Velocities from Cross-Correlation: A Guide for Self-Improvement

The measurement of Doppler velocity shifts in spectra is a ubiquitous theme in astronomy, usually handled by computing the cross-correlation of the signals, and finding the location of its maximum. This paper addresses the problem of the determination of wavelength or velocity shifts among multiple spectra of the same, or very similar, objects. We implement the classical cross-correlation method and experiment with several simple models to determine the location of the maximum of the cross-correlation function. We propose a new technique, 'self-improvement', to refine the derived solutions by requiring that the relative velocity for any given pair of spectra is consistent with all others. By exploiting all available information, spectroscopic surveys involving large numbers of similar objects may improve their precision significantly. As an example, we simulate the analysis of a survey of G-type stars with the SDSS instrumentation. Applying 'self-improvement' refines relative radial velocities by more than 50% at low signal-to-noise ratio. The concept is equally applicable to the problem of combining a series of spectroscopic observations of the same object, each with a different Doppler velocity or instrument-related offset, into a single spectrum with an enhanced signal-to-noise ratio.Comment: 7 pages, 3 figures, uses emulateapj.cls; to appear in the Astronomical Journal; see http://hebe.as.utexas.edu/stools/ to obtain the companion softwar

Quantifying Kinematic Substructure in the Milky Way's Stellar Halo

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative "close pair distribution" (CPD) as a statistic in the 4-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock-observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at $\rm r_{gc} < 20$ kpc.Comment: 29 page, 10 figures, 1 table; accepted by APJ; for related article by another group see arXiv:1011.192

Spectroscopy of Quasar Candidates from SDSS Commissioning Data

The Sloan Digital Sky Survey has obtained images in five broad-band colors for several hundred square degrees. We present color-color diagrams for stellar objects, and demonstrate that quasars are easily distinguished from stars by their distinctive colors. Follow-up spectroscopy in less than ten nights of telescope time has yielded 22 new quasars, 9 of them at $z> 3.65$, and one with $z = 4.75$, the second highest-redshift quasar yet known. Roughly 80% of the high-redshift quasar candidates selected by color indeed turn out to be high-redshift quasars.Comment: 4 pages, 3 figures, to appear in the proceedings of "After the Dark Ages: When Galaxies were Young (the Universe at 2<z<5)", 9th Annual October Astrophysics Conference in Marylan

Can WIMP Spin Dependent Couplings explain DAMA data, in light of Null Results from Other Experiments?

We examine whether the annual modulation found by the DAMA dark matter experiment can be explained by Weakly Interacting Massive Particles (WIMPs), in light of new null results from other experiments. CDMS II has already ruled out most WIMP-nucleus spin-independent couplings as an explanation for DAMA data. Hence we here focus on spin-dependent (axial vector; SD) couplings of WIMPs to nuclei. We expand upon previous work by (i) considering the general case of coupling to both protons and neutrons and (ii) incorporating bounds from all existing experiments. We note the surprising fact that CMDS II places one of the strongest bounds on the WIMP-neutron cross-section, and show that SD WIMP-neutron scattering alone is excluded. We also show that SD WIMP-proton scattering alone is allowed only for WIMP masses in the 5-13 GeV range. For the general case of coupling to both protons and neutrons, we find that, for WIMP masses above 13 GeV and below 5 GeV, there is no region of parameter space that is compatible with DAMA and all other experiments. In the range (5-13) GeV, we find acceptable regions of parameter space, including ones in which the WIMP-neutron coupling is comparable to the WIMP-proton coupling.Comment: 11 pages, 5 figures. v2: added Baksan results, added references, other minor changes. v3: minor changes to match PRD versio

The Chemistry of the Trailing arm of the Sagittarius Dwarf Galaxy

We present abundances of C, O, Ti, and Fe for eleven M-giant stars in the trailing tidal arm of the Sagittarius dwarf (Sgr). The abundances were derived by comparing synthetic spectra with high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. The targeted stars are drawn from two regions of the Sgr trailing arm separated by 66 degrees (5 stars) and 132 degrees (6 stars) from the main body of Sgr. The trailing arm provides a more direct diagnostic of the chemical evolution of Sgr compared to the extensively phase-mixed leading arm. Within our restricted sample of ~2-3 Gyr old stars, we find that the stream material exhibits a significant metallicity gradient of -(2.4\pm0.3)x10^{-3} dex / degree (-(9.4\pm1.1)x10^{-4} dex / kpc) away from the main body of Sgr. The tidal disruption of Sgr is a relatively recently event. We therefore interpret the presence of a metallicity gradient in the debris as indicative of a similar gradient in the progenitor. The fact that such a metallicity gradient survived for almost a Hubble time indicates that the efficiency of radial mixing was very low in the Sgr progenitor. No significant gradient is seen to exist in the [alpha/Fe] abundance ratio along the trailing arm. Our results may be accounted for by a radial decrease in star formation efficiency and/or radial increase in the efficiency of galactic wind-driven metal loss in the chemical evolution of the Sgr progenitor. The [Ti/Fe] and [O/Fe] abundance ratios observed within the stream are distinct from those of the Galactic halo. We conclude that the fraction of the intermediate to metal-rich halo population contributed by the recent dissolution (<3 Gyr) of Sgr-like dwarf galaxies can not be substantial.Comment: 22 pages, 7 figures, ApJ accepte

Imaging of the Host Galaxies of Three X-Ray Selected BL Lacertae Objects

Hubble Space Telescope (HST) WFPC-2 I-band (F814W) images of three X-ray selected BL Lacertae objects (MS1221.8+2452, MS1407.9+5954, & MS2143.4+0704) reveal that each of these BL Lac objects is well-centered in an extended nebulosity that is consistent in brightness and morphology with being light from an elliptical galaxy at the previously reported redshifts of these BL Lac objects. Each of the detected host galaxies have radial surface brightness profiles that are well fit by a DeVaucouleurs' law with effective radii of between 3 to 12 kpc (H_0=50 km s^{-1} Mpc^{-1}, q_0 = 0). The absolute magnitudes of the host galaxies fall in the range -24.7 < M_I < -23.5, in the range of luminosities determined for other BL Lacertae object host galaxies. In addition to allowing the measurement of the host galaxy magnitudes and radial surface brightness profiles, the HST images allow a search for substructure in the host galaxies and the presence of close companion galaxies at spatial resolutions not yet achievable from the ground. While no evidence was found for any ``bars'' or spiral arms, ``boxy'' isophotes are present in the host galaxy of at least one of the three objects observed as part of this study (MS2143.4+0704). The apparent magnitudes and image properties of the companions of the BL Lac objects are catalogued as part of this work. The three BL Lacs appear to occur in diverse environments, from being fairly isolated (MS1221.8+2452) to possibly being a member of a rich group of galaxies (MS1407.9+5954).Comment: Latex, 5 postscript figures, 4 jpeg figures, To appear in The Astrophysical Journal Nov 20, 199