Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared (k P 5 m) continuum of stellar populations is dominated by the composite blackbody curve and peaks at approximately 1.6 m, the ultraviolet to mid-infrared continuum of active galactic nuclei (AGNs) is dominated by a power law. Consequently, with a sufficient wavelength baseline, one can easily distinguish AGNs from stellar populations. Mirroring the tendency of AGNs to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGNs tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. On the basis of this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGNs, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1 galaxies. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGNs and find evidence for a large population of optically obscured active galaxies.

We present early observations of the afterglow of GRB 030329 and the spectroscopic discovery of its associated supernova SN 2003dh. We obtained spectra of the afterglow of GRB 030329 each night from March 30.12 (0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The spectra cover a wavelength range of 350-850 nm. The early spectra consist of a power-law continuum ( ) with narrow emission lines orig-Ϫ0.9

We explore the connection between different classes of active galactic nuclei (AGNs) and the evolution of their host galaxies, by deriving host galaxy properties, clustering, and Eddington ratios of AGNs selected in the radio, X-ray, and infrared (IR) wavebands. We study a sample of 585 AGNs at 0.25 < z < 0.8 using redshifts from the AGN and Galaxy Evolution Survey (AGES). We select AGNs with observations in the radio at 1.4 GHz from the Westerbork Synthesis Radio Telescope, X-rays from the Chandra XBoötes Survey, and mid-IR from the Spitzer IRAC Shallow Survey. The radio, X-ray, and IR AGN samples show modest overlap, indicating that to the flux limits of the survey, they represent largely distinct classes of AGNs. We derive host galaxy colors and luminosities, as well as Eddington ratios, for obscured or optically faint AGNs. We also measure the two-point cross-correlation between AGNs and galaxies on scales of 0.3-10 h −1 Mpc, and derive typical dark matter halo masses. We find that: (1) radio AGNs are mainly found in luminous red sequence galaxies, are strongly clustered (with M halo ∼ 3 × 10 13 h −1 M ⊙ ), and have very low Eddington ratios (λ 10 −3 ); (2) X-rayselected AGNs are preferentially found in galaxies that lie in the "green valley" of color-magnitude space and are clustered similar to typical AGES galaxies (M halo ∼ 10 13 h −1 M ⊙ ), with 10 −3 λ 1; (3) IR AGNs reside in slightly bluer, slightly less luminous galaxies than X-ray AGNs, are weakly clustered (M halo 10 12 h −1 M ⊙ ), and have λ > 10 −2 . We interpret these results in terms of a simple model of AGN and galaxy evolution, whereby a "quasar" phase and the growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a critical mass between ∼10 12 and 10 13 M ⊙ . After this event, star formation ceases and AGN accretion shifts from radiatively efficient (optical-and IR-bright) to radiatively inefficient (optically faint, radio-bright) modes.

In the ΛCDM paradigm, the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with Gaia, is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants atWe identify known structures including the high-α disk, the in situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), Gaia-Sausage-Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (i) Aleph ([Fe/H]=−0.5), a low-eccentricity structure that rises a surprising 10 kpc off the plane, (ii) and (iii) Arjuna ([Fe/H]=−1.2) and I'itoi ([Fe/H]<−2), which comprise the high-energy retrograde halo along with Sequoia, and (iv) Wukong ([Fe/H]=−1.6), a prograde phase-space overdensity chemically distinct from GSE. For each structure, we provide [Fe/H], [α/Fe], and orbital parameters. Stars born within the Galaxy are a major component at | | Z 2 kpc (≈60%), but their relative fraction declines sharply to 5% past 15 kpc. Beyond 15 kpc, >80% of the halo is built by two massive (M å ∼10 8 -10 9 M e ) accreted dwarfs: GSE ([Fe/H]=−1.2) within 25 kpc and Sgr ([Fe/H]=−1.0) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H]≈−1.2). We attribute 95% of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.

We present new high signal-to-noise spectroscopic data on the M31 globular cluster (GC) system, obtained with the Hectospec multifiber spectrograph on the 6.5 m MMT. More than 300 clusters have been observed at a resolution of 5 Å and with a median S/N of 75 per Å, providing velocities with a median uncertainty of 6 km s −1 . The primary focus of this paper is the determination of mean cluster metallicities, ages, and reddenings. Metallicities were estimated using a calibration of Lick indices with [Fe/H] provided by Galactic GCs. These match well the metallicities of 24 M31 clusters determined from Hubble Space Telescope color-magnitude diagrams, the differences having an rms of 0.2 dex. The metallicity distribution is not generally bimodal, in strong distinction with the bimodal Galactic globular distribution. Rather, the M31 distribution shows a broad peak, centered at [Fe/H] = −1, possibly with minor peaks at [Fe/H] = −1.4, −0.7, and −0.2, suggesting that the cluster systems of M31 and the Milky Way had different formation histories. Ages for clusters with [Fe/H] > −1 were determined using the automatic stellar population analysis program EZ_Ages. We find no evidence for massive clusters in M31 with intermediate ages, those between 2 and 6 Gyr. Moreover, we find that the mean ages of the old GCs are remarkably constant over about a decade in metallicity (−0.95 [Fe/H] 0.0).