A substructure analysis of the A3558 cluster complex

The "algorithm driven by the density estimate for the identification of clusters" (DEDICA, Pisani 1993, 1996) is applied to the A3558 cluster complex in order to find substructures. This complex, located at the center of the Shapley Concentration supercluster, is a chain formed by the ACO clusters A3556, A3558 and A3562 and the two poor clusters SC 1327-312 and SC 1329-313. We find a large number of clumps, indicating that strong dynamical processes are active. In particular, it is necessary to use a fully three-dimensional sample(i.e. using the galaxy velocity as third coordinate) in order to recover also the clumps superimposed along the line of sight. Even if a great number of detected substructures were already found in a previous analysis (Bardelli et al. 1998), this method is more efficient and faster when compared with the use of a wide battery of tests and permits the direct estimate of the detection significance. Almost all subclusters previously detected by the wavelet analyses found in the literature are recognized by DEDICA. On the basis of the substructure analysis, we also briefly discuss the origin of the A3558 complex by comparing two hypotheses: 1) the structure is a cluster-cluster collision seen just after the first core-core encounter; 2) this complex is the result of a series of incoherent group-group and cluster-group mergings, focused in that region by the presence of the surrounding supercluster. We studied the fraction of blue galaxies in the detected substructures and found that the bluest groups reside between A3562 and A3558, i.e. in the expected position in the scenario of the cluster-cluster collision.Comment: 10 pages with 12 encapsulated figures; MNRAS in pres

Conditions for electron-cyclotron maser emission in the solar corona

Context. The Sun is an active source of radio emission ranging from long duration radio bursts associated with solar flares and coronal mass ejections to more complex, short duration radio bursts such as solar S bursts, radio spikes and fibre bursts. While plasma emission is thought to be the dominant emission mechanism for most radio bursts, the electron-cyclotron maser (ECM) mechanism may be responsible for more complex, short-duration bursts as well as fine structures associated with long-duration bursts. Aims. We investigate the conditions for ECM in the solar corona by considering the ratio of the electron plasma frequency {\omega}p to the electron-cyclotron frequency {\Omega}e. The ECM is theoretically possible when {\omega}p/{\Omega}e < 1. Methods. Two-dimensional electron density, magnetic field, plasma frequency, and electron cyclotron frequency maps of the off- limb corona were created using observations from SDO/AIA and SOHO/LASCO, together with potential field extrapolations of the magnetic field. These maps were then used to calculate {\omega}p/{\Omega}e and Alfven velocity maps of the off-limb corona. Results. We found that the condition for ECM emission ({\omega}p/{\Omega}e < 1) is possible at heights < 1.07 R_sun in an active region near the limb; that is, where magnetic field strengths are > 40 G and electron densities are greater than 3x10^8 cm-3. In addition, we found comparatively high Alfv\'en velocities (> 0.02 c or > 6000 km s-1) at heights < 1.07 R_sun within the active region. Conclusions. This demonstrates that the condition for ECM emission is satisfied within areas of the corona containing large magnetic fields, such as the core of a large active region. Therefore, ECM could be a possible emission mechanism for high-frequency radio and microwave bursts.Comment: 4 pages, 3 figure

The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems. III. Hubble Space Telescope Imaging of \lya-Absorbing Galaxies at z < 1

We present initial results of a program to obtain and analyze HST WFPC2 images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. We measure properties of 87 galaxies, of which 33 are associated with corresponding \lya absorption systems and 24 do not produce corresponding \lya absorption lines to within sensitive upper limits. Considering only galaxy and absorber pairs that are likely to be physically associated and excluding galaxy and absorber pairs within 3000 \kms of the background QSOs leaves 26 galaxy and absorber pairs and seven galaxies that do not produce corresponding \lya absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations of the galaxy and absorber pairs range from 12.4 to $157.4 h^{-1}$ kpc with a median of $62.4 h^{-1}$ kpc. The primary result of the analysis is that the amount of gas encountered along the line of sight depends on the galaxy impact parameter and B-band luminosity but does not depend strongly on the galaxy average surface brightness, disk-to-bulge ratio, or redshift. This result confirms and improves upon the anti-correlation between \lya absorption equivalent width and galaxy impact parameter found previously by Lanzetta et al. (1995). There is no evidence that galaxy interactions play an important role in distributing tenuous gas around galaxies in most cases. Galaxies might account for all \lya absorption systems with $W > 0.3$ \AA, but this depends on the unknown luminosity function and gaseous cross sections of low-luminosity galaxies as well as on the uncertainties of the observed number density of \lya absorption systems.Comment: Minor changes. Figure 1 stays intact and is available at ftp://ftp.ess.sunysb.edu/pub/lanzetta/wfpc

The Ha Luminosity Function and Star Formation Rate at z\sim 0.2

We have measured the Ha+[N II] fluxes of the I-selected Canada-France Redshift Survey (CFRS) galaxies lying at a redshift z below 0.3, and hence derived the Ha luminosity function. The magnitude limits of the CFRS mean that only the galaxies with M(B) > -21 mag were observed at these redshifts. We obtained a total Ha luminosity density of at least 10^{39.44\pm 0.04} erg/s/Mpc^{3} at a mean z=0.2 for galaxies with rest-fame EW(Ha+[N II]) > 10 Angs. This is twice the value found in the local universe by Gallego et al. 1995. Our Ha star formation rate, derived from Madau (1997) is higher than the UV observations at same z, implying a UV dust extinction of about 1 mag. We found a strong correlation between the Ha luminosity and the absolute magnitude in the B-band: M(B(AB)) = 46.7 - 1.6 log L(Ha). This work will serve as a basis of future studies of Ha luminosity distributions measured from optically-selected spectroscopic surveys of the distant universe, and it will provide a better understanding of the physical processes responsible for the observed galaxy evolution.Comment: Accepted for publication in ApJ, 14 pages, LaTeX (macro aas2pp4.sty), 6 figure