arXiv subjects

C. Ferrari

Publications and source records attributed to C. Ferrari.

158 records · Page 9Linked to original sources

arXiv2008

On the influence of ram-pressure stripping on interacting galaxies in clusters

We investigate the influence of ram pressure on the star-formation rate and the distribution of gas and stellar matter in interacting model galaxies in clusters. To simulate the baryonic and non-baryonic components of interacting disc galaxies moving through a hot, thin medium we use a combined N-body/hydrodynamic code GADGET2 with a description for star formation based on density thresholds. Two identical model spiral galaxies on a collision trajectory with three different configurations were investigated in detail. In the first configuration the galaxies collide without the presence of an ambient medium, in the second configurations the ram pressure acts face on on the interacting galaxies and in the third configuration the ram pressure acts edge on. The ambient medium is thin ($10^{-28}$ g/cm$^3$), hot (3 keV $\approx 3.6\times10^7$K) and has a relative velocity of 1000 km/s, to mimic an average low ram pressure in the outskirts of galaxy clusters. The interaction velocities are comparable to galaxy interactions in groups, falling along filaments into galaxy clusters. The global star formation rate of the interacting system is enhanced in the presence of ram pressure by a factor of three in comparison to the same interaction without the presence of an ambient medium. The tidal tails and the gaseous bridge of the interacting system are almost completely destroyed by the ram pressure. The amount of gas in the wake of the interacting system is $\sim50$% of the total gas of the colliding galaxies after 500 Myr the galaxies start to feel the ram pressure. Nearly $\sim10-15%$ in mass of all newly formed stars are formed in the wake of the interacting system at distances larger than 20 kpc behind the stellar discs. (abrigded)

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arXiv2008

On the influence of ram-pressure stripping on the star formation of simulated spiral galaxies

We investigate the influence of ram-pressure stripping on the star formation and the mass distribution in simulated spiral galaxies. Special emphasis is put on the question where the newly formed stars are located. The stripping radius from the simulation is compared to analytical estimates. Disc galaxies are modelled in combined N-body/hydrodynamic simulations (GADGET-2) with prescriptions for cooling, star formation, stellar feedback, and galactic winds. These model galaxies move through a constant density and temperature gas, which has parameters comparable to the intra-cluster medium (ICM) in the outskirts of a galaxy cluster (T=3 keV ~3.6x10^7 K and rho=10^-28 g/cm^3). With this numerical setup we analyse the influence of ram-pressure stripping on the star formation rate of the model galaxy. We find that the star formation rate is significantly enhanced by the ram-pressure effect (up to a factor of 3). Stars form in the compressed central region of the galaxy as well as in the stripped gas behind the galaxy. Newly formed stars can be found up to hundred kpc behind the disc, forming structures with sizes of roughly 1 kpc in diameter and with masses of up to 10^7 M_sun. As they do not possess a dark matter halo due to their formation history, we name them 'stripped baryonic dwarf' galaxies. We also find that the analytical estimate for the stripping radius from a Gunn & Gott (1972) criterion is in good agreement with the numerical value from the simulation. Like in former investigations, edge-on systems lose less gas than face-on systems and the resulting spatial distribution of the gas and the newly formed stars is different.

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arXiv2008

Observations of extended radio emission in clusters

We review observations of extended regions of radio emission in clusters; these include diffuse emission in `relics', and the large central regions commonly referred to as `halos'. The spectral observations, as well as Faraday rotation measurements of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive and detailed radio measurements; we briefly review prospects for future observations.

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arXiv2007

A2163: Merger events in the hottest Abell galaxy cluster I. Dynamical analysis from optical data

We present a detailed optical analysis of the galaxy cluster A2163 based on new multicolor wide field imaging and medium-to-high resolution (R~2000) spectroscopy. While X-ray observations have revealed that merging processes are present in this cluster, the merging scenario is complex and not well-defined. We undertook a complementary optical analysis, aiming to understand the dynamics of the system, to add constraints to the merging scenario and to test its effect on the properties of galaxies. We provide a spectroscopic catalog of 512 new objects, where 361 galaxies are identified as cluster members. We derived estimates of the mean redshift and velocity dispersion: z= 0.2005 +/- 0.0003 and 1434 +/- 60 km/s, and performed a detailed dynamical analysis of unprecedented accuracy. We detected multiple evidences of merging events: i) strong subclustering in the galaxy density distribution, showing a main central component A2163-A, a northern one A2163-B, visible both in optical and in X-ray, and other substructures detected in optical, ii) bimodality in the density distribution for A2163-A, strong segregation between gaz and galaxies, and between galaxies of different luminosities, iii) low-density structures oriented along specific axes: mainly the EW direction, and the NS one, embedding the main substructures, iv) bimodality in the velocity distribution and a velocity gradient (~1250 km/s) along the NE/SW axis of the cluster. A2163 is exceptionally massive, with Mvir = 3.8 +/- 0.4 10**15 Msol/h70. Our analysis supports a scenario in which A2163-A has undergone a recent (t ~ 0.5 Gyr) merger along a a NE/SW (or E--W) axis, and A2163-B is connected to the main complex, probably infalling on A2163-A. (abridged)

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arXiv2006

Galaxy clusters in the CFHTLS. First matched filter candidate catalogue of the Deep fields

We apply a matched-filter cluster detection algorithm to the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) i-band data for the Deep-1, Deep-2, Deep-3 and Deep-4 fields covering a total of 4square degrees. To test the implemented procedure we carry out simulations for assessing the frequency of noise peaks as well as estimate the recovery efficiency. We estimate that up to z~0.7 the catalogue is essentially complete for clusters of richness class R>~1. The recovered redshifts are in general overestimated by dz=0.1 with a scatter of sigma_dz~0.1, except at redshifts z>~1 where the estimated redshifts are systematically underestimated. The constructed cluster candidate catalogue contains 162 detections over an effective area of 3.112 square degrees corresponding to a density of ~52.1 per square degree. The median estimated redshift of the candidates is z=0.6. The estimated noise frequency is 16.9+-5.4 detections per square degree. From visual inspection we identify systems that show a clear concentration of galaxies with similar colour. These systems have a density of ~20 per square degree.

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arXiv2006

ATCA observations of the galaxy cluster Abell 3921 - I. Radio emission from the central merging sub-clusters

We present the analysis of our 13 and 22 cm ATCA observations of the central region of the merging galaxy cluster A3921 (z=0.094). We investigated the effects of the major merger between two sub-clusters on the star formation (SF) and radio emission properties of the confirmed cluster members. The origin of SF and the nature of radio emission in cluster galaxies was investigated by comparing their radio, optical and X-ray properties. We also compared the radio source counts and the percentage of detected radio galaxies with literature data. We detected 17 radio sources above the flux density limit of 0.25 mJy/beam in the central field of A3921, among which 7 are cluster members. 9 galaxies with star-forming optical spectra were observed in the collision region of the merging sub-clusters. They were not detected at radio wavelengths, giving upper limits for their star formation rate significantly lower than those typically found in late-type, field galaxies. Most of these star-forming objects are therefore really located in the high density part of the cluster, and they are not infalling field objects seen in projection at the cluster centre. Their SF episode is probably related to the cluster collision that we observe in its very central phase. None of the galaxies with post-starburst optical spectra was detected down our 2$σ$ flux density limit, confirming that they are post-starburst and not dusty star-forming objects. We finally detected a narrow-angle tail (NAT) source associated with the second brightest cluster galaxy (BG2), whose diffuse component is a partly detached pair of tails from an earlier period of activity of the BG2 galaxy.

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arXiv2005

Simulations of galactic winds and starbursts in galaxy clusters

We present an investigation of the metal enrichment of the intra-cluster medium (ICM) by galactic winds and merger-driven starbursts. We use combined N-body/hydrodynamic simulations with a semi-numerical galaxy formation model. The mass loss by galactic winds is obtained by calculating transonic solutions of steady state outflows, driven by thermal, cosmic ray and MHD wave pressure. The inhomogeneities in the metal distribution caused by these processes are an ideal tool to reveal the dynamical state of a galaxy cluster. We present surface brightness, X-ray emission weighted temperature and metal maps of our model clusters as they would be observed by X-ray telescopes like XMM-Newton. We show that X-ray weighted metal maps distinguish between pre- or post-merger galaxy clusters by comparing the metallicity distribution with the galaxy-density distribution: pre-mergers have a metallicity gap between the subclusters, post-mergers a high metallicity between subclusters. We apply our approach to two observed galaxy clusters, Abell 3528 and Abell 3921, to show whether they are pre- or post-merging systems. The survival time of the inhomogeneities in the metallicity distribution found in our simulations is up to several Gyr. We show that galactic winds and merger-driven starbursts enrich the ICM very efficiently after z=1 in the central (~ 3 Mpc radius) region of a galaxy cluster.

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arXiv2005

Star formation in the merging galaxy cluster Abell 3921

Through a combined optical and radio analysis, we have investigated the possible connection between the dynamical state of the merging cluster A3921 and its star formation properties, reaching the conclusion that the on-going merger is triggering a SF episode in the collision region.

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arXiv2005

Chandra observation of the multiple merger cluster Abell 521

We present the Chandra analysis of the galaxy cluster A521 (z=0.247). The high resolution of the Chandra observation has allowed us to refine the original merging scenario proposed for A521, and to reveal new features in its X-ray emission. A521 has strongly substructured ICM density and temperature maps. Its X-ray diffuse emission is elongated along a NW/SE direction (SX2) and shows two major components, a main cluster and a northern group of galaxies. This latter is in turn substructured, showing a clump of cold and very dense gas centred on the BCG, and a northern tail aligned in the SX2 direction. A compression of the X-ray isophotes is also observed South of the BCG. We conclude that the northern group is infalling onto the main cluster along the NW/SE direction. This hypothesis is corroborated by the presence of a hot bar in the ICM temperature map located between the southern and northern regions, as the gas could be compressionally heated due to the subclusters' collision. The hot region corresponds to the eastern part of an over-dense ridge of galaxies, along which it was originally suggested that a merging of subclusters has recently occurred along the line of sight. An alternative hypothesis about the origin of the hot central bar is that we could observe in projection the shock fronts due to this older cluster-cluster collision. Two other structures possibly interacting with the main cluster are detected on the West and North-East sides of the BCG. We also uncover the presence of two northern edges in the ICM density. A521 is a spectacular example of a multiple merger cluster made up by several substructures converging at different epochs towards the centre of the system. The very pertubed dynamical state of this cluster is also confirmed by our discovery of a radio relic in its South-East region.

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arXiv2004

Dynamical state and star formation properties of the merging galaxy cluster Abell 3921

We present the results of a new spectroscopic and photometric survey of the central region of the galaxy cluster A3921 (z=0.094). We detect the presence of two dominant clumps of galaxies: a main cluster centred on the BCG (A3921-A), and a NW sub-cluster (A3921-B) hosting the second brightest cluster galaxy. The distorted morphology of the two sub-clusters suggests that they are interacting, while the velocity distribution of 104 confirmed cluster members does not reveal strong signatures of merging. By applying a two-body dynamical formalism to the two sub-clusters of A3921, and by comparing our optical results to the X-ray analysis of A3921 based on XMM observations (Belsole et al. 2004), we conclude that A3921-B is probably tangentially traversing the main cluster along a SW/NE direction. Our estimate of the star formation properties of the cluster members reveals substantial fractions of both emission-line (~13%) and post-star-forming objects (k+a's,~16%). A lack of bright k+a's with respect to higher redshift clusters is observed, while the fraction of k+a's increases towards fainter magnitudes. The spatial and velocity distributions of k+a's galaxies, their red colours and their moderate Balmer absorption lines suggest that their presence is difficult to relate to the on-going merging event. Star forming galaxies, which are mostly concentrated in A3921-B and between the two sub-clusters, share neither the same kinematics, nor the same projected distribution of the passive cluster members. Their spectral properties may be at least partly realted to the ongoing merger.

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arXiv2003

Galaxy Alignments as a Probe of the Dynamical State of Clusters

We present indications, based on a sample of 303 Abell clusters, for a relation between the dynamical state of clusters and the alignments of galaxy members with their parent cluster major axis orientation as well as with the large scale environment within which the clusters are embedded. The statistical results are complemented with a deep, wide-field case study of galaxy alignments in the cluster A521, which is characterised by multiple merging events (Maurogrdato et al 2000, Ferrari et al. 2003) and whose galaxy members show a strong alignment signal out to $\sim 5 h^{-1}$ Mpc. Our results show that galaxy alignments appear to be stronger the more dynamically young is the cluster, especially when found in high-density environments. This relation complements the recently found {``Cluster Substructure - Alignment Connection''} (Plionis & Basilakos 2002) by which dynamically young clusters, found in high-density environments, show stronger cluster-cluster alignments.

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arXiv2002

Multiple merging events in Abell 521

We present a detailed spatial and dynamical analysis of the central $\sim$~2.2~\h~Mpc region of the galaxy cluster Abell~521 (z=0.247), based on 238 spectra obtained at the 3.6~m Telescope of ESO and at the CFHT. From the analysis of the 125 galaxies confirmed members of the cluster, we derive a mean velocity of $74019 ^{+112}_{-125}$ km/s and detect a complex velocity distribution with high velocity dispersion, $1325 ^{+145}_{-100}$ km/s), but clear departure from a single gaussian component. The general structure of the cluster follows a NW/SE direction, crossed by a perpendicular high density ``ridge'' of galaxies in the core region. The northern region of the cluster is characterized by a lower velocity dispersion as compared to the whole cluster value; it hosts the BCG and a dynamically bound complex of galaxies, and it is associated to a group detected in X-ray (Arnaud et al 2000). This region could be in pre-merger stage onto the main cluster nearly in the plane of the sky. These results, taken together with the fact that most of the clumps detected on the isodensity maps, as well as the early type galaxies and the brightest ones are aligned, suggest that this NW/SE direction is the preferred one for the formation of this cluster. The central high dense region shows a lower velocity location ($73625 ^{+344}_{-350}$ km/s) and significantly higher scale ($1780 ^{+234}_{-142}$ km/s) as compared to the whole cluster values. This is due to the presence of a low-velocity group of galaxies with a high fraction of emission line objects. This can be explained in a scenario in which a merging of subclusters has recently occurred along the direction of the ``ridge'' with a significant component along the line of sight.

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arXiv2002

Radio asymmetry in 3C99

The N-galaxy 3C99, a Compact Steep-spectrum Source showing a triple asymmetric structure, has been observed with several arrays of radio telescopes at sub-arcsecond resolution. New images from MERLIN, European VLBI Network and the Very Long Baseline Array show that the source components detected in the central region of 3C99 have steep spectral indices, making it difficult to determine which of them is the core. The asymmetric radio structure of 3C99 is explained using the Scheuer-Baldwin continuous streaming model with a rather large difference in the interstellar medium density on the two sides of the central region. The age of 3C99 has been estimated; the source is found younger than 10^6 years.

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arXiv2000

Detection of arcs in Saturn's F ring during the 1995 Sun ring-plane crossing

Observations of the November 1995 Sun crossing of the Saturn's ring-plane made with the 3.6m CFH telescope, using the UHAO adaptive optics system, are presented here. We report the detection of four arcs located in the vicinity of the F ring. They can be seen one day later in HST images. The combination of both data sets gives accurate determinations of their orbits. Semi-major axes range from 140020 km to 140080 km, with a mean of 140060 +- 60 km. This is about 150 km smaller than previous estimates of the F ring radius from Voyager 1 and 2 data, but close to the orbit of another arc observed at the same epoch in HST images.

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