepted for publication in MNRA
Tracing the stellar component of low surface brightness Milky Way dwarf galaxies to their outskirts
Aims. We present results from deep and very spatially extended CTIO/DECam g and r photometry (reaching out to ~2 mag below the oldest main-sequence turn-off and covering ~20 deg2) around the Sextans dwarf spheroidal galaxy. We aim to use this dataset to study the structural properties of Sextans overall stellar population and its member stars in different evolutionary phases, as well as to search for possible signs of tidal disturbance from the Milky Way, which would indicate departure from dynamical equilibrium.
Methods. We performed the most accurate and quantitative structural analysis to-date of Sextansâ stellar components by applying Bayesian Monte Carlo Markov chain methods to the individual starsâ positions. Surface density maps are built by statistically decontaminating the sample through a matched filter analysis of the colour-magnitude diagram, and then analysed for departures from axisymmetry.
Results. Sextans is found to be significantly less spatially extended and more centrally concentrated than early studies suggested. No statistically significant distortions or signs of tidal disturbances were found down to a surface brightness limit of ~31.8 mag/arcsec2 in V-band. We identify an overdensity in the central regions that may correspond to previously reported kinematic substructure(s). In agreement with previous findings, old and metal-poor stars such as Blue Horizontal Branch stars cover a much larger area than stars in other evolutionary phases, and bright Blue Stragglers (BSs) are less spatially extended than faint ones. However, the different spatial distribution of bright and faint BSs appears consistent with the general age and metallicity gradients found in Sextansâ stellar component. This is compatible with Sextans BSs having formed by evolution of binaries and not necessarily due to the presence of a central disrupted globular cluster, as suggested in the literature. We provide structural parameters for the various populations analysed and make publicly available the photometric catalogue of point-sources as well as a catalogue of literature spectroscopic measurements with updated membership probabilities.</jats:p
Major substructure in the M31 outer halo: distances and metallicities along the giant stellar stream
We present a renewed look at M31's Giant Stellar Stream along with the nearby
structures Stream C and Stream D, exploiting a new algorithm capable of fitting
to the red giant branch (RGB) of a structure in both colour and magnitude
space. Using this algorithm, we are able to generate probability distributions
in distance, metallicity and RGB width for a series of subfields spanning these
structures. Specifically, we confirm a distance gradient of approximately 20
kpc per degree along a 6 degree extension of the Giant Stellar Stream, with the
farthest subfields from M31 lying ~ 120 kpc more distant than the inner-most
subfields. Further, we find a metallicity that steadily increases from
-0.7^{+0.1}_{-0.1} dex to -0.2^{+0.2}_{-0.1} dex along the inner half of the
stream before steadily dropping to a value of -1.0^{+0.2}_{-0.2} dex at the
farthest reaches of our coverage. The RGB width is found to increase rapidly
from 0.4^{+0.1}_{-0.1} dex to 1.1^{+0.2}_{-0.1} dex in the inner portion of the
stream before plateauing and decreasing marginally in the outer subfields of
the stream. In addition, we estimate Stream C to lie at a distance between 794
and 862 kpc and Stream D between 758 kpc and 868 kpc. We estimate the median
metallicity of Stream C to lie in the range -0.7 to -1.6 dex and a metallicity
of -1.1^{+0.3}_{-0.2} dex for Stream D. RGB widths for the two structures are
estimated to lie in the range 0.4 to 1.2 dex and 0.3 to 0.7 dex respectively.
In total, measurements are obtained for 19 subfields along the Giant Stellar
Stream, 4 along Stream C, 5 along Stream D and 3 general M31 spheroid fields
for comparison. We thus provide a higher resolution coverage of the structures
in these parameters than has previously been available in the literature.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society (accepted 29 Feb 2016). 18 pages, 7 figures, 2 table
Major substructure in the M31 Outer Halo: the East Cloud
We present the first detailed analysis of the East Cloud, a highly disrupted
diffuse stellar substructure in the outer halo of M31. The core of the
substructure lies at a projected distance of kpc from the centre of
M31 in the outer halo, with possible extensions reaching right into the inner
halo. Using Pan-Andromeda Archaeological Survey photometry of red giant branch
stars, we measure the distance, metallicity and brightness of the cloud. Using
Hubble Space Telescope data, we independently measure the distance and
metallicity to the two globular clusters coincident with the East Cloud core,
PA-57 and PA-58, and find their distances to be consistent with the cloud. Four
further globular clusters coincident with the substructure extensions are
identified as potentially associated. Combining the analyses, we determine a
distance to the cloud of kpc, a metallicity of , and a brightness of mag. Even allowing for the
inclusion of the potential extensions, this accounts for less than per
cent of the progenitor luminosity implied by the luminosity-metallicity
relation. Using the updated techniques developed for this analysis, we also
refine our estimates of the distance and brightness of the South-West Cloud, a
separate substructure analyzed in the previous work in this series.Comment: 14 pages, 11 figures, 5 table
The large-scale structure of the halo of the Andromeda galaxy II. Hierarchical structure in the Pan-Andromeda Archaeological Survey
The Pan-Andromeda Archaeological Survey is a survey of square degrees
centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided
the most extensive panorama of a galaxy group to large projected
galactocentric radii. Here, we collate and summarise the current status of our
knowledge of the substructures in the stellar halo of M31, and discuss
connections between these features. We estimate that the 13 most distinctive
substructures were produced by at least 5 different accretion events, all in
the last 3 or 4 Gyrs. We suggest that a few of the substructures furthest from
M31 may be shells from a single accretion event. We calculate the luminosities
of some prominent substructures for which previous estimates were not
available, and we estimate the stellar mass budget of the outer halo of M31. We
revisit the problem of quantifying the properties of a highly structured
dataset; specifically, we use the OPTICS clustering algorithm to quantify the
hierarchical structure of M31's stellar halo, and identify three new faint
structures. M31's halo, in projection, appears to be dominated by two
`mega-structures', that can be considered as the two most significant branches
of a merger tree produced by breaking M31's stellar halo into smaller and
smaller structures based on the stellar spatial clustering. We conclude that
OPTICS is a powerful algorithm that could be used in any astronomical
application involving the hierarchical clustering of points. The publication of
this article coincides with the public release of all PAndAS data products.Comment: Accepted for publication in the Astrophysical Journal. 51 pages, 24
figures, 5 tables. Some figures have degraded resolution. All PAndAS data
products are available via the CADC at
http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/pandas/query.html
where you can also find a version of the paper with full resolution figure
The shell game: a panoramic view of Fornax
We present a panoramic study of the Fornax dwarf spheroidal galaxy, using data obtained as part of the VLT Survey Telescope (VST) ATLAS Survey. The data presented here â a subset of the full survey â uniformly cover a region of 25 deg2 centred on the galaxy, in g, r and i bands. This large area coverage reveals two key differences to previous studies of Fornax. First, data extending beyond the nominal tidal radius of the dwarf highlight the presence of a second distinct red giant branch population. This bluer red giant branch appears to be co-eval with the horizontal branch population. Secondly, a shell structure located approximately 1 â . .â 4 from the centre of Fornax is shown to be a mis-identified background overdensity of galaxies. This last result casts further doubt on the hypothesis that Fornax underwent a gas-rich merger in its relatively recent past