Detection of Interstellar C_2 and C_3 in the Small Magellanic Cloud

We report the detection of absorption from interstellar C_2 and C_3 toward the moderately reddened star Sk 143, located in the near 'wing' region of the SMC, in optical spectra obtained with the ESO VLT/UVES. These detections of C_2 (rotational levels J=0-8) and C_3 (J=0-12) absorption in the SMC are the first beyond our Galaxy. The total abundances of C_2 and C_3 (relative to H_2) are similar to those found in diffuse Galactic molecular clouds -- as previously found for CH and CN -- despite the significantly lower average metallicity of the SMC. Analysis of the rotational excitation of C_2 yields an estimated kinetic temperature T_k ~ 25 K and a moderately high total hydrogen density n_H ~ 870 cm^-3 -- compared to the T_01 ~ 45 K and n_H ~ 85-300 cm^-3 obtained from H_2. The populations of the lower rotational levels of C_3 are consistent with an excitation temperature of about 34 K.Comment: accepted to MNRAS; 10 pages, 6 figure

Detection of Ne VIII in the Low-Redshift Warm-Hot IGM

High resolution FUSE and STIS observations of the bright QSO HE 0226-4110 (zem = 0.495) reveal the presence of a multi-phase absorption line system at zabs(O VI) = 0.20701 containing absorption from H I (Ly alpha to Ly theta), C III, O III, O IV, O VI, N III, Ne VIII, Si III, S VI and possibly S V. Single component fits to the Ne VIII and O VI absorption doublets yield logN(Ne VIII) = 13.89+/-0.11 and logN(O VI) = 14.37+/-0.03. The Ne VIII and O VI doublets are detected at 3.9 sigma and 16 sigma significance levels, respectively. This represents the first detection of intergalactic Ne VIII, a diagnostic of gas with temperature in the range from 5x10(5) to 1x10(6) K. The O VI and Ne VIII are not likely created in a low density medium photoionized solely by the extragalactic background at z = 0.2 since the required path length of ~11 Mpc implies the Hubble flow absorption line broadening would be ~10 times greater than the observed line widths. A collisional ionization origin is therefore more likely. Assuming [Ne/H] and [O/H] = -0.5, the value N(Ne VIII)/N(O VI) = 0.33+/-0.10 is consistent with gas in collisional ionization equilibrium near T=5.4x10(5) K with logN(H)= 19.9 and N(H)/N(H I) = 1.7x10(6). The observations support the basic idea that a substantial fraction of the baryonic matter at low redshift exists in hot very highly ionized gaseous structures.Comment: 32 pages text and 9 pages of figures. Accepted by the Astrophysical Journa

Black hole spectroscopy with coherent mode stacking

The measurement of multiple ringdown modes in gravitational waves from binary black hole mergers will allow for testing fundamental properties of black holes in General Relativity, and to constrain modified theories of gravity. To enhance the ability of Advanced LIGO/Virgo to perform such tasks, we propose a coherent mode stacking method to search for a chosen target mode within a collection of multiple merger events. We first rescale each signal so that the target mode in each of them has the same frequency, and then sum the waveforms constructively. A crucial element to realize this coherent superposition is to make use of a priori information extracted from the inspiral-merger phase of each event. To illustrate the method, we perform a study with simulated events targeting the l=m=3 ringdown mode of the remnant black holes. We show that this method can significantly boost the signal-to-noise ratio of the collective target mode compared to that of the single loudest event. Using current estimates of merger rates we show that it is likely that advanced-era detectors can measure this collective ringdown mode with one year of coincident data gathered at design sensitivity.Comment: 11 pages, 4 figure

A Hubble Space Telescope Study of Lyman Limit Systems: Census and Evolution

We present a survey for optically thick Lyman limit absorbers at z<2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs) increasing the number of catalogued LLSs at z<2.6 by a factor of ~10. We compile a statistical sample of 50 tau_LLS > 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z)=dn/dz, at these redshifts is well described by a single power law, l(z) = C1 (1+z)^gamma, with gamma=1.33 +/- 0.61 at z<2.6, or with gamma=1.83 +/- 0.21 over the redshift range 0.2 < z < 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6 Gyr from z=4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ~8 Gyr from z=2.6 to 0.25. We show that the column density distribution function, f(N(HI)), at low redshift is not well fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the column density range 13 17.2. While low and high redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is some evidence that f(N(HI)) evolves with z for log N(HI) < 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ~9 from z~5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L* galaxies.Comment: Accepted by Ap