Leonidas A. Moustakas

Colors and K-Band Counts of Extremely Faint Field Galaxies* **

We combine deep K-band (W. M. Keck Telescope) with V- and I-band (New Technology Telescope) observations of two blank high Galactic latitude fields, surveying a total of ~2 arcmin2. The K-band number-magnitude counts continue to rise above K ? 22 mag, reaching surface densities of few ?105 deg-2. The slope for the galaxy counts is approximately d log (N)/d mag deg-2 = 0.23 ? 0.02 over the range 18-23 mag. While this slope is consistent with other recent deep K-band surveys, there is a definite scatter in the normalizations by about a factor of 2. In particular, our normalization is ~2? greater than the galaxy counts reported by Djorgovski et al. in 1995. Optical-near-infrared color-magnitude and color-color diagrams for all objects detected in the V + I + K image are plotted and discussed in the context of grids of Bruzual-Charlot isochrone synthesis galaxy evolutionary models. The colors of most of the observed galaxies are consistent with a population drawn from a broad redshift distribution. A few galaxies at K ? 19-20 are red in both colors (V - I 3; I - K 2, consistent with being early-type galaxies having undergone a burst of star formation at z 5 and viewed at z ~ 1. At K 20, we find several (approximately eight) red outlier galaxies with I - K 4 and V - I 2.5, whose colors are difficult to mimic by a single evolving or nonevolving stellar population at any redshift unless they either have quite low metallicity or are highly reddened. We compare the data against the evolutionary tracks of second-burst ellipticals and against a grid of models that does not constrain galaxy ages to a particular formation redshift. The red outliers surface density is several per square arcminute, which is so high that they are probably common objects of low luminosity L < L*. Whether these are low-metallicity, dusty dwarf galaxies, or old galaxies at high redshift, they are curious and merit spectroscopic follow-up.

The Discovery of a High-Redshift Quasar without Emission Lines from Sloan Digital Sky Survey Commissioning Data

We report observations of a luminous unresolved object at redshift z=4.62, with a featureless optical spectrum redward of the Lyalpha forest region, discovered from Sloan Digital Sky Survey commissioning data. The redshift is determined by the onset of the Lyalpha forest at lambda approximately 6800 Å and a Lyman limit system at lambda=5120 Å. A strong Lyalpha absorption system with weak metal absorption lines at z=4.58 is also identified in the spectrum. The object has a continuum absolute magnitude of -26.6 at 1450 Å in the rest frame (h0=0.5, q0=0.5) and therefore cannot be an ordinary galaxy. It shows no radio emission (the 3 sigma upper limit of its flux at 6 cm is 60 µJy), indicating a radio-to-optical flux ratio at least as small as that of the radio-weakest BL Lacertae objects known. It is also not linearly polarized to a 3 sigma upper limit of 4% in the observed I band. Therefore, it is either the most distant BL Lac object known to date, with very weak radio emission, or a new type of unbeamed quasar, whose broad emission line region is very weak or absent.

Detecting dark matter substructure spectroscopically in strong gravitational lenses

The cold dark matter (CDM) model for galaxy formation predicts that a significant fraction of mass in the dark matter haloes that surround L ∼ L * galaxies is bound in substructures of mass 10 4 -10 7 M O .. The number of observable baryonic substructures (such as dwarf galaxies and globular clusters) falls short of these predictions by at least an order of magnitude. We present a method for searching for substructure in the haloes of gravitational lenses that produce multiple images of quasi-stellar objects (QSOs), such as four-image Einstein Cross lenses. Current methods based on broad-band flux ratios cannot cleanly distinguish between substructure, differential extinction, scattering in the radio by ionized regions in the lens galaxy, microlensing by stars and, most importantly, ambiguities in the host lens model. These difficulties may be overcome by utilizing the prediction that, when substructure is present, the magnification will be a function of source size. QSO broad-line and narrow-line emission regions are ∼1 pc and > 100 pc in size, respectively. The radio emission region is typically intermediate to these and the continuum emission region is much smaller. When narrow-line region (NLR) features are used as a normalization, the relative intensity and equivalent width of broad-line region (BLR) features will respectively reflect substructure-lensing and microlensing effects. Spectroscopic observations of just a few image pairs would probably be able to extract the desired substructure signature cleanly and distinguish it from microlensing - depending on the actual level of projected mass in substructure. In the rest-optical, the Hβ/[O III] region is ideal, since the narrow wavelength range also largely eliminates differential reddening problems. In the rest-ultraviolet, the region longward of and including Lya may also work. Simulations of Q 2237+0305 are done as an example, to determine the level of substructure that is detectable in this way. Possible systematic difficulties are also discussed. This is an ideal experiment to be carried out with near-infrared integral field unit spectrographs on 8-m class telescopes, and will provide a fundamentally new probe of the internal structure of dark matter haloes.

The Masses, Ancestors, and Descendants of Extremely Red Objects: Constraints from Spatial Clustering

Wide-field near-infrared (IR) surveys have revealed a population of galaxies with very red optical-IR colors, which have been termed extremely red objects (EROs). Modeling suggests that such red colors (R-K > 5) could be produced by galaxies at z 1 with either very old stellar populations or very high dust extinction. Recently, it has been discovered that EROs are strongly clustered. Are these objects the high-redshift progenitors of present-day giant ellipticals (gEs)? Are they already massive at this epoch? Are they the descendants of the z ~ 3 Lyman break galaxies (LBGs), which have also been identified as possible high-redshift progenitors of gEs? We address these questions within the framework of the cold dark matter paradigm, using an analytic model that connects the number density and clustering, or bias, of an observed population with the halo occupation function (the number of observed galaxies per halo of a given mass). We find that EROs reside in massive dark matter halos, with average mass > 1013 h M☉. The occupation function that we derive for EROs is very similar to the one we derive for z = 0, L > L*, early-type galaxies, whereas the occupation function for LBGs is skewed toward much smaller host halo masses ( ≈ 1011-1012 h M☉). We then use the derived occupation function parameters to explore the possible evolutionary connections between these three populations.

An Estimate of H0 from Keck Spectroscopy of the Gravitational Lens System 0957+561

We present long-slit LRIS/Keck spectroscopic observations of the gravitational lens system 0957+561. Averaged over all of our data, the rest-frame velocity dispersion σv of the central lens galaxy G1 is σv = 279 ± 12 km s-1. However, there appears to be a significant decrease in σv as a function of distance from the center of G1 that is not typical of the brightest cluster galaxies. Within 02 of the center of G1, we find the average σv = 316 ± 14 km s-1, whereas for positions greater than 02 from the center of G1, we find the average σv = 266 ± 12 km s-1. A plausible explanation is that G1 contains a central massive dark object of mass MMDO ≈ 4 × 109 h -->−1100 M☉ (h100 = H0/100 km s-1 Mpc-1), which contributes to the central velocity dispersion, and that the outer value of σv is the appropriate measure of the depth of the potential well of G1. The determination of a luminosity-weighted estimate of σv is essential for a determination of H0 from Q0957+561; our accurate measurements remove one of the chief uncertainties in the assumed form of the mass distribution of the lens. Thus, with the recent apparent reduction in the uncertainty in the measurement of the time delay for the images A and B of Q0957+561, ΔτBA = 417 ± 3 days (Kundic et al.), we obtain an estimate for the Hubble constant: H0 = 62 ± 7 km s-1 Mpc-1. If for some reason the trend of σv with slit position is spurious and we should use the dispersion averaged along the slit, then the estimate of H0 increases to 67 ± 8 km s-1 Mpc-1. These standard errors, however, do not include any contribution from any errors in the assumed form of the mass distribution of the lens. In particular, we used the mass model described by Falco, Gorenstein, & Shapiro, as updated by Grogin & Narayan. The reduced χ2 of model fits to the available position and magnification data for this system is relatively high (~4), indicating that the estimate of H0 may have a significant contribution from model errors. Further observations, discussed herein, should allow such errors to be estimated reliably.

Toward the Secondary Bar: Gas Morphology and Dynamics in NGC 4303

We report on the molecular gas properties in the central kiloparsec of the almost face-on double-barred galaxy NGC 4303 (M61), using the 12CO 1-0 line emission observed with the Owens Valley Radio Observatory (OVRO) millimeter interferometer. The bulk of the molecular line emission comes from two straight gas lanes that run north-south along the leading sides of the large-scale primary bar. Velocity deviations of up to 90 km s-1 from the mean rotational velocity are associated with these gas lanes. Inside a radius of ~5 (~400 pc) the molecular gas forms a spiral pattern that, for the northern arm, can be traced to the nucleus. The high angular resolution of our OVRO data (2 ~ 150 pc), together with archival Hubble Space Telescope (HST) data, allows for a comparison with dynamical models of gas flow in the inner kiloparsec of single- and double-barred galaxies. We find that the observed global properties of the molecular gas are in agreement with models for the gas flow in a strong, large-scale bar, and the two-arm spiral structure seen in 12CO in the inner kiloparsec can already be explained by a density wave initiated by the potential of that bar. Only a weak correlation between the molecular gas distribution and the extinction seen in the HST V-H map is found in the inner 400 pc of NGC 4303: The innermost part of one arm of the nuclear 12CO spiral correlates with a weak dust filament in the color map, while the overall dust distribution follows a ring or single-arm spiral pattern well correlated with the UV continuum. This complicated nuclear geometry of the stellar and gaseous components allows for two scenarios: (1) A self-gravitating m = 1 mode is present, forming the spiral structure seen in the UV continuum. In this case, the gas kinematics would be unaffected by the small (~4) inner bar. (2) The UV continuum traces a complete ring that is heavily extincted north of the nucleus. Such a ring forms in hydrodynamic models of double bars, but the models cannot account for the UV emission observed on the leading side of the inner bar. Comparison with other starburst ring galaxies where the molecular gas emission and the star-forming clusters form a ring or tightly wound spiral structure suggests that the starburst ring in NGC 4303 is in an early stage of formation.

The Oxford–Dartmouth Thirty Degree Survey – II. Clustering of bright Lyman break galaxies: strong luminosity‐dependent bias at z= 4

We present measurements of the clustering properties of bright (L > L * ) z ∼ 4 Lyman break galaxies (LBGs) selected from the Oxford-Dartmouth Thirty Degree Survey (ODT). We describe techniques used to select and evaluate our candidates and calculate the angular correlation function, which we find best fitted by a power law, ω(θ) = A w θ -β with A w = 15.4 (with 0 in arcsec), using a constrained slope of β = 0.8. Using a redshift distribution consistent with photometric models, we deproject this correlation function and find a comoving r 0 = 11.4 +1.7 -1.9 h -1 100 Mpc in a Ω m = 0.3 flat A cosmology for i AB ≤ 24.5. This corresponds to a linear bias value of b = 8.1 +2.0 -2.6 (assuming σ 8 = 0.9). These data show a significantly larger r 0 and b than previous studies at z ∼ 4. We interpret this as evidence that the brightest LBGs have a larger bias than fainter ones, indicating a strong luminosity dependence for the measured bias of an LBG sample. Comparing this against recent results in the literature at fainter (subL * ) limiting magnitudes, and with simple models describing the relationship between LBGs and dark matter haloes, we discuss the implications on the implied environments and nature of LBGs. It seems that the brightest LBGs (in contrast with the majority subL * population) have clustering properties, and host dark matter halo masses, which are consistent with them being progenitors of the most massive galaxies today.

Resolving the Stellar Populations in a z=4 Lensed Galaxy*

We present deep near-infrared Keck Near-Infrared Camera (NIRC) imaging of a recently discovered z = 4.04 galaxy. This is lensed by the rich foreground cluster Abell 2390 (z ≈ 0.23) into highly magnified arcs 3-5 in length. Our H and K band NIRC imaging allows us to map the Balmer+4000 A break amplitude. In combination with high-quality archival Hubble Space Telescope (HST) WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which correspond to sites of active star formation. However, there are considerable portions of the arcs that are significantly redder, consistent with being observed 100 Myr after star formation has ceased. Keck Low-Resolution Imaging Spectrograph (LRIS) long-slit spectroscopy along the arcs reveals that the Lyα emission is spatially offset by ≈1 from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring H II regions and that the z = 4 system is unlikely to be an active galactic nucleus.

Keck Spectroscopy of Objects with Lens-like Morphologies in the Hubble Deep Field

We present spectroscopy from the Keck telescope of three sets of objects in the Hubble Deep Field (HDF) that have lens-like morphologies. In the case of J123641+621204, which is composed of four objects with similar colors and a mean separation of 08, we find at least two distinct components at redshifts of z = 3.209 and z = 3.220 that are separated by 05 spatially. Each of these components has narrow Lyα emission and possibly N V emission and Si IV and C IV in absorption or with a P Cygni profile. The second case is J123652+621227, which has an arclike feature offset by 18 to the southwest of a red elliptical-like galaxy and a counterimage offset 14 on the opposite side. We tentatively find a single line at 5301 A at the spatial position of the counterimage and no corresponding emission line at the position of the arc. The colors of the counterimage are consistent with the identification of this line as Lyα at z = 3.36. The colors of the arc are different than those of the counterimage, and thus both the colors and spectra indicate that this object is unlikely to be a gravitational lens. For a third lensing candidate (J123656+621221), which is a blue arc offset by 09 from a red, elliptical-like galaxy, our spectroscopy does not clearly resolve the system spatially, which complicates the interpretation of the spectrum. We discuss possible identifications of a number of absorption features and a very tentative detection of a pair of emission lines at 5650 A and 5664 A, and we find that gravitational lensing remains a possibility in this case. We conclude that the frequency of strong gravitational lensing by galaxies in the HDF appears to be very low. This result is difficult to reconcile with the introduction of a cosmological constant to account for the large number of faint blue galaxies via a large volume element at high redshift and tends to favor models in which very faint galaxies are at fairly modest redshifts.

The Oxford–Dartmouth Thirty Degree Survey – I. Observations and calibration of a wide-field multiband survey

The Oxford-Dartmouth Thirty Degree Survey (ODTS) is a deep, wide, multiband imaging survey designed to cover a total of 30 deg(2) in BV RiZ, with a subset of U- and K-band data, in four separate fields of 5-10 deg(2) centred at 00:18:24 + 34:52, 09:09:45 + 40:50, 13:40:00 +02:30 and 16:39:30 +45:24. Observations have been made using the Wide Field Camera on the 2.5-m Isaac Newton Telescope (INT) in La Palma to average limiting depths (5sigma Vega, aperture magnitudes) of U = 24.8, B = 25.6, V = 25.0, R = 24.6 and i = 23.5, with observations taken in ideal conditions reaching the target depths of U = 25.3, B = 26.2, V = 25.7, R = 25.4 and i = 24.6. The INT Z-band data were found to be severely effected by fringing and, consequently, are now being obtained at the MDM observatory in Arizona. A complementary K-band survey has also been carried out at MDM, reaching an average depth of K 5 approximate to18.5. At present, approximately 23 deg(2) of the ODTS have been observed, with 3.5 deg(2) of the K- band survey completed. This paper details the survey goals, field selection, observation strategy and data reduction procedure, focusing on the photometric calibration and catalogue construction. Preliminary photometric redshifts have been obtained for a subsample of the objects with R less than or equal to 23. These results are presented alongside a brief description of the photometric redshift determination technique used. The median redshift of the survey is estimated to be z approximate to 0.7 from a combination of the ODTS photometric redshifts and comparison with the redshift distributions of other surveys. Finally, galaxy number counts for the ODTS are presented which are found to be in excellent agreement with previous studies.