The relation between gas density and velocity power spectra in galaxy clusters: high-resolution hydrodynamic simulations and the role of conduction

Exploring the ICM power spectrum can help us to probe the physics of galaxy clusters. Using high-resolution 3D plasma simulations, we study the statistics of the velocity field and its relation with the thermodynamic perturbations. The normalization of the ICM spectrum (density, entropy, or pressure) is linearly tied to the level of large-scale motions, which excite both gravity and sound waves due to stratification. For low 3D Mach number M~0.25, gravity waves mainly drive entropy perturbations, traced by preferentially tangential turbulence. For M>0.5, sound waves start to significantly contribute, passing the leading role to compressive pressure fluctuations, associated with isotropic (or slightly radial) turbulence. Density and temperature fluctuations are then characterized by the dominant process: isobaric (low M), adiabatic (high M), or isothermal (strong conduction). Most clusters reside in the intermediate regime, showing a mixture of gravity and sound waves, hence drifting towards isotropic velocities. Remarkably, regardless of the regime, the variance of density perturbations is comparable to the 1D Mach number. This linear relation allows to easily convert between gas motions and ICM perturbations, which can be exploited by Chandra, XMM data and by the forthcoming Astro-H. At intermediate and small scales (10-100 kpc), the turbulent velocities develop a Kolmogorov cascade. The thermodynamic perturbations act as effective tracers of the velocity field, broadly consistent with the Kolmogorov-Obukhov-Corrsin advection theory. Thermal conduction acts to damp the gas fluctuations, washing out the filamentary structures and steepening the spectrum, while leaving unaltered the velocity cascade. The ratio of the velocity and density spectrum thus inverts the downtrend shown by the non-diffusive models, allowing to probe the presence of significant conductivity in the ICM.Comment: Accepted by A&A; 15 pages, 10 figures; added insights and references - thank you for the positive feedbac

Smoothening Transition of a Two-Dimensional Pressurized Polymer Ring

We revisit the problem of a two-dimensional polymer ring subject to an inflating pressure differential. The ring is modeled as a freely jointed closed chain of N monomers. Using a Flory argument, mean-field calculation and Monte Carlo simulations, we show that at a critical pressure, $p_c \sim N^{-1}$, the ring undergoes a second-order phase transition from a crumpled, random-walk state, where its mean area scales as $\sim N$, to a smooth state with $\sim N^2$. The transition belongs to the mean-field universality class. At the critical point a new state of polymer statistics is found, in which $\sim N^{3/2}$. For $p>>p_c$ we use a transfer-matrix calculation to derive exact expressions for the properties of the smooth state.Comment: 9 pages, 8 figure

Are the next-generation households ready for the energy transition? A survey on their positioning and practice with energy management tools

In the last decades, significant effort has been put towards technological advancement in housing for energy transition. Massive retrofitting actions have been called for, and innovative technologies for smart energy management at home have been deployed. However, undesired energy trends in housing suggest that relevant factors have been neglected. Among these, increasing importance is now given to occupants' behaviour, and their capacity to interact with energy management devices available in dwellings. This study investigates what is the position of next-generation users on energy transition at home. Two years ago, the authors launched a survey to explore people's awareness of energy use practices, interaction with metering devices, and user motivation to change when informed. As a pilot survey, over 300 people from the academy were involved to see what was the position of a sample which was supposed to be informed more than the average, in Italy. The test yielded early outcomes on how people become more interested to change as they gain knowledge and are offered suggestions. Despite the expectations, the sample's level of awareness was low. This suggested that a more user-centred approach is needed for wide-scale progress. Especially results from the youngest were below prospects. The questionnaire was relaunched to examine if the pandemic, energy crisis and latest news on climate change have affected positions of the youngsters. A testing session involving university students was performed, and results have been compared with the previous. As a result, reflections on the energy use patterns of the next-generation households are provided

The relation between gas density and velocity power spectra in galaxy clusters: qualitative treatment and cosmological simulations

We address the problem of evaluating the power spectrum of the velocity field of the ICM using only information on the plasma density fluctuations, which can be measured today by Chandra and XMM-Newton observatories. We argue that for relaxed clusters there is a linear relation between the rms density and velocity fluctuations across a range of scales, from the largest ones, where motions are dominated by buoyancy, down to small, turbulent scales: $(\delta\rho_k/\rho)^2 = \eta_1^2 (V_{1,k}/c_s)^2$, where $\delta\rho_k/\rho$ is the spectral amplitude of the density perturbations at wave number $k$, $V_{1,k}^2=V_k^2/3$ is the mean square component of the velocity field, $c_s$ is the sound speed, and $\eta_1$ is a dimensionless constant of order unity. Using cosmological simulations of relaxed galaxy clusters, we calibrate this relation and find $\eta_1\approx 1 \pm 0.3$. We argue that this value is set at large scales by buoyancy physics, while at small scales the density and velocity power spectra are proportional because the former are a passive scalar advected by the latter. This opens an interesting possibility to use gas density power spectra as a proxy for the velocity power spectra in relaxed clusters, across a wide range of scales.Comment: 6 pages, 3 figures, submitted to ApJ Letter

Effect of facade reflectance on outdoor microclimate: An Italian case study

Global warming affects the built environment with relation to its own characteristics, form, density. Heat waves effects would have limited effects if most of the cities would not be affected by Urban Heat Island that strongly increase their impacts (particularly on urban population). Does the choice of façade colours and materials contribute to this issue? The paper reports a research on a case study in Italy that tries to answer to this question comparing the trend in outdoor temperature increase closed to the building façade with relation to its colour and reflectance variations modelled by using Envi-met software. The outcomes point out that there is a correlation between the building façade reflectance and the temperature trend but this has a very limited influence on outdoor microclimate in open spaces as it varies in a range of less than 1°C

Evolución del arbolado urbano en dunas bonaerenses

Los cambios espacio-temporales en la ocupación del territorio son procesos dinámicos que deben ser estudiados porque proporcionan la base para conocer las tendencias de los procesos de urbanización. El objetivo del trabajo fue realizar la zonificación geográfica de la evolución de cambios de la cobertura de territorial a partir de la tendencia temporal al sur del Municipio de Villa Gesell. Se realizó un análisis cluster con SIG de fotografías aéreas e imágenes satelitales de los años 1975 – 1997 y 2015, cuyos resultados expresan el aumento del arbolado urbano. Los cambios encontrados favorecieron el desarrollo del turismo con una mejora del paisaje y aportando un elemento blando de conexión entre las tres localidades del eje.The spatio-temporal changes in the occupation of the territory are dynamic processes that must be studied because they provide the base to know the tendencies of the processes of urbanization. The objective of the work was to carry out the geographical zoning of the evolution of changes in territorial coverage based on the temporal trend south of the Municipality of Villa Gesell. A cluster analysis was carried out with GIS of aerial photographs and satellite images of the years 1975 - 1997 and 2015, whose results express the increase of urban trees. The changes found favored the development of tourism with an improvement of the landscape and contributing a soft element of connection between the three localities.Fil: Gaspari, Fernanda J.. Universidad Nacional de la Plata. Facultad de Ciencias Agrarias y ForestalesFil: Senisterra, Gabriela E.. Universidad Nacional de la Plata. Facultad de Ciencias Agrarias y ForestalesFil: Rodriguez Vagaría, Alfonso. Universidad Nacional de la Plata. Facultad de Ciencias Agrarias y Forestale

Rainfall-runoff and curve number relationship under different conditions of soil´s: : ventania system modal basin, Argentina

La metodología del número de la curva (NC) es la más empleada para transformar la precipitación total en precipitación efectiva. De esta manera se constituye en una herramienta de gran valor para realizar estudios hidrológicos en cuencas hidrográficas, fundamentalmente cuando hay una deficiencia de registros extensos y confiables. Esta metodología requiere del conocimiento del tipo y uso de suelo de la cuenca en estudio y registros pluviográficos. En el presente trabajo se aplicó el procesamiento de imágenes LANDSAT para la zonificación de la vegetación y uso del suelo en la cuenca del Arroyo Pillahuinco Grande (38° LS y 61° 15' LW), ubicada sobre el sistema serrano de La Ventana, en el sudoeste de la provincia de Buenos Aires, Argentina. El análisis de su interrelación generó los valores de NC y coeficiente de escorrentía (CE). El procesamiento digital de la base de datos raster georreferenciada se realizó con aplicación de herramientas de sistema de información geográfica (Idrisi Kilimanjaro). El análisis de regresión múltiple efectuado a las variables generó un R2 que explica el 89,77 % de la variabilidad de CE (a < 0,01). Los resultados se exponen a nivel diagnóstico y zonificación del NC, donde la mayor influencia de la escorrentía se relaciona con las variables cobertura vegetal y uso del suelo.The most used methodology to transformer the storm rainfall in effective rain is the curve number (CN). So, it's un useful tool to hydrologic studies in hydrographic basin when there aren't enough available registers. This methodology requires the knowledge of type and use of soil and pluviographic registers. We processed LANDSAT images to zonification the soil use and vegetation of Pillahuinco Grande basin (38° SL and 61° 15' WL). This basin is sited at Sierra de La Ventana, South West of Buenos Aires Province, Argentina. The analysis of this rainfall-runoff relationship under different conditions of soil's use generated values of CN and runoff coefficient (EC). The digital data processing raster was done using tool of System Information Geographic (Idirisi Kilimanjaro). The multiple regression analysis showed a R2 that explain the 89.77 % of EC variability (a < 0.01). The results are showed at diagnostic level and CN zonation where the most runoff influence is related with both variables vegetal cover and use of soil.Fil: Gaspari, Fernanda J..Fil: Senisterra, Gabriela E..Fil: Marlats, Raúl M.

A textbook example of ram-pressure stripping in the Hydra A/A780 cluster

In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialization of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process

Deep Chandra observations of the stripped galaxy group falling into Abell 2142

In the local Universe, the growth of massive galaxy clusters mainly operates through the continuous accretion of group-scale systems. The infalling group in Abell 2142 is the poster child of such an accreting group, and as such, it is an ideal target to study the astrophysical processes induced by structure formation. We present the results of a deep (200 ks) observation of this structure with Chandra, which highlights the complexity of this system in exquisite detail. In the core of the group, the spatial resolution of Chandra reveals the presence of a leading edge and a complex AGN-induced activity. The morphology of the stripped gas tail appears straight in the innermost 250 kpc, suggesting that magnetic draping efficiently shields the gas from its surroundings. However, beyond $\sim300$ kpc from the core, the tail flares and the morphology becomes strongly irregular, which could be explained by a breaking of the drape, e.g. because of turbulent motions. The power spectrum of surface-brightness fluctuations is relatively flat ($P_{2D}\propto k^{-2.3}$), which indicates that thermal conduction is strongly inhibited even beyond the region where magnetic draping is effective. The amplitude of density fluctuations in the tail is consistent with a mild level of turbulence with a Mach number $M_{3D}\sim0.1-0.25$. Overall, our results show that the processes leading to the thermalization and mixing of the infalling gas are slow and relatively inefficient.Comment: Accepted for publication in A&

Deep Chandra observations of the stripped galaxy group falling into Abell 2142

In the local Universe, the growth of massive galaxy clusters mainly operates through the continuous accretion of group-scale systems. The infalling group in Abell 2142 is the poster child of such an accreting group, and as such, it is an ideal target to study the astrophysical processes induced by structure formation. We present the results of a deep (200 ks) observation of this structure with Chandra that highlights the complexity of this system in exquisite detail. In the core of the group, the spatial resolution of Chandra reveals a leading edge and complex AGN-induced activity. The morphology of the stripped gas tail appears straight in the innermost 250 kpc, suggesting that magnetic draping efficiently shields the gas from its surroundings. However, beyond ~ 300 kpc from the core, the tail flares and the morphology becomes strongly irregular, which could be explained by a breaking of the drape, for example, caused by turbulent motions. The power spectrum of surface-brightness fluctuations is relatively flat (P2D ∝ k⁻²∙³ which indicates that thermal conduction is strongly inhibited even beyond the region where magnetic draping is effective. The amplitude of density fluctuations in the tail is consistent with a mild level of turbulence with a Mach number M3D ~ 0:1 -0:25. Overall, our results show that the processes leading to the thermalization and mixing of the infalling gas are slow and relatively inefficient