Mapping the Inner Halo of the Galaxy with 2MASS-Selected Horizontal-Branch Candidates

Abstract

We use Two Micron All Sky Survey (2MASS) photometry to select blue horizontal-branch (BHB) candidates covering the sky, |b| > 15 . A 12.5 < J 0 < 15.5 sample of BHB stars traces the thick disk and inner halo to d < 9 kpc, with a density comparable to that of M giant stars. We base our sample selection strategy on the Century Survey Galactic Halo Project, a survey that provides a complete, spectroscopically identified sample of blue stars to a similar depth as the 2MASS catalog. We show that a À0.20 < (JÀH ) 0 < 0.10, À0.10 < (HÀK ) 0 < 0.10 colorselected sample of stars is 65% complete for BHB stars and is composed of 47% BHB stars. We apply this photometric selection to the full 2MASS catalog and see no spatial overdensities of BHB candidates at high Galactic latitude, |b| > 50 . We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no $5 wide star stream with density greater than 0.33 objects deg À2 at the 95% confidence level. The absence of observed structure suggests that there have been no major accretion events in the inner halo in the last few Gyr. However, at low Galactic latitudes a two-point angular correlation analysis reveals structure on angular scales P 1 . This structure is apparently associated with stars in the thick disk and has a physical scale of 10-100 pc. Interestingly, such structures are expected by cosmological simulations that predict the majority of the thick disk may arise from accretion and disruption of satellite mergers.

A Search for Field Horizontal-Branch Stars Near the South Galactic Pole

“…In this case, the restriction in b was included in order to decrease contamination by A v stars that populate the Galactic disk at moderate Galactic latitudes. They are also very similar to those proposed by Brown et al (2004) for selecting BHB stars from the 2MASS catalog: −0.20 < (J − H) o < 0.10 and −0.10 < (H − K) o < 0.10. In follow-up spectroscopy of a subset of these candidates (Brown et al 2003(Brown et al , 2004 demonstrated that roughly 47% of the sample are BHB stars; the sample is believed to be 65% complete to a faint limit of V = 16.…”

“…They are also very similar to those proposed by Brown et al (2004) for selecting BHB stars from the 2MASS catalog: −0.20 < (J − H) o < 0.10 and −0.10 < (H − K) o < 0.10. In follow-up spectroscopy of a subset of these candidates (Brown et al 2003(Brown et al , 2004 demonstrated that roughly 47% of the sample are BHB stars; the sample is believed to be 65% complete to a faint limit of V = 16. All of the criteria discussed above select HB candidates according to their color temperature, despite the slight differences among them.…”

“…They also found that the (J − H) o < +0.20 criterium results in 20% of BHB stars, but this percentage rises to 47% if (J − H) o < +0.10. An examination of Table 1 shows that the color-cut of the present sample is at (J − H) o = 0.00 and, according to Brown et al (2004), the percentage of BHB stars bluer than this color is about 55%. This figure would match our results if nine out of the 17 objects presently classified as FHB/AB v were in fact FHB stars.…”

“…Therefore, the total percentage of stars reliably classified as non-FHB is 25%, but the actual figure is certainly larger because 40% of the sample remains near the borderline between the FHB and main-sequence classes. Brown et al (2004) studied the efficacy of finding BHB stars as a function of the (J − H) o color-cut for a sample limited to V < 16 and with a 2MASS counterpart. They concluded that selecting sources with (J − H) o < +0.15 is optimal for completeness, but (J − H) o < +0.10 is actually more efficient and produces less contamination by A v stars.…”

Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1

“…2MASS scanned the sky at 2 00 pixels, using automated 1.3 m telescopes on Mount Hopkins and the Cerro Tololo Inter-American Observatory, taking images at J, H, and K. It includes thousands of BHB candidates with distances up to $8 kpc, which can be selected from their J À H and J À K colors ( Brown et al 2004).…”