arXiv subjects

Marc Davis

Publications and source records attributed to Marc Davis.

At least 73 records · Page 4Linked to original sources

arXiv2002

Deviations from the Local Hubble Flow. I. The Tip of the Red Giant Branch as a Distance Indicator

The properties of the velocity field in the local volume (cz < 550 km/s) have been difficult to constrain due to a lack of a consistent set of galaxy distances. The sparse observations available to date suggest a remarkably quiet flow, with little deviation from a pure Hubble law. However, velocity field models based on the distribution of galaxies in the 1.2 Jy IRAS redshift survey, predict a quadrupolar flow pattern locally with strong infall at the poles of the local Supergalactic plane. We probe this velocity field and begin to establish a consistent set of galactic distances. We have obtained images of nearby galaxies in the I and V band from the W.M. Keck Observatory and in F814W and F555W filters from the Hubble Space Telescope. Where these galaxies are well resolved into stars we may use the Tip of the Red Giant Branch (TRGB) as a distance indicator. Using a maximum likelihood analysis to quantitatively measure the I magnitude of the TRGB we determine precise distances to seven nearby galaxies: Leo I, Sextans B, NGC 1313, NGC 3109, UGC 03755, UGC 06456, and UGC 07577.

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arXiv2001

Three-Dimensional Identification and Reconstruction of Galaxy Systems within Deep Redshift Surveys

We have developed a new geometrical method for identifying and reconstructing a homogeneous and highly complete set of galaxy groups in the next generation of deep, flux-limited redshift surveys. Our method combines information from the three-dimensional Voronoi diagram and its dual, the Delaunay triangulation, to obtain group and cluster catalogs that are remarkably robust over wide ranges in redshift and degree of density enhancement. Using the mock DEEP2 catalogs, we demonstrate that the VDM algorithm can be used to identify a homogeneous set of groups in a magnitude-limited sample ($I\sbr{AB}\le23.5$) throughout the survey redshift window $0.7 < z < 1.2$. The actual group membership can be effectively reconstructed even in the distorted redshift space environment for systems with line of sight velocity dispersion $σ_{los}$ greater than $\approx 200$ \kms. By comparing the galaxy cluster catalog derived from the mock DEEP2 observations to the underlying distribution of clusters found in real space with much fainter galaxies included (which should more closely trace mass in the cluster), we can assess completeness in velocity dispersion directly. We conclude that the recovered DEEP2 group and cluster sample should be statistically complete for $σ_{los} \gtrsim 400$ \kms. Finally, we argue that the reconstructed bivariate distribution of systems as a function of redshift and velocity dispersion reproduces with high fidelity the underlying real space distribution and can thus be used robustly to constrain cosmological parameters

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arXiv2001

Chandra Detection of a TypeII Quasar at z=3.288

We report on observations of a TypeII quasar at redshift z=3.288, identified as a hard X-ray source in a 185 ks observation with the Chandra X-ray Observatory and as a high-redshift photometric candidate from deep, multiband optical imaging. CXOJ084837.9+445352 (hereinafter CXO52) shows an unusually hard X-ray spectrum from which we infer an absorbing column density N(H) = (4.8+/-2.1)e23 / cm2 (90% confidence) and an implied unabsorbed 2-10 keV rest-frame luminosity of L(2-10) = 3.3e44 ergs/s, well within the quasar regime. Hubble Space Telescope imaging shows CXO52 to be elongated with slight morphological differences between the WFPC2 F814W and NICMOS F160W bands. Optical and near-infrared spectroscopy of CXO52 show high-ionization emission lines with velocity widths ~1000 km/s and flux ratios similar to a Seyfert2 galaxy or radio galaxy. The latter are the only class of high-redshift TypeII luminous AGN which have been extensively studied to date. Unlike radio galaxies, however, CXO52 is radio quiet, remaining undetected at radio wavelengths to fairly deep limits, f(4.8GHz) < 40 microJy. High-redshift TypeII quasars, expected from unification models of active galaxies and long-thought necessary to explain the X-ray background, are poorly constrained observationally with few such systems known. We discuss recent observations of similar TypeII quasars and detail search techniques for such systems: namely (1) X-ray selection, (2) radio selection, (3) multi-color imaging selection, and (4) narrow-band imaging selection. Such studies are likely to begin identifying luminous, high-redshift TypeII systems in large numbers. We discuss the prospects for these studies and their implications to our understanding of the X-ray background.

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arXiv2001

3D Identification and Reconstruction of z~1 Clusters: Prospects for the DEEP2 Redshift Survey

We have developed a geometrical method based on 3D Voronoi polyhedra and Delaunay tessellation for identifying and reconstructing clusters of galaxies in the next generation of deep, flux-limited redshift surveys. We here describe this algorithm and tests of it using mock catalogs that simulate the DEEP2/DEIMOS redshift survey, which will begin observations in the Spring of 2002 and will provide a detailed three dimensional map of the large scale structure up to redshift 1.5.

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arXiv2001

Spectral Reduction Software for the DEEP2 Redshift Survey

The DEEP2/DEIMOS redshift survey, which will begin observing in the Spring of 2002, will gather high quality spectra on ~60000 galaxies in order to study the evolution of the properties and large scale clustering of galaxies at z~1. The data rate from DEIMOS will be in excess of 1 Gbyte/hour, and it is therefore imperative to employ completely automated data reduction techniques to manage the analysis. We here describe aspects of our data pipeline, which will make extensive use of B-splines for the sky-subtraction stage and for the combination of multiple frames.

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arXiv2001

Measuring the Cosmic Equation of State with Counts of Galaxies II: Error Budget for the DEEP2 Redshift Survey

In a previous paper, we described a new variant on the classical dN/dz test which could be performed using upcoming redshift surveys. By observing the velocity function of galaxies rather than their luminosity function, it is possible to avoid many of the uncertainties of galaxy evolution while employing a directly measurable quantity. We previously assumed that counting statistics would dominate the errors in this method. Here, we present the results of including cosmic variance and determine the impact of systematic effects on application of this test to the upcoming DEEP2 Redshift Survey. For DEEP2, cosmic variance should yield errors roughly twice those predicted from Poisson statistics. Through Monte Carlo simulations we find that if the functional form, but not the strength, of any of the major systematic effects (baryonic infall, velocity errors, and incompleteness) is known, the systematic may successfully be corrected for using the observed velocity function, leaving a much smaller residual error. The total uncertainty from systematics is comparable to that from cosmic variance, but correlated amongst redshift bins. We find that DEEP2 can provide a much more precise measurement of the cosmic equation of state parameter w than any available today, albeit weaker than some other proposed tests.

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arXiv2001

Measuring the Cosmic Equation of State with Galaxy Clusters in the DEEP2 Redshift Survey

The abundances of groups and clusters of galaxies are expected to have changed substantially since high redshift, with the strength of this evolution dependent upon fundamental cosmological parameters. Upcoming large redshift surveys of distant galaxies will make it possible to measure these quantities at z~1; when combined with the results of local redshift surveys currently underway, the evolution of cluster abundances may be determined. The DEEP2 Redshift Survey, planned to begin in Spring 2002, is particularly well-suited for this work because of the high spectroscopic resolution to be used; velocity dispersions of groups will be readily measurable. In this paper, we determine the constraints on dark energy models that counts of DEEP2 clusters should provide. We find that comparing cosmological models using the simultaneous distribution of clusters in both velocity dispersion and redshift yields significantly stronger constraints than the redshift distribution alone. The method can be made more powerful by employing a value of the fluctuation amplitude sigma_8 determined with upcoming techniques (external to DEEP2) that have no Omega_m degeneracy. The equation-of-state parameter for dark energy models, w=P/rho, can then be measured to ~0.1 from observations of clusters alone.

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arXiv2001

Galaxy Correlation Statistics of Mock Catalogs for the DEEP2 Survey

The DEEP2 project will obtain redshifts for ~60,000 galaxies between z~0.7-1.5 in a comoving volume of 7 10^6 Mpc/h^3 for an LCDM universe. The survey will map four separate 2 by 0.5 degree strips of the sky. To study the expected clustering, we have constructed mock galaxy catalogs from the GIF Virgo Consortium simulations. We present two- and three-point correlation analyses of these mock galaxy catalogs to test how well we will measure these statistics in the presence of selection biases which will limit the surface density of galaxies which we can select for spectroscopy. We find that neither the two-point nor three-point correlation functions are significantly compromised. We will be able to make simple corrections to account for the small amount of bias introduced. We quantify the expected redshift distortions due to random orbital velocities of galaxies within groups and clusters on small scales of ~1 Mpc/h using the pairwise velocity dispersion sigma_12 and galaxy-weighted velocity dispersion sigma_1, which we are able to measure within ~10%. We also estimate the expected large-scale coherent infall of galaxies due to supercluster formation (``Kaiser effect''). From this measure we will be able to constrain beta to within ~0.1 at z=1. For the DEEP2 survey we will combine the correlation statistics with galaxy observables such as spectral type, morphology, absolute luminosity, and linewidth to measure the relative biases in different galaxy types. Here we use a counts-in-cells analysis to measure sigma_8 as a function of redshift and determine the relative bias between galaxy samples based on absolute luminosity. We expect to measure sigma_8 to within 10% and detect the evolution of relative bias with redshift at the 4-5 sigma level.

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arXiv2001

The Structure of Dark Matter Halos in an Annihilating Dark Matter Model

The inability of standard non-interacting cold dark matter (CDM) to account for the small scale structure of individual galaxies has led to the suggestion that the dark matter may undergo elastic and/or inelastic scattering. We simulate the evolution of an isolated dark matter halo which undergoes both scattering and annihilation. Annihilations produce a core that grows with time due to adiabatic expansion of the core as the relativistic annihilation products flow out of the core, lessening the binding energy. An effective annihilation cross section per unit mass equal to $>.03 cm^2 g^{-1} (100 km s^{-1}/v$) with a scattering cross section per unit mass of .6 cm g$^{-1}$ produces a 3 kpc core in a 10$^{10}$ M$_{\sun}$ halo that persists for 100 dynamical times. The same cross section leads to a core of only 120 pc in a rich cluster. In addition to creating to cores, annihilation should erase structure on scales below $\sim 3\times10^8$ M$_{\sun}$. Annihilating dark matter provides a mechanism for solving some of the problems of non-interacting CDM, at the expense of introducing a contrived particle physics model.

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arXiv2001

A Composite Seyfert 2 X-ray Spectrum: Implications for the Origin of the Cosmic X-ray Background

We present a composite 1-10 keV Seyfert 2 X-ray spectrum, derived from ASCA observations of a distance-limited sample of nearby galaxies. All 29 observed objects were detected. Above ~3 keV, the composite spectrum is inverted, confirming that Seyfert 2 galaxies as a class have the spectral properties necessary to explain the flat shape of the cosmic X-ray background spectrum. Integrating the composite spectrum over redshift, we find that the total emission from Seyfert 2 galaxies, combined with the expected contribution from unabsorbed type 1 objects, provides an excellent match to the spectrum and intensity of the hard X-ray background. The principal uncertainty in this procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function. Separate composite spectra for objects in our sample with and without polarized broad optical emission lines are also presented.

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arXiv2001

Mining the Sky with Redshift Surveys

Since the late 1970's, redshift surveys have been vital for progress in understanding large-scale structure in the Universe. The original CfA redshift survey collected spectra of 20-30 galaxies per clear night on a 1.5 meter telescope; over a two year period the project added ~2000 new redshifts to the literature. Subsequent low-z redshift surveys have been up to an order of magnitude larger, and ongoing surveys will yield a similar improvement over the generation preceding them. Full sky redshift surveys have a special role to play as predictors of cosmological flows, and deep pencil beam surveys have provided fundamental constraints on the evolution of properties of galaxies. With the 2DF redshift survey and the SDSS survey, our knowledge of the statistical clustering of low-redshift galaxies will achieve unprecedented precision. Measurements of clustering in the distant Universe are more limited at present, but will become much better in this decade as the VLT/VIRMOS and Keck/DEIMOS projects produce results. As in so many other fields, progress in large scale structure studies, both observational and theoretical, has been made possible by improvements in technologies, especially computing. This review briefly highlights twenty years of progress in this evolving discipline and describes a few novel cosmological tests that will be attempted with the Keck/DEIMOS survey.

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arXiv2001

High-Redshift Galaxy Outflows and the Formation of Dwarf Galaxies

We examine the effects of galaxy outflows on the formation of dwarf galaxies in numerical simulations of the high-redshift Universe. Using a Smoothed Particle Hydrodynamic code, we conduct two detailed simulations of a (5.2 Mpc/h)^3 comoving volume of the Universe. In both simulations we implement simple, well-motivated models of galaxy identification and star formation, while our second simulation also includes a simple ``blow-out'' model of galaxy outflows in which supernova driven winds from newly formed disk galaxies punch-out and shock the intergalactic medium while leaving the host galaxies intact. A direct comparison between these simulations suggests that there are two major mechanisms by which outflows affect dwarf formation. Firstly, the formation of an outflow slows down the further accretion of gas onto a galaxy, causing an overall decrease of approximately 50% in the total gas mass accreted by the objects in our simulations. Additionally, our simulations uncover a significant population of approximately 10^9 solar mass objects whose formation is suppressed by the mechanism of ``baryonic stripping,'' in which outflows from early galaxies strip the gas out of nearby overdense regions that would have otherwise later formed into dwarf galaxies. This mechanism may be important in explaining the observed discrepancy between the number of dwarf galaxies predicted and observed in the local group and provide a natural explanation for the formation of empty halos which may be required by the existence of the extremely gas-poor extra-galactic High-Velocity Clouds.

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arXiv2000

A revised Cepheid distance to NGC 4258 and a test of the distance scale

In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope (HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations and methods then standard for the Key Project on the Extragalactic Distance Scale. Here, we reevaluate the Cepheid distance using the revised Key Project procedures described in Freedman et al. (2001). These revisions alter the zero points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and the treatment of metallicity differences. We also provide herein full information on the Cepheids described in Maoz et al. 1999. Using the refined Key Project techniques and calibrations, we determine the distance modulus of NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag (systematic uncertainties in Key Project distances), corresponding to a metric distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of 7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast (1999), which yield more discrepant results. Additionally, we place weak limits upon the distance to the LMC and upon the effect of metallicity in Cepheid distance determinations.

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arXiv2000

The DEEP2 Redshift Survey

The DEIMOS spectrograph is nearing completion in the UC Lick shops and will be delivered to the Keck Observatory in 2001. Once it is operational, a team of astronomers will initiate DEEP2, a major redshift survey of galaxies that will consume approximately 120 nights at the Keck Observatory over a three year period. Our goal is to gather high-quality spectra of ~60,000 galaxies with Z>0.7 in order to study the properties and large scale clustering of galaxies at $Z \approx 1$. The survey will be done at high spectral resolution, R=λ/Δλ~3500, allowing us to work between the bright OH sky emission lines and to infer linewidths for many of the target galaxies. The linewidth data will facilitate the execution of the classical redshift-volume cosmological test, which can provide a precision measurement of the equation of state of the Universe.

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arXiv2000

Stellar Populations and Galaxy Morphology at High Redshift

In this article we investigate the morphology and stellar populations of high-redshift galaxies through multi-waveband HST imaging and ground-based spatially-resolved spectroscopy. We study the redshift evolution of galaxy morphology in the Hubble Deep Field, using the deep IDT-NICMOS near-infrared HST imaging coupled with spectroscopic and photometric redshifts. Using the multi-waveband data to compare the appearance of galaxies at the same rest-frame wavelengths reveals that "morphological k-corrections" (the change in appearance when viewing high-z objects at shorter rest-frame wavelengths) are only important in a minority of cases, and that galaxies were intrinsically more peculiar at high redshift. One example of significant morphological k-corrections is spiral galaxies, which often show more pronounced barred structure in the near-infrared than in the optical. Therefore, the apparent decline in the fraction of barred spirals at faint magnitudes in the optical HDF may be due to band-shifting effects at the higher redshifts, rather than intrinsic evolution. Using such features as the age-sensitive Balmer+4000Ang break, the spatially- resolved colours of distant galaxies in optical/near-infrared imaging can also be used to study their component stellar populations. We supplement this with deep Keck/LRIS spectroscopy of two extended sources: a chain galaxy at z=2.8 (HDF4-555.1, the "Hot Dog" - the brightest U-drop Lyman-break galaxy in the HDF) and a pair of z=4.04 gravitationally lensed arcs behind the cluster Abell 2390. The absence of measurable rotation across the z=2.8 chain galaxy implies that it is unikely to be a disk viewed edge on. With the resolution enhancement from lensing, we detect stellar populations of different ages in the z=4 arcs.

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arXiv2000

Detection of a Far IR Excess with DIRBE at 60 and 100 Microns

From analysis of the DIRBE weekly averaged sky maps, we have detected substantial flux in the 60 and 100 micron channels in excess of expected zodiacal and Galactic emission. Two methods are used to separate zodiacal light from more distant emission. Method I makes use of the time-dependence of the North-South annual variation observed at the ecliptic poles. This method is robust against errors in the inter-planetary dust (IPD) model, but does not demonstrate isotropy of the background. Method II measures the ecliptic latitude dependence of the dust over a range of ecliptic latitudes at solar elongation e=90 degrees. This allows the excess to be determined in each week of the DIRBE mission for high redundancy, but the results depend weakly on the IPD model. Both methods give consistent results at 60 and 100 microns. The observed signal is consistent with an isotropic background at the level νI_ν= 28.1 +- 1.8 +- 7(syst) nW/m^2/sr at 60 microns and 24.6 +- 2.5 +- 8 nW/m^2/sr at 100 microns. The IR excess detected at 140 and 240 microns by these methods agrees with previous measurements, which are thought to be the cosmic infra-red background (CIB). The detections at 60 and 100 microns are new. While this new excess is not necessarily the CIB, we have ruled out all known sources of emission in the solar system and Galaxy. We therefore tentatively interpret this signal as the CIB and consider the implications of such energy production from the viewpoint of star formation efficiency and black hole accretion efficiency. However, the IR excess exceeds limits on the CIB derived from the inferred opacity of the IGM to observed TeV photons, thus casting doubt on this interpretation. There is currently no satisfactory explanation for the 60-100 micron excess.

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arXiv2000

Measuring the Cosmic Equation of State with Counts of Galaxies

The classical dN/dz test allows the determination of fundamental cosmological parameters from the evolution of the cosmic volume element. This test is applied by measuring the redshift distribution of a tracer whose evolution in number density is known. In the past, ordinary galaxies have been used as such a tracer; however, in the absence of a complete theory of galaxy formation, that method is fraught with difficulties. In this paper, we propose studying instead the evolution of the apparent abundance of dark matter halos as a function of their circular velocity, observable via the linewidths or rotation speeds of visible galaxies. Upcoming redshift surveys will allow the linewidth distribution of galaxies to be determined at both z~1 and the present day. In the course of studying this test, we have devised a rapid, improved semi-analytic method for calculating the circular velocity distribution of dark halos based upon the analytic mass function of Sheth et al. (1999) and the formation time distribution of Lacey & Cole (1993). We find that if selection effects are well-controlled and minimal external constraints are applied, the planned DEEP Redshift Survey should allow the measurement of the cosmic equation-of-state parameter w to 10% (as little as 3% if Omega_m has been well-determined from other observations). This type of test has the potential also to provide a constraint on any evolution of w such as that predicted by ``tracker'' models.

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arXiv1999

Resolving the Stellar Populations in a z=4 Lensed Galaxy

We present deep near-infrared Keck/NIRC imaging of a recently-discovered z=4.04 galaxy (Frye & Broadhurst 1998). This is lensed by the rich foreground cluster Abell~2390 (z~0.23) into highly-magnified arcs 3-5arcsec in length. Our H- and K'-band NIRC imaging allows us to map the Balmer+4000Ang break amplitude. In combination with high-quality archival 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 are significantly redder, consistent with being observed >100Myr after star formation has ceased. Keck/LRIS long-slit spectroscopy along the arcs reveals that the Ly-alpha emission is spatially offset by ~1arcsec from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring HII regions, and that the z=4 system is unlikely to be an AGN.

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