The major contributing observatories in this field are just three: the International Ultraviolet Explorer (IUE), the Hubble Space Telescope (HST), and the Galaxy Evolution Explorer (GALEX), while piecemeal observations have been contributed by other telescopes. Restframe FUV observations began with the IUE in 1977, HST started operation just 13 years later and is still returning Lyα data at z < 1 some 25 years on; the demarcation between history and present is arbitrary of course. For the sake of this review, I will adopt the pre-HST era, which is almost exclusively the IUE and thus is also restricted by method to spectrophotometry of pre-selected targets. In the ‘modern' era we then have two main operational tools. Firstly HST, which like IUE performs pointed observations of individual targets, but in doing so yields data that are always rich with features since both spatial and spectral resolutions are significantly higher. Secondly, the GALEX satellite is similar to the IUE in resolving power but its strength comes instead from its 1.2 degree field of view, which provided the survey efficiency to yield the statistical significance that was not available at low-z with any other telescope.
3.1. The First Lyα Spectra of Active Galactic Nuclei
Even before IUE some extragalactic objects were observed with sounding rocket experiments, which were able to launch small UV telescopes above enough of the atmosphere to observe in the UV. Specifically targeting the first known quasar 3C 273, rocket payloads provided the first measurement of Lyα and Balmer emission lines from any astrophysical body (Davidsen et al., 1977). Combining the measured Lyα flux with optical measurements revealed a Lyα / Hβ ratio of 4, a measurement that then sat in stark contrast to the value of ≈ 40 that was expected for nebular recombinations and a Lyα enhancement from collisions. In 1977 Davidsen et al. discussed the order-of-magnitude Lyα deficit with the following statement:
The most obvious explanation, that the ultraviolet lines are attenuated by absorption by dust similar to that observed in the interstellar medium, seems untenable in view of observations of Paschen α that indicate the hydrogen lines are unreddened. But, dust which is distributed within the line-emitting gas might destroy Lα without having much affect on the Balmer and Paschen lines if the nebula has high optical depth to Lα photons.
Whatever mechanism is at work, an understanding of the reduced Lyα / Hβ ratio may lead to a vastly improved knowledge of the physical conditions in QSO envelopes.
While this review is not focused upon QSOs, replacing “QSO" with “galaxy" in this statement comes precisely to the point. Today Lyα observations play a major role in understanding the interstellar and circumgalactic media of galaxies at effectively all redshifts.
The 1978 launch of IUE opened up the restframe UV to systematic study. IUE had a single 20′′ × 10′′ entrance aperture and Short and Long-Wavelength Prime channels (SWP and LWP, respectively) that could provide R ≈ 250 spectroscopy between 1150 and 3000 Å. First observations were again turned to QSOs and various Seyfert galaxies, and immediately showed Lyα to be weaker than expected for recombination theory, with Lyα / Hβ and Hα / Hβ values almost never falling along characteristic reddening curves, and Lyα always being preferentially suppressed (Oke & Zimmerman, 1979, Wu et al., 1980, Lacy et al., 1982). Interpretations varied, with suggestions that multiple scatterings of Lyα could be responsible, that broad-line regions experience a wide variation in their extinction laws, ionization/excitation from already-excited states, and that there may be no representative intrinsic spectral shape for Seyfert galaxies (Wu et al., 1980). Indeed Lacy et al., 1982 concluded that the low observed Lyα fluxes were likely the result of a combination of reddening, high densities, and high Hi optical depth. All these effects conspire in the same direction but as no single dominant quantities could be identified, it started to become clear that Lyα transfer in true astrophysical objects is a complicated multi-parametric process.
3.2. Star-forming Galaxies
When IUE was first turned to star-forming galaxies, the results directly mirrored those obtained in both nearby QSOs and also those being reported from high-z blind narrowband and spectroscopic surveys (see Pritchet, 1994, for a review, and the forthcoming PASA review in this series by S. Malhotra): Lyα was either absent or unexpectedly weak in all galaxies.
With the intent of studying the analogues of primeval galaxies at high-z, Meier & Terlevich, 1981 found Lyα in emission from just one of three nearby Hii-selected galaxies, and in that single case with a flux well below that expected for the nebular dust content (Figure 1). This led them to conclude that under normal conditions, Lyα emission would be an unlikely phenomenon. It was determined that if a normal prescription for dust attenuation were used to explain the Lyα / Hβ ratio, this would greatly over-predict the Hα/Hβ ratio compared to observation. Similar conclusions were reached by Hartmann et al., 1984, who furthered the discussion, showing how a mixed medium of Hi and dust could preferentially suppress Lyα, and that static Hi columns of density above 1019 cm−2 would be needed to reconcile the line ratios. Further, they raised the issue that, where Hi 21 cm data are available, most blue compact galaxies (BCGs) are found to sit inside extended Hi halos of sufficient column density to reproduce the measured fluxes. All signs pointed towards the fact Lyα emission would not be the good observational beacon to identify primeval galaxies in the early universe that Partridge & Peebles, 1967 had predicted.
Figure 1. The first Lyα spectra of Hii galaxies. Main transitions and cosmic ray hits are labelled. The upper panel shows Mrk 702 (C 0842+163), with a redshift of z = 0.0522 and a metallicity of Z ≈ 0.4 Z⊙. At the wavelength of λ = 1278Å, the galaxy shows only a Lyα absorption feature. The lower panel shows a very different object – C 1543+0907, with a redshift of z = 0.0366 and metallicity of 1/10 the solar value. At the wavelength of λ = 1260Å, the bright high-contrast Lyα emission line is obvious. Reproduced by permission of the AAS, from Meier & Terlevich, 1981.
While the influence of Hi on Lyα visibility was starting to be seen empirically, some correlation with the dust abundance should still be expected, albeit with a large spread. After subsequent data acquisition, the anticorrelation between Lyα EW and gas-phase metallicity (Z) was discovered (Hartmann et al., 1988, Calzetti & Kinney, 1992, Terlevich et al., 1993, Charlot & Fall, 1993), seemingly confirming the prediction. However the spread on the relation was worryingly large, and it was clear that something beyond variations in the extinction law (e.g. Valls-Gabaud, 1993) were behind the wide range of line ratios, with ISM geometry and holes being the most often cited scenarios.
Giavalisco et al., 1996 performed a full reanalysis of the IUE archival data, resulting in several changes. Firstly the IUE data reduction software reached a higher level of maturity, and new spectral extraction and cosmic-ray removal tools were implemented: this reduced the measured WLyα significantly in about half the sample, and completely removed the weak Lyα feature that had been reported in some galaxies. Secondly, these authors performed proper spatial matching between the IUE and apertures used for optical line spectroscopy. When the homogenized reanalysis was complete, both the WLyα and Lyα / Balmer line ratios showed no significant correlation with nebular dust attenuation or the UV continuum colour (β), and only a weak but significant correlation with nebular oxygen abundance. Results concerning the Lyα / Hα line ratios can be seen in Figure 2.
Figure 2. The observed ratio of Lyα / Hα fluxes, shown on a linear scale against nebular metallicity and the Hα/Hβ line ratio for the full sample of star-forming galaxies observed with the IUE, and reprocessed by Giavalisco et al., 1996. Negative Lyα / Hα ratios are the result of the Lyα feature being dominated by ISM absorption. The reference coding is MT81 = Meier & Terlevich, (1981), H84 = Hartmann et al., (1984), H88 = Hartmann et al., (1988), C94 = Calzetti et al., (1994), CK92 = Calzetti & Kinney, (1992), D86 = Deharveng et al., (1986), T93 = Terlevich et al., (1993). The dotted black line shows the effect of the Calzetti et al., (2000) dust attenuation curve, assuming intrinsic ratios of Lyα / Hα = 8.7 and Hα / Hβ = 2.86 and no scattering. Most of the galaxies lie far below this curve.
Regarding these correlations, it is not clear why Lyα throughput should be more strongly correlated with oxygen abundance than nebular attenuation. Metals alone do not absorb Lyα, which can only happen by interaction with dust grains. Thus if the WLyα–Z anti-correlation results from a positive correlation between metal and dust abundance, then a tighter correlation between Lyα / Hα and EB−V would be expected. This correlation is completely absent. Furthermore, when the dust reddening measured from the Balmer decrement is used to correct the observed Lyα for extinction, the Lyα flux does not reach the expected case B recombination value in a single galaxy in the IUE sample. This demonstrates that either some preferential attenuation of Lyα must be at play in every galaxy, or/and that dust attenuation laws, when applied as a screen of absorbing material, are not representative.
When interpreting these analyses of the IUE samples, it is important to keep in mind the selection functions by which the individual samples were established. When effective telescope areas are small and exposure times need to be long, the result is small samples. Some of the blue compact dwarf (BCD) and Hii galaxy studies were designed to study the analogues to ‘primeval' galaxies that are undergoing their first phase of star-formation, and the samples did not include galaxies with strong starbursting nuclei (Hartmann et al., 1984). Yet the primeval stages of galaxy formation, prior to the initial dust and metal production, are expected to be short-lived because the first generations of supernovae will enrich the local ISM on timescales of just a few Myr. Consequently the bulk of the galaxy population we can observe in the high redshift universe is likely to be significantly more metal-enriched than that of primeval galaxies (e.g. Pettini et al., 2002, Shapley et al., 2003), unless observations catch galaxies in very narrow time window. Thus while providing very interesting astrophysical laboratories, the samples are biased and not necessarily in a direction tuned to the real importance of Lyα: probing the faint population of normal galaxies at z > 2. The IUE samples contain few galaxies that can be considered the local analogues of high-z Lyman Break Galaxies (LBG), Lyα-emitters, or primeval galaxy building blocks.