ﻻ يوجد ملخص باللغة العربية
We investigate the UV continuum slope $alpha$ of a large quasar sample from SDSS DR7. By using specific continuum windows, we build two samples at lower ($0.71<z<1.19$) and higher ($1.90<z<3.15$) redshifts, which correspond to the continuum slopes at longer (NUV) and shorter (FUV) rest wavelength ranges respectively. Overall, the average continuum slopes are $-0.36$ and $-0.51$ for $alpha_{rm NUV}$ and $alpha_{rm FUV}$ with similar dispersions $sigma_{alpha} sim 0.5$. For both samples, we confirm the luminosity dependence of the continuum slope, i.e., fainter quasars have redder spectra. We further find that both $alpha_{rm NUV}$ and $alpha_{rm FUV}$ have a common upper limit ($sim 1/3$) which is almost independent of the quasar luminosity $L_{rm bol}$. This finding implies that the intrinsic quasar continuum (or the bluest quasar), at any luminosity, obey the standard thin disk model. We propose that the other quasars with redder $alpha$ are caused by the reddening from the dust {it locally}. With this assumption, we employ the dust extinction scenario to model the observed $L_{rm bol}-alpha$ relation. We find that, a typical value of $E(B-V)sim0.1$ to $0.3$ mag (depending on the types of extinction curve) of the quasar {it local} dust is enough to explain the discrepancy of $alpha$ between the observation ($sim-0.5$) and the standard accretion disk model prediction ($sim 1/3$).
We present a new redshift survey, the 2dF Quasar Dark Energy Survey pilot (2QDESp), which consists of ${approx}10000$ quasars from ${approx}150$ deg$^2$ of the southern sky, based on VST-ATLAS imaging and 2dF/AAOmega spectroscopy. Combining our optic
We determine the slope of the near infrared extinction power law (A$_{lambda} propto lambda^{-alpha}$) for 8 regions of the Galaxy between l$sim27^{circ}$ and $sim100^{circ}$. UKIDSS Galactic Plane Survey data are compared, in colour-colour space, wi
The large majority of extinction sight lines in our Galaxy obey a simple relation depending on one parameter, the total-to-selective extinction coefficient, Rv. Different values of Rv are able to match the whole extinction curve through different env
We present a model for the evolution of the galaxy ultraviolet (UV) luminosity function (LF) across cosmic time where star formation is linked to the assembly of dark matter halos under the assumption of a mass dependent, but redshift independent, ef
Determinations of the UV luminosity function of AGN at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer fr