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Register a new userInduced nuclear activity in galaxy pairs with different morphologies (E+E), (E+S) and (S+S)
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Francisco J. Hernandez-Ibarra
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We analysed 385 galactic spectra from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) that belong to the catalog of isolated pairs of galaxies by Karachentsev. The spectra corresponds to physical pairs of galaxies as defined by V $leq$ 1200 Km/s and a pair separation $leq$ 100 kpc. We search for the incidence of nuclear activity, both thermal (star-forming) and non-thermal -Active Galactic Nuclei (AGN). After a careful extraction of the nuclear spectra, we use diagnostic diagrams and find that the incidence of AGN activity is 48 % in the paired galaxies with emission lines and 40% for the total sample (as compared to $sim$ 43 % and 41% respectively in a sample of isolated galaxies). These results remain after dissecting the effects of morphological type and galactic stellar mass (with only a small, non significant, enhancement of the AGN fraction in pairs of objects). These results suggest that weak interactions are not necessary or sufficient to trigger low-luminosity AGN. Since the fraction of AGN is predominant in early type spiral galaxies, we conclude that the role of a bulge, and a large gas reservoir are both essential for the triggering of nuclear activity. The most striking result is that type 1 galaxies are almost absent from the AGN sample. This result is in conflict with the Unified Model, and suggests that high accretion rates are essential to form the Broad Line Region in active galaxies.
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Galaxy pairs may represent a way station in the evolutionary path from poor groups to giant isolated ellipticals (or fossil groups). To test this evolutionary scenario, we investigated the environment of 4 galaxy pairs composed of a giant elliptical galaxy and its spiral companion. The pairs are very similar from the optical and dynamical point of view, but have very different X-ray properties. The faint galaxy population around the pairs was observed with VIMOS on the VLT. These observations show that the presence of extended diffuse X-ray emission from an IGM is not necessarily connected to the presence of a numerous faint galaxy population. The study of luminosity functions (LFs) indicate that our X-ray luminous pairs are more dynamically evolved than a sample of poor groups with comparable X-ray luminosities from the literature. However, our X-ray faint pairs resemble the LF of those X-ray bright groups and may represent a phase in the dynamical evolution of these groups, where the recent or ongoing interaction, in which the pair E is involved, has destroyed or at least decreased the luminosity of the IGM. The X-ray faint groups LF is also consitent with their evolution into a fossil group.
An $omega$-meson in motion with respect to a nuclear medium can couple to a $sigma$-meson through a particle-hole excitation. This coupling is large. We investigate its consequences for the width of $omega$-mesons in matter and for the s-wave annihilation of pions into lepton pairs which can take place in relativistic heavy ion collisions. We find that the two pion decay of $omega$-mesons, resulting from the $omega-sigma$ transition and the subsequent $2pi$ decay of the $sigma$-meson, leads to a substantial broadening of $omega$-mesons in matter and possibly to an observable effect in experiments measuring the $e^+e^-$ decay of vector mesons produced in nuclei and in relativistic heavy-ion collisions. The inverse process, the s-wave annihilation of pions into $omega$-mesons decaying into $e^+e^-$ pairs, has in general a much smaller cross section than the corresponding p-wave annihilation through $rho$-mesons and is expected to contribute rather little to the total $e^+e^-$ pair production in relativistic heavy ion collisions.
Using 586 $textrm{pb}^{-1}$ of $e^{+}e^{-}$ collision data acquired at $sqrt{s}=4.170$ GeV with the CLEO-c detector at the Cornell Electron Storage Ring, we report the first observation of $D_{s}^{*+} to D_{s}^{+} e^{+} e^{-}$ with a significance of $5.3 sigma$. The ratio of branching fractions $calB(D_{s}^{*+} to D_{s}^{+} e^{+} e^{-}) / calB(D_{s}^{*+} to D_{s}^{+} gamma)$ is measured to be $[ 0.72^{+0.15}_{-0.13} (textrm{stat}) pm 0.10 (textrm{syst})]%$, which is consistent with theoretical expectations.
By analyzing 482 pb$^{-1}$ of $e^+e^-$ collision data collected at $sqrt s=4.009$ GeV with the BESIII detector at the BEPCII collider, we measure the absolute branching fractions for the semileptonic decays $D_{s}^{+}toeta e^{+} u_{e}$ and $D_{s}^{+}to etae^{+} u_{e}$ to be ${B}(D_{s}^{+}rightarroweta e^{+} u_{e})=(2.30pm0.31pm0.08)$% and ${B}(D_{s}^{+}rightarrowetae^{+} u_{e}) = (0.93pm0.30pm0.05)$%, respectively, and their ratio $frac{{B}(D_{s}^{+}rightarrowetae^{+} u_{e})} {{B}(D_{s}^{+}rightarroweta e^{+} u_{e})}=0.40pm0.14pm0.02$, where the first uncertainties are statistical and the second ones are systematic. The results are in good agreement with previous measurements within uncertainties; they can be used to determine the $eta-eta$ mixing angle and improve upon the $D_s^+$ semileptonic branching ratio precision.
We study $D_{s}^{+}$ decays to final states involving the $eta$ with a 482$,$pb$^{-1}$ data sample collected at $sqrt{s}$ = 4.009$,$GeV with the mbox{BESIII} detector at the BEPCII collider. We measure the branching fractions $mathcal{B}(D^+_{s}rightarrow etaX)$ = (8.8$pm$1.8$pm$0.5)$%$ and $mathcal{B}(D_{s}^{+}rightarrow etarho^{+})$ = ($5.8pm1.4pm0.4$)$%$ where the first uncertainty is statistical and the second is systematic. In addition, we estimate an upper limit on the non-resonant branching ratio $mathcal{B}(D_{s}^{+}rightarrow etapi^+pi^0)<5.1%$ at the 90$%$ confidence level. Our results are consistent with CLEOs recent measurements and help to resolve the disagreement between the theoretical prediction and CLEOs previous measurement of $mathcal{B}(D_{s}^{+}rightarrow etarho^{+})$.
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