arXiv subjects

Subhabrata Majumdar

Publications and source records attributed to Subhabrata Majumdar.

88 records · Page 5Linked to original sources

arXiv2010

Using clusters in SZE + x-ray surveys as an ensemble of rulers to constrain cosmology

Ongoing and upcoming surveys in x-rays and SZE are expected to jointly detect many clusters due to the large overlap in sky coverage. We show that, these clusters can be used as an ensemble of rulers to estimate the angular diameter distance, d_A(z). This comes at no extra observational cost, as these clusters form a subset of a much larger sample, assembled to build cluster number counts dn/dz. On using this d_A(z), the dark energy constraints can be improved by factors of 1.5 - 4, over those from just dn/dn. Even in the presence of a mass follow-up of 100 clusters (done for mass calibration), the dark energy constraints can be further tightened by factors of 2 - 3 . Adding d_A(z) from clusters is similar to adding d_L(z), from the SNe observations; for eg., dn/dn (from ACT/SPT) plus d_A(z) is comparable to dn/dz plus d_L(z) in constraining Omega_m and sigma_8.

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arXiv2010

Precision cosmology with a combination of wide and deep Sunyaev-Zeldovich cluster surveys

We show the advantages of a wedding cake design for Sunyaev-Zel'dovich cluster surveys. We show that by dividing up a cluster survey into a wide and a deep survey, one can essentially recover the cosmological information that would be diluted in a single survey of the same duration due to the uncertainties in our understanding of cluster physics. The parameter degeneracy directions of the deep and wide surveys are slightly different, and combining them breaks these degeneracies effectively. A variable depth survey with a few thousand clusters is as effective at constraining cosmological parameters as a single depth survey with a much larger cluster sample.

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arXiv2008

Spectroscopic Confirmation of Two Massive Red-Sequence-Selected Galaxy Clusters at z ~ 1.2 in the SpARCS-North Cluster Survey

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a deep z'-band imaging survey covering the Spitzer SWIRE Legacy fields designed to create the first large homogeneously-selected sample of massive clusters at z > 1 using an infrared adaptation of the cluster red-sequence method. We present an overview of the northern component of the survey which has been observed with CFHT/MegaCam and covers 28.3 deg^2. The southern component of the survey was observed with CTIO/MOSAICII, covers 13.6 deg^2, and is summarized in a companion paper by Wilson et al. (2008). We also present spectroscopic confirmation of two rich cluster candidates at z ~ 1.2. Based on Nod-and-Shuffle spectroscopy from GMOS-N on Gemini there are 17 and 28 confirmed cluster members in SpARCS J163435+402151 and SpARCS J163852+403843 which have spectroscopic redshifts of 1.1798 and 1.1963, respectively. The clusters have velocity dispersions of 490 +/- 140 km/s and 650 +/- 160 km/s, respectively which imply masses (M200) of (1.0 +/- 0.9) x 10^{14} M_{solar} and (2.4 +/- 1.8) x 10^{14} M_{solar}. Confirmation of these candidates as bona fide massive clusters demonstrates that two-filter imaging is an effective, yet observationally efficient, method for selecting clusters at z > 1.

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arXiv2008

Spectroscopic Confirmation of a Massive Red-Sequence-Selected Galaxy Cluster at z = 1.34 in the SpARCS-South Cluster Survey

The Spitzer Adaptation of the Red-sequence Cluster Survey (SpARCS) is a z'-passband imaging survey, consisting of deep (z' ~ 24 AB) observations made from both hemispheres using the CFHT 3.6m and CTIO 4m telescopes. The survey was designed with the primary aim of detecting galaxy clusters at z >~ 1. In tandem with pre-existing 3.6um observations from the Spitzer Space Telescope SWIRE Legacy Survey, SpARCS detects clusters using an infrared adaptation of the two-filter red-sequence cluster technique. The total effective area of the SpARCS cluster survey is 41.9 deg^2. In this paper, we provide an overview of the 13.6 deg^2 Southern CTIO/MOSAICII observations. The 28.3 deg^2 Northern CFHT/MegaCam observations are summarized in a companion paper by Muzzin et al. (2008). In this paper, we also report spectroscopic confirmation of SpARCS J003550-431224, a very rich galaxy cluster at z = 1.335, discovered in the ELAIS-S1 field. To date, this is the highest spectroscopically confirmed redshift for a galaxy cluster discovered using the red-sequence technique. Based on nine confirmed members, SpARCS J003550-431224 has a preliminary velocity dispersion of 1050 +/- 230 km/s. With its proven capability for efficient cluster detection, SpARCS is a demonstration that we have entered an era of large, homogeneously-selected z > 1 cluster surveys.

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arXiv2007

Secondary anisotropies of the CMB

The Cosmic Microwave Background fluctuations provide a powerful probe of the dark ages of the universe through the imprint of the secondary anisotropies associated with the reionisation of the universe and the growth of structure. We review the relation between the secondary anisotropies and and the primary anisotropies that are directly generated by quantum fluctuations in the very early universe. The physics of secondary fluctuations is described, with emphasis on the ionisation history and the evolution of structure. We discuss the different signatures arising from the secondary effects in terms of their induced temperature fluctuations, polarisation and statistics. The secondary anisotropies are being actively pursued at present, and we review the future and current observational status.

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arXiv2007

Dynamics of dwarf-spheroidals and the dark matter halo of the Galaxy

Based on the observed paucity of the dwarf spheroidal (dSph) satellites of the Milky Way at small Galactocentric distances, we put forward the hypothesis that subsequent to the formation of the Milky Way and its satellites, those dSphs that had orbits with small perigalacticons were tidally disrupted, leaving behind a population that now has a relatively larger value of its average perigalacticon to apogalacticon ratio and consequently a larger value of its r.m.s. transverse to radial velocities ratio compared to their values at the time of formation of the dSphs. We analyze the implications of this hypothesis for the phase space distribution of the dSphs and that of the dark matter (DM) halo of the Galaxy within the context of a self-consistent model in which the functional form of the phase space distribution of DM particles follows the King model i.e. the `lowered isothermal' distribution and the potential of the Galaxy is determined self-consistently by including the gravitational cross-coupling between visible matter and DM particles. This analysis, coupled with virial arguments, yields an estimate of $\gsim$ 270 km/s for the circular velocity of any test object at galactocentric distances of $\sim$ 100 kpc, the typical distances of the dSphs. The corresponding self-consistent values of the relevant DM halo model parameters, namely, the local (i.e., the solar neighbourhood) values of the DM density and velocity dispersion in the King model and its truncation radius, are estimated to be $\sim$ 0.3 GeV/cm^3, >350 km/s and $\gsim$ 150 kpc, respectively. Similar self-consistent studies with other possible forms of the DM distribution function will be useful in assessing the robustness of our estimates of the Galaxy's DM halo parameters.

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arXiv2006

Clusters of Galaxies at 1 < z < 2 : The Spitzer Adaptation of the Red-Sequence Cluster Survey

As the densest galaxy environments in the universe, clusters are vital to our understanding of the role that environment plays in galaxy formation and evolution. Unfortunately, the evolution of high-redshift cluster galaxies is poorly understood because of the ``cluster desert'' that exists at 1 < z < 2. The SpARCS collaboration is currently carrying out a 1-passband (z') imaging survey which, when combined with the pre-existing 50 square degree 3.6 micron Spitzer SWIRE Legacy Survey data, will efficiently detect hundreds of clusters in the cluster desert using an infrared application of the well-proven cluster red-sequence technique. We have already tested this 1-color (z' - [3.6]) approach using a 6 square degree ``pilot patch'' and shown it to be extremely successful at detecting clusters at 1 < z < 2. The clusters discovered in this project will be the first large sample of ``nascent'' galaxy clusters which connect the star-forming proto-cluster regions at z > 2 to the quiescent population at z < 1. The existing seven-passband Spitzer data (3.6, 4.5, 5.8, 8.0, 24, 70, 160 micron) will allow us to make the first measurements of the evolution of the cluster red-sequence, IR luminosity function, and the mid-IR dust-obscured star-formation rate for 1 < z < 2 clusters.

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arXiv2006

Cosmological Constraints from the Red-Sequence Cluster Survey

[abridged] We present a first cosmological analysis of a refined cluster catalog from the Red-Sequence Cluster Survey (RCS). The input cluster sample is derived from 72.07 square degrees of imaging data [...] The catalog contains 956 clusters over 0.35<z<0.95, limited by cluster richness and richness error. The calibration of the survey images has been extensively cross-checked against publicly available Sloan Digital Sky Survey imaging [...] We analyze the cluster sample via a general self-calibration technique including scatter in the mass-richness relation [...]. We fit simultaneously for Omega_M and sigma_8, and four parameters describing the calibration of cluster richness to mass, its evolution with redshift, and scatter in the richness-mass relation. The principal goal of this general analysis is to establish the consistency (or lack thereof) between the fitted parameters (both cosmological and cluster mass observables) and available results on both from independent measures. From an unconstrained analysis, Omega_M and sigma_8 are 0.31+0.11-0.10 and 0.67+0.18-0.13 respectively. An analysis including Gaussian priors on the slope and zeropoint of the mass-richness relation gives very similar results: 0.30+0.12-0.11 and 0.70+0.27-0.15. Both analyses are in good agreement with the current literature. The parameters describing the mass-richness relation in the unconstrained fit are also eminently reasonable and agree with existing follow-up data on both the RCS-1 and other cluster samples. Our results directly demonstrate that future surveys (optical and otherwise), with much larger samples of clusters, can give constraints competitive with other probes of cosmology.

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arXiv2004

The hybrid SZ power spectrum: Combining cluster counts and SZ fluctuations to probe gas physics

Sunyaev-Zeldovich (SZ) effect from a cosmological distribution of clusters carry information on the underlying cosmology as well as the cluster gas physics. In order to study either cosmology or clusters one needs to break the degeneracies between the two. We present a toy model showing how complementary informations from SZ power spectrum and the SZ flux counts, both obtained from upcoming SZ cluster surveys, can be used to mitigate the strong cosmological influence (especially that of sigma_8) on the SZ fluctuations. Once the strong dependence of the cluster SZ power spectrum on sigma_8 is diluted, the cluster power spectrum can be used as a tool in studying cluster gas structure and evolution. The method relies on the ability to write the Poisson contribution to the SZ power spectrum in terms the observed SZ flux counts. We test the toy model by applying the idea to simulations of SZ surveys.

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arXiv2003

Point sources in the context of future SZ surveys

We look at the impact of Infra-Red (IR) and radio point sources on upcoming large yield Sunyaev-Zel'dovich (SZ) cluster surveys such as APEX, SPT and ACT. The IR and radio point source counts are based on observations by the the SCUBA and WMAP instruments respectively. We show that the contributions from IR source counts, when extrapolated from the SCUBA frequency of 350GHz to the operating frequencies of these surveys (~100-300GHz), can be a significant source of additional `noise' which needs to be accounted for in order to extract the optimal science from these surveys. Alternatively, these surveys give us an opportunity to study IR sources, their numbers and clustering properties, opening a new window to the high redshift universe. For the radio point sources, the contribution depends on a more uncertain extrapolation from 40GHz but is comparable to the IR near 200GHz. However, the radio signal may be correlated with clusters of galaxies and have a disproportionately larger effect.

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arXiv2003

Self Calibration in Cluster Studies of Dark Energy: Combining the Cluster Redshift Distribution, the Power Spectrum and Mass Measurements

We examine the prospects for measuring the dark energy equation of state parameter w within the context of any uncertain redshift evolution of galaxy cluster structure (building on Majumdar and Mohr, 2003) and show that including the redshift averaged cluster power spectrum, P_cl(k), and direct mass measurements of 100 clusters helps tremendously in reducing cosmological parameter uncertainties. Specifically, we show that when combining the redshift distribution and the power spectrum information for a particular X-ray survey (DUET) and two SZE surveys (SPT & Planck), the constraints on the dark energy equation of state w can be improved by roughly a factor of 4. Because surveys designed to study the redshift distribution of clusters will have all the information necessary to construct P_cl(k), the benefit of adding P_cl(k) in reducing uncertainties comes at no additional observational cost. Combining detailed mass studies of 100 clusters with the redshift distribution improves the parameter uncertainties by a factor of 3-5. The data required for these detailed mass measurements-- assumed to have 1sigma uncertainties of 30-- are accumulating in the the XMM-Newton and Chandra archives. The best constraints are obtained when one combines both the power spectrum constraints and mass measurements with the cluster redshift distribution; when using the survey to extract the parameters and evolution of the mass--observable relations, we estimate the uncertainties on w of ~4% to 6%. These parameter constraints are obtained from self-calibrating cluster surveys alone. In combination with CMB or distance measurements that have different parameter degeneracies, cluster studies of dark energy will provide enhanced constraints and allow for cross--checks of systematics.

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arXiv2002

Importance of Cluster Structural Evolution in Using X-ray and SZE Galaxy Cluster Surveys to Study Dark Energy

We examine the prospects for measuring the dark energy equation of state parameter w within the context of the still uncertain redshift evolution of galaxy cluster structure. We show that for a particular X-ray survey (SZE survey) the constraints on w degrade by roughly a factor of 3 (factor of 2) when one accounts for the possibility of non--standard cluster evolution. With followup measurements of a cosmology independent, mass--like quantity it is possible to measure cluster evolution, improving constraints on cosmological parameters (like w Omega_M). We examine scenarios where 1%, 10% and 100% of detected clusters are followed up, showing that even a modest followup program can enhance the final cosmological constraints. For the case of followup measurements on 1% of the cluster sample with an uncertainty of 30% on individual cluster mass--like quantities, constraints on w are improved by a factor of 2 to 3. For the best case scenario of a zero curvature universe, these particular X-ray and SZE surveys can deliver uncertainties on w of ~4% to 6%.

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arXiv2001

Probing the Evolution of Gas Mass Fraction with Sunyaev-Zel'dovich Effect

Study of the primary anisotropies of the Cosmic Microwave Background (CMB) can be used to determine the cosmological parameters to a very high precision. The power spectrum of the secondary CMB anisotropies due to the thermal Sunyaev-Zel'dovich Effect (SZE) by clusters of galaxies, can then be studied, to constrain more cluster specific properties (like gas mass). We show the SZE power spectrum from clusters to be a sensitive probe of any possible evolution (or constancy) of the gas mass fraction. The position of the peak of the SZE power spectrum is a strong discriminatory signature of different gas mass fraction evolution models. For example, for a flat universe, there can be a difference in the $l$ values (of the peak) of {\it as much as 3000} between a constant gas mass fraction model and an evolutionary one. Moreover, observational determination of power spectrum, from blank sky surveys, is devoid of any selection effects that can possibly affect targeted X-ray or radio studies of gas mass fractions in galaxy clusters.

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arXiv2000

Sunyaev-Zel'dovich distortion from early galactic winds

We consider the distortion in the cosmic microwave background (CMB) due to galactic winds at high redshift. Winds outflowing from galaxies have been hypothesized to be possible sources of metals in the intergalactic medium, which is known to have been enriched to $10^{-2.5}$ Z$_{\odot}$ at $z \sim 3$. We model these winds as functions of mass of the parent galaxy and redshift, assuming they activate at a common initial redshift, $z_{in}$, and calculate the mean $y$ distortion and the angular power spectrum of the distortion in the CMB. We find that the thermal Sunyaev-Zel'dovich (SZ) effect due to the winds to be consistent with previous estimates. The distortion due to the kinetic SZ (kSZ) effect is, however, found to be more important than the thermal SZ (tSZ) effect. We find that the distortion due to galactic winds is an important contribution to the power spectrum of distortion at very small angular scales $(l \sim 10^4)$. We also find that the power spectrum due to clustering dominates the Poisson power spectrum for $l \le 4\hbox{--}5 \times 10^5$. We show explicitly how the combined power spectrum from wind dominates over that of clusters at 217 GHz, relevant for PLANCK. We also show how these constraints change when the efficiency of the winds is varied.

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arXiv2000

On cooling flows and the Sunyaev-Zel'dovich effect

We study the effect of cooling flows in galaxy clusters on the Sunyaev-Zel'dovich (SZ) distortion and the possible cosmological implications. The SZ effect, alongwith X-ray observations of clusters, is used to determine the Hubble constant, $H_{\circ}$. Blank sky surveys of SZ effect are being planned to constrain the geometry of the universe through cluster counts. It is also known that a significant fraction of clusters has cooling flows in them, which changes the pressure profile of intracluster gas. Since the SZ decrement depends essentially on the pressure profile, it is important to study possible changes in the determination of cosmological parameters in the presence of a cooling flow. We build several representative models of cooling flows and compare the results with the corresponding case of gas in hydrostatic equilibrium. We find that cooling flows can lead to an overestimation of the Hubble constant. Specifically, we find that for realistic models of cooling flow with mass deposition (varying $\dot m$ with radius), there is of the order $\sim 10 %$ deviation in the estimated value of the Hubble constant (from that for gas without a cooling flow) even after excluding $ \sim 80 %$ of the cooling flow region from the analysis. We also discuss the implications of using cluster counts from SZ observations to constrain other cosmological parameters, in the presence of clusters with cooling flows.

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arXiv1999

Constraints on structure formation models from Sunyaev-Zel'dovich Effect

In the context of cold dark matter (CDM) cosmological models, we have simulated images of the brightness temperature fluctuations in the cosmic microwave background (CMB)sky owing to the Sunyaev Zel$^{\prime}$dovich (S-Z) effect in a cosmological distribution of clusters. We compare the image statistics with recent ATCA limits on arcmin-scale CMB anisotropy. The S-Z effect produces a generically non-Gaussian field and we compute the variance in the simulated temperature-anisotropy images, after convolution with the ATCA beam pattern, for different cosmological models. All the models are normalised to the 4-year {\it COBE} data. We find an increase in the simulated-sky temperature variance with increase in the cosmological density parameter $Ω_0$. A comparison with the upper limits on the sky variance set by the ATCA appears to rule out our closed-universe model: low-$Ω_0$ open-universe models are preferred. The result is independent of any present day observations of $σ_8$.

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