Spectroscopic studies of magnetic cataclysmic variables

In this thesis I present the results of optical spectroscopic analyses of four Polars and the spectra of five new systems. The results of four candidate magnetic cataclysmic variables, which were selected from the ROSAT Bright Survey, are also presented. Time resolved spectra are used for the dual purpose of conducting Doppler tomographical studies and searching for any evidence of cyclotron beaming. The first five chapters are introductory and background sections, including a review of all individual objects with magnetic field strengths determined by cyclotron spectroscopy. In Chapter 6 I discuss the observations, data reduction and analysis techniques while the results of individual objects are presented in Chapter 7. Results are presented for the two Polars HU Aqr and QS Tel, which were selected as test objects to confirm the techniques of Doppler tomography and cyclotron spectroscopy, respectively. Doppler maps, similar to those previously obtained, are produced for HU Aqr. Two cyclotron humps are also detected in the HU Aqr spectra corresponding to a magnetic field of 39 MG, again similar to values quoted in the literature. A complex cyclotron spectrum for QS Tel indicates that the second magnetic pole is more dominant than in previous studies. Spectra are presented for the new Polars RX J1313-32, RX J1141-64, RX J1610+03, RX J0153-59, RX J0501-03, RX J0512-72 and.RX J0515+01. More extensive observations were made for RX Jl313-32 and RX Jll41-64, with Doppler maps derived for RX J1313-32 while the RX J1141-64 spectra show a rise in the spectral slope, indicative of cyclotron radiation. Four ROSAT Bright Survey sources, RBS 0324, RBS 0372, RBS 0490 & RBS 1969, were selected to search for any evidence of them being magnetic cataclysmic variables. More detailed spectroscopy and photometry was obtained for RBS 0324, the only RBS source which shows evidence of being a magnetic system. The other RBS systems are all probably dwarf novae in quiescence

Cosmological simulations in MOND: the cluster scale halo mass function with light sterile neutrinos

We use our Modified Newtonian Dynamics (MOND) cosmological particle-mesh N-body code to investigate the feasibility of structure formation in a framework involving MOND and light sterile neutrinos in the mass range 11 - 300 eV: always assuming that \Omega_{\nu_s}=0.225 for H_o=72 \kms Mpc^{-1}. We run a suite of simulations with variants on the expansion history, cosmological variation of the MOND acceleration constant, different normalisations of the power spectrum of the initial perturbations and interpolating functions. Using various box sizes, but typically with ones of length 256 Mpc/h, we compare our simulated halo mass functions with observed cluster mass functions and show that (i) the sterile neutrino mass must be larger than 30 eV to account for the low mass (M_{200}<10^{14.6} solar masses) clusters of galaxies in MOND and (ii) regardless of sterile neutrino mass or any of the variations we mentioned above, it is not possible to form the correct number of high mass (M_{200}>10^{15.1} solar masses) clusters of galaxies: there is always a considerable over production. This means that the ansatz of considering the weak-field limit of MOND together with a component of light sterile neutrinos to form structure from z ~ 200 fails. If MOND is the correct description of weak-field gravitational dynamics, it could mean that subtle effects of the additional fields in covariant theories of MOND render the ansatz inaccurate, or that the gravity generated by light sterile neutrinos (or by similar hot dark matter particles) is different from that generated by the baryons.Comment: 10 pages, 9 figures, accepted for publication in MNRA

N-body simulations of the Carina dSph in MOND

The classical dwarf spheroidals (dSphs) provide a critical test for Modified Newtonian Dynamics (MOND) because they are observable satellite galactic systems with low internal accelerations and low, but periodically varying, external acceleration. This varying external gravitational field is not commonly found acting on systems with low internal acceleration. Using Jeans modelling, Carina in particular has been demonstrated to require a V-band mass-to-light ratio greater than 5, which is the nominal upper limit for an ancient stellar population. We run MOND N-body simulations of a Carina-like dSph orbiting the Milky Way to test if dSphs in MOND are stable to tidal forces over the Hubble time and if those same tidal forces artificially inflate their velocity dispersions and therefore their apparent mass-to-light ratio. We run many simulations with various initial total masses for Carina, and Galactocentric orbits (consistent with proper motions), and compare the simulation line of sight velocity dispersions (losVDs) with the observed losVDs of Walker et al. (2007). We find that the dSphs are stable, but that the tidal forces are not conducive to artificially inflating the losVDs. Furthermore, the range of mass-to-light ratios that best reproduces the observed line of sight velocity dispersions of Carina is 5.3 to 5.7 and circular orbits are preferred to plunging orbits. Therefore, some tension still exists between the required mass-to-light ratio for the Carina dSph in MOND and those expected from stellar population synthesis models. It remains to be seen whether a careful treatment of the binary population or triaxiality might reduce this tension.Comment: 17 pages, 12 figures, accepted for publication in MNRA

Ram pressure statistics for bent tail radio galaxies

In this paper we use the MareNostrum Universe Simulation, a large scale, hydrodynamic, non-radiative simulation in combination with a simple abundance matching approach to determine the ram pressure statistics for bent radio sources (BRSs). The abundance matching approach allows us to determine the locations of all galaxies with stellar masses $> 10^{11} MSol$ in the simulation volume. Assuming ram pressure exceeding a critical value causes bent morphology, we compute the ratio of all galaxies exceeding the ram pressure limit (RPEX galaxies) relative to all galaxies in our sample. According to our model 50% of the RPEX galaxies at $z = 0$ are found in clusters with masses larger than $10^{14.5}MSol$ the other half resides in lower mass clusters. Therefore, the appearance of bent tail morphology alone does not put tight constraints on the host cluster mass. In low mass clusters, $M < 10^{14}MSol$, RPEX galaxies are confined to the central 500 kpc whereas in clusters of $> 10^{15}Msol$ they can be found at distances up to 1.5Mpc. Only clusters with masses $> 10^{15}MSol$ are likely to host more than one BRS. Both criteria may prove useful in the search for distant, high mass clusters.Comment: 10 pages, 10 figures, Submitted to the Monthly Notices of the Royal Astronomical Societ

Properties of X-ray detected far-IR AGN in the green valley

In this study, we analysed active galactic nuclei in the "green valley" by comparing active and non-active galaxies using data from the COSMOS field. We found that most of our X-ray detected active galactic nuclei with far-infrared emission have star formation rates higher than the ones of normal galaxies of the same stellar mass range.Comment: Proceedings paper of the IAU symposium "Nuclear Activity in Galaxies Across Cosmic Time" (Ethiopia) accepted to be published under the Cambridge University Press, eds. M. Povic, P. Marziani, J. Masegosa, H. Netzer, S. H. Negu, and S. B. Tessem

The X-ray synchrotron emission of RCW 86 and the implications for its age

We report here X-ray imaging spectroscopy observations of the northeastern shell of the supernova remnant RCW 86 with Chandra and XMM-Newton. Along this part of the shell the dominant X-ray radiation mechanism changes from thermal to synchrotron emission. We argue that both the presence of X-ray synchrotron radiation and the width of the synchrotron emitting region suggest a locally higher shock velocity of V_s = 2700 km/s and a magnetic field of B = 24+/-5 microGauss. Moreover, we also show that a simple power law cosmic ray electron spectrum with an exponential cut-off cannot explain the broad band synchrotron emission. Instead a concave electron spectrum is needed, as predicted by non-linear shock acceleration models. Finally, we show that the derived shock velocity strengthens the case that RCW 86 is the remnant of SN 185.Comment: 5 pages, 4 figures. The last figure is intended as a color plate. Accepted by ApJ Letter