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We combine Gaia data release 1 astrometry with Sloan Digital Sky Survey (SDSS) images taken some ~10-15 years earlier, to measure proper motions of stars in the halo of our Galaxy. The SDSS-Gaia proper motions have typical statistical errors of 2 mas/yr down to r ~ 20 mag, and are robust to variations with magnitude and colour. Armed with this exquisite set of halo proper motions, we identify RR Lyrae, blue horizontal branch (BHB), and K giant stars in the halo, and measure their net rotation with respect to the Galactic disc. We find evidence for a gently rotating prograde signal (< $V_phi$ > ~ 5-25 km/s) in the halo stars, which shows little variation with Galactocentric radius out to 50 kpc. The average rotation signal for the three populations is < $V_phi$ > = 14 +/- 2 +/- 10 (syst.) km/s. There is also tentative evidence for a kinematic correlation with metallicity, whereby the metal richer BHB and K giant stars have slightly stronger prograde rotation than the metal poorer stars. Using the Auriga simulation suite we find that the old (T >10 Gyr) stars in the simulated halos exhibit mild prograde rotation, with little dependence on radius or metallicity, in general agreement with the observations. The weak halo rotation suggests that the Milky Way has a minor in situ halo component, and has undergone a relatively quiet accretion history.
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We investigate the origin of the abundance ratios and scatter of $alpha$- and neutron-capture elements of old, metal-poor stars, using cosmological, hydrodynamical simulations of galaxy formation. For this, we implement a novel treatment for the production and distribution of chemical products of Type II supernovae, which considers the effects of the rotation of massive stars on the chemical yields and the effects of the different life-times of stars that are progenitors of this type of supernovae. We focus on the stellar halo of a Milky Way-mass galaxy, studying the abundances and scatter of [O/Fe], [Mg/Fe], [Si/Fe], [Sr/Fe], [Eu/Fe] and [Ba/Fe]. Our model is able, for the first time in a cosmological simulation, to describe at the same time the low scatter in the abundances of $alpha$-elements and the higher scatter associated to neutron-capture elements in the halo stars, as suggested by observations of the Milky Way. We also reproduce the scatter observed in the [Sr/Ba] ratio, which results from the treatment of the fast-rotating stars and the dependence of the chemical yields on the metallicity, mass and rotational velocities. Our simulations show that such scatter patterns appear naturally if the different ejection times associated to stars of different mass are properly described, without the need to invoke for additional mixing mechanisms or a distinct treatment of the alpha- and neutron-capture elements. Simulations of this type will help characterizing and identifying the past accretion debris as well as the pristine in-situ populations in the Galaxy unveiled by Gaia and spectroscopic data.
The stellar halos of large galaxies represent a vital probe of the processes of galaxy evolution. They are the remnants of the initial bouts of star formation during the collapse of the proto-galactic cloud, coupled with imprint of ancient and on-going accretion events. Previously, we have reported the tentative detection of a possible, faint, extended stellar halo in the Local Group spiral, the Triangulum Galaxy (M33). However, the presence of substructure surrounding M33 made interpretation of this feature difficult. Here, we employ the final data set from the Pan-Andromeda Archaeological Survey (PAndAS), combined with an improved calibration and a newly derived contamination model for the region to revisit this claim. With an array of new fitting algorithms, fully accounting for contamination and the substantial substructure beyond the prominent stellar disk in M33, we reanalyse the surrounds to separate the signal of the stellar halo and the outer halo substructure. Using more robust search algorithms, we do not detect a large scale smooth stellar halo and place a limit on the maximum surface brightness of such a feature of ${mu}_V$ = 35.5 mags per square arcsec, or a total halo luminosity of $L < 10^6L_{odot}$.
Determining the properties of old stellar populations (those with age >1 Gyr) has long involved the comparison of their integrated light, either in the form of photometry or spectroscopic indexes, with empirical or synthetic templates. Here we reevaluate the properties of old stellar populations using a new set of stellar population synthesis models, designed to incorporate the effects of binary stellar evolution pathways as a function of stellar mass and age. We find that single-aged stellar population models incorporating binary stars, as well as new stellar evolution and atmosphere models, can reproduce the colours and spectral indices observed in both globular clusters and quiescent galaxies. The best fitting model populations are often younger than those derived from older spectral synthesis models, and may also lie at slightly higher metallicities.
We examine metallicities, ages and orbital properties of halo stars in a Milky-Way like disk galaxy formed in the cosmological hydrodynamical MaGICC simulations. Halo stars were either accreted from satellites or they formed in situ in the disk or bulge of the galaxy and were then kicked up into the halo (in situ/ kicked-up stars). Regardless of where they formed both types show surprisingly similar orbital properties: the majority of stars of both types are on short-axis tubes with the same sense of rotation as the disk -- implying that a large fraction of satellites are accreted onto the halo with the same sense of angular momentum as the disk.
Models of galaxy formation in a cosmological context predict that massive disk galaxies should have structured extended stellar halos. Recent studies in integrated light, however, report a few galaxies, including the nearby disk galaxy M101, that have no measurable stellar halos to the detection limit. We aim to quantify the stellar content and structure of M101s outskirts by resolving its stars. We present the photometry of its stars based on deep F606W and F814W images taken with Hubble Space Telescope as part of the GHOSTS survey. The constructed CMDs of stars reach down to two magnitudes below the tip of the red giant branch. We derived radial number density profiles of the bright red giant branch (RGB) stars. The mean color of the RGB stars at $R sim$ 40 -- 60 kpc is similar to those of metal-poor globular clusters in the Milky Way. We also derived radial surface brightness profiles using the public image data provided by the Dragonfly team. Both the radial number density and surface brightness profiles were converted to radial mass density profiles and combined. We find that the mass density profiles show a weak upturn at the very outer region, where surface brightness is as faint as $mu_gapprox 34$ mag arcsec$^{-1}$. An exponential disk + power-law halo model on the mass density profiles finds the total stellar halo mass of $M_{halo}=8.2_{-2.2}^{+3.5}times 10^7M_odot$. The total stellar halo mass does not exceed $M_{halo} = 3.2 times 10^8$ $M_{odot}$ when strongly truncated disk models are considered. Combining the halo mass with the total stellar mass of M101, we obtain the stellar halo mass fraction of $M_{halo}/M_{gal} = 0.20_{-0.08}^{+0.10}%$ with an upper limit of 0.78%. We compare the halo properties of M101 with those of six GHOSTS survey galaxies as well as the Milky Way and M31 and find that M101 has an anemic stellar halo.
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