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Ph. Jetzer

Publications and source records attributed to Ph. Jetzer.

At least 73 records · Page 4Linked to original sources
arXiv2002

Microlensing towards the Large Magellanic Cloud

The nature and the location of the lenses discovered in the microlensing surveys done so far towards the LMC remain unclear. Motivated by these questions we compute the optical depth and particularly the number of expected events for self-lensing for both the MACHO and EROS2 observations. We calculate these quantities also for other possible lens populations such as thin and thick disk and galactic spheroid. Moreover, we estimate for each of these components the corresponding average event duration and mean mass using the mass moment method. By comparing the theoretical quantities with the values of the observed events it is possible to put some constraints on the location and the nature of the MACHOs. Clearly, given the large uncertainties and the few events at disposal it is not possible to draw sharp conclusions, nevertheless we find that certainly at least 3-4 MACHO events are due to lenses in LMC, which are most probably low mass stars, but that hardly all events can be due to self-lensing. This conclusions is even stronger when considering the EROS2 events, due to their spatial distribution. The most plausible solution is that the events observed so far are due to lenses belonging to different intervening populations: low mass stars in the LMC, in the thick disk, in the spheroid and possibly some true MACHOs in the halo.

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arXiv2002

Morphology of Galaxy Clusters and Sunyaev-Zel'dovich effect

Observations of the X-ray band wavelength reveal an evident ellipticity of many galaxy clusters atmospheres. The modeling of the intracluster gas with an ellipsoidal $β$-model leads to different estimates for the total gravitational mass and the gas mass fraction of the cluster than those one finds for a spherical beta-model. An analysis of a recent Chandra image of the galaxy cluster RBS797 indicates a strong ellipticity and thus a pronounced aspherical geometry. A preliminary investigation which takes into account an ellipsoidal shape for this cluster gives different mass estimates than by assuming spherical symmetry. We have also investigated the influence of aspherical geometries of galaxy clusters, and of polytropic profiles of the temperature on the estimate of the Hubble constant through the Sunyaev-Zel'dovich effect. We find that the non-inclusion of such effects can induce errors up to 40 per cent on the Hubble constant value.

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arXiv2001

Microlensing by Compact Objects associated to Gas Clouds

We investigate gravitational microlensing of point-like lenses surrounded by diffuse gas clouds. Besides gravitational bending, one must also consider refraction and absorption phenomena. According to the cloud density, the light curves may suffer small to large deviations from Paczynski curves, up to complete eclipses. Moreover, the presence of the cloud endows this type of microlensing events with a high chromaticity and absorption lines recognizable by spectral analysis. It is possible that these objects populate the halo of our galaxy, giving a conspicuous contribution to the fraction of the baryonic dark matter. The required features for the extension and the mass of the cloud to provide appreciable signatures are also met by several astrophysical objects.

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arXiv2001

Microlensing search towards M31

We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M31), aimed to the detection of gravitational microlensing effects as a probe of the presence of dark matter in our and in M31 halo. The analysis is performed using the pixel lensing technique, which consists in the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape analysis by demanding that the detected flux variations be achromatic and compatible with a Paczynski light curve. We apply the Durbin-Watson hypothesis test to the residuals. Furthermore, we consider the background of variables sources. Finally five candidate microlensing events emerge from our selection. Comparing with the predictions of a Monte Carlo simulation, assuming a standard spherical model for the M31 and Galactic haloes, and typical values for the MACHO mass, we find that our events are only marginally consistent with the distribution of observable parameters predicted by the simulation.

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arXiv2001

Sunyaev-Zel'dovich Effect and Morphology of Galaxy Clusters

We investigate the influence of the finite extension and the aspherical geometry of a galaxy cluster on the estimate of the Hubble constant through the Sunyaev-Zel'dovich effect. An analysis of a recent CHANDRA image of the galaxy cluster RBS797 indicates a strong ellipticity and thus a pronounced aspherical geometry. We estimate the total mass of RBS797 assuming spherical or ellipsoidal geometry and show that in the latter case the mass is about 10-17 % less than the one inferred for a spherical shape.

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arXiv2000

Asphericity of galaxy clusters and Sunyaev-Zel'dovich effect

In this paper we investigate the Sunyaev-Zel'dovich (SZ) effect and the X-ray surface brightness for clusters of galaxies with a non-spherical mass distribution. In particular, we consider the influence of the shape and the finite extension of a cluster as well as of a polytropic thermal profile on the Compton parameter, the X-ray surface brightness and on the determination of the Hubble constant. We find that the the non-inclusion of such effects can induce errors up to 30 per cent in the various parameters and in particular on the Hubble constant value, when compared with results obtained under the isothermal, infinitely extended and spherical shape assumptions.

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arXiv2000

Gamma ray emission from a baryonic dark halo

A recent re-analysis of EGRET data by Dixon et al. has led to the discovery of a statistically significant diffuse $γ$-ray emission from the galactic halo. We show that this emission can naturally be accounted for within a previously-proposed model for baryonic dark matter, according to which dark clusters of brown dwarfs and cold self-gravitating $H_2$ clouds populate the outer galactic halo and can show up in microlensing observations. Basically, cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark clusters, giving rise to the observed $γ$-ray flux. We derive maps for the corresponding intensity distribution, which turn out to be in remarkably good agreement with those obtained by Dixon et al. We also address future prospects to test our predictions.

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arXiv1999

Cold clouds in cooling flows

In many clusters of galaxies there is evidence for cooling flows which deposit large quantities of cool gas in the central regions. A fraction of this gas might accumulate as dense cool clouds. The aim of this communication is to discuss the minimum temperature achievable by clouds in cooling flows of different cluster of galaxies.

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arXiv1999

Thermal equilibrium of cold clouds in PKS 0745-191

PKS 0745-191 is a powerful radio source with one of the largest known cooling flows (De Grandi, Molendi 1999). A fraction of the cool gas could accumulate into small molecular clouds. We studied the minimum temperature which can be reached by sub-clouds (resulting from the fragmentation of bigger clouds) in the cooling flow region.

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arXiv1999

Molecular clouds in cooling flow cluster of galaxies

In many clusters of galaxies there is evidence for cooling flow in the central regions. A possibility is that a fraction of the gas forms cold molecular clouds. We investigate the minimum temperature which can be reached by clouds in cooling flows by computing the cooling function due to H2, HD and CO molecules. As an example, we determine the minimum temperature achievable by clouds in the cooling flow of PKS 0745-191.

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arXiv1999

Dark Matter in the Dwarf Galaxy NGC 247

Dwarf galaxies are dominated by dark matter even in the innermost regions and, therefore, provide excellent probes for the investigation of dark halos. To that purpose, we analyse ROSAT PSPC-data of the dwarf galaxy NGC 247. We focus in particular on the diffuse X-ray emission in the $1/4 $keV band. Assuming an isothermal density profile, we find that the mass of the hot emitting gas is about $10^8 {\rm M_{\odot}}$, corresponding to $\lesssim 0.5%$ of the total dynamical mass of the galaxy. The total mass of NGC 247, as derived from the X-ray data agrees quite well with the value obtained from the measured rotation curve (Burlak). The X-ray profile in the $3/4 $keV and $1.5 $keV band shows an excess at a radial distance of about $15 $ arcmin from the center. Such a ``hump'' in the radial X-ray profile can be explained by the presence of a cluster of young low mass stars or brown dwarfs. Therefore, NGC 247 offers the possibility to observe the formation of a halo of MACHOs.

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arXiv1999

Gravitational microlensing and dark matter in the galactic halo

We present the basics of microlensing and give an overview of the results obtained so far. We also describe a scenario in which dark clusters of MACHOs (Massive Astrophysical Compact Halo Objects) and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10-20 kpc. Moreover, we discuss various experimental tests of this picture in particular a $γ$-ray emission from the clouds due to the scattering of high-energy cosmic-ray protons. Our estimate for the $γ$-ray flux turns out to be in remarkably good agreement with the recent discovery by Dixon et al. of a possible $γ$-ray emission from the halo using EGRET data.

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arXiv1999

Gamma ray astronomy and baryonic dark matter

Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse $γ$-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which $γ$-rays are produced through the interaction of high-energy cosmic-ray protons with cold $H_2$ clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo $γ$-ray flux turns out to be in remarkably good agreement with the discovery by Dixon et al. We also address future prospects to test our predictions.

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arXiv1998

MACHOs as brown dwarfs

Recent observations of microlensing events in the Large Magellanic Cloud suggest that a sizable fraction of the galactic halo is in the form of Massive Astrophysical Compact Halo Objects (MACHOs). Although the average MACHO mass is presently poorly known, the value $\sim 0.1 M_{\odot}$ looks as a realistic estimate, thereby implying that brown dwarfs are a viable and natural candidate for MACHOs. We describe a scenario in which dark clusters of MACHOs and cold molecular clouds (mainly of $H_2$) naturally form in the halo at galactocentric distances larger than 10-20 kpc. Moreover, we discuss various experimental tests of this picture.

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arXiv1998

Binary brown dwarfs in the galactic halo?

Microlensing events towards the Large Magellanic Cloud entail that a sizable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs. Various reasons strongly suggest that a large amount of MACHOs should actually consist of binary brown dwarfs. Yet, this circumstance looks in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with a more accurate photometric observation can resolve binary brown dwarfs.

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arXiv1998

Halo dark clusters of brown dwarfs and molecular clouds

The discovery of Massive Astrophysical Compact Halo Objects (MACHOs) in microlensing experiments makes it compelling to understand their physical nature, as well as their formation mechanism. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs, and the present paper deals with this option. According to a recently proposed scenario, brown dwarfs are clumped along with cold molecular clouds into dark clusters -- in several respects similar to globular clusters -- which form in the outer part of the galactic halo. Here, we analyze the dynamics of these dark clusters and we address the possibility that a sizable fraction of MACHOs can be binary brown dwarfs. Moreover, we point out that Ly-$α$ absorption systems naturally fit within the present picture.

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