z2p5 updates raef/jv Dear Gang, As we (Joel!) work on the analysis of the z ~ 2.5 spectropolarimetry, we thought it would be a good idea to keep informal notes on what we see and do. This will help to keep us all in touch and will also help in writing the paper. We start with the few notes we already wrote for the preparation of the latest observing proposal and continue from there. 1) from Mon, 9 Mar 1998 We have splendid data on 6 z~2.5 objects - which is what we asked for. I worry a bit about the selection process - but basically we did more-or-less everything that was available so there was not much we could do about it even if we could think of selection criteria. Our objects span a broad range in polarization and Lyman-a strength - but the excitation states measured by the carbon, helium and nitrogen lines are rather similar. We see this interesting anticorrelation between the Ly-a/CIV and P and this extends to 4C41.17. In fact, if we include 4C 41.17 in the set, then it looks like one of the family. I believe that, with some spectral decomposition, we will be able to identify 2 or 3 continuum components which, in different proportions, will build our continua. The spatial and velocity variations are rich in information and it will be especially interesting to look at the Ly-a absorption. I think we can make a very strong case to complete this programme with observations of 4 or 5 (3 or 4?) objects with z>3, ie. the far side of the agn space density peak when quasars were newly invented. We can then see if there is really a trend in the relative proportions of the continua. We can also look for systematic effects in the emission line spectra - do the abundances change - does the dust/gas ratio change? Bob ------------- 2) from Wed, 11 Mar 1998 Reductions and preliminary analysis. The data from the December 1997 run have been fully reduced for preliminary analysis althogh some refinements in the spectral extraction are still needed. Emission line fluxes have been measured and are being compared with the dusty photoionization models. Several absorption lines are seen but the most prominent can be attributed to interstellar absorption. There is no clear detection of any unambiguously stellar photospheric line. The continuum polarization, measured just longward of Lyman-$\alpha$, ranges up to 20\% with only one object being unpolarized at the 1\% level. There is a clear anticorrelation between the degree of continuum polarization and the strength of the Lyman-$\alpha$\ emission relative to other lines, eg, CIV. see: http://ecf.hq.eso.org/~rfosbury/research/z2p5/z2p5_pol-Lya.gif We believe that this is more likely to be an indicator of source geometry than be simply related to the total quantity of extended dust. The two objects with the highest polarization show significant broad emission line wings. The continuum colors, measured between rest-frame wavelengths of 1320 and 2070\AA, span only half a magnitude and the continuum shapes are similar with a peak in $F_\lambda$\ at around 1400\AA. This continuum shape, including the minimum at 2200\AA, appears to be consistent with an SED which is approximately flat in $F_\lambda$\ and reddened with a Galactic extinction curve with $E_{B-V}\sim 0.1$. There is no correlation between color and polarization. [The spectra are rich in kinematic and spatial information which we have not yet studied.] Bob ------------- 3) Thu, 19 Mar 1998 Continuum --------- As remarked in (2), the continuum shape appears rather similar in most of the objects, Joel has fitted a reddened power law to 0211 and 0731 and 0943, see: http://ecf.hq.eso.org/~rfosbury/research/z2p5/0211_cont.gif http://ecf.hq.eso.org/~rfosbury/research/z2p5/0731_cont.gif http://ecf.hq.eso.org/~rfosbury/research/z2p5/0943_cont.gif where it really does look as if we see the 2200A dip and the fit to the continuum shape shortward of this looks pretty good. Very roughly, we have a power law with f_nu ~ nu^-1 or so and E_B-V ~ 0.1 or so. We will send the values when all the fits have been done. This is clearly just a way of characterising the spectra at the moment. Proper decompositions will have to be done in the end... It does, however, cause us to ponder the reason for this similarity. We think the answer is both simple and important. Consider the simplest possible model for our RG: a hidden quasar surrounded by an 'atmosphere' of dust and gas. If we start with a dust to gas ratio of zero, there will be no (dust) scattering and we see nothing other than emission lines, nebular continuum, starlight etc. As we increase the dust content, the scattered light increases linearly with the amount of dust while the atmosphere remains optically thin. As the dust content increases further, we start seeing extinction as well from the same dust which does the scattering. This goes like F_scat ~ tau_s * exp(-tau_e) where the scattering tau and the extinction tau can be different - but are proportional to one another. This function has a maximum at tau_e = 1, so we will always see most of the scttered light (even from a complex source) from regions where tau ~ 1. Looking at the standard extinction curve, tau = 1 at 1300A corresponds to E_B-V = 0.1. Hey Presto. This is all very textbookish and simple, but the important thing is that it implies we are seeing scattering by DUST and the same dust is doing the extinction. It also implies that the dust is not too different from Galactic dust in that it exhibits the 2200A absorption feature. We should be able to model this quite well. Taking this a little bit further, we wonder if this goes some way to explaining the Ly-a - polarization relationship. Even if we have a very complex, highly obscured source (like TXS0211?), the continuum we do see will come from tau ~ 1 and have the same spectrum as the other sources. Such a geometry may produce a higher polarization - because the scattered flux is integrated over a smaller range of angles - and will produce more opportunities for Ly-a destruction - since most of the sky at the source of a Ly-a photon will be covered by dusty gas. Emission lines -------------- Montse is working on the modelling of the strong lines. The average spectrum (see: http://ecf.hq.eso.org/~rfosbury/research/z2p5/z2p5_ave_em.gif shows a host of fainter lines which we have not completely identified yet. Absorption lines ---------------- Joel has used the line list in the recent paper by Heckman et al. http://xxx.lanl.gov/abs/astro-ph/9803185 to look for the interstellar, wind and photospheric lines. You can look at the average spectrum: http://ecf.hq.eso.org/~rfosbury/research/z2p5/z2p5_ave_abs.gif but beware that, in individual objects, some of these lines are affected by strong night sky lines. We are currently going through and checking them all. Si III, 1892 looks a promising photospheric line which may be present. All for now. Any comments? Regards, Bob and Joel... PS we will try to make a password-protected website for exchange of plots, tables etc.