WNM via Stacking Analysis:
(from Murray et al. 2014: http://arxiv.org/abs/1401.1215)
To increase our sensitivity to shallow, broad absorption features from the WNM, we conduct a spectral stacking analysis on the first 24 completed 21SPONGE spectra. We detect a widespread warm neutral medium (WNM) component with excitation temperature <Ts> = 7200 (+1800, -1200) K (68% conﬁdence). This temperature lies above theoretical predictions based on collisional excitation alone, implying that Ly-α scattering, the most probable additional source of excitation, is more important in the interstellar medium (ISM) than previously assumed. Our results demonstrate that Hi absorption can be used to constrain the Ly-α radiation ﬁeld, a critical quantity for studying the energy balance in the ISM and intergalactic medium yet notoriously diﬃcult to model because of its complicated radiative transfer, in and around galaxies nearby and at high redshift.
(from Begum et al. 2010: http://iopscience.iop.org/0004-637X/725/2/1779/pdf/apj_725_2_1779.pdf)
Figure 1: HI absorption spectra towards 3C286, demonstrating the increased sensitivity of the EVLA (right) vs. VLA (left) in our survey. The bandpass is significantly improved, allowing for the detection of spectral lines with very low optical depth. In the right figure, the deepest absorption figure corresponds to an optical depth of ~ 6 × 10-3.
Figure 2: Some example H I absorption spectra from pilot observations, with corresponding emission spectra from Arecibo (from Begum et al. 2010). Fitted profiles correspond to components of cold neutral medium (CNM) in both emission and absorption and warm neutral medium (WNM) in emission along the line of sight. In the direction of source P0347, previous studies (Heiles & Troland 2003) found a narrow emission feature without corresponding absorption, resulting in a thermally unstable WNM. Here we show our EVLA absorption spectrum for this source (left): our detection of an additional absorption feature at a velocity of −0.5 km s-1 results in the best-fit solution without any thermally-unstable WNM. Thus, weak absorption lines could decrease the estimated fraction of the thermally unstable WNM. For several sources, e.g. 3C273, P0347 (left), and J0022 (right), we detected very wide absorption components (FWHM > 7.0 km s−1 ). This implies Tk,max > 1000 K (the kinetic temperature in the case in which there is no non-thermal broadening), suggesting that we have already detected several WNM features in absorption. Using the OSRO2 spectral-line mode with the WIDAR correlator, with a bandwith of 0.5 MHz, 256 frequency channels, and two polarization products, the observing setup results in a velocity resolution of 0.4 km s-1, and a velocity coverage of107 km s-1.