We have compiled L 0 (3.4-4.1 m) and M 0 (4.6-4.8 m) photometry of 63 single and binary M, L, and T dwarfs obtained at the United Kingdom Infrared Telescope using the Mauna Kea Observatory filter set. This compilation includes new L 0 measurements of eight L dwarfs and 13 T dwarfs and new M 0 measurements of seven L dwarfs, five T dwarfs, and the M1 dwarf Gl 229A. These new data increase by factors of 0.6 and 1.6, respectively, the numbers of ultracool dwarfs (T eff P 2400 K) for which L 0 and M 0 measurements have been reported. We compute L bol , BC K , and T eff for 42 dwarfs whose flux-calibrated JHK spectra, L 0 photometry, and trigonometric parallaxes are available, and we estimate these quantities for nine other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC K is a well-behaved function of near-infrared spectral type with a dispersion of $0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T eff declines steeply and monotonically for types M6-L7 and T4-T9, but it is nearly constant at $1450 K for types L7-T4 with assumed ages of $3 Gyr. This constant T eff is evidenced by nearly unchanging values of L 0 -M 0 between types L6 and T3. It also supports recent models that attribute the changing near-infrared luminosities and spectral features across the L-T transition to the rapid migration, disruption, and/or thinning of condensate clouds over a narrow range of T eff . The L 0 and M 0 luminosities of early-T dwarfs do not exhibit the pronounced humps or inflections previously noted in the I through K bands, but insufficient data exist for types L6-T5 to assert that M L 0 and M M 0 are strictly monotonic within this range of types. We compare the observed K, L 0 , and M 0 luminosities of L and T dwarfs in our sample with those predicted by precipitating-cloud and cloud-free models for varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M 0 luminosities of late-T dwarfs are 1.5-2.5 times larger than those derived empirically for the late-T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 m by CO, which is not expected under the condition of thermochemical equilibrium assumed in the models. Our photometry and bolometric calculations indicate that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57À041403.5 are probable binary systems. We compute log (L bol /L ) = À5.73 AE 0.05 and T eff = 600-750 K for the T9 dwarf 2MASSI J0415195À093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.

We present a spectroscopic analysis of nearly 8000 late-type dwarfs in the Sloan Digital Sky Survey. Using the Hα emission line as an activity indicator, we investigate the fraction of active stars as a function of spectral type and find a peak near type M8, confirming previous results. In contrast to past findings, we find that not all M7-M8 stars are active. We show that this may be a selection effect of the distance distributions of previous samples, as the active stars appear to be concentrated near the Galactic Plane. We also examine the activity strength (ratio of the luminosity emitted in Hα to the bolometric luminosity) for each star, and find that the mean activity strength is constant over the range M0-M5 and declines at later types. The decline begins at a slightly earlier spectral type than previously found. We explore the effect that activity has on the broadband photometric colors and find no significant differences between active and inactive stars. We also carry out a search for subdwarfs using spectroscopic metallicity indicators, and find 60 subdwarf candidates. Several of these candidates are near the extreme subdwarf boundary. The spectroscopic subdwarf candidates are redder by ∼ 0.2 magnitudes in g − r compared to disk dwarfs at the same r − i color.

We announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of ∼60 kpc. It was found in a systematic search for stellar overdensities in the north Galactic cap using Sloan Digital Sky Survey Data Release 5. The color-magnitude diagram shows a well-defined turnoff, red giant branch, and extended horizontal branch. Its absolute magnitude is mag, which makes it one of the faintest M ∼ Ϫ5.8 V galaxies known. The half-light radius is ∼220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.

We study 16,707 quasar spectra from the Sloan Digital Sky Survey (SDSS) (an early version of the First Data Release; DR1) using the Karhunen-Loève (KL) transform (or Principal Component Analysis, PCA). The redshifts of these quasars range from 0.08 to 5.41, the i-band absolute magnitudes from −30 to −22, and the resulting restframe wavelengths from 900Å to 8000Å. The quasar eigenspectra of the full catalog reveal the following: 1st order -the mean spectrum; 2nd order -a host-galaxy component; 3rd order -the UV-optical continuum slope; 4th order -the correlations of Balmer emission lines. These four eigenspectra account for 82 % of the total sample variance. Broad absorption features are found not to be confined in one particular order but to span a number of higher orders. We find that the spectral classification of quasars is redshift and luminosity dependent, as such there does not exist a compact set (i.e., less than ≈ 10 modes) of eigenspectra (covering 900Å to 8000Å) which can describe most variations (i.e., greater than ≈ 95 %) of the entire catalog. We therefore construct several sets of eigenspectra in different redshift and luminosity bins. From these eigenspectra we find that quasar spectra can be classified (by the first two eigenspectra) into a sequence that is defined by a simple progression in the steepness of the slope of the continuum. We also find a dependence on redshift and luminosity in the eigencoefficients. The dominant redshift effect is a result of the evolution of the blended Fe II emission (optical) and the Balmer continuum (the "small bump", λ rest ≈ 2000−4000Å). A luminosity dependence is also present in the eigencoefficients and is related to the Baldwin

We perform an objective classification of 170,000 galaxy spectra in the Sloan Digital Sky Survey (SDSS) using the Karhunen-Loève (KL) transform. With about one-sixth of the total set of galaxy spectra that will be obtained by the survey, we are able to carry out the most extensive analysis of its kind to date. The formalism proposed by Connolly and Szalay is adopted to correct for gappy regions in the spectra and to derive eigenspectra and eigencoefficients. From this analysis, we show that this gap-correction formalism leads to a converging set of eigenspectra and KL-repaired spectra. Furthermore, KL eigenspectra of galaxies are found to be convergent not only as a function of iteration, but also as a function of the number of randomly selected galaxy spectra used in the analysis. From these data a set of 10 eigenspectra of galaxy spectra are constructed, with rest-wavelength coverage 3450-8350 8. The eigencoefficients describing these galaxies naturally place the spectra into several classes defined by the plane formed by the first three eigencoefficients of each spectrum. Spectral types corresponding to different Hubble types and galaxies with extreme emission lines are identified for the 170,000 spectra and are shown to be complementary to existing spectral classifications. From a nonparametric classification technique, we find that the population of galaxies can be divided into three classes that correspond to early late-type through intermediate late-type galaxies. This finding is believed to be related to the color separation of SDSS galaxies discussed in earlier works. Bias in the spectral classifications due to the aperture spectroscopy in the SDSS is small and within the signal-to-noise limit for a majority of galaxies, except for the reddest nearby galaxies and large galaxies (>30 kpc) with prominent emissions. The mean spectra and eigenspectra derived from this work can be downloaded from the SDSS Web site.