We show that a combination of infrared photometry from WISE and 2MASS surveys can yield highly pure samples of M giant stars. We take advantage of the new WISE$cap$2MASS M giant selection to trace the Sagittarius trailing tail behind the Galactic dis
k in the direction of the anti-centre. The M giant candidates selected via broad-band photometry are confirmed spectroscopically using AAOmega on the AAT in 3 fields around the extremity of the Sgr trailing tail in the Southern Galactic hemisphere. We demonstrate that at the Sgr longitude $tilde Lambda_{odot} = 204^{circ}$, the line-of-sight velocity of the trailing tail starts to deviate from the track of the Law & Majewski (2010) model, confirming the prediction of Belokurov et al. (2014). This discovery serves to substantiate the measurement of low differential orbital precession of the Sgr stream which in turn may imply diminished dark matter content within 100 kpc.
We report the discovery of a narrow stellar stream crossing the constellations of Sculptor and Fornax in the Southern celestial hemisphere. The portion of the stream detected in the Data Release 1 photometry of the ATLAS survey is at least 12 degrees
long, while its width is $approx$ 0.25 deg. The Color Magnitude Diagram of this halo sub-structure is consistent with a metal-poor [Fe/H] $lesssim -1.4$ stellar population located at a heliocentric distance of 20 $pm$ 2 kpc. There are three globular clusters that could tentatively be associated with the stream: NGC 7006, NGC 7078 (M15) and Pyxis, but NGC 7006 and 7078 seem to have proper motions incompatible with the stream orbit.
We announce the discovery of a new Galactic companion found in data from the ESO VST ATLAS survey, and followed up with deep imaging on the 4m William Herschel Telescope. The satellite is located in the constellation of Crater (the Cup) at a distance
of $sim$ 170 kpc. Its half-light radius is $r_h=30$ pc and its luminosity is $M_V=-5.5$. The bulk of its stellar population is old and metal-poor. We would probably have classified the newly discovered satellite as an extended globular cluster were it not for the presence of a handful of Blue Loop stars and a sparsely populated Red Clump. The existence of the core helium burning population implies that star-formation occurred in Crater perhaps as recently as 400 Myr ago. No globular cluster has ever accomplished the feat of prolonging its star-formation by several Gyrs. Therefore, if our hypothesis that the blue bright stars in Crater are Blue Loop giants is correct, the new satellite should be classified as a dwarf galaxy with unusual properties. Note that only ten degrees to the North of Crater, two ultra-faint galaxies Leo IV and V orbit the Galaxy at approximately the same distance. This hints that all three satellites may once have been closely associated before falling together into the Milky Way halo.
We employ a large sample of 20171 optically-selected groups and clusters at 0.15 < z < 0.4 in the SDSS to investigate how the stacked stellar mass fraction varies across a wide range of total mass, $M_{500}$. Our study improves upon previous observat
ional studies in a number of important ways, including having a much larger sample size, an explicit inclusion of the intracluster light (ICL) component, and a thorough examination of the accuracy of our total mass estimates via comparisons to simulations and weak lensing observations. We find that the stellar mass fraction depends only weakly on total mass and that the contribution of ICL to the total stellar mass fraction is significant (typically 20-40 per cent). Both of these findings are in excellent accordance with the predictions of cosmological simulations. Under the assumption of a Chabrier (Salpeter) IMF, the derived star formation efficiency ($f_{star}$/$f_{b}$, where $f_b=Omega_b/Omega_m$) is relatively low at 8 per cent (14 per cent) and is consistent with the global star formation efficiency of semi-analytic models that reproduce the galaxy stellar mass function. When our measured stellar mass fractions are combined with the observed relation between hot gas mass fraction and total mass from X-ray observations, our results imply that galaxy groups have significantly lower baryon fractions than massive clusters. Ejection of gas due to energetic AGN feedback (most likely at high redshift) provides a plausible mechanism for explaining the trends we observe.
Using a variety of stellar tracers -- blue horizontal branch stars, main-sequence turn-off stars and red giants -- we follow the path of the Sagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our study presents new Sgr debris d
etections, accurate distances and line-of-sight velocities that together help to shed new light on the puzzle of the Sgr tails. For both the leading and the trailing tail, we trace the points of their maximal extent, or apo-centric distances, and find that they lie at $R^L$ = 47.8 $pm$ 0.5 kpc and $R^T$ = 102.5 $pm$ 2.5 kpc respectively. The angular difference between the apo-centres is 93.2 $pm$ 3.5 deg, which is smaller than predicted for logarithmic haloes. Such differential orbital precession can be made consistent with models of the Milky Way in which the dark matter density falls more quickly with radius. However, currently, no existing Sgr disruption simulation can explain the entirety of the observational data. Based on its position and radial velocity, we show that the unusually large globular cluster NGC 2419 can be associated with the Sgr trailing stream. We measure the precession of the orbital plane of the Sgr debris in the Milky Way potential and show that, surprisingly, Sgr debris in the primary (brighter) tails evolves differently to the secondary (fainter) tails, both in the North and the South.
The structure of the Sagittarius stream in the Southern Galactic hemisphere is analysed with the Sloan Digital Sky Survey Data Release 8. Parallel to the Sagittarius tidal track, but ~ 10deg away, there is another fainter and more metal-poor stream.
We provide evidence that the two streams follow similar distance gradients but have distinct morphological properties and stellar populations. The brighter stream is broader, contains more metal-rich stars and has a richer colour-magnitude diagram with multiple turn-offs and a prominent red clump as compared to the fainter stream. Based on the structural properties and the stellar population mix, the stream configuration is similar to the Northern bifurcation. In the region of the South Galactic Cap, there is overlapping tidal debris from the Cetus Stream, which crosses the Sagittarius stream. Using both photometric and spectroscopic data, we show that the blue straggler population belongs mainly to Sagittarius and the blue horizontal branch stars belong mainly to the Cetus stream in this confused location in the halo.
The narrow GD-1 stream of stars, spanning 60 deg on the sky at a distance of ~10 kpc from the Sun and ~15 kpc from the Galactic center, is presumed to be debris from a tidally disrupted star cluster that traces out a test-particle orbit in the Milky
Way halo. We combine SDSS photometry, USNO-B astrometry, and SDSS and Calar Alto spectroscopy to construct a complete, empirical 6-dimensional phase-space map of the stream. We find that an eccentric orbit in a flattened isothermal potential describes this phase-space map well. Even after marginalizing over the stream orbital parameters and the distance from the Sun to the Galactic center, the orbital fit to GD-1 places strong constraints on the circular velocity at the Suns radius V_c=224 pm 13 km/s and total potential flattening q_Phi=0.87^{+0.07}_{-0.04}. When we drop any informative priors on V_c the GD-1 constraint becomes V_c=221 pm 18 km/s. Our 6-D map of GD-1 therefore yields the best current constraint on V_c and the only strong constraint on q_Phi at Galactocentric radii near R~15 kpc. Much, if not all, of the total potential flattening may be attributed to the mass in the stellar disk, so the GD-1 constraints on the flattening of the halo itself are weak: q_{Phi,halo}>0.89 at 90% confidence. The greatest uncertainty in the 6-D map and the orbital analysis stems from the photometric distances, which will be obviated by Gaia.
We revisit the well known discrepancy between the observed number of Milky Way (MW) dwarf satellite companions and the predicted population of cold dark matter (CDM) sub-halos, in light of the dozen new low luminosity satellites found in SDSS imaging
data and our recent calibration of the SDSS satellite detection efficiency, which implies a total population far larger than these dozen discoveries. We combine a dynamical model for the CDM sub-halo population with simple, physically motivated prescriptions for assigning stellar content to each sub-halo, then apply observational selection effects and compare to the current observational census. As expected, models in which the stellar mass is a constant fraction F(Omega_b/Omega_m) of the sub-halo mass M_sat at the time it becomes a satellite fail for any choice of F. However, previously advocated models that invoke suppression of gas accretion after reionization in halos with circular velocity v_c <~ 35 km/s can reproduce the observed satellite counts for -15 < M_V < 0, with F ~ 10^{-3}. Successful models also require strong suppression of star formation BEFORE reionization in halos with v_c <~ 10 km/s; models without pre-reionization suppression predict far too many satellites with -5 < M_V < 0. Our models also reproduce the observed stellar velocity dispersions ~ 5-10 km/s of the SDSS dwarfs given the observed sizes of their stellar distributions, and model satellites have M(<300 pc) ~ 10^7 M_sun as observed even though their present day total halo masses span more than two orders of magnitude. Our modeling shows that natural physical mechanisms acting within the CDM framework can quantitatively explain the properties of the MW satellite population as it is presently known, thus providing a convincing solution to the `missing satellite problem.
We report the discovery of two extremely low luminosity globular clusters in the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters,
Koposov 1 and Koposov 2, are located at $sim 40-50$ kpc and appear to have old stellar populations and luminosities of only $M_V sim -1$ mag. Their observed sizes of $sim 3$ pc are well within the expected tidal limit of $sim$10 pc at that distance. Together with Palomar 1, AM 4 and Whiting 1, these new clusters are the lowest luminosity globulars orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only $sim 70$ and $sim 55$ Myr respectively (2 orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as $sim 0.1 t_{rm Hubble}$. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.
