The Century Survey Galactic Halo Project II: Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars

We discuss a 175 deg^2 spectroscopic survey for blue horizontal branch (BHB) stars in the Galactic halo. We use the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to select BHB candidates, and find that the 2MASS and SDSS color-selection is 38% and 50% efficient, respectively, for BHB stars. Our samples include one likely run-away B7 star 6 kpc below the Galactic plane. The global properties of the BHB samples are consistent with membership in the halo population: the median metallicity is [Fe/H]=-1.7, the velocity dispersion is 108 km/s, and the mean Galactic rotation of the BHB stars 3<|z|<15 kpc is -4 +- 30 km/s. We discuss the theoretical basis of the Preston, Shectman & Beers M_V-color relation for BHB stars, and conclude that intrinsic shape of the BHB M_V-color relation results from the physics of stars on the horizontal branch. We calculate the luminosity function for the field BHB star samples using the Efstathiou, Ellis, & Peterson maximum-likelihood method which is unbiased by density variations. The field BHB luminosity function exhibits a steep rise at bright luminosities, a peak between 0.8 < M_V < 1.0, and a tail at faint luminosities. We compare the field BHB luminosity functions with the luminosity functions derived from sixteen different globular cluster BHBs. Kolmogorov-Smirnov tests suggest that field BHB stars and BHB stars in globular clusters share a common distribution of luminosities, with the exception of globular clusters with extended BHBs.Comment: 14 pages, including 16 figures, accepted for publication in A

Do the nearby BHB stars belong to the Thick Disk or the Halo?

We study the Milky Way region Z<3.0 kpc, where the thick disk and inner halo overlap, by using the kinematics of local blue horizontal branch (BHB) stars (within 1 kpc) and new samples of BHB stars and A-type stars from the Century Survey. We derive Galactic U,V,W velocities for these BHB and A-type star samples using proper motions from the NOMAD catalog. The mean velocities and the velocity dispersions of the BHB samples (Z<3 kpc) are characteristic of the halo, while those of the Century Survey A-type stars are characteristic of the thick disk. There is no evidence from our samples that the BHB stars rotate with the thick disk in the region Z<3 kpc. Nearly a third of the nearby local RR Lyrae stars have disk kinematics and are more metal-rich than [Fe/H]~-1. Only a few percent of the Century Survey BHB stars have these properties. Only one nearby BHB star (HD 130201) is likely to be such a disk star but selection based on high proper motions will have tended to exclude such stars from the local sample. The scale height derived from a sample of local RR Lyrae stars agrees with that of the Century Survey BHB stars. The local samples of BHB stars and metal-weak red giants are too incomplete for a similar comparison.Comment: 14 pages, accepted to A

Supermassive black holes coalescence mediated by massive perturbers: implications for gravitational waves emission and nuclear cluster formation

A large fraction of galactic nuclei is expected to host supermassive black hole binaries (BHB), likely formed during the early phase of galaxies assembly and merging. In this paper, we use a large set of state-of-art numerical models to investigate the interplay between a BHB and a massive star cluster (GCs) driven toward the galactic centre by dynamical friction. Varying the BHB mass and mass ratio and the GC orbit, we show that the reciprocal feedback exerted between GCs and the BHB shapes their global properties. We show that, at GC-to-BHB mass ratios above 0.1, the GC affects notably the BHB orbital evolution, possibly boosting its coalescence. This effect is maximized if the GC moves on a retrograde orbit, and for a non-equal mass BHB. We show that the GC debris dispersed around the galactic nucleus can lead to the formation of a nuclear cluster, depending on the BHB tidal field, and that the distribution of compact remnants resulting from the GC disruption can carry information about the BHB orbital properties. We find that red giant stars delivered by the spiralling GC can be disrupted at a rate of $\simeq (0.7-7)\times 10^{-7}$ yr$^{-1}$ for BHB masses $\sim 10^7{\rm M}_\odot$, while tens to hundreds of stars can be possibly observed in the galactic halo as high-velocity stars, with velocities up to $\sim 2000$ km s$^{-1}$, depending on the BHB orbital properties.Comment: 26 pages, 27 figures, 2 tables. Resubmitted to MNRAS after minor revisio

The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo

We present a complete spectroscopic survey of 2414 2MASS-selected blue horizontal branch (BHB) candidates selected over 4300 deg^2 of the sky. We identify 655 BHB stars in this non-kinematically selected sample. We calculate the luminosity function of field BHB stars and find evidence for very few hot BHB stars in the field. The BHB stars located at a distance from the Galactic plane |Z|<4 kpc trace what is clearly a metal-weak thick disk population, with a mean metallicity of [Fe/H]= -1.7, a rotation velocity gradient of dv_{rot}/d|Z|= -28+-3.4 km/s in the region |Z|<6 kpc, and a density scale height of h_Z= 1.26+-0.1 kpc. The BHB stars located at 5<|Z|<9 kpc are a predominantly inner-halo population, with a mean metallicity of [Fe/H]= -2.0 and a mean Galactic rotation of -4+-31 km/s. We infer the density of halo and thick disk BHB stars is 104+-37 kpc^-3 near the Sun, and the relative normalization of halo to thick-disk BHB stars is 4+-1% near the Sun.Comment: 12 pages in emulateapj format, accepted for publication in February A

Stellar Population Variations in the Milky Way's Stellar Halo

If the stellar halos of disk galaxies are built up from the disruption of dwarf galaxies, models predict highly structured variations in the stellar populations within these halos. We test this prediction by studying the ratio of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor populations) to main-sequence turn-off stars (MSTO stars; a feature of all populations) in the stellar halo of the Milky Way using data from the Sloan Digital Sky Survey. We develop and apply an improved technique to select BHB stars using ugr color information alone, yielding a sample of ~9000 g<18 candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across ~1/4 of the sky at the distance resolution permitted by the absolute magnitude distribution of MSTO stars. We find large variations of BHB/MSTO star ratio in the stellar halo. Previously identified, stream-like halo structures have distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo features, e.g., the low-latitude structure, appear to be almost completely devoid of BHB stars, whereas other structures appear to be rich in BHB stars. The Sagittarius tidal stream shows an apparent variation in BHB/MSTO ratio along its extent, which we interpret in terms of population gradients within the progenitor dwarf galaxy. Our detection of coherent stellar population variations between different stellar halo substructures provides yet more support to cosmologically motivated models for stellar halo growth.Comment: Astronomical Journal, in press. 10 pages, 5 color figures. Much better printed in colo

Vertical stratification of iron in atmospheres of blue horizontal-branch stars

The aim of this study is to search for observational evidence of vertical iron stratification in the atmosphere of fourteen blue horizontal-branch (BHB) stars. We have found from our numerical simulations that five BHB stars: B22, B186 in the globular cluster NGC 288, WF2-820, WF2-2692 in M13 and B203 in M15 show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere. Two other BHB stars (B334 in M15 and B176 in M92) also show possible iron stratification in their atmosphere. A dependence of the slope of iron stratification on the effective temperature was also discovered. It is found that the vertical stratification of iron is strongest in BHB stars with Teff around 11,500K. The slope of iron abundance decreases as Teff increases and becomes negligible for the BHB stars with Teff= 14,000K. These results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with Teff > 11,500K.Comment: 6 pages, 2 figures, 3 table

Protein detection using hydrogel-based molecularly imprinted polymers integrated with dual polarisation interferometry

A polyacrylamide-based molecularly imprinted polymer (MIP) was prepared for bovine haemoglobin (BHb). A 3 mg/ml solution of BHb was injected over a dual polarisation interferometer (DPI) sensor to form a physisorbed layer typically of 3.5 ± 0.5 nm thickness. Onto the pre-adsorbed protein layer, MIP and NIP (non-imprinted polymer) were separately injected to monitor the interaction of BHb MIP or NIP particles under different loading conditions with the pre-adsorbed protein layer. In the case of NIP flowing of the protein layer, there was negligible surface stripping of the pre-adsorbed protein. When a protein-eluted sample of MIP particles was flowed over a pre-adsorbed protein layer on the sensor chip, the sensor detected significant decreases in both layer thickness and mass, suggestive that protein was being selectively bound to MIP after being stripped-off from the sensor surface. We also integrated thin-film MIPS for BHb and BSA onto the DPI sensor surface and were able to show that whereas BHb bound selectively and strongly to the BHb MIP thin film (resulting in a sustained increase in thickness and mass), the BHb protein only demonstrated transient and reversible binding on the BSA MIP. MIPs were also tested after biofouling with plasma or serum at various dilutions. We found that serum at 1/100 dilution allowed the MIP to still function selectively. This is the first demonstration of MIPs being integrated with DPI in the development of synthetic receptor-based optical protein sensors. © 2012 Elsevier B.V. All rights reserved

Quantifying Kinematic Substructure in the Milky Way's Stellar Halo

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative "close pair distribution" (CPD) as a statistic in the 4-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock-observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at $\rm r_{gc} < 20$ kpc.Comment: 29 page, 10 figures, 1 table; accepted by APJ; for related article by another group see arXiv:1011.192