Abstract We present the discovery of `A Big Ring on the Sky' (BR), the second ultra-large large-scale structure (uLSS) found in Mg II -absorber catalogues, following the previously reported Giant Arc (GA). In cosmological terms the BR is close to the GA — at the same redshift z ∼ 0.8 and with a separation on the sky of only ∼ 12°. Two extraordinary uLSSs in such close configuration raises the possibility that together they form an even more extraordinary cosmological system. The BR is a striking circular, annulus-like, structure of diameter ∼ 400 Mpc (proper size, present epoch). The method of discovery is as described in the GA paper, but here using the new Mg II -absorber catalogues restricted to DR16Q quasars. Using the Convex Hull of Member Spheres (CHMS) algorithm, we estimate that the annulus and inner absorbers of the BR have departures from random expectations, at the density of the control field, of up to 5.2σ. We present the discovery of the BR, assess its significance using the CHMS, Minimal Spanning Tree (MST), FilFinder and Cuzick & Edwards (CE) methods, discuss it in the context of the GA+BR system, and suggest some implications for the origins of uLSS and for our understanding of cosmology. For example, it may be that unusual geometric patterns, such as these uLSSs, have an origin in cosmic strings.
We present 13 spectra and 31 photometric observations covering the first 150 days of SN 1991bg in NGC 4374 (M 84). Although SN 1991bg was a type Ia supernova displaying the characteristic Si II absorption at 6150 A near maximum and the Fe emission lines at late phases, it varied from the well-defined norm for SNe Ia in several important respects. The peculiarities include faster declines in the B and V light curves after maximum, a distinct color evolution, a very red B - V color near maximum, relatively faint peak luminosity, a distinct spectral evolution, and a short peak phase. The narrow peak of the luminosity and the rapid declines of the light curves suggest a smaller mass in the ejecta and larger energy losses than for most SNe Ia. The unusually red color at maximum is not a result of normal extinction, since SN 1991bg was as blue as other SNe Ia at late times and no narrow interstellar lines are observed in the spectra. The faint absolute magnitude of SN 1991bg is established beyond doubt by comparison with SN 1957B, another type Ia supernova in the same galaxy, which was 2.5 magnitudes brighter than SN 1991bg. The spectral evolution reveals minor differences near maximum compared to other well-observed SNe Ia, mainly in relative line strengths. At later phases several wavelength regions display discrepancies when compared to spectra of normal SNe Ia. Although other SNe Ia, such as SN 1986G and SN 1939B, have light curves with fast decline rates, SN 1991bg is unique, with deviations in both light curves and spectra. In particular SN 1991bg is the only SN Ia observed to date with a distinct spectrum at ~40 days past maximum. Although SN 1991bg is an extreme case, with unusual photometric and spectroscopic properties, we believe it can be understood in the context of exploding white dwarf models, and is properly grouped with type Ia. SN 1991bg demonstrates the need for detailed observations of SNe Ia as part of their use as standard candles for cosmology. While there is a well-defined prototype with homogeneous properties, unusual cases like SN 1991bg must be identified and separated to avoid misleading results.
We present a search for spatial and redshift correlations in a 2 A resolution spectroscopic survey of the Lyα forest at 2.15 < z < 3.37 toward 10 QSOs concentrated within a 1° diameter field. We find a signal at 2.7 σ significance for correlations of the Lyα absorption-line wavelengths between different lines of sight over the whole redshift range. The significance rises to 3.2 σ if we restrict the redshift range to 2.60 < z < 3.37, and to 4.0 σ if we further restrict the sample to lines with rest equivalent widths 0.1 ≤ W0 < 0.9 A. We conclude that a significant fraction of the Lyα forest arises in structures whose correlation length extends at least over 30' (~26 h-1 comoving Mpc at z = 2.6 for H0 ≡ 100 h km s-1 Mpc-1, Ω = 1.0, Λ = 0). We have also calculated the three-dimensional two-point correlation function for Lyα absorbers; we do not detect any significant signal in the data. However, we note that line blending prevents us from detecting the signal produced by a 100% overdensity of Lyα absorbers in simulated data. We find that the Lyα forest redshift distribution provides a more sensitive test for such clustering than the three-dimensional two-point correlation function.
Proto‐planetary and transitional disks which are detected in scattered light provide a critical test of the interpretation of circumstellar disks based on the IR spectral energy distribution (SED) alone. The disk inclination to the line‐of‐sight, outter radius, and surface brightness (SB) maps or radial SB distributions provided by spatially resolved imaging remove most of the degeneracies inherent in fitting IR SEDs without such observational constraints. We have imaged the disk of SAO 206462 (HD 135344 B) in 1.1 and 1.6 μm scattered light with HST/NICMOS and can trace the essentially face‐on disk out to 1.05". The cavity detected in sub‐mm observations lies entirely under the NICMOS coronagraphic spot, a result consistent with the SED fitting if the star is at d = 140 pc. The SED had previously been classified as a Meeus Group I SED and interpreted as arising in a flared disk. Neither the 1.1 nor the 1.6 μm radial surface brightness profiles are consistent with a flared disk. A FUSE FUV spectrum demonstrates the presence of excess light in this system, confirming the accretion rate estimated from Brγ. Collectively, these data strengthen the interpretation of this system as a transitional disk.
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