The Dominance of Metal-Rich Streams in Stellar Halos: A Comparison Between Substructure in M31 and Lambda-CDM Models

Extensive photometric and spectroscopic surveys of the Andromeda galaxy (M31) have discovered tidal debris features throughout M31's stellar halo. We present stellar kinematics and metallicities in fields with identified substructure from our on-going SPLASH survey of M31 red giant branch stars with the DEIMOS spectrograph on the Keck II 10-m telescope. Radial velocity criteria are used to isolate members of the kinematically-cold substructures. The substructures are shown to be metal-rich relative to the rest of the dynamically hot stellar population in the fields in which they are found. We calculate the mean metallicity and average surface brightness of the various kinematical components in each field, and show that, on average, higher surface brightness features tend to be more metal-rich than lower surface brightness features. Simulations of stellar halo formation via accretion in a cosmological context are used to illustrate that the observed trend can be explained as a natural consequence of the observed dwarf galaxy mass-metallicity relation. A significant spread in metallicity at a given surface brightness is seen in the data; we show that this is due to time effects, namely the variation in the time since accretion of the tidal streams' progenitor onto the host halo. We show that in this theoretical framework a relationship between the alpha-enhancement and surface brightness of tidal streams is expected, which arises from the varying times of accretion of the progenitor satellites onto the host halo. Thus, measurements of the alpha-enrichment, metallicity, and surface brightness of tidal debris can be used to reconstruct the luminosity and time of accretion onto the host halo of the progenitors of tidal streams.Comment: 11 pages, 6 figures, published in Ap

Taking the Milky Way for a spin: disc formation in the ARTEMIS simulations

We investigate the formation (spin-up) of galactic discs in the ARTEMIS simulations of Milky Way-mass galaxies. In almost all galaxies discs spin up at higher [Fe/H] than the Milky Way (MW). Those that contain an analogue of the Gaia Sausage-Enceladus (GSE) spin up at a lower average metallicity than those without. We identify six galaxies with spin-up metallicity similar to that of the MW, which form their discs $\sim 8-11$ Gyr ago. Five of these experience a merger similar to the GSE. The spin-up times correlate with the halo masses at early times: galaxies with early spin-up have larger virial masses at a lookback time $t_L=12$ Gyr. The fraction of stars accreted from outside the host galaxy is smaller in galaxies with earlier spin-ups. Accreted fractions small enough to be comparable to the MW are only found in galaxies with the earliest disc formation and large initial virial masses ($M_\mathrm{200c} \approx2\times10^{11}M_\odot$ at $t_L=12$ Gyr). We find that discs form when the halo's virial mass reaches a threshold of $M_\mathrm{200c}\approx(6\pm3)\times10^{11}M_\odot$, independent of the spin-up time. We also find that discs form when the central potential is not particularly steep. Our results indicate that the MW assembled its mass and formed its disc earlier than the average galaxy of a similar mass.Comment: 8 pages, 6 figures, submitted to MNRA

Environment from cross-correlations: connecting hot gas and the quenching of galaxies

The observable properties of galaxies depend on both internal processes and the external environment. In terms of the environmental role, we still do not have a clear picture of the processes driving the transformation of galaxies. The use of proxies for environment (e.g., host halo mass, distance to the N^th nearest neighbour, etc.), as opposed to the real physical conditions (e.g., hot gas density) may bear some responsibility for this. Here we propose a new method that directly links galaxies to their local environments, by using spatial cross-correlations of galaxy catalogues with maps from large-scale structure surveys (e.g., thermal Sunyaev-Zel'dovich [tSZ] effect, diffuse X-ray emission, weak lensing of galaxies or the CMB). We focus here on the quenching of galaxies and its link to local hot gas properties. Maps of galaxy overdensity and quenched fraction excess are constructed from volume-limited SDSS catalogs, which are cross-correlated with tSZ effect and X-ray maps from Planck and ROSAT, respectively. Strong signals out to Mpc scales are detected for most cross-correlations and are compared to predictions from the EAGLE and BAHAMAS cosmological hydrodynamical simulations. The simulations successfully reproduce many, but not all, of the observed power spectra, with an indication that environmental quenching may be too efficient in the simulations. We demonstrate that the cross-correlations are sensitive to both the internal (e.g., AGN and stellar feedback) and external processes (e.g., ram pressure stripping, harassment, strangulation, etc.) responsible for quenching. The methods outlined in this paper can be adapted to other observables and, with upcoming surveys, will provide a stringent test of physical models for environmental transformation.Comment: 23 pages, 11 figures, MNRAS, in pres

Merger-induced galaxy transformations in the ARTEMIS simulations

Using the ARTEMIS set of 45 high-resolution cosmological simulations, we investigate a range of merger-induced dynamical transformations of Milky Way-like galaxies. We first identify populations of accreted stars on highly radial orbits, similar to the 'Gaia Sausage' in the Milky Way. We show that $\approx1/3$ of the ARTEMIS galaxies contain a similar feature, and confirm that they usually comprise stellar debris from the most massive accreted satellite. Selecting these 15 galaxies, we study their changes around the times of the GS-like mergers. Dark matter haloes of many of these exhibit global changes in shape and orientation. Focusing on the galaxies themselves, we find multiple examples of stellar discs whose angular momentum (AM) axes change orientation at rapid rates of $\sim60$ degrees Gyr$^{-1}$. By calculating the orbital angular momentum axes of the satellites before they are accreted, we show that there is a tendency for the disc's AM to become more aligned with this axis after the merger. We also investigate the origin of in situ retrograde stars, analogous to the 'Splash' in the Milky Way. Tracing them back to earlier snapshots, we demonstrate that they were often disrupted onto their extreme orbits by multiple early mergers. We also find that the total mass of these stars outside the central regions positively correlates with the total accreted stellar mass. Finally, we conduct a brief investigation into whether bars form soon after the mergers. In a few galaxies we find a bar-like feature whose emergence coincides with a significant merger.Comment: 20 pages, 18 figures, submitted to MNRA

Dynamics and Stellar Content of the Giant Southern Stream in M31. II. Interpretation

We examine the nature of the progenitor of the giant stellar stream in M31 using as constraints new radial velocity measurements of stream red giant stars (presented in the companion paper by Guhathakurta et al. 2005, astro-ph/0406145) along with other M31 data sets available in the literature. We find that the observations are best fit by orbits that are highly eccentric and close to edge-on, with apo- to peri-center ratios of order 25 - 30, and with apocenters at or only slightly beyond the southern edge of the current data. Among these orbits, we are able to find a few that plausibly connect the stream with the northern spur or with the low-surface-brightness feature of similar high metallicity as the stream (originally reported by Ferguson et al. 2002) to the east of M31's center. In the latter case, if the connection is real, then the eastern debris should lie well in front of M31 near the apocenter of the orbit. Both the width of the debris and velocity dispersion measurements imply a rough lower limit on the mass of the progenitor of 10^8 M_sun. We use this limit and our orbits to discuss which of M31's satellites could be plausibly associated with the stream. In addition, we predict that the width of the stream should increase beyond the southern edge of the current data around the apocenter of the orbit and that the line-of-sight velocity dispersion should exhibit significant variations along the stream.Comment: 20 pages, 6 figures, 1 table; submitted to the Astronomical Journal; includes revisions suggested by the refere

High-resolution synthetic UV-submm images for Milky Way-mass simulated galaxies from the ARTEMIS project

We present redshift-zero synthetic dust-aware observations for the 45 Milky Way-mass simulated galaxies of the ARTEMIS project, calculated with the SKIRT radiative transfer code. The post-processing procedure includes components for star-forming regions, stellar sources, and diffuse dust. We produce and publicly release realistic high-resolution images for 50 commonly-used broadband filters from ultraviolet to sub-millimetre wavelengths and for 18 different viewing angles. We compare the simulated ARTEMIS galaxies to observed galaxies in the DustPedia database with similar stellar mass and star formation rate, and to synthetic observations of the simulated galaxies of the Auriga project produced in previous work using a similar post-processing technique. In all cases, global galaxy properties are derived using SED fitting. We find that, similar to Auriga, the post-processed ARTEMIS galaxies generally reproduce the observed scaling relations for global fluxes and physical properties, although dust extinction at FUV/UV wavelengths is underestimated and representative dust temperatures are lower than observed. At a resolved scale, we compare multi-wavelength non-parametric morphological properties of selected disc galaxies across the data sets. We find that the ARTEMIS galaxies largely reproduce the observed morphological trends as a function of wavelength, although they appear to be more clumpy and less symmetrical than observed. We note that the ARTEMIS and Auriga galaxies occupy adjacent regions in the specific star formation versus stellar mass plane, so that the synthetic observation data sets supplement each other.Comment: Accepted for publication by MNRA

Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories

This paper explores the mapping between the observable properties of a stellar halo in phase- and abundance-space and the parent galaxy's accretion history in terms of the characteristic epoch of accretion and mass and orbits of progenitor objects. The study utilizes a suite of eleven stellar halo models constructed within the context of a standard LCDM cosmology. The results demonstrate that coordinate-space studies are sensitive to the recent (0-8 Gyears ago) merger histories of galaxies (this timescale corresponds to the last few to tens of percent of mass accretion for a Milky-Way-type galaxy). Specifically, the {\it frequency, sky coverage} and {\it fraction of stars} in substructures in the stellar halo as a function of surface brightness are indicators of the importance of recent merging and of the luminosity function of infalling dwarfs. The {\it morphology} of features serves as a guide to the orbital distribution of those dwarfs. Constraints on the earlier merger history (> 8 Gyears ago) can be gleaned from the abundance patterns in halo stars: within our models, dramatic differences in the dominant epoch of accretion or luminosity function of progenitor objects leave clear signatures in the [alpha/Fe] and [Fe/H] distributions of the stellar halo - halos dominated by very early accretion have higher average [alpha/Fe], while those dominated by high luminosity satellites have higher [Fe/H]. This intuition can be applied to reconstruct much about the merger histories of nearby galaxies from current and future data sets.Comment: 21 pages, 20 figures. To appear in the Astrophysical Journa