Evalyn Gates

University of Chicago, Chicago, IL, United States

Are you Evalyn Gates?

Claim your profile

Publications (34)167.99 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We obtained low-resolution spectroscopy of 98 targets using the 9.2m HET in the queue mode and the Marcario Low Resolution Spectrograph (LRS). Our observations were performed between 2004 October and 2006 July. We obtained JHK photometry of 126 WDs using the Near Infra-Red Imager and Spectrometer (NIRI) on Gemini-North, the 0.8-5.4um medium-resolution spectrograph and imager (SpeX) on the Infrared Telescope Facility (IRTF), and the Wide-Field Camera (WFCAM) on the United Kingdom Infra-Red Telescope (UKIRT). The IRTF observations were obtained on several observing runs between 2004 December and 2006 April. The UKIRT observations (of J0146+1404 and J2239+0018) were performed on UT 2007 September 20. (3 data files).
    VizieR Online Data Catalog. 09/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs (WDs) in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool WD candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool WDs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope data sets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km s–1. Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool WD candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool WDs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo WD candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.
    The Astrophysical Journal Supplement Series 08/2010; 190(1):77. · 16.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11,663 deg^2 of imaging data, with most of the ~2000 deg^2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes repeat photometry on a 120° long, 2°.5 wide stripe along the celestial equator in the Southern Galactic Cap, with some regions covered by as many as 90 individual imaging runs. We include a co-addition of the best of these data, going roughly 2 mag fainter than the main survey over 250 deg^2. The survey has completed spectroscopy over 9380 deg^2; the spectroscopy is now complete over a large contiguous area of the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including 930,000 galaxies, 120,000 quasars, and 460,000 stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog, reducing the rms statistical errors at the bright end to 45 milliarcseconds per coordinate. We further quantify a systematic error in bright galaxy photometry due to poor sky determination; this problem is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities.
    The Astrophysical Journal Supplement Series 05/2009; 182(2):543. · 16.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass. We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages.
    04/2009;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Modern cosmology has sharpened questions posed for millennia about the origin of our cosmic habitat. The age-old questions have been transformed into two pressing issues primed for attack in the coming decade: How did the Universe begin? and What physical laws govern the Universe at the highest energies? The clearest window onto these questions is the pattern of polarization in the Cosmic Microwave Background (CMB), which is uniquely sensitive to primordial gravity waves. A detection of the special pattern produced by gravity waves would be not only an unprecedented discovery, but also a direct probe of physics at the earliest observable instants of our Universe. Experiments which map CMB polarization over the coming decade will lead us on our first steps towards answering these age-old questions.
    03/2009;
  • Source
    Evalyn I. Gates, Geza Gyuk, and Michael S. Turner
    [Show abstract] [Hide abstract]
    ABSTRACT: For almost 20 years models of the Galaxy have included a dark halo responsible for supporting a substantial fraction of the local rotation velocity and a flat rotation curve at large distances. Estimates of the local halo density range from 2 × 10-25 g cm-3 to 10 × 10-25 g cm-3. By careful modeling of the Galaxy, taking account of the evidence that dark halos are flattened and recent microlensing data, we arrive at a more quantitative estimate, 9.2+ 3.8−3.1 × 10-25 g cm-3. Microlensing toward the LMC indicates that only a small fraction, less than ~30%, can be in the form of MACHOs, which is consistent with the idea that most of the halo consists of cold dark matter particles.
    The Astrophysical Journal 01/2009; 449(2):L123. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: One interpretation of the more than dozen microlensing events seen in the direction of the LMC is a halo population of 0.5 M☉ MACHOs that accounts for about half of the mass of the Galaxy. Such an interpretation is not without its problems, and we show that LMC microlensing can be explained by a combination of dark components of the thick disk and spheroid in a viable and detailed model of the Galaxy. In our models, the total mass within 50 kpc is ~1011 M☉, about 60% of the value based on halo MACHO models, and the estimate for the lens mass is lower, 0.3 M☉ versus 0.5 M☉. The chemical evolution problems associated with the MACHO progenitors are not resolved. However, since the MACHO distribution does not trace the 1/r2 dark halo, which extends significantly beyond 50 kpc, the total baryon mass fraction of the Galaxy is reduced considerably, which lessens the problem.
    The Astrophysical Journal 01/2009; 500(2):L145. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11,663 deg2 of imaging data, with most of the ~2000 deg2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes repeat photometry on a 120° long, 2fdg5 wide stripe along the celestial equator in the Southern Galactic Cap, with some regions covered by as many as 90 individual imaging runs. We include a co-addition of the best of these data, going roughly 2 mag fainter than the main survey over 250 deg2. The survey has completed spectroscopy over 9380 deg2 the spectroscopy is now complete over a large contiguous area of the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including 930,000 galaxies, 120,000 quasars, and 460,000 stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog, reducing the rms statistical errors at the bright end to 45 milliarcseconds per coordinate. We further quantify a systematic error in bright galaxy photometry due to poor sky determination; this problem is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities.
    The Astrophysical Journal Supplement Series, v.182, 543-558 (2009). 01/2009;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Sloan Extension for Galactic Understanding and Exploration (SEGUE) Survey obtained ≈240,000 moderate-resolution (R ~ 1800) spectra from 3900 Što 9000 Šof fainter Milky Way stars (14.0 < g < 20.3) of a wide variety of spectral types, both main-sequence and evolved objects, with the goal of studying the kinematics and populations of our Galaxy and its halo. The spectra are clustered in 212 regions spaced over three quarters of the sky. Radial velocity accuracies for stars are \sigma (\rm RV) \sim 4 \:\rm km\; s^{-1} at g < 18, degrading to \sigma (\rm RV) \sim 15\rm \:km\; s^{-1} at g ~ 20. For stars with signal-to-noise ratio >10 per resolution element, stellar atmospheric parameters are estimated, including metallicity, surface gravity, and effective temperature. SEGUE obtained 3500 deg2 of additional ugriz imaging (primarily at low Galactic latitudes) providing precise multicolor photometry (sigma(g, r, i) ~ 2%), (sigma(u, z) ~ 3%) and astrometry (≈0farcs1) for spectroscopic target selection. The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of Sloan Digital Sky Survey Data Release 7.
    The Astronomical Journal 01/2009; · 4.97 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system—its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310–00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.
    The Astrophysical Journal 12/2008; 679(1):697. · 6.73 Impact Factor
  • Source
    Evalyn I. Gates, Geza Gyuk
    [Show abstract] [Hide abstract]
    ABSTRACT: We suggest a new component of the Milky Way galaxy that can account for both the optical depth and the event durations obtained by the MACHO microlensing survey toward the Large Magellanic Cloud. This component is consistent with recent evidence for a significant population of faint white dwarf stars, detected in a proper-motion study of the Hubble Deep Field, which cannot be accounted for by stars in the disk or spheroid. This new component, which represents less than 4% of the total dark halo mass, consists of (mostly) old white dwarf stars distributed in a highly extended (very thick) disk configuration. It extends beyond the traditional thin and thick disks, but well within the dark, roughly spherical cold dark matter (CDM) halo. We explore the distribution of proper motions expected for white dwarfs in this model and the ability of future surveys to differentiate between models. Finally, we argue that such a component is reasonable, natural, and consistent with a variety of observations, and many of the problems associated with a significant halo population of white dwarfs are ameliorated.
    The Astrophysical Journal 12/2008; 547(2):786. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper describes the Sixth Data Release of the Sloan Digital Sky Survey. With this data release, the imaging of the northern Galactic cap is now complete. The survey contains images and parameters of roughly 287 million objects over 9583 deg^2, including scans over a large range of Galactic latitudes and longitudes. The survey also includes 1.27 million spectra of stars, galaxies, quasars, and blank sky (for sky subtraction) selected over 7425 deg^2. This release includes much more stellar spectroscopy than was available in previous data releases and also includes detailed estimates of stellar temperatures, gravities, and metallicities. The results of improved photometric calibration are now available, with uncertainties of roughly 1% in g, r, i, and z, and 2% in u, substantially better than the uncertainties in previous data releases. The spectra in this data release have improved wavelength and flux calibration, especially in the extreme blue and extreme red, leading to the qualitatively better determination of stellar types and radial velocities. The spectrophotometric fluxes are now tied to point-spread function magnitudes of stars rather than fiber magnitudes. This gives more robust results in the presence of seeing variations, but also implies a change in the spectrophotometric scale, which is now brighter by roughly 0.35 mag. Systematic errors in the velocity dispersions of galaxies have been fixed, and the results of two independent codes for determining spectral classifications and redshifts are made available. Additional spectral outputs are made available, including calibrated spectra from individual 15 minute exposures and the sky spectrum subtracted from each exposure. We also quantify a recently recognized underestimation of the brightnesses of galaxies of large angular extent due to poor sky subtraction; the bias can exceed 0.2 mag for galaxies brighter than r = 14 mag.
    The Astrophysical Journal Supplement Series 04/2008; · 16.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The coolest and dimmest white dwarfs may be among the oldest objects in the Galaxy and can be used to date galactic populations. In the last decade, cooling curves for these objects have been extended to much lower temperatures, indicating a unique spectral profile, and hence colors, caused by collisionally-induced absorption (CIA) by molecular hydrogen at temperatures below about 5000K. So far, however, only a handful of these coolest white dwarfs have been observed. We have begun a program of systematic search and follow-up to significantly increase the number of known ultra-cool white dwarfs with temperatures below 4000K. Utilizing Sloan Digital Sky Survey spectroscopic and photometric data we have identified a pool of candidates showing colors, proper motions and visible light spectroscopic signatures consistent with very cool white dwarfs. We are performing J, H, and Ks band near-IR follow-up with the APO 3.5 meter to confirm the existence of a strong CIA feature expected for ultra-cool white dwarfs. So far around 10 new very cool white dwarfs have been found in the SDSS. In addition, many more show colors and temperatures for which CIA is just becoming effective. The discovery and characterization of many more very cool and ultra-cool white dwarfs with temperatures near and below 4000 K will provide needed constraints to white dwarf cooling models at the end of the cooling sequence.
    05/2007;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the discovery of five very cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Four are ultracool, exhibiting strong collision induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250 and LHS 1402. The fifth, an ultracool white dwarf candidate, shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (T_{eff} ~ 5000K) white dwarf companion.
    The Astrophysical Journal 05/2004; · 6.73 Impact Factor
  • Source
    Evalyn Gates, Geza Gyuk
    [Show abstract] [Hide abstract]
    ABSTRACT: We suggest a new component of the Milky Way that can account for both the optical depth and event durations implied by microlensing searches targeting the Large Magellanic Cloud. This component, which represents less than 4% of the total dark matter halo mass, consists of mainly old white dwarf stars in a distribution that extends beyond the disk of the galaxy, but lies well within the dark matter halo. It is consistent with recent evidence for a significant population of white dwarfs detected in proper motion studies of the Hubble Deep Field that cannot be accounted for by stars in the disk or spheroid. Further, it evades all of the current observational constraints that restrict a halo population of white dwarfs.
    05/2000;
  • Source
    Evalyn I. Gates, Geza Gyuk
    [Show abstract] [Hide abstract]
    ABSTRACT: We suggest a new component of the Milky Way galaxy that can account for both the optical depth and the event durations obtained by the MACHO microlensing survey toward the Large Magellanic Cloud. This component is consistent with recent evidence for a significant population of faint white dwarf stars, detected in a proper motion study of the Hubble Deep Field, which cannot be accounted for by stars in the disk or spheroid. This new component consists of (mostly) old white dwarf stars distributed in a highly extended (very thick) disk configuration. It extends beyond the traditional thin and thick disks, but well within the dark, roughly spherical CDM halo. The total mass in this component is $\sim 7-9 \times 10^{10} M_\odot$. We argue that such a component is reasonable, natural, consistent with a variety of observations, and many of the problems associated with a significant halo population of white dwarfs are ameliorated.
    12/1999;
  • Source
    Geza Gyuk, Evalyn Gates
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the implications of a very thick (scaleheight 1.5--3.0 kpc) disc population of MACHOs. Such a population represents a reasonable alternative to standard halo configurations of a lensing population. We find that very thick disc distributions can lower the lens mass estimate derived from the microlensing data towards the LMC, although an average lens mass substantially below 0.3 M⊙ is unlikely. Constraints from direct searches for such lenses imply very low-luminosity objects; thus thick discs do not solve the microlensing lens problem. We discuss further microlensing consequences of very thick disc populations, including an increased probability for parallax events.
    Monthly Notices of the Royal Astronomical Society 04/1999; 304(2):281 - 287. · 5.52 Impact Factor
  • Source
    Geza Gyuk, N. Wyn Evans, Evalyn I. Gates
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper investigates the hypothesis that the lensing objects towards the Large Magellanic Cloud (LMC) are brown dwarfs by analysing the effects of velocity anisotropy on the inferred microlensing masses. To reduce the masses, the transverse velocity of the lenses with respect to the microlensing tube must be minimised. In the outer halo, radial anisotropy is best for doing this; closer to the solar circle, azimuthal anisotropy is best. By using a constraint on the total kinetic energy of the tracer population from the Jeans equations, the microlensing mass is minimised over orientations of the velocity dispersion tensor. This minimum mass is about a tenth of a solar mass, which lies above the hydrogen burning limit. This demonstrates explicitly that populations of brown dwarfs with smoothly decreasing densities and dynamically mixed velocity distributions cannot be responsible for the microlensing events. Brown dwarfs are no white knights! There is one caveat. If there are demons sitting on the microlensing tube, they can drop brown dwarfs so as to reproduce the microlensing data-set exactly. Such a distribution is not smooth and does not give well-mixed velocities in phase space. It is a permissible solution only if the outer halo is dynamically young and lumpy. In such a case, theorists cannot rule out brown dwarfs. Only exorcists can! Comment: 15 pages, 3 figures, in press at The Astrophysical Journal (Letters)
    The Astrophysical Journal 05/1998; · 6.73 Impact Factor
  • Source
    Evalyn Gates, Geza Gyuk, N. Wyn Evans
    [Show abstract] [Hide abstract]
    ABSTRACT: Current microlensing data in the direction of the LMC indicate that in the context of a spherical isothermal model with a Maxwellian velocity distribution, some significant fraction of the Galactic halo is composed of MACHOs with masses roughly in the range 0.1 to 1.0 Msun . Such masses are consistent with several astrophysical candidates for MACHOs -- white dwarfs, neutron stars, and black holes -- each of which presents serious challenges for stellar formation and evolution theories. However, the MACHO component of the halo, if it is not the major component, as in Cold Dark Matter scenarios, may have a very different distribution from the typically assumed spherical isothermal model. The MACHO distribution may be in a significantly flattened halo and/or, due to dissipation, more centrally condensed. In addition, such a distribution is likely to be anisotropic and may have a significant rotational component. The velocity dispersion and mass density distribution which describe the halo model are crucial input parameters in extracting an estimate of the lens mass from the data. We investigate several non-standard halo models and their implications for the lens mass derived from event timescales. We examine two classes of models with a bulk rotational component of the halo velocity distribution: a highly flattened 1/r(2) halo, and a spheroid-like population with whose density falls off as 1/r(3.5) . The highly flattened 1/r(2) models can decrease the implied average MACHO mass only marginally and the spheroid models not at all. Generally, rotational models cannot bring the MACHO mass implied by the current microlensing data down to the sub-stellar range. We also analyze the effects of velocity anisotropy on the inferred microlensing masses and find that populations of brown dwarfs with smoothly decreasing densities and dynamically mixed velocity distributions cannot be responsible for the microlensing events.
    04/1998; 30:873.
  • Source
    Geza Gyuk, Evalyn Gates
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the implications of a very thick (scale height 1.5 - 3.0 kpc) disk population of MACHOs. Such a population represents a reasonable alternative to standard halo configurations of a lensing population. We find that very thick disk distributions can lower the lens mass estimate derived from the microlensing data toward the LMC, although an average lens mass substantially below $0.3\Msol$ is unlikely. Constraints from direct searches for such lenses imply very low luminosity objects: thus thick disks do not solve the microlensing lens problem. We discuss further microlensing consequences of very thick disk populations, including an increased probability for parallax events.
    04/1998;

Publication Stats

1k Citations
167.99 Total Impact Points

Institutions

  • 1993–2010
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, IL, United States
  • 2009
    • The University of Tokyo
      • Institute for Cosmic Ray Research
      Edo, Tōkyō, Japan
    • University of Washington Seattle
      • Department of Astronomy
      Seattle, Washington, United States
  • 1998–2009
    • Adler Planetarium
      Chicago, Illinois, United States
  • 1996–1997
    • Fermi National Accelerator Laboratory (Fermilab)
      Batavia, Illinois, United States