arXiv subjects

N. Malavasi

Publications and source records attributed to N. Malavasi.

27 records · Page 2Linked to original sources

arXiv2024

The SRG/eROSITA All-Sky Survey: Exploring halo assembly bias with X-ray selected superclusters

We explore halo assembly bias on cluster scales using large samples of superclusters. Leveraging the largest-ever X-ray galaxy cluster and supercluster samples obtained from the first SRG/eROSITA all-sky survey, we construct two subsamples of galaxy clusters which consist of supercluster members (SC) and isolated clusters (ISO) respectively. After correcting the selection effects on redshift, mass, and survey depth, we compute the excess in the concentration of the intracluster gas of isolated clusters with respect to supercluster members, defined as $δc_{\rm gas} \equiv c_{\rm gas,ISO}/c_{\rm gas,SC}-1$, to investigate the environmental effect on the concentration of clusters, an inference of halo assembly bias on cluster scales. We find that the average gas mass concentration of isolated clusters is a few percent higher than that of supercluster members, with a maximum significance of $2.8σ$. The result on $δc_{\rm gas}$ varies with the overdensity ratio $f$ in supercluster identification, cluster mass proxies, and mass ranges, but remains positive in all the measurements. We measure slightly larger $δc_{\rm gas}$ when adopting a higher $f$ in supercluster identification. $δc_{\rm gas}$ is also larger for low-mass and low-redshift clusters. We perform weak lensing analyses to compare the total mass concentration of the two classes and find a similar trend as obtained from gas mass concentration. Our results are consistent with the prediction of HAB on cluster scales, where halos located in denser environments are less concentrated, and this trend is stronger for halos with lower mass and at lower redshifts. These phenomena can be interpreted by the fact that clusters in denser environments such as superclusters have experienced more mergers than isolated clusters in their assembling history.

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arXiv2024

Detecting Galaxy Groups and AGNs populating the local Universe in the eROSITA era

The eROSITA will deliver an unprecedented volume of X-ray survey observations, 20-30 times more sensitive than ROSAT in the soft band (0.5-2 keV) and for the first time imaging in the hard band (2-10 keV) including galaxy clusters and groups along with obscured and unobscured AGNs. This calls for a powerful theoretical effort to control the systematics and biases that may affect the data analysis. We investigate the detection technique and selection effects in the galaxy group and AGN populations of a mock eROSITA survey at the depth of eRASS:4. We create a $30\times 30$ deg$^{2}$ mock observation based on the cosmological hydrodynamical simulation Magneticum Pathfinder within z=0-0.2. We combine a physical background extracted from the real eFEDS background analysis with realistic simulations of X-ray emission for the hot gas, AGNs and X-ray binaries. We apply a detection procedure equivalent to the reduction done on eRASS data and evaluate the completeness and contamination to reconstruct the luminosity functions of the extended and point sources in the catalogue. We assess the completeness of extended detections as a function of the input X-ray flux and halo. We achieve full recovery of the brightest (most massive) clusters and AGNs. However, a significant fraction of galaxy groups remains undetected. Examining the gas properties between the detected and undetected galaxy groups at fixed halo mass, we observe that the detected population exhibits, on average, higher X-ray brightness compared to the undetected ones. Moreover, we find that X-ray luminosity primarily correlates with the hot gas fraction, rather than temperature or metallicity. Our simulation suggests the presence of a systematic selection effect in current surveys, resulting in X-ray survey catalogues predominantly composed of the lowest-entropy, gas-richest, and highest surface brightness halos on galaxy group scales.

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arXiv2024

The Galaxy Group Merger Origin of the Cloverleaf Odd Radio Circle System

Odd radio circles (ORCs) are a newly discovered class of extended faint radio sources of unknown origin. We report the first detection of diffuse X-ray gas at the location of a low-redshift ORC (z=0.046) known as Cloverleaf ORC. This observation was performed with the XMM-Newton X-ray telescope. The physical extent of the diffuse X-ray emission corresponds to a region of approximately 230 kpc by 160 kpc, lying perpendicular to the radio emission detected by ASKAP. The X-ray spectrum shows characteristics of thermal multiphase gas with temperatures of 1.10+/-0.08 keV and 0.22+/-0.01 keV and a central density of $(4.9\pm0.6)\times10^{-4}$ cm$^{-3}$, indicating that the Cloverleaf ORC resides in a low-mass galaxy group. Using X-ray observations, with hydrostatic equilibrium and isothermal assumptions, we measure the galaxy group to have a gas mass and a total mass of $(7.7\pm 0.8) \times 10^{11}$ M$_{\rm sun}$ and $2.6\pm0.3\times10^{13}$~$M_{\rm sun}$ within the overdensity radius R$_{500}$. The presence of a high-velocity subgroup identified in optical data, the orientation of the brightest cluster galaxy, the disturbed morphologies of galaxies toward the east of the Cloverleaf ORC, and the irregular morphology of the X-ray emission suggest that this system is undergoing a galaxy group merger. The radio power of the ORC could be explained by the shock reacceleration of fossil cosmic rays generated by a previous episode of black hole activity in the central active galactic nucleus.

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arXiv2023

The X-ray invisible Universe. A look into the halos undetected by eROSITA

The paper presents the analysis of optically selected GAMA groups and clusters in the SRG/eROSITA X-ray map of eFEDS (eROSITA Final Equatorial Depth Survey), in the halo mass range $10^{13}-5{\times}10^{14}$ $M_{\odot}$ and at $z < 0.2$. All X-ray detections have a clear GAMA counterpart, but most of the GAMA groups in the halo mass range $10^{13}-10^{14}$ $M_{\odot}$ remain undetected. We compare the X-ray surface brightness profiles of the eROSITA detected groups with the mean stacked profile of the undetected low-mass halos at fixed halo mass. Overall, we find that the undetected groups exhibit less concentrated X-ray surface brightness, dark matter, and galaxy distributions with respect to the X-ray detected halos. The mean gas mass fraction profiles are consistent in the two samples within 1.5$σ$, indicating that the gas follows the dark matter profile. The low mass concentration and the magnitude gap indicate that these systems are young. They reside with a higher probability in filaments while X-ray detected groups favor the nodes of the Cosmic Web. Because of the lower central emission, the undetected systems tend to be X-ray under-luminous at fixed halo mass and to lie below the $L_X-M_{halo}$ relation. Interestingly, the X-ray detected systems inhabiting the nodes scatter the less around the relation, while those in filaments tend to lie below it. We do not observe any strong relationship between the system X-ray appearance and the AGN activity. We cannot exclude the role of the past AGN feedback in affecting the gas distribution over the halo lifetime. However, the data suggests that the observed differences might be related to the halo assembly bias.

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arXiv2020

First detection of stacked X-ray emission from cosmic web filaments

We report the first statistical detection of X-ray emission from cosmic web filaments in ROSAT data. We selected 15,165 filaments at 0.2<z<0.6 ranging from 30 Mpc to 100 Mpc in length, identified in the Sloan Digital Sky Survey (SDSS) survey. We stacked the X-ray count-rate maps from ROSAT around the filaments, excluding resolved galaxy groups and clusters above the mass of ~3 * 10^13 Msun as well as the detected X-ray point sources from the ROSAT, Chandra, and XMM-Newton observations. The stacked signal results in the detection of the X-ray emission from the cosmic filaments at a significance of 4.2 sigma in the energy band of 0.56-1.21 keV. The signal is interpreted, assuming the Astrophysical Plasma Emission Code (APEC) model, as an emission from the hot gas in the filament-core regions with an average gas temperature of 0.9(+1.0-0.6) keV and a gas overdensity of ~30 at the center of the filaments. Furthermore, we show that stacking the SRG/eROSITA data for ~2,000 filaments only would lead to a ~5 sigma detection of their X-ray signal, even with an average gas temperature as low as ~0.3 keV.

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arXiv2020

Density and temperature of cosmic-web filaments on scales of tens of megaparsecs

We studied physical properties of matter in 24,544 filaments ranging from 30 to 100 Mpc in length, identified in the Sloan Digital Sky Survey (SDSS). We stacked the Comptonization y map produced by the Planck Collaboration around the filaments, excluding the resolved galaxy groups and clusters above a mass of ~3*10^13 Msun. We detected the thermal Sunyaev-Zel'dovich signal for the first time at a significance of 4.4 sigma in filamentary structures on such a large scale. We also stacked the Planck cosmic microwave background (CMB) lensing convergence map in the same manner and detected the lensing signal at a significance of 8.1 sigma. To estimate physical properties of the matter, we considered an isothermal cylindrical filament model with a density distribution following a beta-model (beta=2/3). Assuming that the gas distribution follows the dark matter distribution, we estimate that the central gas and matter overdensity and gas temperature are overdensity = (19.0 +27.3 -12.1) and temperature = (1.2 +- 0.4)*10^6 K, which results in a measured baryon fraction of (0.080 +0.116 -0.051) * Omega_b.

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arXiv2019

Filament profiles from WISExSCOS galaxies as probes of the impact of environmental effects

The role played by the large-scale structures in the galaxy evolution is not quite well understood yet. In this study, we investigate properties of galaxy in the range 0.1<z<0.3 from a value-added version of the WISExSCOS catalogue around cosmic filaments detected with DisPerSE. We have fitted a profile of galaxy over-density around cosmic filaments and found a typical radius of r_m = 7.5+-0.2 Mpc. We have measured an excess of passive galaxies near the filament's spine, higher than the excess of transitioning and active galaxies. We have also detected SFR and Mstar gradients pointing towards the filament's spine. We have investigated this result and found an Mstar gradient for each type of galaxies: active, transitioning, and passive, and a positive SFR gradient for passive galaxies. We also link the galaxy properties and the gas content in the Cosmic Web. To do so, we have investigated the quiescent fraction fQ profile of galaxies around the cosmic filaments. Based on recent studies about the effect of the gas and of the Cosmic Web on galaxy properties, we have modeled fQ with a beta model of gas pressure. The slope obtained here, beta=0.54+-0.18, is compatible with the scenario of projected isothermal gas in hydrostatic equilibrium (beta=2/3), and with the profiles of gas fitted in SZ.

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arXiv2017

Galaxy evolution in the metric of the Cosmic Web

The role of the cosmic web in shaping galaxy properties is investigated in the GAMA spectroscopic survey in the redshift range $0.03 \leq z \leq 0.25$. The stellar mass, $u - r$ dust corrected colour and specific star formation rate (sSFR) of galaxies are analysed as a function of their distances to the 3D cosmic web features, such as nodes, filaments and walls, as reconstructed by DisPerSE. Significant mass and type/colour gradients are found for the whole population, with more massive and/or passive galaxies being located closer to the filament and wall than their less massive and/or star-forming counterparts. Mass segregation persists among the star-forming population alone. The red fraction of galaxies increases when closing in on nodes, and on filaments regardless of the distance to nodes. Similarly, the star-forming population reddens (or lowers its sSFR) at fixed mass when closing in on filament, implying that some quenching takes place. Comparable trends are also found in the state-of-the-art hydrodynamical simulation Horizon-AGN. These results suggest that on top of stellar mass and large-scale density, the traceless component of the tides from the anisotropic large-scale environment also shapes galactic properties. An extension of excursion theory accounting for filamentary tides provides a qualitative explanation in terms of anisotropic assembly bias: at a given mass, the accretion rate varies with the orientation and distance to filaments. It also explains the absence of type/colour gradients in the data on smaller, non-linear scales.

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arXiv2016

The VIMOS Public Extragalactic Redshift Survey (VIPERS): Galaxy segregation inside filaments at $z \simeq 0.7$

We present the first quantitative detection of large-scale filamentary structure at $z \simeq 0.7$ in the large cosmological volume probed by the VIMOS Public Extragalactic Redshift Survey (VIPERS). We use simulations to show the capability of VIPERS to recover robust topological features in the galaxy distribution, in particular the filamentary network. We then investigate how galaxies with different stellar masses and stellar activities are distributed around the filaments and find a significant segregation, with the most massive or quiescent galaxies being closer to the filament axis than less massive or active galaxies. The signal persists even after down-weighting the contribution of peak regions. Our results suggest that massive and quiescent galaxies assemble their stellar mass through successive mergers during their migration along filaments towards the nodes of the cosmic web. On the other hand, low-mass star-forming galaxies prefer the outer edge of filaments, a vorticity rich region dominated by smooth accretion, as predicted by the recent spin alignment theory. This emphasizes the role of large scale cosmic flows in shaping galaxy properties.

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