Maximum-likelihood estimation prevents unphysical Mueller matrices

We show that the method of maximum-likelihood estimation, recently introduced in the context of quantum process tomography, can be applied to the determination of Mueller matrices characterizing the polarization properties of classical optical systems. Contrary to linear reconstruction algorithms, the proposed method yields physically acceptable Mueller matrices even in presence of uncontrolled experimental errors. We illustrate the method on the case of an unphysical measured Mueller matrix taken from the literature.Comment: 3 pages, 1 figur

Classical and MgII-selected Damped Lyman-alpha Absorbers: impact on Omega_HI at z<1.7

The Damped Lyman-alpha systems (DLAs), seen in absorption in the spectrum of quasars, are believed to contain a large fraction of the neutral gas in the Universe. Paradoxically, these systems are more difficult to observe at z_abs<1.7, since they are rare and their HI feature then falls in UV spectra. Rao & Turnshek (2000) pioneered a method based on MgII-selected DLAs, that is absorbers discovered thanks to our knowledge of their MgII feature in optical spectra. We use new observations undertaken at the TNG as well as a careful literature & archival search to build samples of low redshift absorbers classified according to the technique used for their discovery. We successfully recover N(HI) and equivalent widths of FeII 2600, MgII 2796, MgII 2803 and MgII 2852 for a sample of 36 absorbers, 21 of which are MgII-selected. We find that the MgII-selected sample contains a marginally larger fraction of absorbers with log N(HI)>21.0 than seen otherwise at low redshift. If confirmed, this property will in turn affect estimates of Omega_HI which is dominated by the highest HI column densities. We find that log N(HI) does not correlate significantly with metal equivalent widths. Similarly, we find no evidence that gravitational lensing, the fraction of associated systems or redshift evolution affect the absorber samples in a different way. We conclude that the hint of discrepancies in N(HI) distributions most likely arises from small number statistics. Therefore, further observations are required to better clarify the impact of this discrepancy on estimates of Omega_HI at low redshift.Comment: 13 pages, 8 figures. Accepted for publication in MNRA

Mg II Absorption Systems in SDSS QSO Spectra

We present the results of a MgII absorption-line survey using QSO spectra from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than 0.3\AA and redshifts $0.366 \le z \le 2.269$ were identified and measured. We find that the $\lambda2796$ rest equivalent width ($W_0^{\lambda2796}$) distribution is described very well by an exponential function $\partial N/\partial W_0^{\lambda2796} = \frac{N^*}{W^*} e^{-\frac{W_0}{W^*}}$, with $N^*=1.187\pm0.052$ and $W^*=0.702\pm0.017$\AA. Previously reported power law fits drastically over-predict the number of strong lines. Extrapolating our exponential fit under-predicts the number of $W_0 \le 0.3$\AA systems, indicating a transition in $dN/dW_0$ near $W_0 \simeq 0.3$\AA. A combination of two exponentials reproduces the observed distribution well, suggesting that MgII absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of $W_0^{\lambda2796} \ge 0.3$\AA lines: $N^*=1.001\pm0.132(1+z)^{0.226\pm0.170}$ and $W^*=0.443\pm0.032(1+z)^{0.634\pm 0.097}$\AA. We find that the distribution steepens with decreasing redshift, with $W^*$ decreasing from $0.80\pm0.04$\AA at $z=1.6$ to $0.59\pm0.02$\AA at $z=0.7$. The incidence of moderately strong MgII $\lambda2796$ lines does not show evidence for evolution with redshift. However, lines stronger than $\approx 2$\AA show a decrease relative to the no-evolution prediction with decreasing redshift for $z \lesssim 1$. The evolution is stronger for increasingly stronger lines. Since $W_0$ in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low z.Comment: 50 pages, 26 figures, accepted for publication in Ap

A new 21-cm absorber identified with an L∼L⋆L \sim L^\star galaxy

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted 21-cm absorption from the $z=0.437$ metal line absorption system towards PKS 1243-072. HI absorption is clearly detected; the absorption profile has a velocity spread of $\sim 20$ km/s. Detection of 21-cm absorption indicates that the absorber has an HI column density large enough to be classified as a damped Lyman-$\alpha$ system. Follow up ground based optical imaging and spectroscopy allow us to identify the absorber with an $L \sim L^\star$ galaxy at an impact parameter of $\sim 9.8$ kpc from the line of sight to the QSO. The absorbing galaxy is unusual in that it has bright emission lines. On the basis of the optical spectrum we are unable to uniquely classify the galaxy since its emission line ratios lie in the transition region between starburst and Seyfert II type spectra.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy & Astrophysic

Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample

We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs in the UV spectra of quasars selected from the SDSS Early Data Release. These quasars have strong intervening MgII-FeII systems which are known signatures of high column density neutral gas. In total, UV observations of Ly-alpha absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW) W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36 lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M = 0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is constant for 0.5<z<5.0 to within the uncertainties. This is larger than Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution does not change significantly with redshift. However, the low redshift distribution is marginally flatter due to the higher fraction of high N(HI) systems in our sample. (6) Finally, using the precision of MgII survey statistics, we find that there may be evidence of a decreasing Omega_{DLA} from z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very high columns of neutral gas might be missed by DLA surveys because of their very small cross sections, and therefore, that Omega_{DLA} might not include the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure

The z=0.0912 and z=0.2212 Damped Lyman Alpha Galaxies Along the Sight-Line Toward the Quasar OI 363

New optical and infrared observations along the sight-line toward the quasar OI 363 (0738+313) are presented and discussed. Excluding systems which lack confirming UV spectroscopic observations of the actual Lyman alpha line, this sight-line presently contains the two lowest-redshift classical damped Lyman alpha (DLA) quasar absorption line systems known (i.e. with N(HI) \ge 2 x 10^{20} atoms cm^{-2}), one at z(abs)=0.0912 and the other at z(abs)=0.2212. The z=0.09 DLA galaxy appears to be an extended low surface brightness galaxy which is easily visible only in infrared images and shows rich morphological structure. Subtraction of the quasar nuclear and host light yields L_K \approx 0.08L_K* at z=0.09. The impact parameter between the galaxy and quasar sight-line is very small, b<3.6 kpc (<2 arcsec), which makes measurements difficult. The z=0.22 DLA galaxy is an early-type dwarf with a K-band luminosity of L_K \approx 0.1L_K* at impact parameter b=20 kpc. In general, these results serve to support mounting evidence that DLA galaxies are drawn from a wide variety of gas-rich galaxy types. (Abridged)Comment: 27 pages, 6 figures, 2 in color. Submitted to Ap

Representations of Double Affine Lie algebras

We study representations of the double affine Lie algebra associated to a simple Lie algebra. We construct a family of indecomposable integrable representations and identify their irreducible quotients. We also give a condition for the indecomposable modules to be irreducible, this is analogous to a result in the representation theory of quantum affine algebras. Finally, in the last section of the paper, we show, by using the notion of fusion product, that our modules are generically reducible