We present new moderate-resolution, far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 1ES 1553+113 covering the wavelength range 1135 Å < λ < 1795 Å. The data show a smooth continuum with a wealth of narrow (b < 100 km s−1) absorption features arising in the interstellar medium and intergalactic medium. These features include 41 Lyα absorbers at 0 < zabs < 0.43, 14 of which are detected in multiple Lyman lines and 6 of which show absorption in one or more metal lines. We analyze a metal-rich triplet (Δcz ∼ 1000 km s−1) of Lyα absorbers at zabs ≈ 0.188 in which O vi, N v, and C iii absorption is detected. Silicon ions (Si iii, Si iv) are not detected to fairly strong upper limits and we use the measured Si iii/C iii upper limit to derive an abundance limit (C/Si) ⩾ 4(C/Si)☉ for the strongest component of the absorber complex. Galaxy redshift surveys show a number of massive galaxies at approximately the same redshift as this absorption complex, suggesting that it arises in a large-scale galaxy filament. As one of the brightest extragalactic X-ray and γ-ray sources, 1ES 1553+113 is of great interest to the high-energy astrophysics community. With no intrinsic emission or absorption features, 1ES 1553+113 has no direct redshift determination. We use intervening Lyα absorbers to place a direct limit on the redshift: zem>0.395 based on a confirmed Lyα+O vi absorber and zem>0.433 based on a single-line detection of Lyα. The current COS data are only sensitive to Lyα absorbers at z < 0.47, but we present statistical arguments that zem ≲ 0.58 (at a 1σ confidence limit) based on the non-detection of any Lyβ absorbers at z>0.4.
We report on the detection of Ne viii in the Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectrum of the intervening absorption system at z = 0.495096 toward PKS 0405−123 (zem = 0.5726). The high signal-to-noise COS spectrum also covers absorption from H i, C iii, O iii, O iv, and O vi associated with this multiphase system. The Ne viii is detected with high significance in both lines of the doublet, with integrated column densities of log Na(Ne viii 770) = 13.96 ± 0.06 and log Na(Ne viii 780) = 14.08 ± 0.07. We find the origin of Ne viii consistent with collisionally ionized gas at T ∼ 5 × 105 K with a large baryonic column density of N(H) ∼ 1019–1020 cm−2. The metallicity in the Ne viii gas phase is estimated to be [Ne/H] ∼−0.6 ± 0.3 dex. The intermediate ions such as C iii, O iii, O iv, and H i are consistent with photoionization in lower ionization gas at T ∼ 104 K. The O v and O vi in this absorber can have contributions from both the photoionized and collisionally ionized gas phases. The absorber is at |Δv| = 180 km s−1 systematic velocity and ρ = 110h−170 kpc projected separation from an MR = −19.6 galaxy of extended morphology. The collisionally ionized gas at T ∼ 5 × 105 K detected in Ne viii and O vi points to an origin in multiphase gas embedded in the hot halo of the galaxy, or in a nearby WHIM structure. The high-sensitivity UV spectroscopy afforded by COS has opened up new opportunities for discovering large reservoirs of missing baryons in the low-z universe through the detection of Ne viii systems.
A total of 717 sources from the Australia Telescope Compact Array (ATCA) catalogue of the Small Magellanic Cloud (SMC) have been classified in Paper II (of this series) by Payne et al. Here, we present a statistical analysis of all 16 confirmed supernova remnants (SNRs) and five new candidate remnants. Included is a detailed discussion of the latter and three other sources that have some SNR characteristics. We have also found a new microquasar candidate (ATCA J005523-721055) in addition to ATCA J004718-723947 reported in Paper II. Source diameter comparisons suggest that SNRs as a group are of similar size in radio, optical and X-ray with surface brightness values in the range of Galactic remnants. Remnant spectral indices, α[defined as Sυ∝υα, with Sυ (flux density) and υ (frequency)], have a mean of −0.63 [standard deviation (s.d.) = 0.43] and ROSAT X-ray hardness ratios confirm them to be soft X-ray sources compared to background objects. We could not find any meaningful correlation between SNR surface brightness and diameter; we also discuss the number-diameter relation. A Venn diagram summarizes that most SNRs emit radiation in all three of the radio, optical and X-ray domains. H ii region diameter comparisons between radio and optical sources show them to be a very diverse group that defies any simple relationship, preventing any meaningful calculation of flux density or spectral index. To better understand environments containing SNRs, we have scaled Hα images of four SMC regions and subtracted their flux from the ATCA 2.37-GHz radio image. These 2.37 GHz-Hα subtraction (or difference) images reveal some new sources with predominantly non-thermal emission, exposing SNRs confused with H ii regions.
