arXiv subjects

Arlin Crotts

Publications and source records attributed to Arlin Crotts.

25 records · Page 2Linked to original sources

arXiv2000

On the Emergence and Discovery of Hot Spots in Supernova Remnant 1987A

We present the discovery of several new regions of interaction between the ejecta and equatorial circumstellar ring of SN1987A, an interaction leading to a much expanded development of the supernova remnant. We also trace the development of the first such "hot spot," discovered in 1997, back to 1995. Later hot spots seem to have emerged by early 1999. We discuss mechanisms for the long delay between the first and later spots.

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arXiv1999

Expectations from Realistic Microlensing Models of M31. I: Optical Depth

We provide a set of microlensing optical depth maps for M31. Optical depths towards Andromeda were calculated on the basis of a four component model of the lens and source populations: disk and bulge sources lensed by bulge, M31 halo and Galactic halo lenses. We confirm the high optical depth and the strong optical depth gradient along the M31 minor axis due to a dark halo of lenses and also discuss the magnitude of the self-lensing due to the bulge. We explore how the shape of the optical depth maps to M31 vary with the halo parameters core radius and flattening.

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arXiv1998

Reobservation of Close QSO Groups: The Size Evolution and Shape of Lyman Alpha Forest Absorbers

Probing the size and shape of the absorbers that lie in front of the QSOs, in particular the Ly alpha forest, we analyze 785 absorption lines in spectra of five QSOs in close groupings: a pair and a triplet. Both QSO groups have been observed before, but these data are a drastic improvement over earlier data, and provide a qualitatively different view of the nature of the absorbers. The pair samples a scale critical in determining the size upper bound of Ly alpha absorbers, with significant leverage in redshift compared to previous studies. The triplet data represents the spatially densest sample of Ly alpha forest ever studied, and an almost ideally-suited probe of the shape of absorbers. We observe a significant number of Ly alpha lines in common between the triplet sightlines, for lines stronger than rest equivalent width W > 0.4A (and no detected metal lines) and velocity differences up to 200 km/s, corresponding to a two-point correlation function xi = 1.88 +0.78/-0.50 on scales 0.5 to 0.8 Mpc/h with < z > = 2.14, and inconsistent at the 99.999% level with the absence of any clustering. These data also show that a significant fraction of the W > 0.4A Ly alpha forest absorbers span all three sightlines to the triplet, indicating that the strong-lined absorbers are consistent with nearly round shapes, chosen from a range of possible cylinders of different elongations. This may be inconsistent with results from hydrodynamic/gravitational simulations of H I in the early Universe indicating that the theoretical counterparts of Ly alpha forest clouds are long and filamentary. We conclude, tentatively, that the W > 0.4A Ly alpha forest objects are sheetlike. This is supported by the uniformity of linestrengths between the three sightlines, for W > 0.4A. (Abstract continues.)

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arXiv1997

Rings and Echoes: An Overview (of SN 1987A's Environment)

The volume around the SN 1987A contains a variety of structures, including but certainly not limited to the three-ring nebula glowing in recombination lines. Many of these structures are revealed by light echoes, hence have been mapped in three dimensions by our optical imaging monitoring of the field around the SN. The three rings are part of a bipolar nebula which contains them at its waist and crowns, and which is itself contained in a larger, diffuse nebula with a detectable equatorial overdensity. This diffuse nebula terminates in a boundary overdensity which most likely marks the inner edge of a bubble blown by the main sequence wind of the progenitor star and its neighbors. Beyond this bubble is a rich collection of interstellar structures revealed by light echoes. In addition to detecting and mapping these structures, we add dynamical and age information by establishing the kinematics of the gas, both on interstellar and circumstellar scales. These reveal, for instance, a timescale for the outer circumstellar rings which is in close agreement with the inner ring. The presence of these structures, their ages and morphologies should be included in any model explaining the evolution of the progenitor star and its mass loss envelope.

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arXiv1997

Ly alpha Cloud Size Evolution and Shape from QSO Groups

We present results from Keck HIRES, KPNO 4m/RC Spec and HST FOS spectra of a low-redshift, close QSO pair and a close, higher-redshift QSO triplet in order to explore the size evolution, shape and clustering of Ly alpha absorbers. The results we highlight here indicate that there is no strong evidence for evolution of Ly $α$ cloud size with redshift, that strong (W > 0.4A) Ly alpha forest absorbers are probably not filaments, but probably more sheet- like. For C IV absorbers a large component of clustering on 200 km/s < Delta v < 600 km/s scales along single sightlines is due to internal velocities of single absorbers, not clustering between absorbers.

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arXiv1997

The Weakness of C IV Absorber Clustering in Keck HIRES Spectra of Adjacent QSO Sightlines

We observe with Keck/HIRES the z ~ 2.5 QSO triplet 1623+27 in order to explore on the scale of a Megaparsec the spatial clustering of C IV absorbers between adjacent sightlines. We find this signal to be significantly weaker than the clustering in velocity on corresponding scales along single sightlines, assuming that the relative velocity of absorbers is dominated by the Hubble flow. This indicates that small-scale clustering (200 km/s < Delta v < 600 km/s) of the C IV absorbers cannot be interpreted in terms of the positions of the absorbers in space, but must be considered as internal motions within individual absorbers, or within clusters of absorbers whose internal velocities dominate over Hubble expansion across the cluster scale. If the single- sightline signal is due to spatial clustering, it is caused by absorber clusters smaller than would be implied by their velocities if a Hubble flow is assumed. The spatial clustering of C IV absorbers at z ~ 2 is consistent with data on Ly alpha forest clustering measured in the same way at the same redshifts. However, present-day galaxy clustering, evolved back to z ~ 2, is consistent with C IV spatial clustering but perhaps not with that of the Ly alpha forest. Even so, one cannot as yet distinguish the two absorber populations on the basis of spatial clustering on these small scales.

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arXiv1994

A New Method for Measuring the Hi Gunn-Peterson Effect at High Redshift

In order to carry out a systematic and thorough measurement of the HI Gunn-Peterson effect at high redshift, a quantitatively testable and repeatable procedure, in particular, a robust statistical weighting technique, is developed. It is applied to an echelle spectrum of resolution 15 km s$^{-1}$ of PKS 1937-101 with z=3.787. A weighted intensity distribution which is derived overwhelmingly from pixels close to the continuum level in the Ly alpha forest region is constructed by the evaluation of how closely correlated each pixel is with its neighboring pixels. The merit of the distribution is its stronger and narrower peak compared to the unweighted, as well as its smaller dependence on uncertainty of strong absorption lines and noise spikes. By comparison to the weighted intensity distribution of synthetic Ly alpha forest spectrum with various chosen diffuse HI opacities, a chi square fit is performed. In addition to a weak line population with power law N_H distribution $β=1.7$ extrapolated down to 10$^{12}$ cm$^{-2}$, a best chi square fit requires a GP opacity of $0.115 \pm 0.025$ at average z=3.4 with estimation of the contribution from the variance of $β$. Although no evidence of more than 1-2\% error is seen in the continuum extrapolation, the possible systematic overestimation due to the slope can be as high as the level of the $χ^2$ fit, which is investigated by splitting the \Lya forest region into subsamples to check the continuum drop's dependence on absorber redshift.

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