📝 Original Info
- Title: Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array
- ArXiv ID: 0903.0672
- Date: 2009-11-13
- Authors: Researchers from original ArXiv paper
📝 Abstract
This paper reports dual-epoch, Very Long Baseline Array observations of H I absorption toward 3C 147. One of these epochs (2005) represents new observations while one (1998) represents the reprocessing of previous observations to obtain higher signal-to-noise results. Significant H I opacity and column density variations, both spatially and temporally, are observed with typical variations at the level of \Delta\tau ~ 0.20 and in some cases as large as \Delta\tau ~ 0.70, corresponding to column density fluctuations of order 5 x 10^{19} cm^{-2} for an assumed 50 K spin temperature. The typical angular scale is 15 mas; while the distance to the absorbing gas is highly uncertain, the equivalent linear scale is likely to be about 10 AU. Approximately 10% of the face of the source is covered by these opacity variations, probably implying a volume filling factor for the small-scale absorbing gas of no more than about 1%. Comparing our results with earlier results toward 3C 138 (Brogan et al.), we find numerous similarities, and we conclude that small-scale absorbing gas is a ubiquitous phenomenon, albeit with a low probability of intercept on any given line of sight. Further, we compare the volumes sampled by the line of sight through the Galaxy between our two epochs and conclude that, on the basis of the motion of the Sun alone, these two volumes are likely to be substantially different. In order to place more significant constraints on the various models for the origin of these small-scale structures, more frequent sampling is required in any future observations.
💡 Deep Analysis
Deep Dive into Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array.
This paper reports dual-epoch, Very Long Baseline Array observations of H I absorption toward 3C 147. One of these epochs (2005) represents new observations while one (1998) represents the reprocessing of previous observations to obtain higher signal-to-noise results. Significant H I opacity and column density variations, both spatially and temporally, are observed with typical variations at the level of \Delta\tau ~ 0.20 and in some cases as large as \Delta\tau ~ 0.70, corresponding to column density fluctuations of order 5 x 10^{19} cm^{-2} for an assumed 50 K spin temperature. The typical angular scale is 15 mas; while the distance to the absorbing gas is highly uncertain, the equivalent linear scale is likely to be about 10 AU. Approximately 10% of the face of the source is covered by these opacity variations, probably implying a volume filling factor for the small-scale absorbing gas of no more than about 1%. Comparing our results with earlier results toward 3C 138 (Brogan et al.)
📄 Full Content
Beginning with a two-antenna very long baseline interferometric (VLBI) observation of 3C 147 by Dieter et al. (1976), a variety of H I absorption studies over the past three decades have found AU-scale optical depth variations in the Galactic interstellar medium (ISM). The initial detections were confirmed by Diamond et al. (1989), and the first images of the small-scale H I in the direction of 3C 138 and 3C 147 were made by Davis et al. (1996) using MERLIN. Faison et al. (1998) and Faison & Goss (2001) used the Very Long Baseline Array (VLBA) to improve the resolution toward a number of sources to approximately 20 mas (∼ 10 AU). Significant variations were detected in the direction of 3C 138 and 3C 147, while no significant variations in H I opacity were found in the direction of five other compact radio sources.
An independent means of probing small-scale neutral structures is multi-epoch H I absorption measurements of high proper motion pulsars (Frail et al. 1994;Johnston et al. 2003;Stanimirović et al. 2003). While early pulsar observations suggested that small-scale structure might be ubiquitous, more recent observations suggest that it could be more sporadic. A significant advantage of VLBI observations is that they provide 2-D images of the opacity variations, rather than 1-D samples as in the case of pulsars observations. Brogan et al. (2005) revisited the observations of 3C 138, by re-analyzing the 1995 VLBA observations (Faison et al. 1998) and by obtaining two new epochs of observations (1999 and 2002). They confirmed the initial results of Faison & Goss (2001), that there are small-scale opacity changes along the line of sight to 3C 138 at the level of ∆τ max = 0.50 ± 0.05, with typical sizes of roughly 50 mas (∼ 25 AU). However, with multiple epochs and improvements in data analysis techniques (yielding an increase of a factor of 5 in the sensitivity of the 1995 epoch), they reached a number of additional significant conclusions:
They found clear evidence for temporal variations in the H I opacity over the sevenyear time span of the three epochs, consistent with structures moving across the line of sight at velocities of a few tens of kilometers per second, though the infrequent sampling in time means that they could not determine whether these structures were persistent.
They found no evidence for a drop in the H I spin temperature, as would be evidenced by a narrowing of line widths at small scales compared to single dish measurements. In turn, a constant H I spin temperature implies that the small-scale opacity variations are due to density enhancements, although these enhancements would necessarily be extremely over-pressured relative to the mean interstellar pressure, far from equilibrium, and likely of relatively short duration.
For the first time they determined that the plane of sky covering fraction of the smallscale H I gas is roughly 10%. In turn, this small covering fraction suggests that the volume filling factor of such gas, within the cold neutral medium, is quite low ( 1%), in agreement with HST observations of highpressure gas in the ISM (Jenkins & Tripp 2001;Jenkins 2004).
They simulated pulsar observations that have been used to search for H I opacity variations and showed that the existing pulsar observations have generally been too sparsely sampled (in time) to be useful in studying the details of small-scale H I opacity variations.
While the multi-epoch study of Brogan et al. (2005) represented a substantial improvement, nonetheless their conclusions rested on observations of only one line of sight. In light of this sample of one, their conclusions might seem rather audacious, particularly given the larger sample observed by Faison et al. (1998) and Faison & Goss (2001), in which most of the objects did not show variations in the H I absorption. The 3C 138 study has shown that the key to a successful small scale H I study is a background source with both high surface brightness ( 60 mJy beam -1 ) and large angular extent (> 100 mas). The quasar 3C 147 is one of the few sources that shares these characteristics with 3C 138. This paper presents dual-epoch observations of 3C 147 that were designed specifically to confront the conclusions of Brogan et al. (2005) with a second line of sight. Section 2 of this paper describes the observations, focussing on the new observations acquired for the second epoch, §3 discusses the results, and §4 presents our conclusions and recommendations for future work.
We have observed the Galactic H I absorption (near -10 km s -1 ) toward the quasar 3C 147 at two epochs. Epoch I was 1998 October 22, and the results from those observations have been published previously by Faison & Goss (2001). Epoch II consists of new data observed on 2005 August 21. Table 1 summarizes the basic observing parameters for the two epochs.
For both epochs the data were obtained using the 10 antennas of the Very Long Baseline Array combined with th
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