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Michael A. Strauss

Publications and source records attributed to Michael A. Strauss.

At least 325 records · Page 18Linked to original sources
arXiv2000

Discovery of a Close Pair of z = 4.25 Quasars from the Sloan Digital Sky Survey

We report the discovery of a pair of z = 4.25 quasars with a separation of 33 arcseconds. The brighter of the two objects was identified as a high-redshift quasar candidate from Sloan Digital Sky Survey multicolor imaging data, and the redshift was measured from a spectrum obtained with the Hobby-Eberly Telescope. The slit orientation of this observation {\it by chance} included another quasar, approximately one magnitude fainter and having the same redshift as the target. This is the third serendipitous discovery of a z > 4 quasar. The differences in the relative strengths and profiles of the emission lines suggest that this is a quasar pair and not a gravitational lens. The two objects are likely to be physically associated; the projected physical separation is approximately 210 $h_{50}^{-1}$ kpc and the redshifts are identical to $\approx$ 0.01, implying a radial physical separation of 950 $h_{50}^{-1}$ kpc or less. The existence of this pair is strong circumstantial evidence that $z \sim 4$ quasars are clustered.

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arXiv2000

Dust Emission from High Redshift QSOs

We present detections of emission at 250 GHz (1.2 mm) from two high redshift QSOs from the Sloan Digital Sky Survey sample using the bolometer array at the IRAM 30m telescope. The sources are SDSSp 015048.83+004126.2 at z = 3.7, and SDSSp J033829.31+002156.3 at z = 5.0, which is the third highest redshift QSO known, and the highest redshift mm emitting source yet identified. We also present deep radio continuum imaging of these two sources at 1.4 GHz using the Very Large Array. The combination of cm and mm observations indicate that the 250 GHz emission is most likely thermal dust emission, with implied dust masses of 1e8 M_solar. We consider possible dust heating mechanisms, including UV emission from the active nucleus (AGN), and a massive starburst concurrent with the AGN, with implied star formation rates > 1e3 M_solar/year.

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arXiv1999

Questions and Controversies in the Measurement and Interpretation of Large-Scale Flows

This introductory talk to the 1999 Victoria Conference on Large-Scale Flows will present the ``big questions'' which will be discussed in the conference: (a) Does the velocity field converge on the largest scales? (b) Why can't we agree on the value of beta? (c) How can we properly measure the small-scale velocity dispersion? (d) Just how complicated can biasing be? (e) How universal are the distance indicators we are using? (f) How do we design our next generation of surveys to answer the above questions?

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arXiv1999

The Physical Origin of Scale Dependent Bias in Cosmological Simulations

Using large-scale hydrodynamic simulations with heuristic criteria for galaxy formation, we investigate how the galaxy field is related to the mass density and the gas temperature. We find that the bias b(R) = sigma_g(R)/sigma(R), where sigma_g(R) is the variance of galaxy counts in spheres of radius R and sigma(R) is the same for mass, varies from 2.6 at 1 h^{-1} Mpc to 1.2 at 30 h^{-1} Mpc. Including the dependence of the galaxy density on local gas temperature as well as on local mass density can fully account for this scale dependence. Galaxy density depends on temperature because gas which is too hot cannot cool to form galaxies; this causes scale dependence of b(R) because local gas temperature is related to the gravitational potential, and thus contains information about the large scale density field. We show that temperature dependence generally causes b(R) to vary on quasilinear and nonlinear scales, indicating that scale dependence of bias may be a generic effect in realistic galaxy formation scenarios. We find that the relationship between the galaxy and mass density fields is also a function of galaxy age. On large scales, the older galaxies are highly biased (b ~ 1.7) and highly correlated (r ~ 1.0) with the mass density field; younger galaxies are not biased (b ~ 0.8) and are poorly correlated (r ~ 0.5) with the mass. We argue that linear bias is inadequate to describe the relationship between galaxies and mass. (Abridged)

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arXiv1999

The Discovery of a Field Methane Dwarf from Sloan Digital Sky Survey Commissioning Data

We report the discovery of the coolest field dwarf yet known, selected as a stellar object with extremely red colors from commissioning imaging data of the Sloan Digital Sky Survey. Its spectrum from 0.8 to 2.5 microns is dominated by strong bands of H_2 O and CH_4. Its spectrum and colors over this range are very similar to those of Gliese 229B, the only other known example of a methane dwarf. It is roughly 1.2 mag fainter than Gliese 229B, implying that it lies at a distance of roughly 10 pc. Such a cool object must have a mass well below the hydrogen-burning limit of 0.08 solar masses, and therefore is a genuine brown dwarf, with a probable mass in the range 0.015-0.06 solar masses for an age range of 0.3-5 Gyr.

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arXiv1999

Time Evolution of Galaxy Formation and Bias in Cosmological Simulations

The clustering of galaxies relative to the mass distribution declines with time because: first, nonlinear peaks become less rare events; second, the densest regions stop forming new galaxies because gas there becomes too hot to cool and collapse; third, after galaxies form, they are gravitationally ``debiased'' because their velocity field is the same as the dark matter. To show these effects, we perform a hydrodynamic cosmological simulation and examine the density field of recently formed galaxies as a function of redshift. We find the bias b_* of recently formed galaxies (the ratio of the rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around 1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_* between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at z=0. As gas in the universe heats up and prevents star formation, star-forming galaxies become poorer tracers of the mass density field. After galaxies form, the linear continuity equation is a good approximation to the gravitational debiasing, even on nonlinear scales. The most interesting observational consequence of the simulations is that the linear regression of the star-formation density field on the galaxy density field evolves from about 0.9 at z=1 to 0.35 at z=0. These effects also provide a possible explanation for the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega, finding that while Omega(z) increases with z, one's estimate Omega_est(z) decreases. (Abridged)

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arXiv1998

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.

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arXiv1998

Model-Independent Measurement of the Primordial Power Spectrum

In inflationary models with minimal amount of gravity waves, the primordial power spectrum of density fluctuations, $A_S^2(k)$, together with the basic cosmological parameters, completely specify the predictions for the cosmic microwave background (CMB) anisotropy and large scale structure. Here we show how we can strongly constrain both $A_S^2(k)$ and the cosmological parameters by combining the data from the Microwave Anisotropy Probe (MAP) and the galaxy redshift survey from the Sloan Digital Sky Survey (SDSS). We allow $A_S^2(k)$ to be a free function, and thus probe features in the primordial power spectrum on all scales. MAP and SDSS have scale-dependent measurement errors that decrease in opposite directions on astrophysically interesting scales; they complement each other and allow the measurement of the primordial power spectrum independent of inflationary models, giving us valuable information on physics in the early Universe, and providing clues to the correct inflationary model.

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arXiv1998

Large-Scale Flows As A Cosmological Probe

We review the use of peculiar velocities of galaxies as a probe of cosmological models. We put particular emphasis on comparison of the peculiar velocity and density fields, focussing on the discrepancies between various recent analyses. We discuss the limitations of the commonly used linear bias model, which may lie at the heart of some of the current controversies in the field.

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arXiv1998

Constraining the Dark Matter Distribution With Large-Scale Structure Observations

We discuss the use of galaxies to trace the large-scale structure of the universe and thereby to make cosmological inferences. We put special emphasis on our lack of knowledge about the relative distribution of galaxies and the dynamically important dark matter. We end with a discussion of the increasing importance of infrared astronomy to large-scale structure studies.

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arXiv1998

The Velocity Field Predicted by the Optical Redshift Survey

We have used the Optical Redshift Survey (ORS; Santiago et al. 1995) to construct the gravity field due to fluctuations in the galaxy density field out to distances of 8000 km/s. At large scales where linear theory applies, the comparison of this gravity field with the observed peculiar velocity field offers a powerful cosmological probe, because the predicted flow field is proportional to the parameter Omega^{0.6}/b, where Omega is the matter density and b is the bias of the galaxy distribution. The more densely sampled ORS gravity field, to excellent approximation, matches that of the earlier IRAS 1.2-Jy redshift survey (Fisher et al. 1995), provided beta is reduced by a factor b(opt)/b(IRAS)=1.4. Apart from this scaling, the most significant difference between the ORS and IRAS fields is induced by differing estimates of the over-density of the Virgo cluster. Neither of these gravity fields is consistent with the peculiar velocity field constructed from the full Mark III (Willick et al. 1997) sample. We find that a simple but plausible non-linear bias algorithm for the galaxy distribution relative to the mass has a negligible effect on the derived fields. We conclude that the substitution of optical for IRAS catalogues cannot alone resolve the discrepancies between the IRAS gravity field and the Mark III peculiar velocity field.

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arXiv1998

Cosmology in the Next Millennium: Combining MAP and SDSS Data to Constrain Inflationary Models

The basic cosmological parameters and the primordial power spectrum together completely specify predictions for the cosmic microwave background radiation anisotropy and large scale structure. Here we show how we can strongly constrain both $A_S^2(k)$ and the cosmological parameters by combining the data from the Microwave Anisotropy Probe (MAP) and the galaxy redshift survey from the Sloan Digital Sky Survey (SDSS). We allow $A_S^2(k)$ to be a free function, and thus probe features in the primordial power spectrum on all scales. The primordial power spectrum in 20 steps in $\log k$ to $k\leq 0.5h$Mpc$^{-1}$ can be determined to $\sim 16%$ accuracy for $k\sim 0.01h$Mpc$^{-1}$, and to $\sim 1%$ accuracy for $k\sim 0.1h$Mpc$^{-1}$. The uncertainty in the primordial power spectrum increases by a factor up to 3 on small scales if we solve simultaneously for the dimensionless Hubble constant $h$, the cosmological constant $Λ$, the baryon fraction $Ω_b$, the reionization optical depth $τ_{ri}$, and the effective bias between the matter density field and the redshift space galaxy density field $b_{\it eff}$. Alternately, if we restrict $A_S^2(k)$ to be a power law, we find that inclusion of the SDSS data breaks the degeneracy between the amplitude of the power spectrum and the optical depth inherent in the MAP data, significantly reduces the uncertainties in the determination of the matter density and the cosmological constant, and allows a determination of the galaxy bias parameter. Thus, combining the MAP and SDSS data allows the independent measurement of important cosmological parameters, and a measurement of the primordial power spectrum independent of inflationary models, giving us valuable information on physics in the early Universe, and providing clues to the correct inflationary model.

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arXiv1998

Large-Scale Structure in the Distribution of Galaxies as a Probe of Cosmological Models

The last 20 years have seen an explosion in our understanding of the large-scale distribution and motions of galaxies in the nearby universe. The field has moved from a largely qualitative, morphological description of the structures seen in the galaxy distribution, to a rich and increasingly rigorous statistical description, which allows us to constrain cosmological models. New surveys just now getting underway will be unprecedented in their uniformity and volume surveyed. The study of the evolution of large-scale structure with time is now becoming feasible.

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arXiv1998

Maximum-Likelihood Comparisons of Tully-Fisher and Redshift Data. II. Results from an Expanded Sample

This is the second in a series of papers in which we compare Tully-Fisher (TF) data from the Mark III Catalog with predicted peculiar velocities based on the IRAS galaxy redshift survey and gravitational instability theory, using a rigorous maximum likelihood method called VELMOD. In Paper I (Willick et al. 1997b), we we applied the method to a $cz_{LG} \leq 3000$ km/sec, 838-galaxy TF sample and found $β_I=0.49\pm 0.07,$ where $β_I\equiv Ω^{0.6}/b_I$ and $b_I$ is the linear biasing parameter for IRAS galaxies. In this paper we increase the redshift limit to $cz_{LG}=7500$ km/sec, thereby enlarging the sample to 1876 galaxies. The expanded sample now includes the W91PP and CF subsamples of the Mark III catalog, in addition to the A82 and MAT subsamples already considered in Paper I. We implement VELMOD using both the forward and inverse forms of the TF relation, and allow for a more general form of the quadrupole velocity residual detected in Paper I. We find $β_I=0.50\pm 0.04$ (1-sigma error) at 300 km/sec smoothing of the IRAS-predicted velocity field. The fit residuals are spatially incoherent for $β_I=0.5,$ indicating that the IRAS plus quadrupole velocity field is a good fit to the TF data. If we eliminate the quadrupole we obtain a worse fit, but a similar value for $β_I$ of $0.54\pm 0.04.$ Changing the IRAS smoothing scale to 500 km/sec has almost no effect on the best $β_I.$ We find evidence for a density-dependence of the small-scale velocity dispersion, $σ_v(δ_g)\simeq (100 + 35 δ_g)$ km/sec.

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arXiv1997

Measuring High-Order Moments of the Galaxy Distribution from Counts in Cells -- The Edgeworth Approximation

To probe the weakly non-linear regime, past the point where simple linear theory is sufficient to describe the statistics of the density distribution, we measure the skewness (S_3) and kurtosis (S_4) of the Count Probability Distribution Function (CPDF) of the IRAS 1.2 Jy sample obtained from counts in cells. These quantities are free parameters in a maximum likelihood fit of an Edgeworth expansion convolved with a Poissonian to the observed CPDF. This method, applicable on scales greater than 5 Mpc, is appreciably less sensitive to the tail of the distribution than are measurements of S_3 and S_4 from moments of the CPDF. We measure S_3 and S_4 to l~50 h^{-1} Mpc; the data are consistent with scale invariance, yielding averages of = 2.83 +/- 0.09, and = 6.89 +/- 0.68. These values are higher than those found using the moments method on the same data set, = 1.5 +/- 0.5 and = 4.4 +/- 3.7, due to lack of correction in the latter work for finite-volume effects. Unlike the moments method, our results are quite robust to the fact that IRAS galaxies are under-represented in cluster cores. We use N-body simulations to show that our method yields unbiased results.

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arXiv1997

Using Cluster Abundances and Peculiar Velocities to Test the Gaussianity of the Cosmological Density Field

(Abridged) By comparing the frequency of typical events with that of unusual events, one can test whether the cosmological density distribution function is consistent with the normally made assumption of Gaussianity. To this end, we compare the consistency of the tail-inferred (from clusters) and measured values (from large-scale flows) of the rms level of mass fluctuations for two distribution functions: a Gaussian, and a texture (positively-skewed) PDF. Averaging the recent large-scale flow measurements, we find that observations of the rms and the tail at the 10 h^-1 Mpc scale disfavor a texture PDF at ~1.5 sigma in all cases. However, taking only the most recent measurement of the rms, that from Willick et al. (1997b), the comparison disfavors textures for low Omega_0=0.3, and disfavors Gaussian models if Omega_0=1 (again at ~1.5 sigma). Predictions for evolution of high temperature clusters can also be made for the models considered, and strongly disfavor Omega_0=1 in Gaussian models and marginally disfavor Omega_0=1 in texture models. Only Omega_0=0.3 Gaussian models are consistent with all the data considered.

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arXiv1997

The Galaxy Pairwise Velocity Dispersion as a Function of Local Density

The standard method of measuring the galaxy pairwise velocity dispersion on small scales is heavily weighted by the densest regions in a way that is difficult to calibrate. We propose a new statistic which measures the small-scale velocity dispersion as an explicit function of density. Computing this statistic for a volume-limited subsample of the Optical Redshift Survey, we find that the small-scale velocity dispersion rises from 220 to 760 km/s as density increases. We calculate this statistic for a series of mock catalogs drawn from a hydrodynamic simulation of an Ωh = 0.5 Cold Dark Matter universe (standard CDM), and find that the observed velocity distribution lies ~1 σbelow the simulations in each of eight density bins, formally ruling out this model at the 7.4 σlevel, quantifying the well-known problem that this model produces too high a velocity dispersion. This comparison is insensitive to the normalization of the power spectrum, although it is quite sensitive to the density and velocity bias of galaxies relative to dark matter on small scales. (abridged)

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