Searcharxiv⌕ Search

arXiv subjects

Vijay K. Narayanan

Publications and source records attributed to Vijay K. Narayanan.

At least 19 recordsLinked to original sources

ALBench: A Framework for Evaluating Active Learning in Object Detection

Active learning is an important technology for automated machine learning systems. In contrast to Neural Architecture Search (NAS) which aims at automating neural network architecture design, active learning aims at automating training data selection. It is especially critical for training a long-tailed task, in which positive samples are sparsely distributed. Active learning alleviates the expensive data annotation issue through incrementally training models powered with efficient data selection. Instead of annotating all unlabeled samples, it iteratively selects and annotates the most valuable samples. Active learning has been popular in image classification, but has not been fully explored in object detection. Most of current approaches on object detection are evaluated with different settings, making it difficult to fairly compare their performance. To facilitate the research in this field, this paper contributes an active learning benchmark framework named as ALBench for evaluating active learning in object detection. Developed on an automatic deep model training system, this ALBench framework is easy-to-use, compatible with different active learning algorithms, and ensures the same training and testing protocols. We hope this automated benchmark system help researchers to easily reproduce literature's performance and have objective comparisons with prior arts. The code will be release through Github.

cs.CV↗

The near-IR properties and continuum shapes of high redshift quasars from the Sloan Digital Sky Survey

We present J-H-K' photometry for a sample of 45 high redshift quasars found by the Sloan Digital Sky Survey. The sample was originally selected on the basis of optical colors and spans a redshift range from 3.6 to 5.03. Our photometry reflects the rest-frame SED longward of Ly alpha for all redshifts. The results show that the near-IR colors of high redshift quasars are quite uniform. We have modelled the continuum shape of the quasars (from just beyond Ly alpha to ~4000 A) with a power law of the form f_nu \propto nu^alpha, and find =-0.57 with a scatter of 0.33. This value is similar to what is found for lower redshift quasars over the same restframe wavelength range, and we conclude that there is hardly any evolution in the continuum properties of optically selected quasars up to redshift 5. The spectral indices found by combining near-IR with optical photometry are in general consistent but slightly flatter than what is found for the same quasars using the optical spectra and photometry alone, showing that the continuum region used to determine the spectral indices can somewhat influence the results.

astro-ph↗

A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6

We present the discovery of three new quasars at z>6 in 1300 deg^2 of SDSS imaging data, J114816.64+525150.3 (z=6.43), J104845.05+463718.3 (z=6.23) and J163033.90+401209.6 (z=6.05). The first two objects have weak Ly alpha emission lines; their redshifts are determined from the positions of the Lyman break. They are only accurate to 0.05 and could be affected by the presence of broad absorption line systems. The last object has a Ly alpha strength more typical of lower redshift quasars. Based on a sample of six quasars at z>5.7 that cover 2870 deg^2 presented in this paper and in Paper I, we estimate the comoving density of luminous quasars at z 6 and M_{1450} < -26.8 to be (8 +/- 3)x10^{-10} Mpc^{-3} (for H_0 = 50 km/s/Mpc, Omega = 1). HST imaging of two z>5.7 quasars and high-resolution ground-based images (seeing 0.4'') of three additional z>5.7 quasars show that none of them is gravitationally lensed. The luminosity distribution of the high-redshfit quasar sample suggests the bright end slope of the quasar luminosity function at z 6 is shallower than Psi L^{-3.5} (2-sigma), consistent with the absence of strongly lensed objects.

astro-ph↗

Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Main Galaxy Sample

We describe the algorithm that selects the main sample of galaxies for spectroscopy in the Sloan Digital Sky Survey from the photometric data obtained by the imaging survey. Galaxy photometric properties are measured using the Petrosian magnitude system, which measures flux in apertures determined by the shape of the surface brightness profile. The metric aperture used is essentially independent of cosmological surface brightness dimming, foreground extinction, sky brightness, and the galaxy central surface brightness. The main galaxy sample consists of galaxies with r-band Petrosian magnitude r < 17.77 and r-band Petrosian half-light surface brightness < 24.5 magnitudes per square arcsec. These cuts select about 90 galaxy targets per square degree, with a median redshift of 0.104. We carry out a number of tests to show that (a) our star-galaxy separation criterion is effective at eliminating nearly all stellar contamination while removing almost no genuine galaxies, (b) the fraction of galaxies eliminated by our surface brightness cut is very small (0.1%), (c) the completeness of the sample is high, exceeding 99%, and (d) the reproducibility of target selection based on repeated imaging scans is consistent with the expected random photometric errors. (abridged)

astro-ph↗

Evolution of the Ionizing Background and the Epoch of Reionization from the Spectra of z~6 Quasars

We study the process of cosmic reionization and estimate the ionizing background in the IGM using the Lyman series absorption in the spectra of the four quasars at 5.7 10^{-3} at z~6. At this redshift, the mass-averaged neutral hydrogen fraction is larger than 1%; the mildly overdense regions (delta > 3) are still mostly neutral and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z~6 are typical of those in the era at the end of the overlap stage of reionization when the individual HII regions merge. Thus, z~6 marks the end of the reionization epoch. The redshift of reionization constrains the small scale power of the mass density fluctuations and the star forming efficiency of the first generation of objects.

astro-ph↗

Early-type Galaxy Distances from the Fundamental Plane and Surface Brightness Fluctuations

We compare two methods for deriving distances to early-type galaxies: fundamental plane (FP) and surface brightness fluctuations (SBF) distances for 170 galaxies. A third set of distances is provided by predictions derived from the density field of the IRAS redshift survey. Overall there is good agreement. However, several nearby, low-luminosity, mainly S0 galaxies have systematically low FP distances; we conclude that this is at least partly due to recent star formation and consequently low mass-to-light ratios. The tie between the Cepheid-calibrated SBF distances (Mpc) and the far-field calibrated FP distances (km/s) yields a Hubble constant H_0 = 68 +/- 3 km/s/Mpc, while the comparison between SBF and the IRAS-reconstructed distances yields H_0 = 74 +/- 2 (internal errorbars). Possible explanations for the marginal inconsistency include systematic errors in the redshift survey completeness estimates or in the FP aperture corrections, but at this point, the best estimate of H_0 from early-type galaxies may be a simple average of the above two estimates. After revising the SBF distances to be in agreement with the final set of Key Project Cepheid distances, we conclude H_0 = 73 +/- 4 +/- 11, where the second errorbar represents the total systematic uncertainty in the distance zero point. We also discuss the recently introduced "fluctuation star count" parameter Nbar as a less demanding alternative to (V-I) for calibrating SBF distances. The Nbar-calibrated SBF method is akin to a hybrid SBF-FP distance indicator, and we find that the use of Nbar actually improves the SBF distances. Further study of Nbar would provide useful constraints for elliptical galaxy formation models. (abridged)

astro-ph↗

Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar

We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99 and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly Alpha absorption in the spectra of these quasars evolves strongly with redshift. To z~5.7, the Ly Alpha absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest redshift object, SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is 0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A (5.95 150, and is consistent with zero flux in the Ly Alpha forest region immediately blueward of the Ly Alpha emission line, compared with a drop by a factor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, tau(eff) > 20, on the optical depth to Ly Alpha absorption at z=6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly Alpha forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z~5 to 6, and the universe is approaching the reionization epoch at z~6.

astro-ph↗

A Survey of z>5.8 Quasars in the Sloan Digital Sky Survey I: Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z~6

We present the results from a survey of i-dropout objects selected from ~1550 deg^2 of multicolor imaging data from the Sloan Digital Sky Survey, to search for luminous quasars at z>5.8. Objects with i*-z*>2.2 and z*<20.2 are selected, and follow-up J band photometry is used to separate L and T type cool dwarfs from high-redshift quasars. We describe the discovery of three new quasars, at z=5.82, 5.99 and 6.28, respectively. Their spectra show strong and broad Ly alpha+NV emission lines, and very strong Ly alpha absorption, with a mean continuum decrement D_A > 0.90. The ARC 3.5m spectrum of the z=6.28 quasar shows that over a range of 300 A immediately blueward of the Ly alpha emission, the average transmitted flux is only 0.003 +/-0.020 times that of the continuum level, consistent with zero flux, and suggesting a tentative detection of the complete Gunn-Peterson trough. The existence of strong metal lines suggests early chemical enrichment in the quasar enviornment. The three new objects, together with the previously published z=5.8 quasar form a complete color-selected flux-limited sample at z>5.8. We estimate that at $z=6$, the comoving density of luminous quasars at M_1450 < -26.89 (h=0.5, Omega=1)is 1.1x10^-9 Mpc^-3. This is a factor of ~2 lower than that at z~5, and is consistent with an extrapolation of the observed quasar evolution at low-z. We discuss the contribution of quasars to the ionizing background at z~6. The luminous quasars discussed in the paper have central black hole masses of several times 10^9 M_sun by the Eddington argument. Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift. (Abridged)

astro-ph↗

The 3D Power Spectrum from Angular Clustering of Galaxies in Early SDSS Data

Early photometric data from the Sloan Digital Sky Survey (SDSS) contain angular positions for 1.5 million galaxies. In companion papers, the angular correlation function $w(θ)$ and 2D power spectrum $C_l$ of these galaxies are presented. Here we invert Limber's equation to extract the 3D power spectrum from the angular results. We accomplish this using an estimate of $dn/dz$, the redshift distribution of galaxies in four different magnitude slices in the SDSS photometric catalog. The resulting 3D power spectrum estimates from $w(θ)$ and $C_l$ agree with each other and with previous estimates over a range in wavenumbers $0.03 < k/{\rm h Mpc}^{-1} < 1$. The galaxies in the faintest magnitude bin ($21 < \rstar < 22$, which have median redshift $z_m=0.43$) are less clustered than the galaxies in the brightest magnitude bin ($18 < \rstar < 19$ with $z_m=0.17$), especially on scales where nonlinearities are important. The derived power spectrum agrees with that of Szalay et al. (2001) who go directly from the raw data to a parametric estimate of the power spectrum. The strongest constraints on the shape parameter $Γ$ come from the faintest galaxies (in the magnitude bin $21 < \rstar < 22$), from which we infer $Γ= 0.14^{+0.11}_{-0.06}$ (95% C.L.).

astro-ph↗

Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data

We study the optical colors of 147,920 galaxies brighter than g* = 21, observed in five bands by the Sloan Digital Sky Survey (SDSS) over ~100 sq. deg. of high Galactic latitude sky along the Celestial Equator. The distribution of galaxies in the g*-r* vs. u*-g* color--color diagram is strongly bimodal, with an optimal color separator of u*-r* = 2.22. We use visual morphology and spectral classification of subsamples of 287 and 500 galaxies respectively, to show that the two peaks correspond roughly to early (E, S0, Sa) and late (Sb, Sc, Irr) type galaxies, as expected from their different stellar populations. We also find that the colors of galaxies are correlated with their radial profiles, as measured by the concentration index and by the likelihoods of exponential and de Vaucouleurs' profile fits. While it is well known that late type galaxies are bluer than early type galaxies, this is the first detection of a local minimum in their color distribution. In all SDSS bands, the counts vs. apparent magnitude relations for the two color types are significantly different, and indicate that the fraction of blue galaxies increases towards the faint end.

astro-ph↗

Galaxy Number Counts from the Sloan Digital Sky Survey Commissioning Data

We present bright galaxy number counts in five broad bands ($u', g', r', i', z'$) from imaging data taken during the commissioning phase of the Sloan Digital Sky Survey (SDSS). The counts are derived from two independent stripes of imaging scans along the Celestial Equator, one each toward the North and the South Galactic cap, covering about 230 and 210 square degrees, respectively. A careful study is made to verify the reliability of the photometric catalog. For galaxies brighter than $r^* = 16$, the catalog produced by automated software is examined against eye inspection of all objects. Statistically meaningful results on the galaxy counts are obtained in the magnitude range $12 \le r^* \le 21$, using a sample of 900,000 galaxies. The counts from the two stripes differ by about 30% at magnitudes brighter than $r^*= 15.5$, consistent with a local $2σ$ fluctuation due to large scale structure in the galaxy distribution. The shape of the number counts-magnitude relation brighter than $r^* = 16$ is well characterized by $N \propto 10^{0.6m}$, the relation expected for a homogeneous galaxy distribution in a ``Euclidean'' universe. In the magnitude range $ 16 < r^* < 21$, the galaxy counts from both stripes agree very well, and follow the prediction of the no-evolution model, although the data do not exclude a small amount of evolution. We use empirically determined color transformations to derive the galaxy number counts in the $B$ and $I_{814}$ bands. We compute the luminosity density of the universe at zero redshift in the five SDSS bands and in the $B$ band. We find ${\cal L}_{B} = 2.4 \pm 0.4 \times 10^8L_\odot h $Mpc$^{-3}$, for a reasonably wide range of parameters of the Schechter luminosity function in the $B$ band.

astro-ph↗

The Merger History of Supermassive Black Holes in Galaxies

The ubiquity of supermassive black holes (SMBHs) at the centers of nearby luminous galaxies can arise from the multiple mergers experienced by dark matter halos in hierarchical structure formation models, even if only a small fraction of these galaxies harbor SMBHs at high redshifts. We illustrate this possibility using cosmological Monte Carlo simulations of the merger history of dark matter halos and their associated SMBHs. In our most extreme models, in order to populate nearly every bright galaxy with a SMBH at z=0, only a few percent of the halos with virial temperatures above 10^4 K are required to harbor a SMBH at high redshift. This possibility must be included in studies of the luminosity function and the clustering properties of quasars. We predict the number of SMBH merger events that are detectable by the gravitational wave experiment LISA, as a function of redshift, out to z=5. Although the event rates can be significantly reduced in scenarios with rare SMBHs, a minimum of \~10 detectable merger events per year is predicted if SMBH binaries coalesce efficiently. The observed distribution of events with redshift could yield valuable information on the SMBH formation process. If SMBH binaries do not coalesce, we find that at least several SMBH slingshot ejections probably occurred from z=5 to the present in each galaxy more massive than ~10^11 Msun at z=0. Although our results are sensitive to the minimum cooling mass assumed for the formation of SMBHs, we expect the qualitative predictions of our models to be robust.

astro-ph↗

Biased Galaxy Formation And Measurements Of Beta

Measurements of the cosmological density parameter Omega using techniques that exploit the gravity-induced motions of galaxies constrain, in linear perturbation theory, the degenerate parameter combination beta = Omega^{0.6}/b, where the linear bias parameter b is the ratio of the fluctuation amplitudes of the galaxy and mass distributions. However, the relation between the galaxy and mass density fields depends on the complex physics of galaxy formation, and it can in general be non-linear, stochastic, and perhaps non-local. The one-parameter linear bias model is almost certainly oversimplified, which leads to the obvious question: What is the quantity beta that is actually measured by different techniques? To address this question, we estimate beta from galaxy distributions that are constructed by applying a variety of locally biased galaxy formation models to cosmological N-body simulations. We compare the values of beta estimated using three different techniques: a density-density comparison similar to the POTENT analysis, a velocity-velocity comparison similar to the VELMOD analysis, and an anisotropy analysis of the redshift-space power spectrum. In most cases, we find that beta estimated using all three methods is similar to the asymptotic value of Omega^{0.6}/b_{sigma}(R) at large R, where b_{sigma}(R) is the ratio of rms galaxy fluctuations to rms mass fluctuations on scale R. Thus, something close to the conventional interpretation of beta continues to hold even for complex bias models. Moreover, we find that beta estimates made using these three methods should, in principle, agree with each other. It is thus unlikely that non-linear or scale-dependent bias is responsible for the discrepancies that exist among current measurements of beta from different techniques.

astro-ph↗

Lyman-alpha Forest Constraints on the Mass of Warm Dark Matter and the Shape of the Linear Power Spectrum

High resolution N-body simulations of cold dark matter (CDM) models predict that galaxies and clusters have cuspy halos with excessive substructure. Observations reveal smooth halos with central density cores. One possible resolution of this conflict is that the dark matter is warm (WDM); this will suppress the power spectrum on small scales. The Lyman-alpha forest is a powerful probe of the linear power spectrum on these scales. We use collisionless N-body simulations to follow the evolution of structure in WDM models, and analyze artificial Lyman-alpha forest spectra extracted from them. By requiring that there is enough small-scale power in the linear power spectrum to reproduce the observed properties of the Lyman-alpha forest in quasar spectra, we derive a lower limit to the mass of the WDM particle of 750 eV. This limit is robust to reasonable uncertainties in our assumption about the temperature of the mean density gas (T0) at z=3. We argue that any model that suppresses the CDM linear theory power spectrum more severely than a 750 eV WDM particle cannot produce the Lyman-alpha forest.

astro-ph↗

Biased Estimates of Omega from Comparing Smoothed Predicted Velocity Fields to Unsmoothed Peculiar Velocity Measurements

We show that a regression of unsmoothed peculiar velocity measurements against peculiar velocities predicted from a smoothed galaxy density field leads to a biased estimate of the cosmological density parameter Omega, even when galaxies trace the underlying mass distribution and galaxy positions and velocities are known perfectly. The bias arises because the errors in the predicted velocities are correlated with the predicted velocities themselves. We investigate this bias using cosmological N-body simulations and analytic arguments. In linear perturbation theory, for cold dark matter power spectra and Gaussian or top hat smoothing filters, the bias in Omega is always positive, and its magnitude increases with increasing smoothing scale. This linear calculation reproduces the N-body results for Gaussian smoothing radii R_s > 10 Mpc/h, while non-linear effects lower the bias on smaller smoothing scales, and for R_s < 3 Mpc/h Omega is underestimated rather than overestimated. The net bias in Omega for a given smoothing filter depends on the underlying cosmological model. The effect on current estimates of Omega from velocity-velocity comparisons is probably small relative to other uncertainties, but taking full advantage of the statistical precision of future peculiar velocity data sets will require either equal smoothing of the predicted and measured velocity fields or careful accounting for the biases discussed here.

astro-ph↗

Reconstruction Analysis of the IRAS Point Source Catalog Redshift Survey

We present the results of reconstruction analysis of the galaxy distribution in a spherical region of radius 50 Mpc/h centered on the Local Group, as mapped by the IRAS Point Source Catalog Redshift Survey (PSCz). We reconstruct it using 15 different models for structure formation in the universe, each consisting of a set of assumptions regarding the value of the density parameter Omega_m, and the amplitude and nature of the biasing between IRAS galaxies and mass. For every model, we also reconstruct ten mock PSCz catalogs derived from the outputs of numerical simulations that have the appropriate assumptions about Omega_m and bias. We use a variety of statistics to compare the accuracy of each reconstruction of the PSCz catalog to the accuracy expected based on the corresponding mock catalog reconstructions. We find that gravitational instability of Gaussian mass density fluctuations can account for the galaxy distribution in the PSCz catalog, at least for some plausible assumptions about the value of Omega_m and the bias. Unbiased models in which IRAS galaxies trace mass fail to reconstruct the PSCz catalog accurately, both for Omega_m=0.4 and for Omega_m=1. Low Omega_m models in which IRAS galaxies are antibiased with respect to the mass are the most successful in reconstructing the PSCz catalog. In particular, a model with Omega_m=0.4 and IRAS galaxies related to the mass according to the predictions of a semi-analytic galaxy formation model is very successful in reconstructing the PSCz galaxy distribution.

astro-ph↗

What is Beta?

Measurements of the cosmological density parameter (Omega) using techniques that exploit the gravity-induced motions of galaxies yield, in linear perturbation theory, the degenerate parameter combination beta=Omega^{0.6}/b, where the linear bias parameter b is the ratio of the fluctuation amplitudes of the galaxy and mass distributions. However, the relation between the mass and the galaxy density fields depends on the complex physics of galaxy formation, and it can in general be non-linear, non-local, and stochastic. There is a growing consensus that the one-parameter model for bias is oversimplified. This leaves us with the following question: What is the quantity that is actually being measured by the different techniques? In order to address this question, we present estimates of beta from galaxy distributions constructed by applying a variety of biased galaxy formation models to cosmological N-body simulations. We compare the values of beta estimated using two different techniques: the anisotropy of the redshift-space power spectrum and an idealized version of the POTENT method. In most cases, we find that the bias factor b=Omega^{0.6}/beta derived from redshift-space anisotropy or POTENT is similar to the large-scale value of b_{sigma}(R) defined by the ratio of rms fluctuation amplitudes of the galaxy and mass distributions. However, non-linearity of bias and residual effects of non-linear gravitational evolution both influence beta estimates at the 10-20% level.

astro-ph↗

A Precision test of Hipparcos systematics towards the Hyades

We propose and apply a test that can detect any systematic errors in the Hipparcos parallaxes towards the Hyades cluster at the level of 0.3 mas. We show that the statistical parallax method subsumes the classical moving cluster methods and provides more accurate and robust estimates of the distance and the first two moments of the velocity distribution of the Hyades cluster namely, its bulk space velocity and the velocity dispersion tensor. We predict the parallaxes of Hyades cluster members using the common cluster space velocity derived from the statistical parallax method and their individual Hipparcos proper motions. We show that the parallaxes determined in this manner (pi_pm) are consistent at the 1-sigma level with the parallaxes (pi_orb) of three Hyades spectroscopic binary systems with orbital solutions. We find that <(pi_pm - pi_orb)> = 0.52 +- 0.47 mas, where the error is dominated by the errors in the orbital parallaxes. A reduction in this error would allow a test of the systematic errors in the Hipparcos parallaxes at the 0.3 mas level. If the Hyades distance scale is fixed by Hipparcos parallaxes, then its bulk velocity in equatorial coordinates is (Vx,Vy,Vz) = (-5.70 +- 0.20, 45.62 +- 0.11, 5.65 +- 0.08)km/s, its velocity dispersion is 320 +- 39 m/s, and the distance modulus to the centroid of our sample of 43 cluster members is 3.34 +- 0.02 mag.

astro-ph↗