Lyman-Alpha-Emitter

As Lyman-alpha emitter (LAE ), also Lyα emitter, a class of extragalactic objects is called, which are due to their emission of radiation in the Lyman - α emission line, a spectral line of hydrogen, found. Currently, more than 500 Lyman-alpha emitters are known to have been mainly at high redshifts z> 2, have been found. So there are objects from the time of the origin and early evolution of galaxies around 1-2 billion years after the Big Bang.

A few per cent of the known Lyman-alpha emitter will have substantial angular diameter of up to 15 or even 30 seconds of arc, corresponding to a physical diameter of up to 500,000 light years. That these objects, the name of Lyman-alpha blobs; introduced (LAB from the English blob for "blob ").

Physical background and possible explanations

The Lyman - alpha spectral line is formed in the recombination of electrons with ionized hydrogen atoms when the electron from the first excited state to the ground state. Conditions for a strong emission in the Lyman - alpha line are, for example, in star-forming regions, where hydrogen gas is ionized by the intense UV radiation of young stars.

While the majority of the Lyman-alpha emitters should be associated with areas of intense star formation in young galaxies, the current models for Lyman-alpha blobs are assuming here that the Lyman - alpha radiation of large amounts of " cool gas " ( temperatures 10,000 to 20,000 Kelvin) is due either to flow in a very massive galaxies or be ejected from it. The spectroscopically measured line widths of these objects correspond to high speeds of up to 2,000 km / s It can be concluded that the gas is not in hydrostatic equilibrium in the gravitational field.

Observed Lyman-alpha emitters

Partridge and Peeples (1967 ) first suggested, to find redshift galaxies using their Lyman - alpha line. This can be exploited by the so-called narrow-band technique. For this, two filters are used, a narrowband filter and a broadband filter. If the Lyman -alpha galaxy has the right redshift and the Lyman -alpha line falls in the narrow-band filter, it ensures a strong signal in narrow-band filter. Of the wide-band filter covers the continuum, which is significantly weaker. Lyman -alpha galaxies can now be found quickly and efficiently by comparing the fluxes in the narrow-band filters and broadband filters. The first successful systematic search using the narrowband technology began in the mid 1990s ( Hu, McMahon 1996, Nature, 382, 281). Meanwhile, Lyman -alpha emitters are hundreds with this technique found (eg Ouchi et al. 2008, ApJS, 176, 301). Particularly successful is the method at very high redshifts (z > 5 ), eg the galaxy with the highest ever redshift was found ( Iye et al. 2006 Nature, 443, 186).

The Lyman - alpha line should be very strong in star-forming galaxies and thus be relatively easy to observe. The Lyman - alpha line is in the ultraviolet region of the spectrum at 121.6 nanometers. Since the Earth's atmosphere very effectively absorbs UV photons can be observed from the surface only Lyman-alpha photons are redshifted into the optical wavelength range. Therefore, only Lyman-alpha emitters with ground-based telescopes found that have a redshift of about z = 2. Red shifts in about z = 7 Lyman -alpha line migrates in the near infrared region, which is in turn observed worse. Meanwhile, Lyman -alpha emitters at z = 0.3 have been using the Satellite Galex found ( Deharveng et al., 2008 ApJ, 680, 1072 ).

In the search above not only normal Lyman -alpha emitters have been found, but also the so-called Lyman -alpha blobs. Two of the most well-known Lyman-alpha blobs were in 2000 by Steidel et al. at the Palomar Observatory, discovered in San Diego, California. Thus a range has been studied in which there are many Lyman -break galaxies at redshift 2.7

Matsuda et al. discovered in the same field using the Subaru Telescope of the National Astronomical Observatory of Japan and more than 30, slightly smaller LABs. These Lyman-alpha emitters together form an entity that has an extension of more than 200 million light years.

More Lyman-alpha blobs were by Francis et al. (2001), Dey et al. (2005), Nilsson, et al. (2006), Iye et al. ( 2006), and Smith & Jarvis et al. (2007) discovered. The most recently discovered LAB ( Himiko ) was founded in 2009 by Masami Ouchi et al. described.

Open Questions

Lyman-alpha emitters are found by their strong Lyman - alpha line. Because otherwise have little information about the objects present, can so far not much to be said about them. Observations can but conclude that the vast number of compact Lyman-alpha emitters are young galaxies that are in their infancy and are producing very many new stars. Lyman-alpha emitters should be closely related to the so-called Lyman -break galaxies. Whether this is really true and if it applies at each redshift, is the subject of current research.

The extended Lyman-alpha emitter, the so-called Lyman-alpha blobs, however, are poorly understood. It is currently not known whether Lyman-alpha blobs, the density distribution of galaxies in the far - red-shifted universe trace ( as is the case for example with far - redshift radio galaxies, the broad Lyman-alpha haloes possess ), or which mechanisms the emission lines produce, nor how the Lyman-alpha blobs are connected to the surrounding galaxies. Lyman-alpha blobs could provide further insights into the formation of galaxies.