arXiv subjects

Alan McConnachie

Publications and source records attributed to Alan McConnachie.

70 records · Page 4Linked to original sources

arXiv2014

Comparing M31 and Milky Way Satellites: The Extended Star Formation Histories of Andromeda II and Andromeda XVI

We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (M$_{V} = -$12.0; log(M$_{\star}$/M$_{\odot}$) $\sim$ 6.7) and Andromeda XVI (M$_{V} = -$7.5; log(M$_{\star}$/M$_{\odot}$) $\sim$ 4.9) yielding color-magnitude diagrams that extend at least 1 magnitude below the oldest main sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar SFHs: both formed 50-70% of their total stellar mass between 12.5 and 5 Gyr ago (z$\sim$5-0.5) and both were abruptly quenched $\sim$ 5 Gyr ago (z$\sim$0.5). The predominance of intermediate age populations in And XVI makes it qualitatively different from faint companions of the MW and clearly not a pre-reionization fossil. Neither And II nor And XVI appears to have a clear analog among MW companions, and the degree of similarity in the SFHs of And II and And XVI is not seen among comparably faint-luminous pairs of MW satellites. These findings provide hints that satellite galaxy evolution may vary substantially among hosts of similar stellar mass. Although comparably deep observations of more M31 satellites are needed to further explore this hypothesis, our results underline the need for caution when interpreting satellite galaxies of an individual system in a broader cosmological context.

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arXiv2013

The Orbital Ellipticity of Satellite Galaxies and the Mass of the Milky Way

We use simulations of Milky Way-sized dark matter haloes from the Aquarius Project to investigate the orbits of substructure haloes likely, according to a semi-analytic galaxy formation model, to host luminous satellites. These tend to populate the most massive subhaloes and are on more radial orbits than the majority of subhaloes found within the halo virial radius. One reason for this (mild) kinematic bias is that many low-mass subhaloes have apocentres that exceed the virial radius of the main host; they are thus excluded from subhalo samples identified within the virial boundary, reducing the number of subhalos on radial orbits. Two other factors contributing to the difference in orbital shape between dark and luminous subhaloes are their dynamical evolution after infall, which affects more markedly low-mass (dark) subhaloes, and a weak dependence of ellipticity on the redshift of first infall. The ellipticity distribution of luminous satellites exhibits little halo-to-halo scatter and it may therefore be compared fruitfully with that of Milky Way satellites. Since the latter depends sensitively on the total mass of the Milky Way we can use the predicted distribution of satellite ellipticities to place constraints on this important parameter. Using the latest estimates of position and velocity of dwarfs compiled from the literature, we find that the most likely Milky Way mass lies in the range $6 \times 10^{11} M_{\odot} < M_{200} < 3.1 \times 10^{12} M_{\odot}$, with a best fit value of $M_{200} = 1.1 \times 10^{12} M_{\odot}$. This value is consistent with Milky Way mass estimates based on dynamical tracers or the timing argument.

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arXiv2013

Spectroscopy in the Era of LSST

This report summarizes the results of the 'Spectroscopy in the Era of LSST' workshop held April 11-12, 2013 in Tucson, Arizona. At the workshop, there were breakout sessions covering four broad science topics. These were: time domain science, Galactic structure and stellar populations, galaxies and AGN, and dark energy and cosmology. We present the science cases discussed in these breakout sessions and provide a synthesis of highly desired capabilities that meet needs across all four broad topics. We also present a table that will be useful to characterize the needs of specific science cases in a format that provides a general framework for discussion of future spectroscopic capabilities.

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arXiv2013

The Next Generation Virgo Cluster Survey - Infrared (NGVS-IR): I. A new Near-UV/Optical/Near-IR Globular Cluster selection tool

The NGVS-IR project (Next Generation Virgo Survey - Infrared) is a contiguous near-infrared imaging survey of the Virgo cluster of galaxies. It complements the optical wide-field survey of Virgo (NGVS). The current state of NGVS-IR consists of Ks-band imaging of 4 deg^2 centered on M87, and J and Ks-band imaging of 16 deg^2 covering the region between M49 and M87. In this paper, we present the observations of the central 4 deg^2 centered on Virgo's core region. The data were acquired with WIRCam on the Canada-France-Hawaii Telescope and the total integration time was 41 hours distributed in 34 contiguous tiles. A survey-specific strategy was designed to account for extended galaxies while still measuring accurate sky brightness within the survey area. The average 5σlimiting magnitude is Ks=24.4 AB mag and the 50% completeness limit is Ks=23.75 AB mag for point source detections, when using only images with better than 0.7" seeing (median seeing 0.54"). Star clusters are marginally resolved in these image stacks, and Virgo galaxies with μ_Ks=24.4 AB mag arcsec^-2 are detected. Combining the Ks data with optical and ultraviolet data, we build the uiK color-color diagram which allows a very clean color-based selection of globular clusters in Virgo. This diagnostic plot will provide reliable globular cluster candidates for spectroscopic follow-up campaigns needed to continue the exploration of Virgo's photometric and kinematic sub-structures, and will help the design of future searches for globular clusters in extragalactic systems. Equipped with this powerful new tool, future NGVS-IR investigations based on the uiK diagram will address the mapping and analysis of extended structures and compact stellar systems in and around Virgo galaxies.

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arXiv2013

A peculiar faint satellite in the remote outer halo of M31

We present Hubble Space Telescope imaging of a newly-discovered faint stellar system, PAndAS-48, in the outskirts of the M31 halo. Our photometry reveals this object to be comprised of an ancient and very metal-poor stellar population with age > 10 Gyr and [Fe/H] < -2.3. Our inferred distance modulus of 24.57 +/- 0.11 confirms that PAndAS-48 is most likely a remote M31 satellite with a 3D galactocentric radius of 149 (+19 -8) kpc. We observe an apparent spread in color on the upper red giant branch that is larger than the photometric uncertainties should allow, and briefly explore the implications of this. Structurally, PAndAS-48 is diffuse, faint, and moderately flattened, with a half-light radius rh = 26 (+4 -3) pc, integrated luminosity Mv = -4.8 +/- 0.5, and ellipticity = 0.30 (+0.08 -0.15). On the size-luminosity plane it falls between the extended globular clusters seen in several nearby galaxies, and the recently-discovered faint dwarf satellites of the Milky Way; however, its characteristics do not allow us to unambiguously class it as either type of system. If PAndAS-48 is a globular cluster then it is the among the most elliptical, isolated, and metal-poor of any seen in the Local Group, extended or otherwise. Conversely, while its properties are generally consistent with those observed for the faint Milky Way dwarfs, it would be a factor ~2-3 smaller in spatial extent than any known counterpart of comparable luminosity.

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arXiv2010

The impact of dark matter cusps and cores on the satellite galaxy population around spiral galaxies

(Abridged) We use N-body simulations to study the effects that a divergent (i.e. "cuspy") dark matter (DM) profile introduces on the tidal evolution of dwarf spheroidal galaxies (dSphs). Our models assume cosmologically-motivated initial conditions where dSphs are DM-dominated systems on eccentric orbits about a host galaxy composed of a dark halo and a baryonic disc. We find that the resilience of dSphs to tidal stripping is extremely sensitive to the halo cuspiness; whereas dwarfs with a cored profile can be easily destroyed by the host disc, those with cusps always retain a bound remnant. For a given halo profile the evolution of the structural parameters as driven by tides is controlled solely by the total amount of mass lost. This information is used to construct a semi-analytic code that simulates the hierarchical build-up of spiral galaxies assuming different halo profiles and disc masses. We find that tidal encounters with discs tend to decrease the average mass of satellites at all galactocentric radii. Interestingly, satellites accreted before re-ionization (z>6), which may be singled out by anomalous metallicity patterns, survive only if haloes are cuspy. We show that the size-mass relation established from Milky Way (MW) dwarfs strongly supports the presence of cusps in the majority of these systems, as cored models systematically underestimate the masses of the known Ultra-Faint dSphs. Our models also indicate that a massive M31 disc may explain why many of its dSphs fall below the size-mass relationship derived from MW dSphs. We use our models to constrain the mass threshold below which star formation is suppressed in DM haloes, finding that luminous satellites must be accreted with masses above 10^8--10^9 M_sol in order to explain the size-mass relation observed in MW dwarfs.

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arXiv2010

Evidence for an accretion origin for the outer halo globular cluster system of M31

We use a sample of newly-discovered globular clusters from the Pan-Andromeda Archaeological Survey (PAndAS) in combination with previously-catalogued objects to map the spatial distribution of globular clusters in the M31 halo. At projected radii beyond ~30 kpc, where large coherent stellar streams are readily distinguished in the field, there is a striking correlation between these features and the positions of the globular clusters. Adopting a simple Monte Carlo approach, we test the significance of this association by computing the probability that it could be due to the chance alignment of globular clusters smoothly distributed in the M31 halo. We find the likelihood of this possibility is low, below 1%, and conclude that the observed spatial coincidence between globular clusters and multiple tidal debris streams in the outer halo of M31 reflects a genuine physical association. Our results imply that the majority of the remote globular cluster system of M31 has been assembled as a consequence of the accretion of cluster-bearing satellite galaxies. This constitutes the most direct evidence to date that the outer halo globular cluster populations in some galaxies are largely accreted.

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arXiv2008

Compact groups in theory and practice - III. Compact groups of galaxies in the Sixth Data Release of the Sloan Digital Sky Survey

We present the largest publicly available catalogue of compact groups of galaxies identified using the original selection criteria of Hickson, selected from the Sixth Data Release (DR6) of the Sloan Digital Sky Survey (SDSS). We identify 2297 compact groups down to a limiting magnitude of r = 18 (~0.24groups degree^{-2}), and 74791 compact groups down to a limiting magnitude of r = 21 (~6.7groups degree^{-2}). This represents 0.9% of all galaxies in the SDSS DR6 at these magnitude levels. Contamination due to gross photometric errors has been removed from the bright sample of groups, and we estimate it is present in the large sample at the 14% level. Spectroscopic information is available for 4131 galaxies in the bright catalogue (43% completeness), and we find that the median redshift of these groups is z_{med} = 0.09. The median line-of-sight velocity dispersion within the compact groups from the bright catalogue is sigma_{LOS} ~ 230km/s and their typical inter-galactic separations are of order 50 - 100kpc. We show that the fraction of groups with interloping galaxies identified as members is in good agreement with the predictions from our previous study of a mock galaxy catalogue, and we demonstrate how to select compact groups such that the interloper fraction is well defined and minimized. This observational dataset is ideal for large statistical studies of compact groups, the role of environment on galaxy evolution, and the effect of galaxy interactions in determining galaxy morphology.

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arXiv2008

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.

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arXiv2007

The Cold Dark Matter Halos of Local Group Dwarf Spheroidals

We examine the dynamics of stellar systems embedded within cold dark matter (CDM) halos in order to assess observational constraints on the dark matter content of Local Group dwarf spheroidals (dSphs). Our analysis shows that the total mass within the luminous radius is reasonably well constrained and approximately independent of the luminosity of the dwarf, highlighting the poor correspondence between luminosity and halo mass. This result implies that the average density of dark matter is substantially higher in physically small systems such as Draco and Sculptor than in larger systems such as Fornax. For example, our results imply that Draco formed in a halo 5 times more massive than Fornax's despite being roughly 70 times fainter. Stellar velocity dispersion profiles, sigma_p(R), provide further constraints; flat sigma_p(R) profiles imply that stars are deeply embedded within their cold dark matter halos and so quite resilient to tidal disruption. We estimate that halos would need to lose more than 90% of their original mass before tides begin affecting the kinematics of stars. We estimate that V_max is about 3 times higher than the central velocity dispersion of the stars, which alleviates significantly the CDM ``substructure crisis''. We use these results to interpret the size differences between the M31 and Milky Way (MW) dSph population. Our modeling indicates that this difference should be reflected in their kinematics, and predicts that M31 dwarfs should have velocity dispersions up to a factor of ~ 2 higher than their MW counterparts. This CDM-motivated prediction may be verified with present observational capabilities.

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arXiv2007

Deconstructing dwarf galaxies: a Suprime-Cam survey of Andromeda II

(Abridged) We present deep, sub-horizontal branch, multi-colour photometry of the Andromeda II dwarf spheroidal (And II dSph) taken with Subaru Suprime-Cam. We identify a red clump population in this galaxy, the first time this feature has been detected in a M31 dSph, which are normally characterized as having no significant intermediate age populations. We construct radial profiles for the various stellar populations and show that the horizontal branch has a nearly constant density spatial distribution out to large radius, whereas the reddest red giant branch stars are centrally concentrated in an exponential profile. We argue that these populations trace two distinct structural components in And II and show that this assumption provides a good match to the overall radial profile of this galaxy. The extended component dominates the stellar populations at large radius, whereas the exponential component dominates the inner few arcminutes. We show that the two components have very different stellar populations; the exponential component has an average age of $\sim 7 - 10$ Gyrs old, is relatively metal-rich ([Fe/H] $\sim -1$) but with a significant tail to low metallicities, and possesses a red clump. The extended component, on the other hand, is ancient ($\sim 13$ Gyrs), metal-poor ([Fe/H] $\sim -1.5$) with a narrower dispersion $σ_{\rm [Fe/H]} \simeq 0.28$, and has a well developed blue horizontal branch. The extended component contains approximately three-quarters of the light of And II and its unusual density profile is unique in Local Group dwarf galaxies. This suggests that its formation and/or evolution may have been quite different to other dwarf galaxies.

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arXiv2006

On the formation of extended galactic disks by tidally disrupted dwarf galaxies

We explore the possibility that extended disks, such as that recently discovered in M31, are the result of a single dwarf (10^9--10^10 Msun) satellite merger. We carry out N-body simulations of dwarf NFW halos with embedded spheriodal stellar components on co-planar, prograde orbits in a M31-like host galaxy. As the orbit decays due to dynamical friction and the system is disrupted, the stellar particles form an extended, exponential disk-like structure that spans the radial range 30--200 kpc once they have relaxed. The disk scale-length correlates with the initial extent of the stellar component within the satellite halo: the more embedded the stars, the smaller the resulting disk scale-length. If the progenitors start on circular orbits, the kinematics of the stars that make up the extended disk have an average rotational motion that is 30--50 km/s lower than the host's circular velocity. For dwarf galaxies moving on highly eccentric orbits (e=0.7), the stellar debris exhibits a much lower rotational velocity. Our results imply that extended galactic disks might be a generic feature of the hierarchical formation of spiral galaxies such as M31 and the Milky Way.

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arXiv2005

Structural parameters for the M31 dwarf spheroidals

(Abridged) The projected structures and integrated properties of the Andromeda I, II, III, V, VI, VII and Cetus dwarf spheroidal galaxies are analysed based upon resolved counts of red giant branch stars. For each object, we have derived isopleth maps, surface brightness profiles, absolute magnitudes, central surface brightnesses, and a large number of other morphological parameters. Our analysis probes to larger radius and fainter surface brightnesses than most previous studies and as a result we find that the galaxies are generally larger and brighter than has previously been recognised. In particular, the luminosity of Andromeda V is found to be consistent with the higher metallicity value which has been derived for it. Andromeda I shows strong evidence of tidal disruption and S-shaped tidal tails are clearly visible. On the other hand, Cetus does not show any evidence of tidal truncation, let alone disruption. Andromeda II shows compelling evidence of a large excess of stars at small radius and suggests that this galaxy consists of a secondary core component. Comparing the M31 dwarf spheroidal population with the Galactic population, we find that the scale radii of the M31 population are larger than those for the Galactic population by at least a factor of two, for all absolute magnitudes. We also find that the two populations are offset from one another in the central surface brightness - luminosity relation. Finally, we find that the M31 dwarf spheroidals show the same correlation with distance-from-host as shown by the Galactic population, such that dwarf spheroidals with a higher central surface brightness are found further from their host. This again suggests that environment plays a significant role in dwarf galaxy evolution.

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arXiv2005

The satellite distribution of M31

(Abridged) The spatial distribution of the Galactic satellite system plays an important role in Galactic dynamics and cosmology, where its successful reproduction is a key test of simulations of galaxy halo formation. Here, we examine its representative nature by conducting an analysis of the 3-dimensional spatial distribution of the M31 subgroup of galaxies. We begin by a discussion of distance estimates and incompleteness concerns, before revisiting the question of membership of the M31 subgroup. Comparison of the distribution of M31 and Galactic satellites relative to the galactic disks suggests that the Galactic system is probably modestly incomplete at low latitudes by ~20%. We find that the radial distribution of satellites around M31 is more extended than the Galactic subgroup; 50% of the Galactic satellites are found within ~100kpc of the Galaxy, compared to ~200kpc for M31. We search for ``ghostly streams'' of satellites around M31, in the same way others have done for the Galaxy, and find several. The lack of M31-centric kinematic data, however, means we are unable to probe whether these streams represent real physical associations. Finally, we find that the M31 satellites are asymmetrically distributed with respect to our line-of-sight to this object, so that the majority of its satellites are on its near side with respect to our line-of-sight. We quantify this result and find it to be significant at the ~3 sigma level. Until such time as a satisfactory explanation for this finding is presented, our results warn against treating the M31 subgroup as complete, unbiased and relaxed.

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arXiv2005

A Keck/DEIMOS Kinematic Study of Andromeda IX: dark matter on the smallest galactic scales

We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And-IX, which appears to be the lowest surface brightness and also the faintest galaxy (M_V = -8.3) found to date. Using Keck/DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion sigma_v = 6.8+3.0-2.0km/s, which coupled with the low luminosity implies a very high mass to V-band light ratio, M/L ~ 93+120-50msun/lsun (M/L > 17msun/lsun at 99% confidence). Unless strong tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter dominated system.

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arXiv2004

A Keck/Deimos Survey of Red Giant Branch Stars in the Outskirts of M31

We are using the DEIMOS multi-object spectrograph on the Keck II 10m telescope to conduct a spectroscopic survey of red giant branch stars in the outskirts of M31. To date, velocities have been obtained for most of the major substructures in the halo as well as at several positions in the far outer disk and inner halo. First results concerning the giant stellar stream and major axis substructures are presented here.

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