Direct Optical Coupling to an Unoccupied Dirac Surface State in the Topological Insulator Bi2_2Se3_3

We characterize the occupied and unoccupied electronic structure of the topological insulator Bi$_2$Se$_3$ by one-photon and two-photon angle-resolved photoemission spectroscopy and slab band structure calculations. We reveal a second, unoccupied Dirac surface state with similar electronic structure and physical origin to the well-known topological surface state. This state is energetically located 1.5 eV above the conduction band, which permits it to be directly excited by the output of a Ti:Sapphire laser. This discovery demonstrates the feasibility of direct ultrafast optical coupling to a topologically protected, spin-textured surface state.Comment: Accepted to Physical Review Letter

EVOLUTION OF IR-SELECTED GALAXIES IN Z~0.4 CLUSTERS

Wide-field optical and near--IR ($JHK$) imaging is presented for two rich galaxy clusters: Abell~370 at $z=0.374$ and Abell~851 (Cl0939+47) at $z=0.407$. Galaxy catalogs selected from the near--IR images are 90\% complete to approximately 1.5 mag below $K^\ast$ resulting in samples with $\sim$100 probable member galaxies per cluster in the central $\sim$2 Mpc. Comparison with $HST$ WFPC images yields subsamples of $\sim$70 galaxies in each cluster with morphological types. Analysis of the complete samples and the $HST$ subsamples shows that the $z\sim 0.4$ E/S0s are bluer than those in the Bower et al.\ (1992) Coma sample in the optical$-K$ color by $0.13$~mag for Abell~370 and by $0.18$~mag for Abell~851. If real, the bluing of the E/S0 populations at moderate redshift is consistent with that calculated from the Bruzual and Charlot (1993) models of passive elliptical galaxy evolution. In both clusters the intrinsic scatter of the known E/S0s about their optical$-K$ color--mag relation is small ($\sim 0.06$ mag) and not significantly different from that of Coma E/S0s as given by Bower et al.\ (1992), indicating that the galaxies within each cluster formed at the same time at an early epoch.Comment: uuencoded gzipped tar file containing latex files of manuscript (42 pages) plus tables (9 pages); figures available by anonymous ftp at ftp://ipac.caltech.edu//pub/pickup/sed ; accepted for publication in the Ap

The kinematics and morphology of NGC 520: One, two, or three galaxies

The peculiar galaxy NGC 520 (Arp 157) is often interpreted as an interacting pair of galaxies. The identification of the two bulges and overall morphology of the two galaxies has long been a puzzle which the authors attempt to solve here. New longslit optical spectroscopy and near-infrared images of NGC 520 are presented. These data suggest that the northwest peak is the bulge of one of two galaxies in the system. The other larger bulge is clearly evident in the K band image in the middle of the dust lane. The stellar radial velocity profile in the central 10 seconds of the larger bulge is consistent with counterrotation seen in the molecular gas component. This kinematic subsystem could be the remains of a merged gas-rich irregular

Brunnian links are determined by their complements

If L_1 and L_2 are two Brunnian links with all pairwise linking numbers 0, then we show that L_1 and L_2 are equivalent if and only if they have homeomorphic complements. In particular, this holds for all Brunnian links with at least three components. If L_1 is a Brunnian link with all pairwise linking numbers 0, and the complement of L_2 is homeomorphic to the complement of L_1, then we show that L_2 may be obtained from L_1 by a sequence of twists around unknotted components. Finally, we show that for any positive integer n, an algorithm for detecting an n-component unlink leads immediately to an algorithm for detecting an unlink of any number of components. This algorithmic generalization is conceptually simple, but probably computationally impractical.Comment: Published by Algebraic and Geometric Topology at http://www.maths.warwick.ac.uk/agt/AGTVol1/agt-1-7.abs.htm

Near-IR imaging of moderate redshift galaxy clusters

We have obtained near-IR imaging of 3 moderate-z clusters on the 1.3 m at KPNO with SQIID, a new camera offering wide-field (5.5 arcmin) simultaneous JHK band imaging. Our photometry on a sample of approximately 100 likely member galaxies in one of the clusters, Abell 370 at z = 0.37, shows that we can obtain magnitudes good to 20 percent down to K = 18, considerably below the estimated K* = 16.5 at this redshift. These data indicate that there are no systematic problems in obtaining photometry at faint levels with SQIID. With the development of larger arrays, the field is open to progress. The resulting J, H, and K data for three clusters are combined with previously obtained multiband optical photometry. We present an investigation of the spectral properties and evolution of the dominant cold stellar populations by comparing optical-to-IR colors and color-magnitude diagrams to predictions from population synthesis models and galaxy spectral evolution codes

Discovery of a Radio-loud/Radio-quiet Binary Quasar

We report the discovery of a small separation quasar pair (z=0.586, O=18.4, 19.2, sep. = 2.3 arcsec) associated with the radio source FIRST J164311.3+315618 (S_1400 = 120 mJy). The spectrum of the brighter quasar (A) has a much stronger narrow emission-line spectrum than the other (B), and also stronger Balmer lines relative to the continuum. The continuum ratio of the spectra is flat in the blue at about 2.1, but falls to 1.5 at longer wavelengths. A K' image shows two unresolved sources with a flux ratio of 1.3. The different colors appear to result from the contribution of the host galaxy of B, which is evident from Ca II and high-order Balmer absorption lines indicative of a substantial young stellar population. New 3.6 cm VLA observations show that the compact radio source is coincident with quasar A (B is only marginally detected). We rule out the lensing hypothesis because the optical flux ratio is A/B = 1.2 to 2, while the radio flux ratio is A/B > 40, and conclude that this system is a binary. Moreover, the radio-loud quasar is a compact steep spectrum source. FIRST J164311.3+315618A, B is the lowest redshift and smallest separation binary quasar yet identified.Comment: 8 pages, 3 figures, to appear in Astrophysical Journal Letter