Michelle M. Collins

THE LARGE-SCALE STRUCTURE OF THE HALO OF THE ANDROMEDA GALAXY. I. GLOBAL STELLAR DENSITY, MORPHOLOGY AND METALLICITY PROPERTIES*

We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite the presence of copious substructures, the global halo populations follow closely power-law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component (defined as the population that cannot be resolved into spatially distinct substructures with PAndAS). Fitting a three-dimensional halo model reveals that the most metal-poor populations () are distributed approximately spherically (slightly prolate with ellipticity c/a = 1.09 ± 0.03), with only a relatively small fraction residing in discernible stream-like structures (f stream = 42%). The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams, with f stream becoming as high as 86% for . The space density of the smooth metal-poor component has a global power-law slope of γ = –3.08 ± 0.07, and a non-parametric fit shows that the slope remains nearly constant from 30 kpc to ~300 kpc. The total stellar mass in the halo at distances beyond 2° is ~1.1 × 1010 M ☉, while that of the smooth component is ~3 × 109 M ☉. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly ~8 × 109 M ☉. We detect a substantial metallicity gradient, which declines from [Fe/H] = –0.7 at R = 30 kpc to [Fe/H] = –1.5 at R = 150 kpc for the full sample, with the smooth halo being ~0.2 dex more metal poor than the full sample at each radius. While qualitatively in line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.

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 cannot be found to fit all of the observations well. Further, the best-fit dark matter density profile measured solely for the Milky Way dSphs is marginally discrepant with that of the Andromeda dSphs (at just beyond the 1? level), 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 ( 600 pc) and low-velocity dispersions (? v < 5 km s?1), are omitted from the sample. We argue that the unusual properties of these outliers are likely caused by tidal interactions with the host galaxy.

A tidally distorted dwarf galaxy near NGC 4449

NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of −18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8 megaparsecs (ref. 3). It is wreathed in an extraordinary neutral hydrogen (H i) complex, which includes rings, shells and a counter-rotating core, spanning ∼90 kiloparsecs (kpc; refs 1, 4). NGC 4449 is relatively isolated, although an interaction with its nearest known companion—the galaxy DDO 125, some 40 kpc to the south—has been proposed as being responsible for the complexity of its H i structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of −13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxy’s centre.

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII and And XIII

We present the first spectroscopic analysis of the faint M31 satellite galaxies, And XI and And XIII, as well as a re-analysis of existing spectroscopic data for two further faint companions, And IX (correcting for an error in earlier geometric modelling that caused a misclassification of member stars in previous work) and And XII. By combining data obtained using the Deep Imaging Multi-Object Spectrograph (DEIMOS) mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have identified the most probable members for each of the satellites based on their radial velocities (precise to several km s^(-1) down to i ~22), distance from the centre of the dwarf spheroidal galaxies (dSphs) and their photometric [Fe/H]. Using both the photometric and spectroscopic data, we have also calculated global properties for the dwarfs, such as systemic velocities, metallicities and half-light radii. We find each dwarf to be very metal poor ([Fe/H]~−2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity–metallicity relationship established with brighter dwarfs. We are unable to resolve dispersion for And XI due to small sample size and low signal-to-noise ratio, but we set a 1σ upper limit of σ_v < 4.5 km s^(−1). For And IX, And XII and And XIII we resolve velocity dispersions of σ_v= 4.5^(+3.6)_(−3.4), 2.6^(+5.1)_(−2.6) and 9.7^(+8.9)_(−4.5) km s^(−1), though we note that the dispersion for And XIII is based on just three stars. We derive masses within the half-light radii for these galaxies of 6.2^(+5.3)_(−5.1)× 10^6, 2.4^(+6.5)_(−2.4)× 10^6 and 1.1^(+1.4)_(−0.7)× 10_7 M_⊙, respectively. We discuss each satellite in the context of the Mateo relations for dSphs, and in reference to the universal halo profiles established for Milky Way dwarfs. Both And IX and And XII fall below the universal halo profiles of Walker et al., indicating that they are less massive than would be expected for objects of their half-light radius. When combined with the findings of McConnachie & Irwin, which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit haloes that with regard to their central densities are significantly different from those of the Milky Way.

A Keck/DEIMOS spectroscopic survey of the faint M 31 satellites And XV and And XVI

We present the results of a spectroscopic survey of the recently discovered M31 satellites And XV and And XVI, lying at projected distances from the centre of M31 of 93 and 130 kpc, respectively. These satellites lie to the south of M31, in regions of the stellar halo which wide-field imaging has revealed as relative voids (compared to the ~degree-scale coherent stream-like structures). Using the Deep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ~6 km s^(−1) down to i ~ 21.5. While the distance to And XVI remains the same as previously reported (525 ± 50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch in And XV are in fact Galactic, and And XV is actually likely to be much more distant at 770 ± 70 kpc (compared to the previous 630 kpc), increasing the luminosity from M_V ≈ −9.4 to −9.8. The And XV velocity dispersion is resolved with v_r = −339^(+7)_(−6) km s^(−1) and σ_v = 11^(+7)_(−5) km s^(−1). The And XVI dispersion is not quite resolved at 1σ with v_r = −385^(+5)_(−6) km s^(−1) and σ = 0^(+10)_(−indef) km s^(−1) . Using the photometry of the confirmed member stars, we find metallicities of And XV (median [Fe/H] = −1.58 , interquartile range ±0.08) and And XVI (median [Fe/H] = −2.23, interquartile range ±0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H] = −1.8 (−2.1) for And XV (And XVI), with an uncertainty of ~0.2 dex in both cases. Our measurements of And XV reasonably resolve its mass (~10^8 M_⊙) and suggest a polar orbit, while the velocity of And XVI suggests it is approaching the M31 escape velocity given its large M31 centric distance.

The scatter about the ‘Universal’ dwarf spheroidal mass profile: a kinematic study of the M31 satellites And V and And VI

While the satellites of the Milky Way (MW) have been shown to be largely consistent in terms of their mass contained within one half-light radius (M_half) with a ‘universal’ mass profile, a number of M31 satellites are found to be inconsistent with these relations, and seem kinematically colder in their central regions than their MW cousins. In this work, we present new kinematic and updated structural properties for two M31 dwarf spheroidals (dSph), And V and And VI, using data from the Keck Low Resolution Imaging Spectrograph (LRIS) and the DEep Imaging Multi-Object Spectrograph (DEIMOS) instruments and the Subaru Suprime-Cam imager. We measure systemic velocities of v_r=−393.1 ± 4.2 and −344.8 ± 2.5 km s^(−1), and dispersions of σ_v= 11.5^(+5.3)_(−4.4) and 9.4^(+3.2)_(−2.4) km s^(−1) for And V and And VI, respectively, meaning these two objects are consistent with the trends in σv and r_half set by their MW counterparts. We also investigate the nature of this scatter about the MW dSph mass profiles for the ‘classical’ (i.e. M_V < −8) MW and M31 dSph. When comparing both the ‘classical’ MW and M31 dSph to the best-fitting mass profiles in the size–velocity dispersion plane, we find general scatter in both the positive (i.e. hotter) and negative (i.e. colder) directions from these profiles. However, barring one exception (CVnI) only the M31 dSph are found to scatter towards a colder regime, and, excepting the And I dSph, only MW objects scatter to hotter dispersions. The scatter for the combined population is greater than expected from measurement errors alone. We assess this divide in the context of the differing disc-to-halo mass (i.e. stars and baryons to total virial mass) ratios of the two hosts and argue that the underlying mass profiles for dSph differ from galaxy to galaxy, and are modified by the baryonic component of the host.

THRESHING IN ACTION: THE TIDAL DISRUPTION OF A DWARF GALAXY BY THE HYDRA I CLUSTER ∗

We report on the discovery of strong tidal features around a dwarf spheroidal galaxy in the Hydra I galaxy cluster, indicating its ongoing tidal disruption. This very low surface brightness object, HCC-087, was originally classified as an early-type dwarf in the Hydra Cluster Catalogue (HCC), but our re-analysis of the ESO-VLT/FORS images of the HCC unearthed a clear indication of an S-shaped morphology and a large spatial extent. Its shape, luminosity (MV = –11.6 mag), and physical size (at a half-light radius of 3.1 kpc and a full length of ~5.9 kpc) are comparable to the recently discovered NGC 4449B and the Sagittarius dwarf spheroidal, all of which are undergoing clear tidal disruption. Aided by N-body simulations we argue that HCC-087 is currently at its first apocenter, at 150 kpc, around the cluster center and that it is being tidally disrupted by the galaxy clusters potential itself. An interaction with the nearby (50 kpc) S0 cluster galaxy HCC-005, at M * ~ 3 × 1010 M ☉ is rather unlikely, as this constellation requires a significant amount of dynamical friction and thus low relative velocities. The S-shaped morphology and large spatial extent of the satellite would, however, also appear if HCC-087 would orbit the cluster center. These features appear to be characteristic properties of satellites that are seen in the process of being tidally disrupted, independent of the environment of the destruction. An important finding of our simulations is an orientation of the tidal tails perpendicular to the orbit.

Accretion in action: Phase space coherence of stellar debris and globular clusters in andromeda's south-west cloud

A central tenet of the current cosmological paradigm is that galaxies grow over time through the accretion of smaller systems. Here, we present new kinematic measurements near the centre of one of the densest pronounced substructures, the South-West Cloud, in the outer halo of our nearest giant neighbour, the Andromeda galaxy. These observations reveal that the kinematic properties of this region of the South-West Cloud are consistent with those of PA-8, a globular cluster previously shown to be co-spatial with the stellar substructure. In this sense the situation is reminiscent of the handful of globular clusters that sit near the heart of the Sagittarius dwarf galaxy, a system that is currently being accreted into the Milky Way, confirming that accretion deposits not only stars but also globular clusters into the halos of large galaxies.

Spectroscopy of the Three Distant Andromedan Satellites Cassiopeia III, Lacerta I, and Perseus I

We present Keck II/DEIMOS spectroscopy of the three distant dwarf galaxies of M31 Lacerta I, Cassiopeia III, and Perseus I, recently discovered within the Pan-STARRS1 3\pi imaging survey. The systemic velocities of the three systems (v_{r,helio} = -198.4 +/- 1.1 km/s, -371.6 +/- 0.7 km/s, and -326 +/- 3 km/s, respectively) confirm that they are satellites of M31. In the case of Lacerta I and Cassiopeia III, the high quality of the data obtained for 126 and 212 member stars, respectively, yields reliable constraints on their global velocity dispersions (\sigma_{vr} = 10.3 +/- 0.9 km/s and 8.4 +/- 0.6 km/s, respectively), leading to dynamical-mass estimates for both of ~4x10^7 Msun within their half-light radius. These translate to V-band mass-to-light ratios of 15^{+12}_{-9} and 8^{+9}_{-5} in solar units. We also use our spectroscopic data to determine the average metallicity of the 3 dwarf galaxies ([Fe/H] = -2.0 +/- 0.1, -1.7 +/- 0.1, and -2.0 +/- 0.2, respectively). All these properties are typical of dwarf galaxy satellites of Andromeda with their luminosity and size.

Expression patterns of hormones, signaling molecules, and transcription factors during adenohypophysis development in the chick embryo

The chick embryo is an ideal model to study pituitary cell‐type differentiation. Previous studies describing the temporal appearance of differentiated pituitary cell types in the chick embryo are contradictory. To resolve these controversies, we used RT‐PCR to define the temporal onset and in situ hybridization and immunohistochemistry to define the spatial localization of hormone expression within the pituitary. RT‐PCR detected low levels of Fshβ (gonadotropes) and Pomc (corticotropes, melanotropes) mRNA at E4 and Gh (somatotropes), Prl (lactotropes), and Tshβ (thyrotropes) mRNA at E8. For all hormones, sufficient accumulation of mRNA and/or protein to permit detection by in situ hybridization or immunohistochemistry was observed ∼3 days later and in all cases corresponded to a notable increase in RT‐PCR product. We also describe the expression patterns of signaling (Bmp2, Bmp4, Fgf8, Fgf10, Shh) and transcription factors (Pitx1, Pitx2, cLim3) known to be important for pituitary organogenesis in other model organisms. Developmental Dynamics 239:1197–1210, 2010.