Alan W. McConnachie

The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action*

We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of the CFHT and INT telescopes. The surveyed area covers the inner 50kpc of the galaxy and the Southern quadrant out to a projected distance of ∼ 150kpc. A survey extension to M33 at > 200kpc probes the interface between the halos of these two galaxies. This survey is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of largescale structures of low surface brightness, including several streams. Significant variations in stellar populations due to intervening stream-like structures are detected in the inner halo along the minor axis. This, together with the fact that the light profile between 0 ◦ .5 < R < 1 ◦ .3 follows the exponential “extended disk”, is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Two new relatively luminous (MV ∼ −9) dwarf galaxies And XV and XVI are found in the study; And XVI is a particularly interesting specimen being located 270kpc in front of M31, towards the Milky Way. Underlying the many substructures that we have uncovered lies a faint, smooth and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. This is consistent with recent claims based on spectroscopy of a small sample of stars. We find that the smooth halo component in M31 has a radially-decreasing profile that can be fit with a Hernquist model of immense scale radius ∼ 55kpc, almost a factor

GALAXY PAIRS IN THE SLOAN DIGITAL SKY SURVEY. I. STAR FORMATION, ACTIVE GALACTIC NUCLEUS FRACTION, AND THE LUMINOSITY/MASS-METALLICITY RELATION

We present a sample of 1716 galaxies with companions within Δv <500 km s–1, rp < 80 h–1 70 kpc and stellar mass ratio 0.1 < M 1/M 2 < 10 from the Sloan Digital Sky Survey Data Release 4. The galaxy pairs are selected from the Main Galaxy Sample using stringent and well-understood criteria for redshift, spectral quality, available stellar masses, and metallicities. In agreement with previous studies, we find an enhancement in the star-formation rate (SFR) of galaxy pairs at projected separations <30-40 h–1 70 kpc. In addition, we find that this enhancement is highest (and extends to the greatest separations) for galaxies of approximately equal mass, the so-called major pairs. However, SFR enhancement can still be detected for a sample of galaxy pairs whose masses are within a factor of 10 of each other. Based on these results, we define a sample of close pairs (Δv <500 km s–1, rp < 30 h–1 70 kpc, and 0.1 < M 1/M 2 < 10) which we use to investigate interaction-induced effects in the luminosity-metallicity (LZ) relation. In agreement with the one previous study of the LZ relation in paired galaxies, we find an offset to lower metallicities (by ~0.1 dex) for a given luminosity for galaxies in pairs compared to the control sample. We also present the first mass-metallicity (MZ) relation comparison between paired galaxies and the field and again find an offset to lower metallicities (by ~0.05 dex) for a given mass. The smaller offset in the MZ relation indicates that both higher luminosities and lower metallicities may contribute to the shift of pairs relative to the control in the LZ relation. We show that the offset in the LZ relation depends on galaxy half-light radius, rh . Galaxies with rh 3 h–1 70 kpc and with a close companion show a 0.05-0.1 dex downward offset in metallicity compared to control galaxies of the same size. Larger galaxies do not show this offset and have LZ and MZ relations consistent with the control sample. We investigate the physical impetus behind this empirical dependence on rh and consider the galaxys dynamical time and bulge fractions as possible causes. We conclude that the former is unlikely to be a fundamental driver of the offset in the LZ relation for paired galaxies, but that bulge fraction may play a role. Finally, we study the active galactic nucleus (AGN) fraction in both the pair and control sample and find that whilst selecting galaxies in different cuts of color and asymmetry yields different AGN fractions, the fraction for pairs and the control sample are consistent for a given set of selection criteria. This indicates that if AGNs are ignited as a result of interactions, this activity begins later than the close pairs stage (i.e. once the merger is complete).

The Cold Dark Matter Halos of Local Group Dwarf Spheroidals

We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically motivated model in which dSphs are approximated by a King model embedded in an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius decrease monotonically. Remarkably, we find that the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius. Of all parameters, the core radius is the least affected: after losing 99% of the stars, -->Rc decreases by just a factor of ~2. Interestingly, tides tend to make dSphs more dark matter dominated because the tightly bound central dark matter cusp is more resilient to disruption than the cored King profile. We examine whether the extremely large mass-to-light ratios of the newly discovered ultrafaint dSphs might have been caused by tidal stripping of once brighter systems. Although dSph tidal evolutionary tracks parallel the observed scaling relations in the luminosity-radius plane, they predict too steep a change in velocity dispersion compared with the observational estimates hitherto reported in the literature. The ultrafaint dwarfs are thus unlikely to be the tidal remnants of systems like Fornax, Draco, or Sagittarius. Despite spanning four decades in luminosity, dSphs appear to inhabit halos of comparable peak circular velocity, lending support to scenarios that envision dwarf spheroidals as able to form only in halos above a certain mass threshold.

Clues to the Origin of the Mass-Metallicity Relation: Dependence on Star Formation Rate and Galaxy Size

We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey Data Release 4 to study the systematic effects of specific star formation rate (SSFR) and galaxy size (as measured by the half-light radius, -->rh) on the mass-metallicity relation. We find that galaxies with high SSFR or large -->rh for their stellar mass have systematically lower gas-phase metallicities (by up to 0.2 dex) than galaxies with low SSFR or small -->rh. We discuss possible origins for these dependencies, including galactic winds/outflows, abundance gradients, environment, and star formation rate efficiencies.

Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy

We present the discovery of three faint dwarf galaxies and a globular cluster in the halo of the Andromeda galaxy (M31), found in our MegaCam survey that spans the southern quadrant of M31, from a projected distance of ~50 kpc to \~150 kpc. Though the survey covers 57 sq. degrees, the four satellites lie within 2\deg of one another. We estimate that the globular cluster lies at a total distance of 175+/-55 kpc from M31, making it the farthest M31 globular cluster known. It also shows the typical characteristics of a bright globular cluster, with a half-light radius of 2.3+/-0.2 pc and an absolute magnitude in the V band of M_{V,0}=-8.5+/-0.3 and contains a very old population with a metallicity of [Fe/H]~-1.3. The three dwarf galaxies are all very faint, with absolute magnitudes in the range -7.3<M_{V,0}<-6.4, and show strikingly similar characteristics with metallicities of [Fe/H]~-1.4 and half-light radii of \~120+/-45 pc, making these dwarf galaxies two to three times smaller than the smallest previously known satellites of M31. We estimate them to be between 740 and 955 kpc from the Sun. Extrapolating from the coverage of the survey, we estimate that up to 45+/-20 satellites brighter than M_V~-6.5 should be orbiting M31. Hence faint dwarf galaxies cannot alone account for the missing satellites that are predicted by

A Kinematically Selected, Metal-poor Stellar Halo in the Outskirts of M31

\Lambda

Taking measure of the Andromeda halo: a kinematic analysis of the giant stream surrounding M31

CDM models, unless they reside in dark matter mini-halos that are more massive than the typical masse of 10^7 solar masses currently inferred from their central radial velocity dispersion. (abridged)

A Trio of New Local Group Galaxies with Extreme Properties

We present evidence for a metal-poor, [Fe/H] ~- 1:4,σ = 0.2 dex, stellar halo component detectable at radii from 10 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 (deprojected). The stars lie in 54 spectroscopic fields spread over an 8 deg^2 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 km s^(-1) decreasing by 0.90 km s^(-1) kpc^(-1). 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 x 10^(11) M_☉ 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.

Investigating the Andromeda stream – III. A young shell system in M31

We present a spectroscopic survey of the giant stellar stream found in the halo of the Andromeda galaxy. Taken with the DEIMOS multi-object spectrograph on the Keck2 telescope, these data display a narrow velocity dispersion of 11 ± 3k m s −1 , with a steady radial velocity gradient of 245 km s −1 over the 125-kpc radial extent of the stream studied so far. This implies that the Andromeda galaxy possesses a substantial dark matter halo. We fit the orbit of the stream in different galaxy potential models. In a simple model with a composite bulge, disc and halo, where the halo follows a universal profile that is compressed by the formation of the baryonic components, we find that the kinematics of the stream require a total mass inside 125 kpc of M 125 = 7.5 +2.5

Structural properties of the M31 dwarf spheroidal galaxies

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 southwestern quadrant of M31, extending our extant survey area to include the majority of the southern hemisphere of M31s 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 rh ~ 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 MV ~ − 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 prerequisite for successfully and consistently linking these systems to the predicted cosmological dark matter substructure.