arXiv subjects

A. Kravtsov

Publications and source records attributed to A. Kravtsov.

At least 19 recordsLinked to original sources

arXiv2025

Bursting at the seams: the star-forming main sequence and its scatter at z=3-9 using NIRCam photometry from JADES

We present a comprehensive study of the star-forming main sequence (SFMS) and its scatter at redshifts $3 \leq z \leq 9$, using NIRCam photometry from the JADES survey in the GOODS-S and GOODS-N fields. Our analysis is based on a sample of galaxies that is stellar mass complete down to $\log \left(M_{\star}/M_{\odot}\right) \approx 8.1$. The redshift evolution of the SFMS at an averaging timescale of 10 Myr follows a relation, quantified by the specific star-formation rates (sSFR$_{10}$), of $\mathrm{sSFR}\propto(1+z)^{\mu}$ with $\mu = 2.30^{+0.03}_{-0.01}$, in good agreement with theoretical predictions and the specific mass accretion rate of dark matter halos. We find that the SFMS normalisation varies in a complex way with the SFR averaging timescale, reflecting the combined effects of bursty star formation and rising star formation histories (SFHs). We quantify the scatter of the SFMS, revealing that it decreases with longer SFR averaging timescales, from $\sigma_{\rm{int}} \approx 0.4-0.5~\mathrm{dex}$ at 10 Myr to $\sigma_{\rm{int}} \approx 0.2~\mathrm{dex}$ at 100 Myr, indicating that shorter-term fluctuations dominate the scatter, although long-term variations in star formation activity are also present. Our findings suggest that bursty SFHs are more pronounced at lower stellar masses. Furthermore, we explore the implications of our results for the observed over-abundance of UV-bright galaxies at $z > 10$, concluding that additional mechanisms, such as top-heavy initial mass functions, increased star-formation efficiencies, or increased burstiness in star formation are needed to explain these observations. Finally, we emphasize the importance of accurate stellar mass completeness limits when fitting the SFMS, especially for galaxies with bursty SFHs.

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arXiv2020

Milky Way Satellite Census. II. Galaxy--Halo Connection Constraints Including the Impact of the Large Magellanic Cloud

The population of Milky Way (MW) satellites contains the faintest known galaxies and thus provides essential insight into galaxy formation and dark matter microphysics. Here we combine a model of the galaxy--halo connection with newly derived observational selection functions based on searches for satellites in photometric surveys over nearly the entire high Galactic latitude sky. In particular, we use cosmological zoom-in simulations of MW-like halos that include realistic Large Magellanic Cloud (LMC) analogs to fit the position-dependent MW satellite luminosity function. We report decisive evidence for the statistical impact of the LMC on the MW satellite population due to an estimated $6\pm 2$ observed LMC-associated satellites, consistent with the number of LMC satellites inferred from Gaia proper-motion measurements, confirming the predictions of cold dark matter models for the existence of satellites within satellite halos. Moreover, we infer that the LMC fell into the MW within the last $2\ \rm{Gyr}$ at high confidence. Based on our detailed full-sky modeling, we find that the faintest observed satellites inhabit halos with peak virial masses below $3.2\times 10^{8}\ M_{\rm{\odot}}$ at $95\%$ confidence, and we place the first robust constraints on the fraction of halos that host galaxies in this regime. We predict that the faintest detectable satellites occupy halos with peak virial masses above $10^{6}\ M_{\rm{\odot}}$, highlighting the potential for powerful galaxy formation and dark matter constraints from future dwarf galaxy searches.

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arXiv2020

Stellar mass measurements in Abell 133 with Magellan / IMACS

We present the analysis of deep optical imaging of the galaxy cluster Abell 133 with the IMACS instrument on Magellan. Our multi-band photometry enables stellar mass measurements in the cluster member galaxies down to a mass limit of $M_\star=3\times10^8\,M_\odot$ ($\approx 0.1$ of the Large Magellanic Cloud stellar mass). We observe a clear difference in the spatial distribution of large and dwarf galaxies within the cluster. Modeling these galaxies populations separately, we can confidently track the distribution of stellar mass locked in the galaxies to the cluster's virial radius. The extended envelope of the cluster's brightest galaxy can be tracked to $\sim 200$ kpc. The central galaxy contributes $\sim 1/3$ of the the total cluster stellar mass within the radius $r_{500c}$.

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arXiv2019

Measurement of the Splashback Feature around SZ-selected Galaxy Clusters with DES, SPT and ACT

We present a detection of the splashback feature around galaxy clusters selected using their Sunyaev-Zel'dovich (SZ) signal. Recent measurements of the splashback feature around optically selected galaxy clusters have found that the splashback radius, $r_{\rm sp}$, is smaller than predicted by N-body simulations. A possible explanation for this discrepancy is that $r_{\rm sp}$ inferred from the observed radial distribution of galaxies is affected by selection effects related to the optical cluster-finding algorithms. We test this possibility by measuring the splashback feature in clusters selected via the SZ effect in data from the South Pole Telescope SZ survey and the Atacama Cosmology Telescope Polarimeter survey. The measurement is accomplished by correlating these clusters with galaxies detected in the Dark Energy Survey Year 3 data. The SZ observable used to select clusters in this analysis is expected to have a tighter correlation with halo mass and to be more immune to projection effects and aperture-induced biases than optically selected clusters. We find that the measured $r_{\rm sp}$ for SZ-selected clusters is consistent with the expectations from simulations, although the small number of SZ-selected clusters makes a precise comparison difficult. In agreement with previous work, when using optically selected redMaPPer clusters, $r_{\rm sp}$ is $\sim$ $2σ$ smaller than in the simulations. These results motivate detailed investigations of selection biases in optically selected cluster catalogs and exploration of the splashback feature around larger samples of SZ-selected clusters. Additionally, we investigate trends in the galaxy profile and splashback feature as a function of galaxy color, finding that blue galaxies have profiles close to a power law with no discernible splashback feature, which is consistent with them being on their first infall into the cluster.

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arXiv2018

DES Y1 Results: Validating cosmological parameter estimation using simulated Dark Energy Surveys

We use mock galaxy survey simulations designed to resemble the Dark Energy Survey Year 1 (DES Y1) data to validate and inform cosmological parameter estimation. When similar analysis tools are applied to both simulations and real survey data, they provide powerful validation tests of the DES Y1 cosmological analyses presented in companion papers. We use two suites of galaxy simulations produced using different methods, which therefore provide independent tests of our cosmological parameter inference. The cosmological analysis we aim to validate is presented in DES Collaboration et al. (2017) and uses angular two-point correlation functions of galaxy number counts and weak lensing shear, as well as their cross-correlation, in multiple redshift bins. While our constraints depend on the specific set of simulated realisations available, for both suites of simulations we find that the input cosmology is consistent with the combined constraints from multiple simulated DES Y1 realizations in the $Ω_m-σ_8$ plane. For one of the suites, we are able to show with high confidence that any biases in the inferred $S_8=σ_8(Ω_m/0.3)^{0.5}$ and $Ω_m$ are smaller than the DES Y1 $1-σ$ uncertainties. For the other suite, for which we have fewer realizations, we are unable to be this conclusive; we infer a roughly 70% probability that systematic biases in the recovered $Ω_m$ and $S_8$ are sub-dominant to the DES Y1 uncertainty. As cosmological analyses of this kind become increasingly more precise, validation of parameter inference using survey simulations will be essential to demonstrate robustness.

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arXiv2018

The Splashback Feature around DES Galaxy Clusters: Galaxy Density and Weak Lensing Profiles

Splashback refers to the process of matter that is accreting onto a dark matter halo reaching its first orbital apocenter and turning around in its orbit. The cluster-centric radius at which this process occurs, r_sp, defines a halo boundary that is connected to the dynamics of the cluster. A rapid decline in the halo profile is expected near r_sp. We measure the galaxy number density and weak lensing mass profiles around redMaPPer galaxy clusters in the first year Dark Energy Survey (DES) data. For a cluster sample with mean M_200m mass ~2.5 x 10^14 M_sun, we find strong evidence of a splashback-like steepening of the galaxy density profile and measure r_sp=1.13 +/- 0.07 Mpc/h, consistent with earlier SDSS measurements of More et al. (2016) and Baxter et al. (2017). Moreover, our weak lensing measurement demonstrates for the first time the existence of a splashback-like steepening of the matter profile of galaxy clusters. We measure r_sp=1.34 +/- 0.21 Mpc/h from the weak lensing data, in good agreement with our galaxy density measurements. For different cluster and galaxy samples, we find that consistent with LCDM simulations, r_sp scales with R_200m and does not evolve with redshift over the redshift range of 0.3--0.6. We also find that potential systematic effects associated with the redMaPPer algorithm may impact the location of r_sp. We discuss progress needed to understand the systematic uncertainties and fully exploit forthcoming data from DES and future surveys, emphasizing the importance of more realistic mock catalogs and independent cluster samples.

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arXiv2017

Cosmology from Cosmic Shear with DES Science Verification Data

We present the first constraints on cosmology from the Dark Energy Survey (DES), using weak lensing measurements from the preliminary Science Verification (SV) data. We use 139 square degrees of SV data, which is less than 3\% of the full DES survey area. Using cosmic shear 2-point measurements over three redshift bins we find $σ_8 (Ω_{\rm m}/0.3)^{0.5} = 0.81 \pm 0.06$ (68\% confidence), after marginalising over 7 systematics parameters and 3 other cosmological parameters. We examine the robustness of our results to the choice of data vector and systematics assumed, and find them to be stable. About $20$\% of our error bar comes from marginalising over shear and photometric redshift calibration uncertainties. The current state-of-the-art cosmic shear measurements from CFHTLenS are mildly discrepant with the cosmological constraints from Planck CMB data; our results are consistent with both datasets. Our uncertainties are $\sim$30\% larger than those from CFHTLenS when we carry out a comparable analysis of the two datasets, which we attribute largely to the lower number density of our shear catalogue. We investigate constraints on dark energy and find that, with this small fraction of the full survey, the DES SV constraints make negligible impact on the Planck constraints. The moderate disagreement between the CFHTLenS and Planck values of $σ_8 (Ω_{\rm m}/0.3)^{0.5}$ is present regardless of the value of $w$.

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arXiv2017

Digital receivers for low-frequency radio telescopes UTR-2, URAN, GURT

This paper describes digital radio astronomical receivers used for decameter and meter wavelength observations. This paper describes digital radio astronomical receivers used for decameter and meter wavelength observations. Since 1998, digital receivers performing on-the-fly dynamic spectrum calculations or waveform data recording without data loss have been used at the UTR-2 radio telescope, the URAN VLBI system, and the GURT new generation radio telescope. Here we detail these receivers developed for operation in the strong interference environment that prevails in the decameter wavelength range. Data collected with these receivers allowed us to discover numerous radio astronomical objects and phenomena at low frequencies, a summary of which is also presented.

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arXiv2016

The Role of Penetrating Gas Streams in Setting the Dynamical State of Galaxy Clusters

We utilize cosmological simulations of 16 galaxy clusters at redshifts \zeq{0} and \zeq{0.6} to study the effect of inflowing streams on the properties of the X-ray emitting intracluster medium. We find that the mass accretion occurs predominantly along streams that originate from the cosmic web and consist of heated gas. Clusters that are unrelaxed in terms of their X-ray morphology are characterized by higher mass inflow rates and deeper penetration of the streams, typically into the inner third of the virial radius. The penetrating streams generate elevated random motions, bulk flows and cold fronts. The degree of penetration of the streams may change over time such that clusters can switch from being unrelaxed to relaxed over a time-scale of several giga years.

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arXiv2016

Cosmic Shear Measurements with DES Science Verification Data

We present measurements of weak gravitational lensing cosmic shear two-point statistics using Dark Energy Survey Science Verification data. We demonstrate that our results are robust to the choice of shear measurement pipeline, either ngmix or im3shape, and robust to the choice of two-point statistic, including both real and Fourier-space statistics. Our results pass a suite of null tests including tests for B-mode contamination and direct tests for any dependence of the two-point functions on a set of 16 observing conditions and galaxy properties, such as seeing, airmass, galaxy color, galaxy magnitude, etc. We furthermore use a large suite of simulations to compute the covariance matrix of the cosmic shear measurements and assign statistical significance to our null tests. We find that our covariance matrix is consistent with the halo model prediction, indicating that it has the appropriate level of halo sample variance. We compare the same jackknife procedure applied to the data and the simulations in order to search for additional sources of noise not captured by the simulations. We find no statistically significant extra sources of noise in the data. The overall detection significance with tomography for our highest source density catalog is 9.7sigma. Cosmological constraints from the measurements in this work are presented in a companion paper (DES et al. 2015).

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arXiv2013

Bias from gas inhomogeneities in the pressure profiles as measured from X-ray and SZ observations

X-ray observations of galaxy clusters provide emission measure weighted spectra, arising from a range of density and temperature fluctuations in the intra-cluster medium (ICM). This is fitted to a single temperature plasma emission model to provide an estimate of the gas density and temperature, which are sensitive to the gas inhomogeneities. Therefore, X-ray observations yield a potentially biased estimate of the thermal gas pressure, P_X. At the same time Sunyaev-Zeldovich (SZ) observations directly measure the integrated gas pressure, P_SZ. If the X-ray pressure profiles are strongly biased with respect to the SZ, then one has the possibility to probe the gas inhomogeneities, even at scales unresolved by the current generation of telescopes. At the same time, a weak bias has implications for the interchangeable use of mass proxies like Y_SZ and Y_X as cosmological probes. In this paper we investigate the dependence of the bias, defined as b_P(r)=P_X(r)/P_SZ(r)-1, on the characteristics of fluctuations in the ICM taking into account the correlation between temperature and density fluctuations. We made a simple prediction of the irreducible bias in idealised X-ray vs SZ observations using multi-temperature plasma emission model. We also provide a simple fitting form to estimate the bias given the distribution of fluctuations. Analysing a sample of 16 clusters extracted from hydrodynamical simulations, we find that the median value of bias is within +/-3% within R_500, it decreases to -5% at R_500 ~ 2R_500. The scatter of b_P(r) between individual relaxed clusters is small -- at the level of <0.03 within R_500, but turns significantly larger (0.25) and highly skewed at r>~ 1.5R_500. For any relaxed cluster we find |b_P(r)| < 15% within R_500, across different implementations of input physics in the simulations. [abridged]

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arXiv2012

Quantifying properties of ICM inhomogeneities

We present a new method to identify and characterize the structure of the intracluster medium (ICM) in simulated galaxy clusters. The method uses the median of gas properties, such as density and pressure, which we show to be very robust to the presence of gas inhomogeneities. In particular, we show that the radial profiles of median gas properties are smooth and do not exhibit fluctuations at locations of massive clumps in contrast to mean and mode properties. It is shown that distribution of gas properties in a given radial shell can be well described by a log-normal PDF and a tail. The former corresponds to a nearly hydrostatic bulk component, accounting for ~99% of the volume, while the tail corresponds to high density inhomogeneities. We show that this results in a simple and robust separation of the diffuse and clumpy components of the ICM. The FWHM of the density distribution grows with radius and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2r_500 in relaxed clusters. The small scatter in the width between relaxed clusters suggests that the degree of inhomogeneity is a robust characteristic of the ICM. It broadly agrees with the amplitude of density perturbations in the Coma cluster. We discuss the origin of ICM density variations in spherical shells and show that less than 20% of the width can be attributed to the triaxiality of the cluster gravitational potential. As a link to X-ray observations of real clusters we evaluated the ICM clumping factor with and without high density inhomogeneities. We argue that these two cases represent upper and lower limits on the departure of the observed X-ray emissivity from the median value. We find that the typical value of the clumping factor in the bulk component of relaxed clusters varies from ~1.1-1.2 at r_500 up to ~1.3-1.4 at r_200, in broad agreement with recent observations.

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arXiv2012

Constraints on the ICM velocity power spectrum from the X-ray lines width and shift

Future X-ray observations of galaxy clusters by high spectral resolution missions will provide spatially resolved measurements of the energy and width for the brightest emission lines in the intracluster medium (ICM) spectrum. In this paper we discuss various ways of using these high resolution data to constrain velocity power spectrum in galaxy clusters. We argue that variations of these quantities with the projected distance R in cool core clusters contain important information on the velocity field length scales in the ICM. The effective length $l_{\rm eff}$ along the line of sight, which provides dominant contribution to the line flux, increases with R, allowing one to probe the amplitude of the velocity variations at different spatial scales. In particular, we show that the width of the line as a function of R is closely linked to the structure function of the 3D velocity field. Yet another easily obtainable proxy of the velocity field length scales is the ratio of the amplitude of the projected velocity field (line energy) variations to the dispersion of the velocity along the line of sight (line width). Finally the projected velocity field can be easily converted into 3D velocity field, especially for clusters like Coma with an extended flat core in the surface brightness. Under assumption of a homogeneous isotropic Gaussian 3D velocity field we derived simple expressions relating the power spectrum of the 3D velocity field (or structure function) and the observables. The uncertainties in the observables, caused by stochastic nature of the velocity field, are estimated by making multiple realizations of the random Gaussian velocity field and evaluating the scatter in observables. If large scale motions are present in the ICM these uncertainties may dominate the statistical errors of line width and shift measurements.

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arXiv2011

Constraining halo occupation properties of X-ray AGNs using clustering of Chandra sources in the Bootes survey region

We present one of the most precise measurement to date of the spatial clustering of X-ray selected AGNs using a sample derived from the Chandra X-ray Observatory survey in the Bootes field. The real-space two-point correlation function over a redshift interval from z=0.17 to z~3 is well described by the power law, xi(r)=(r/r0)^-gamma, for comoving separations r<~20h^-1 Mpc. We find gamma=1.84+-0.12 and r0 consistent with no redshift trend within the sample (varying between r0=5.5+-0.6 h^-1 Mpc for =0.37 and r0=6.9+-1.0 h^-1 Mpc for =1.28). Further, we are able to measure the projections of the two-point correlation function both on the sky plane and in the line of sight. We use these measurements to show that the Chandra/Bootes AGNs are predominantly located at the centers of dark matter halos with the circular velocity Vmax>320 km/s or M_200 > 4.1e12 h^-1 Msun, and tend to avoid satellite galaxies in halos of this or higher mass. The halo occupation properties inferred from the clustering properties of Chandra/Bootes AGNs --- the mass scale of the parent dark matter halos, the lack of significant redshift evolution of the clustering length, and the low satellite fraction --- are broadly consistent with the Hopkins et al. scenario of quasar activity triggered by mergers of similarly-sized galaxies.

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arXiv2009

Clusters and Large-Scale Structure: the Synchrotron Keys

For over four decades, synchrotron-radiating sources have played a series of pathfinding roles in the study of galaxy clusters and large scale structure. Such sources are uniquely sensitive to the turbulence and shock structures of large-scale environments, and their cosmic rays and magnetic fields often play important dynamic and thermodynamic roles. They provide essential complements to studies at other wavebands. Over the next decade, they will fill essential gaps in both cluster astrophysics and the cosmological growth of structure in the universe, especially where the signatures of shocks and turbulence, or even the underlying thermal plasma itself, are otherwise undetectable. Simultaneously, synchrotron studies offer a unique tool for exploring the fundamental question of the origins of cosmic magnetic fields. This work will be based on the new generation of m/cm-wave radio telescopes now in construction, as well as major advances in the sophistication of 3-D MHD simulations.

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arXiv2009

Galaxy Cluster Astrophysics and Cosmology: Questions and Opportunities for the Coming Decade

We are learning much about how structure forms, in particular how clusters as nodes in the cosmic web evolve and accrete matter, and about the physical processes within these objects. In the next decade, the study of clusters will enable us to tackle important questions regarding the nature of Dark Matter and Dark Energy, how clusters co-evolve with super-massive black holes at their centers, and to advance our knowledge about fundamental plasma astrophysics. This science white paper outlines the key questions and research opportunities in cluster astrophysics that are emerging in the coming decade and beyond, and serves as an overview to other cluster related white papers.

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arXiv2009

Towards the 2020 vision of the baryon content of galaxy groups and clusters

Groups and clusters of galaxies occupy a special position in the hierarchy of large-scale cosmic structures because they are the largest and the most massive (from ~10^13 Msun to over 10^15 Msun) objects in the universe that have had time to undergo gravitational collapse. The large masses of clusters imply that their contents have been accreted from regions of ~8-40 comoving Mpc in size and should thus be representative of the mean matter content of the universe. During the next decade sensitive multi-wavelength observations should be able to map the radial distributions of all main mass components (stars, cold, warm, and hot gas and total mass) at z<~ 1 out to the virial radius. At the same time, comparative studies of real and simulated cluster samples sould allow us to use clusters as veritable astrophysical laboratories for studying galaxy formation, as well as testing our theoretical models of structure formation and underlying assumptions about fundamental physics governing the universe.

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arXiv2009

Diffuse baryonic matter beyond 2020

The hot, diffuse gas that fills the largest overdense structures in the Universe -- clusters of galaxies and a web of giant filaments connecting them -- provides us with tools to address a wide array of fundamental astrophysical and cosmological questions via observations in the X-ray band. Clusters are sensitive cosmological probes. To utilize their full potential for precision cosmology in the following decades, we must precisely understand their physics -- from their cool cores stirred by jets produced by the central supermassive black hole (itself fed by inflow of intracluster gas), to their outskirts, where the infall of intergalactic medium (IGM) drives shocks and accelerates cosmic rays. Beyond the cluster confines lies the virtually unexplored warm IGM, believed to contain most of the baryonic matter in the present-day Universe. As a depository of all the matter ever ejected from galaxies, it carries unique information on the history of energy and metal production in the Universe. Currently planned major observatories, such as Astro-H and IXO, will make deep inroads into these areas, but to see the most interesting parts of the picture will require an almost science-fiction-grade facility with tens of m^2 of effective area, subarcsecond angular resolution, a matching imaging calorimeter and a super high-dispersion spectrograph, such as Generation-X.

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