MILKY WAY RED DWARFS IN THE BoRG SURVEY; GALACTIC SCALE-HEIGHT AND THE DISTRIBUTION OF DWARF STARS IN WFC3 IMAGING

Abstract

We present a tally of Milky Way late-type dwarf stars in 68 Wide Field Camera 3 (WFC3) pure-parallel fields (227 arcmin 2) from the Brightest of Reionizing Galaxies survey for high-redshift galaxies. Using spectroscopically identified M-dwarfs in two public surveys, the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey and the Early Release Science mosaics, we identify a morphological selection criterion using the half-light radius (r 50), a near-infrared J − H, G − J color region where M-dwarfs are found, and a V − J relation with M-dwarf subtype. We apply this morphological selection of stellar objects, color-color selection of M-dwarfs, and optical-near-infrared color subtyping to compile a catalog of 274 M-dwarfs belonging to the disk of the Milky Way with a limiting magnitude of m F 125W < 24(AB). Based on the M-dwarf statistics, we conclude that (1) the previously identified north-south discrepancy in M-dwarf numbers persists in our sample; there are more M-dwarfs in the northern fields on average than in southern ones, (2) the Milky Way's single disk scale-height for M-dwarfs is 0.3-4 kpc, depending on subtype, (3) the scale-height depends on M-dwarf subtype with early types (M0-4) high scale-height (z 0 = 3-4 kpc) and later types M5 and above in the thin disk (z 0 = 0.3-0.5 kpc), (4) a second component is visible in the vertical distribution, with a different, much higher scale-height in the southern fields compared to the northern ones. We report the M-dwarf component of the Sagittarius stream in one of our fields with 11 confirmed M-dwarfs, seven of which are at the stream's distance. In addition to the M-dwarf catalog, we report the discovery of 1 T-dwarfs and 30 L-dwarfs from their near-infrared colors. The dwarf scale-height and the relative low incidence in our fields of Land T-dwarfs in these fields makes it unlikely that these stars will be interlopers in great numbers in color-selected samples of high-redshift galaxies. The relative ubiquity of M-dwarfs however will make them ideal tracers of Galactic halo substructure with EUCLID and reference stars for James Webb Space Telescope observations.

A non-uniform distribution of the nearest brown dwarfs

“…(iv) If this observed non-uniform substellar distribution remains unchanged after future searches, then either some fortuitous random aggregation or some dynamics affecting primarily the lower mass objects, could be invoked. We note that pencil beam deep surveys of the Milky Way suggests there might be slightly more M-type dwarfs in the Galactic northern hemisphere (b > 0 deg) than in the southern hemisphere (b < 0 deg; see Holwerda et al 2014 and references therein) and hints at a reversal occuring for midto late M-type dwarfs, which would be more numerous in the southern hemisphere. However, as already mentioned in Sect.…”

The galaxy luminosity function atz≃ 6and evidence for rapid evolution in the bright end fromz≃ 7to5

“…Previous searches for M, L and T-type stars have commonly assumed a single disk model (e.g. Holwerda et al 2014;Ryan et al 2011) to describe the observed number of stars, as the small samples of objects preclude a more complicated analysis. We follow such an approach using a single exponential disk model as described in Caballero et al (2008).…”

“…We adopt the main parameters of the galactic thin disk model from Chen et al (2001), with Z = 27 ± 4pc, R = 8600 ± 200pc and hR = 2250 ± 1000 pc. The most relevant quantity for this study is the galactic vertical scale height, where we use two values at hZ = 300 pc and hZ = 400 pc to reflect the uncertainty in this quantity (Holwerda et al 2014;Ryan et al 2011;Pirzkal et al 2009). Fig.…”

HST Imaging of the Brightest z ∼ 8–9 Galaxies from UltraVISTA: The Extreme Bright End of the UV Luminosity Function

“…To this aim, we opted for not adopting SExtractor class_star parameter because the classification becomes uncertain at low S/N (e.g., Bertin & Arnouts 1996). To overcome this, other star/galaxy separation criteria based on SExtractor have been developed (see e.g., Holwerda et al 2014). One of the most reliable is the effective radius versus magnitude (Ryan et al 2011).…”