A specialized data mining algorithm has been developed using wide-field photometry catalogues, enabling systematic and efficient searches for resolved, extremely low surface brightness satellite galaxies in the halo of the Milky Way (MW). Tested and calibrated with the Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) we recover all fifteen MW satellites recently detected in SDSS, six known MW/Local Group dSphs in the SDSS footprint, and 19 previously known globular and open clusters. In addition, 30 point source overdensities have been found that correspond to no cataloged objects. The detection efficiencies of the algorithm have been carefully quantified by simulating more than three million model satellites embedded in star fields typical of those observed in SDSS, covering a wide range of parameters including galaxy distance, scale-length, luminosity, and Galactic latitude. We present several parameterizations of these detection limits to facilitate comparison between the observed Milky Way satellite population and predictions. We find that all known satellites would be detected with > 90% efficiency over all latitudes spanned by DR6 and that the MW satellite census within DR6 is complete to a magnitude limit of M V ≈ −6.5 and a distance of 300 kpc. Assuming all existing MW satellites contain an appreciable old stellar population and have sizes and luminosities comparable to currently known companions, we predict lower and upper limit totals of 52 and 340 Milky Way dwarf satellites within ∼ 260 kpc if they are uniformly distributed across the sky. This result implies that many MW satellites still remain undetected. Identifying and studying these elusive satellites in future survey data will be fundamental to test the dark matter distribution on kpc scales.

We present an investigation of the transmission spectrum of the 6.5 M ⊕ planet GJ 1214b based on new ground-based observations of transits of the planet in the optical and near-infrared, and on previously published data. Observations with the VLT + FORS and Magellan + MMIRS using the technique of multi-object spectroscopy with wide slits yielded new measurements of the planet's transmission spectrum from 0.61 to 0.85 μm, and in the J, H, and K atmospheric windows. We also present a new measurement based on narrow-band photometry centered at 2.09 μm with the VLT + HAWKI. We combined these data with results from a reanalysis of previously published FORS data from 0.78 to 1.00 μm using an improved data reduction algorithm, and previously reported values based on Spitzer data at 3.6 and 4.5 μm. All of the data are consistent with a featureless transmission spectrum for the planet. Our K-band data are inconsistent with the detection of spectral features at these wavelengths reported by Croll and collaborators at the level of 4.1σ . The planet's atmosphere must either have at least 70% H 2 O by mass or optically thick high-altitude clouds or haze to be consistent with the data.

As part of preparations for a southern sky search for faint Milky Way dwarf galaxy satellites, we report the discovery of a stellar overdensity in the Sloan Digital Sky Survey Data Release 5, lying at an angular distance of only 1.5Њ from the recently discovered Boötes dwarf. The overdensity was detected well above statistical noise by employing a sophisticated data-mining algorithm and does not correspond to any cataloged object. Overlaid isochrones using stellar population synthesis models show that the color-magnitude diagram of that region has the signature of an old (12 Gyr), metal-poor ( ) stellar population at a tentative distance of 60 kpc, evidently Fe/H ≈ Ϫ2.0 the same heliocentric distance as the Boötes dwarf. We estimate the new object to have a total magnitude of mag and a half-light radius of ( pc), placing it in an apparent 40 pc ! M ∼ Ϫ3.1 ‫ע‬ 1.1 r p 4.1 ‫ע‬ 1.6 72 ‫ע‬ 28 V h r h ! 100 pc void between globular clusters and dwarf galaxies, occupied only by another recently discovered Milky Way satellite, Coma Berenices.

We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find the half-light radius of Segue 3 is 26 ± 5 (2.1 ± 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere M V = 0.0 ± 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0 +1.5 −0.4 Gyr and an [Fe/H] of −1.7 +0.07 −0.27 . Line-of-sight velocities from the spectra are combined with the photometry to determine a sample of 32 stars which are likely associated with Segue 3. The member stars within three half-light radii have a velocity dispersion of 1.2 ± 2.6 km s −1 . Photometry of the members indicates the stellar population has a spread in [Fe/H] of ∼ < 0.3 dex. These facts, together with the small physical size of Segue 3, imply the object is likely an old, faint stellar cluster which contains no significant dark matter. We find tentative evidence for stellar mass loss in Segue 3 through the eleven candidate member stars outside of three half-light radii, as expected from dynamical arguments. Interpretation of the data outside of three half-light radii, is complicated by the object's spatial coincidence with a previously known halo substructure, which may enhance contamination of our member sample.