Alis J. Deason

Mismatch and misalignment: dark haloes and satellites of disc galaxies

We study the phase-space distribution of satellite galaxies associated with late-type galaxies in the GIMIC suite of simulations. GIMIC consists of resimulations of five cosmologically representative regions from the Millennium Simulation, which have higher resolution and incorporate baryonic physics. Whilst the disc of the galaxy is well aligned with the inner regions (r ∼ 0.1r200) of the dark matter halo, both in shape and angular momentum, there can be substantial misalignments at larger radii (r ∼r200). Misalignments of >45 ◦ are seen in ∼30 per cent of our sample. We find that the satellite population aligns with the shape (and angular momentum) of the outer dark matter halo. However, the alignment with the galaxy is weak owing to the mismatch between the disc and dark matter halo. Roughly 20 per cent of the satellite systems with 10 bright galaxies within r200 exhibit a polar spatial alignment with respect to the galaxy – an orientation reminiscent of the classical satellites of the Milky Way. We find that a small fraction (∼10 per cent) of satellite systems show evidence for rotational support which we attribute to group infall. There is a bias towards satellites on prograde orbits relative to the spin of the dark matter halo (and to a lesser extent with the angular momentum of the disc). This preference towards co-rotation is stronger in the inner regions of the halo where the most massive satellites accreted at relatively early times are located. We attribute the anisotropic spatial distribution and angular momentum bias of the satellites at z = 0 to their directional accretion along the major axes of the dark matter halo. The satellite galaxies have been accreted relatively recently compared to the dark matter mass and have experienced less phase-mixing and relaxation – the memory of their accretion history can remain intact to z = 0. Understanding the phase-space distribution of the z = 0 satellite population is key for studies that estimate the host halo mass from the line-of-sight velocities and projected positions of satellite galaxies. We quantify the effects of such systematics in estimates of the host halo mass from the satellite population.

Global structure and kinematics of stellar haloes in cosmological hydrodynamic simulations

We use the Galaxies–Intergalactic Medium Interaction Calculation (GIMIC) suite of cosmological hydrodynamical simulations to study the global structure and kinematics of stellar spheroids of Milky Way mass disc galaxies. Font et al. have recently demonstrated that these simulations are able to successfully reproduce the satellite luminosity functions and the metallicity and surface brightness profiles of the spheroids of the Milky Way and M31. A key to the success of the simulations is a significant contribution to the spheroid from stars that formed in situ. While the outer halo is dominated by accreted stars, stars formed in the main progenitor of the galaxy dominate at r≲ 30 kpc. In the present study, we show that this component was primarily formed in a protodisc at high redshift and was subsequently liberated from the disc by dynamical heating associated with mass accretion. As a consequence of its origin, the in situ component of the spheroid has different kinematics (namely net prograde rotation with respect to the disc) than that of the spheroid component built from the disruption of satellites. In addition, the in situ component has a flattened distribution, which is due in part to its rotation. We make comparisons with measurements of the shape and kinematics of local galaxies, including the Milky Way and M31, and stacked observations of more distant galaxies. We find that the simulated disc galaxies have spheroids of the correct shape (oblate with a median axial ratio of ∼0.6 at radii of ≲30 kpc, but note there is significant system-to-system scatter in this quantity) and that the kinematics show evidence for two components (due to in situ versus accreted), as observed. Our findings therefore add considerable weight to the importance of dissipative processes in the formation of stellar haloes and to the notion of a ‘dual stellar halo’.

The cold veil of the Milky Way stellar halo

We build a sample of distant (D > 80 kpc) stellar halo stars with measured radial velocities. Faint (20 < g < 22) candidate blue horizontal branch (BHB) stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky Survey photometry. Follow-up spectroscopy for these A-type stars was performed using the Very Large Telescope (VLT) FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument. We classify stars according to their Balmer line profiles, and find that seven are bona fide BHB stars and 31 are blue stragglers (BS). Owing to the magnitude range of our sample, even the intrinsically fainter BS stars can reach out to D ∼ 90 kpc. We complement this sample of A-type stars with intrinsically brighter, intermediate-age, asymptotic giant branch stars. A set of four distant cool carbon stars is compiled from the literature and we perform spectroscopic follow-up on a further four N-type carbon stars using the William Herschel Telescope (WHT) Intermediate dispersion Spectrograph and Imaging System (ISIS) instrument. Altogether, this provides us with the largest sample to date of individual star tracers out to r ∼ 150 kpc. We find that the radial velocity dispersion of these tracers falls rapidly at large distances and is surprisingly cold (σr ≈ 50–60 km s−1) between 100 and 150 kpc. Relating the measured radial velocities to the mass of the Milky Way requires knowledge of the (unknown) tracer density profile and anisotropy at these distances. Nonetheless, by assuming the stellar halo stars between 50 and 150 kpc have a moderate density fall-off (with power-law slope α < 5) and are on radial orbits (graphic), we infer that the mass within 150 kpc is less than 1012 M⊙ and suggest it probably lies in the range (5–10) × 1011 M⊙. We discuss the implications of such a low mass for the Milky Way.

Broken degeneracies: the rotation curve and velocity anisotropy of the Milky Way halo

We use distant blue horizontal branch stars with Galactocentric distances 16 < r < 48 kpc as kinematic tracers of the Milky Way dark halo. We model the tracer density as an oblate, power law embedded within a spherical power-law potential. Using a distribution function method, we estimate the overall power-law potential and the velocity anisotropy of the halo tracers. We measure the slope of the potential to be γ ∼ 0.4, and the overall mass within 50 kpc is ∼4 × 10 11 M� . The tracer velocity anisotropy is radially biased with β ∼ 0.5, which is in good agreement with local solar neighbourhood studies. Our results provide an accurate outer circular velocity profile for the Milky Way and suggest a relatively high-concentration dark matter halo (cvir ∼ 20).

Rotation of halo populations in the Milky Way and M31

We search for signs of rotation in the subsystems of the Milky Way and M31 that are defined by their satellite galaxies, their globular cluster populations and their Blue Horizontal Branch (BHB) stars. A set of simple distribution functions is introduced to describe anisotropic and rotating stellar populations embedded in dark haloes of approximate Navarro-Frenk-White form. The BHB stars in the Milky Way halo exhibit a dichotomy between a prograde-rotating, comparatively metal-rich component ([Fe/H] > -2) and a retrograde-rotating, comparatively metal-poor ([Fe/H] < -2) component. The prograde metal-rich population may be associated with the accretion of a massive satellite (∼10 9 M ⊙ ). The metal-poor population may characterize the primordial stellar halo and the net retrograde rotation could then reflect an underestimate in our adopted local standard of rest circular velocity ⊙ 0 . If ⊙ 0 is ≈240 km s -1 , then the metal-poor component has no rotation and there is a net prograde rotation signal of ≈45 km s -1 in the metal-rich component. There is reasonable evidence that the Milky Way globular cluster and satellite galaxy systems are rotating with 〈v o 〉 ≈ 50 and 40 km s -1 , respectively. Furthermore, a stronger signal is found for the satellite galaxies when the angular momentum vector of the satellites is inclined with respect to the normal of the disc. The dwarf spheroidal satellites of M31 exhibit prograde rotation relative to the M31 disc with (v o ) ≈ 40 km s -1 . We postulate that this group of dwarf spheroidals may share a common origin. We also find strong evidence for systemic rotation in the globular clusters of M31 particularly for the most metal-rich.

ELLIPTICAL GALAXY MASSES OUT TO FIVE EFFECTIVE RADII: THE REALM OF DARK MATTER

We estimate the masses of elliptical galaxies out to five effective radii using planetary nebulae and globular clusters as tracers. A sample of 15 elliptical galaxies with a broad variation in mass is compiled from the literature. A distribution function-maximum likelihood analysis is used to estimate the overall potential slope, normalization, and velocity anisotropy of the tracers. We assume power-law profiles for the potential and tracer density and a constant velocity anisotropy. The derived potential power-law indices lie in between the isothermal and Keplerian regime and vary with mass: there is tentative evidence that the less massive galaxies have steeper potential profiles than the more massive galaxies. We use stellar mass-to-light ratios appropriate for either a Chabrier/KTG (Kroupa, Tout & Gilmore) or Salpeter initial mass function to disentangle the stellar and dark matter components. The fraction of dark matter within five effective radii increases with mass, in agreement with several other studies. We employ simple models to show that a combination of star formation efficiency and baryon extent are able to account for this trend. These models are in good agreement with both our measurements out to five effective radii and recent Sloan Lens ACS Survey measurements within one effective radii when a universal Chabrier/KTG initial mass function is adopted.

Halo mass and assembly history exposed in the faint outskirts: the stellar and dark matter haloes of Illustris galaxies

We use the Illustris Simulations to gain insight into the build-up of the outer, low-surface brightness regions which surround galaxies. We characterize the stellar haloes by means of the logarithmic slope of the spherically-averaged stellar density profiles, alphaSTARS at z=0, and we relate these slopes to the properties of the underlying Dark-Matter (DM) haloes, their central galaxies, and their assembly histories. We analyze a sample of ~5,000 galaxies resolved with more than 5x10^4 particles each, and spanning a variety of morphologies and halo masses (3x10^11 < Mvir < 10^14 Msun). We find a strong trend between stellar halo slope and total halo mass, where more massive objects have shallower stellar haloes than the less massive ones (-5.5 \pm 0.5 < alphaSTARS <-3.5 \pm 0.2 in the studied mass range). At fixed halo mass, we show that disk-like, blue, young, and more massive galaxies are surrounded by significantly steeper stellar haloes than elliptical, red, older, and less massive galaxies. Overall, the stellar density profiles fall off much more steeply than the underlying DM, and no clear trend holds between stellar slope and DM halo concentration. However, DM haloes which formed more recently, or which accreted larger fractions of stellar mass from infalling satellites, exhibit shallower stellar haloes than their older analogs with similar masses, by up to Delta(alphaSTARS) ~ 0.5-0.7. Our findings, combined with the most recent measurements of the strikingly different stellar power-law indexes for M31 and the Milky Way, appear to favour a massive M31, and a Milky Way characterized by a much quieter accretion history over the past 10 Gyrs than its companion.

SATELLITE DWARF GALAXIES IN A HIERARCHICAL UNIVERSE: THE PREVALENCE OF DWARF-DWARF MAJOR MERGERS

Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the Local Group remains poorly understood. We investigate the frequency of major mergers between dwarf galaxies in the Local Group using the ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way- and M31-like host halos. We find that ~10% of satellite dwarf galaxies with Mstar > 10^6 M_☉ that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z = 1, with a lower fraction for lower mass dwarf galaxies. Recent merger remnants are biased toward larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo. Satellite–satellite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group. The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the Local Group are the most likely to have experienced a recent major merger. We discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive black holes in dwarf galaxies.

Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50?kpc

We use A-type stars selected from Sloan Digital Sky Survey data release 9 photometry to measure the outer slope of the Milky Way stellar halo density profile beyond 50 kpc. A likelihood-based analysis is employed that models the ugr photometry distribution of blue horizontal branch and blue straggler stars. In the magnitude range 18.5 < g < 20.5, these stellar populations span a heliocentric distance range of: 10 D BS/kpc 75, 40 D BHB/kpc 100. Contributions from contaminants, such as QSOs, and the effect of photometric uncertainties, are also included in our modeling procedure. We find evidence for a very steep outer halo profile, with power-law index α ~ 6 beyond Galactocentric radii r = 50 kpc, and even steeper slopes favored (α ~ 6-10) at larger radii. This result holds true when stars belonging to known overdensities, such as the Sagittarius stream, are included or excluded. We show that, by comparison to numerical simulations, stellar halos with shallower slopes at large distances tend to have more recent accretion activity. Thus, it is likely that the Milky Way has undergone a relatively quiet accretion history over the past several gigayears. Our measurement of the outer stellar halo profile may have important implications for dynamical mass models of the Milky Way, where the tracer density profile is strongly degenerate with total mass estimates.

The nature of H i absorbers in gamma‐ray burst afterglows: clues from hydrodynamic simulations

In recent work, we have shown that it is possible to link quantitatively many aspects of damped Lyman α (DLA) absorbers in the spectra of quasars to high-resolution simulations of galaxy formation. Using runs from the same series of hydrodynamic numerical studies, we consider the expected properties of intrinsic Lyman α absorbers seen in the spectra of high-redshift (z > 2) gamma-ray burst afterglows (GRB–DLAs). If GRBs are associated with the death of massive stars, their afterglows provide insights into otherwise unprobed regions of protogalactic objects, but detailed physical interpretations are currently embryonic. We find that median impact parameters (measured from the potential minimum) are approximately 1 kpc for GRBs compared with 4 kpc for quasi-stellar object–DLA (QSO–DLA). However, an equally important difference is that GRB–DLAs are predominantly associated with haloes of mass 10^(10) < M_(vir)/M_⊙ < 10^(12) , an order of magnitude larger than the hosts of QSO–DLAs. Accordingly, there are differences in the stellar properties of hosts. For instance, mean star formation rates are higher: <M(overdot)_★ ≃ 10 M_⊙ yr^(-1) for GRB–DLAs compared with <M(overdot)_★ ≃ 1 M_⊙ yr^(-1) for QSO–DLAs. Our simulations accurately predict the form of the GRB–DLA H I column density distribution, producing quantitative agreement for N_(H I) > 10^(19) cm^(−2) , but they somewhat underpredict the incidence of low column densities N_(H I_ < 10^(19) cm^(−2) . This is reflected in our estimate of the ionizing photon escape fraction, f_(esc) ≃ 1 per cent, which is lower than the observational GRB-derived escape fraction (2 per cent). Line-of-sight neutral gas metallicities predicted by our simulations (10^(−2) < Z/Z_⊙ < 1) are consistent with the modest observational constraints. Because of large internal dispersions in gas metallicities, this agreement is not significantly compromised by imposing a cut-off on the metallicity of stars able to launch GRBs (Z_★ < Z_⊙/3) , confounding claims that the observed metallicity of GRB–DLAs poses a severe challenge to current GRB models.