Lyman alpha emitter

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As Lyman-alpha emitter ( LAE ), also Lyα emitter , is a class of extragalactic designated objects, due to their radiation emission in the Lyman-α-emission line , a spectral line of hydrogen to be found. More than 500 Lyman alpha emitters are currently known, most of which were found at high redshifts ( z  > 2). These are objects from the time of the formation and early development of galaxies , approx. 1–2 billion years after the Big Bang .

A few percent of the known Lyman alpha emitters have considerable angular diameters of 15 or even 30  arc seconds , which correspond to a physical diameter of up to 500,000  light years . This has given these objects the name Lyman Alpha Blobs ( LAB ; from the English blob for "blob").

Physical background and possible explanations

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

While the majority of the Lyman-Alpha emitters are likely to be associated with areas of intense star formation in the young galaxies, the current models for Lyman-Alpha-Blobs assume that the Lyman-Alpha radiation here comes from large amounts of “cool gas “(Temperatures 10,000 to 20,000  Kelvin ), which either flow into very massive galaxies or are ejected from them. The spectroscopically measured line widths of these objects correspond to speeds of up to 2,000 km / s. From this it can be concluded that the gas in the gravitational field is not in hydrostatic equilibrium .

Lyman alpha emitters observed

Partridge and Peebles (1967) first suggested finding high-redshift galaxies using their Lyman-alpha line. One can take advantage of this with the narrow band technique. Two filters are used for this, a narrow band filter and a broad band filter. If the Lyman-alpha galaxy has the correct redshift and the Lyman-alpha line falls into the narrow-band filter, it provides a strong signal there; the broadband filter covers the continuum , which is significantly weaker. Lyman alpha galaxies can now be found quickly and efficiently by comparing the fluxes in the narrowband filter and broadband filter. The first successful systematic searches using the narrow band technique began in the mid-1990s (Hu, McMahon 1996, Nature, 382, ​​281). In the meantime, hundreds of Lyman-alpha emitters have been found using this technology (e.g. Ouchi et al. 2008, ApJS, 176, 301). The method is particularly successful at very high redshifts (z> 5), e.g. For example, the galaxy with the highest redshift to date was found in this way (Iye et al. 2006, Nature, 443, 186).

The Lyman alpha line should be very strong in star-forming galaxies and therefore relatively easy to observe. It is in the ultraviolet region of the spectrum at 121.6  nanometers . Since the earth's atmosphere absorbs UV photons very effectively, only Lyman alpha photons that are red-shifted into the optical wavelength range can be observed from the earth's surface . Therefore, with ground-based telescopes, only Lyman alpha emitters are found that have a redshift of over z = 2. With redshifts above z = 7, the Lyman-alpha line migrates into the near infrared range , which in turn is more difficult to observe. In the meantime, Lyman-alpha emitters at z = 0.3 have also been found using the GALEX satellite (Deharveng et al., 2008 ApJ, 680, 1072)

The above searches not only found normal Lyman-alpha emitters, but also Lyman-alpha blobs. Two of the best-known Lyman alpha blobs were discovered in 2000 by Steidel et al. discovered at the Palomar Observatory near San Diego . An area was investigated in which there are many Lyman break galaxies with redshift 2.7 <  z  <3.4. The narrowband technique described above was used to search for Lyman alpha emitters at a redshift of z + 3.09. In addition to many compact Lyman-alpha emitters, two Lyman-alpha blobs were found. One of these objects is in the vicinity of a quasar with z  = 3.083.

Matsuda et al. discovered over 30 other, somewhat smaller LABs in the same field using the Subaru telescope of the National Astronomical Observatory of Japan . These Lyman alpha emitters together form a structure that is more than 200 million light years in size.

Additional Lyman alpha blobs were reported 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 largest recently discovered LAB ( Himiko ) was in 2009 by Masami Ouchi et al. described.

Open questions

Lyman alpha emitters are found by their strong Lyman alpha line. Since otherwise little information is available about the objects, not much can be said about them so far. However, observations suggest that the vast majority of compact Lyman alpha emitters are young galaxies that are at the beginning of their evolution and are currently producing a large number of new stars. Lyman alpha emitters are said to be closely related to Lyman break galaxies . Whether this picture is really correct and whether it applies to every redshift is the subject of current research.

Lyman alpha blobs, on the other hand, are largely not understood. It is currently not known whether they trace the density distribution of galaxies in the far redshifted universe (as is the case, for example, with far redshifted radio galaxies that have wide Lyman alpha halos ), or which mechanisms produce the emission lines, nor how the Lyman alpha blobs are connected to the surrounding galaxies. Lyman alpha blobs could provide further insight into the formation of galaxies.

See also

Individual evidence

  1. ↑ Nilsson, KK et al .: Understanding Lyman-alpha emitters . 2008, arxiv : 0904.3335 .
  2. ^ Charles C. Steidel et al .: Lyα Imaging of a Proto-Cluster Region at z = 3.09 . In: The Astrophysical Journal . tape 532 , 2000, pp. 170-182 , doi : 10.1086 / 308568 .
  3. ↑ Yuichi Matsuda et al .: A Subaru Search for Ly-alpha Blobs in and around the Proto-cluster Region at Redshift z = 3.1 . In: The Astronomical Journal . tape 128 , 2004, pp. 569 , doi : 10.1086 / 422020 , arxiv : astro-ph / 0405221 .
  4. ^ Subaru Telescope, National Astronomical Observatory of Japan: Giant Gas Clouds Illuminate Universe's Largest Structure. Retrieved May 10, 2009 .
  5. ↑ Paul J. Francis et al .: A Pair of Compact Red Galaxies at Redshift 2.38, Immersed in a 100 Kiloparsec Scale Lyα Nebula . In: The Astronphysical Journal . tape 554 , 2001, p. 1001-1011 , doi : 10.1086 / 321417 .
  6. ↑ Arjun Dey et al .: Discovery of a Large ~ 200 kpc Gaseous Nebula at z 2.7 with the Spitzer Space Telescope . In: The Astronphysical Journal . tape 629 , 2005, p. 654-666 , doi : 10.1086 / 430775 .
  7. ^ Nilsson et al., 2006, A&A , 452, 23.
  8. ↑ Masanori Iye et al .: A galaxy at a redshift z = 6.96 . In: Nature . tape 443 , 2006, pp. 186-188 , arxiv : astro-ph / 0609393v1 .
  9. ↑ Smith et al. , 2007, MNRAS, 378, 49.
  10. ↑ a b Masami Ouchi et al .: Discovery of a giant Lyα Emitter near the Reionization Epoch . In: The Astrophysical Journal . tape 696 , 2009, p. 1164–1175 , doi : 10.1088 / 0004-637X / 696/2/1164 , arxiv : 0807.4174v2 .