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Register a new userObserving the stellar halo of Andromeda in cosmological simulations: the Auriga2PAndAS pipeline
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We present a direct comparison of the Pan-Andromeda Archaeological Survey (PAndAS) observations of the stellar halo of M31 with the stellar halos of 6 galaxies from the Auriga simulations. We process the simulated halos through the Auriga2PAndAS pipeline and create PAndAS-like mocks that fold in all observational limitations of the survey data (foreground contamination from the Milky Way stars, incompleteness of the stellar catalogues, photometric uncertainties, etc). This allows us to study the survey data and the mocks in the same way and generate directly comparable density maps and radial density profiles. We show that the simulations are overall compatible with the observations. Nevertheless, some systematic differences exist, such as a preponderance for metal-rich stars in the mocks. While these differences could suggest that M31 had a different accretion history or has a different mass compared to the simulated systems, it is more likely a consequence of an under-quenching of the star formation history of galaxies, related to the resolution of the Auriga simulations. The direct comparison enabled by our approach offers avenues to improve our understanding of galaxy formation as they can help pinpoint the observable differences between observations and simulations. Ideally, this approach will be further developed through an application to other stellar halo simulations. To facilitate this step, we release the pipeline to generate the mocks, along with the six mocks presented and used in this contribution.
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We present a suite of high-resolution cosmological zoom-in simulations to $z=4$ of a $10^{12},{rm M}_{odot}$ halo at $z=0$, obtained using seven contemporary astrophysical simulation codes widely used in the numerical galaxy formation community. Physics prescriptions for gas cooling, heating, and star formation, are similar to the ones used in our previous {it AGORA} disk comparison but now account for the effects of cosmological processes. In this work, we introduce the most careful comparison yet of galaxy formation simulations run by different code groups, together with a series of four calibration steps each of which is designed to reduce the number of tunable simulation parameters adopted in the final run. After all the participating code groups successfully completed the calibration steps, we reach a suite of cosmological simulations with similar mass assembly histories down to $z=4$. With numerical accuracy that resolves the internal structure of a target halo, we find that the codes overall agree well with one another in e.g., gas and stellar properties, but also show differences in e.g., circumgalactic medium properties. We argue that, if adequately tested in accordance with our proposed calibration steps and common parameters, the results of high-resolution cosmological zoom-in simulations can be robust and reproducible. New code groups are invited to join this comparison by generating equivalent models by adopting the common initial conditions, the common easy-to-implement physics package, and the proposed calibration steps. Further analyses of the simulations presented here will be in forthcoming reports from our Collaboration.
We present a quantitative measurement of the amount of clustering present in the inner $sim30$ kpc of the stellar halo of the Andromeda galaxy (M31). For this we analyse the angular positions and radial velocities of the carefully selected Planetary Nebulae (PNe) in the M31 stellar halo. We study the cumulative distribution of pair-wise distances in angular position and line of sight velocity space, and find that the M31 stellar halo contains substantially more stars in the form of close pairs as compared to that of a featureless smooth halo. In comparison to a smoothed/scrambled distribution we estimate that the clustering excess in the M31 inner halo is roughly $40%$ at maximum and on average $sim 20%$. Importantly, comparing against the 11 stellar halo models of cite{2005ApJ...635..931B}, which were simulated within the context of the $Lambda{rm CDM}$ cosmological paradigm, we find that the amount of substructures in the M31 stellar halo closely resembles that of a typical $Lambda{rm CDM}$ halo.
We present a photometric survey of the stellar halo of the Andromeda galaxy, using Suprime-Cam on the Subaru Telescope. A detailed analysis of VI color-magnitude diagrams of the resolved stellar population is used to measure properties such as line-of-sight distance, surface brightness, metallicity, and age, and these are used to isolate and characterize different components of the M31 halo: (1) several substructures, and (2) the smooth halo. First, we study M31s halo substructure along the north-west/south-east minor axis out to R ~ 100 kpc and the south-west major axis region at R ~ 60 kpc. We confirm two substructures in the south-east halo reported by Ibata et al. (2007) and discover two overdense substructures in the north-west halo. We investigate the properties of these four substructures as well as other structures including the western shelf and find that differences in stellar populations among these systems, thereby suggesting each has a different origin. Our statistical analysis implies that the M31 halo as a whole may contain at least 16 substructures, each with a different origin. Second, we investigate the properties of an underlying, smooth and extended halo component out to R > 100 kpc. We find that the surface density of this smooth halo can be fitted to a Hernquist model of scale radius ~ 17 kpc or a power-law profile with ~ R^{-2.17 +/- 0.15}. In contrast to the relative smoothness of the halo density profile, its metallicity distribution appears to be spatially non-uniform with non-monotonic variations with radius, suggesting that the halo population has not had sufficient time to dynamically homogenize the accreted populations. Further implications for the formation of the M31 halo are discussed.
We study the gas distribution in the Milky Way and Andromeda using a constrained cosmological simulation of the Local Group (LG) within the context of the CLUES (Constrained Local UniversE Simulations) project. We analyse the properties of gas in the simulated galaxies at $z=0$ for three different phases: `cold, `hot and HI, and compare our results with observations. The amount of material in the hot halo ($M_{hot}approx 4-5times10^{10},$M$_{odot}$), and the cold ($M_{cold}(rlesssim10,$kpc$)approx10^{8},$M$_{odot}$) and HI ($M_{HI}(rlesssim50,$kpc$)approx 3-4times10^8,$M$_{odot}$) components display a reasonable agreement with observations. We also compute the accretion/ejection rates together with the HI (radial and all-sky) covering fractions. The integrated HI accretion rate within $r=50,$kpc gives $sim$$0.2-0.3,$M$_{odot},$yr$^{-1}$, i.e. close to that obtained from high-velocity clouds in the Milky Way. We find that the global accretion rate is dominated by hot material, although ionized gas with $Tlesssim10^5,$K can contribute significantly too. The $net$ accretion rates of $all$ material at the virial radii are $6-8,$M$_{odot},$yr$^{-1}$. At $z=0$, we find a significant gas excess between the two galaxies, as compared to any other direction, resulting from the overlap of their gaseous haloes. In our simulation, the gas excess first occurs at $zsim1$, as a consequence of the kinematical evolution of the LG.
We study the components of cool and warm/hot gas in the circumgalactic medium (CGM) of simulated galaxies and address the relative production of OVI by photoionization versus collisional ionization, as a function of halo mass, redshift, and distance from the galaxy halo center. This is done utilizing two different suites of zoom-in hydro-cosmological simulations, VELA (6 halos; $z>1$) and NIHAO (18 halos; to $z=0$), which provide a broad theoretical basis because they use different codes and physical recipes for star formation and feedback. In all halos studied in this work, we find that collisional ionization by thermal electrons dominates at high redshift, while photoionization of cool or warm gas by the metagalactic radiation takes over near $zsim2$. In halos of $sim 10^{12}M_{odot}$ and above, collisions become important again at $z<0.5$, while photoionization remains significant down to $z=0$ for less massive halos. In halos with $M_{textrm v}>3times10^{11}~M_{odot}$, at $zsim 0$ most of the photoionized OVI is in a warm, not cool, gas phase ($Tlesssim 3times 10^5$~K). We also find that collisions are dominant in the central regions of halos, while photoionization is more significant at the outskirts, around $R_{textrm v}$, even in massive halos. This too may be explained by the presence of warm gas or, in lower mass halos, by cool gas inflows.
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