Lyman break technique
With the Lyman break technique , an observation method of modern astrophysics , large amounts of strongly redshifted galaxies can be found very efficiently . As is often the case in modern astronomy , these galaxies are named after their discovery method: Lyman break galaxies .
Galaxies with a high proportion of massive stars show strong emissions in the ultraviolet range of their spectra . This part of the spectrum shows two noticeable leaps:
- one at 912 angstroms (resting wavelength ), d. H. at the Lyman limit; this jump is also called the Lyman break , it is the namesake of the method;
- one at 1216 angstroms; here is the Lyman line with the highest wavelength, it is called Lyman-alpha .
Between these two wavelengths there is a decrease in flux caused by the discrete absorption lines of the hydrogen- Lyman series ( Lyman-alpha forest , resonance absorption ). These lines do not come primarily from the galaxy itself, but from the intergalactic medium .
Light with a wavelength below 912 Angstroms ionizes neutral hydrogen , which means that this short-wave light of the galaxy is practically completely absorbed by interstellar and intergalactic gas .
Filters used
The redshift shifts the two UV continuum jumps into the easily observable range of optical wavelengths. Now the spectral properties of these galaxies can be used to find them. These galaxies can be distinguished from other objects with three color filters :
- the first filter lets light through below the Lyman limit
- the second filter lets light through between the Lyman limit and the Lyman alpha line
- the third filter lets light through above the Lyman alpha line.
For each object, the measured flows in the three filters are compared with one another. In this way the galaxies can be identified relatively easily. The choice of filters determines the redshift range in which the selected galaxies are located. For example, the filters U (corresponds to the color UV or violet), B (corresponds approximately to the color blue) and V (corresponds to the color green) select galaxies with a redshift of z ~ 3.
Advantages and disadvantages
The advantage of the process is that you no longer have to examine the spectrum of each galaxy individually (which is difficult with faint galaxies), but can search entire areas of the sky at the same time. The method has so far also proven to be robust; the proportion of incorrect identifications is relatively low. Tens of thousands of galaxies have so far been found using this method.
One disadvantage of the method is that it only finds one type of galaxy: the galaxy must have massive stars that produce enough UV flux. However, massive stars have a short lifespan. The Lyman Break galaxies must therefore have, or have recently had, a high rate of star formation . Lyman break galaxies are therefore not representative of the entire galaxy population at high redshifts.
Similar procedures
The technique can also be used at redshifts higher than z = 3. However, the spectrum of a typical UV-bright galaxy changes here: the Lyman-alpha forest is getting stronger and stronger. Practically only the jump at 1216 Angstroms can be seen, because below it the spectrum of the galaxy is almost completely absorbed. The galaxies are now only found using the drop-out technique : they can no longer be seen in the short-wave filter, but they can be seen in the long-wave filter.
Analogous to the Lyman break technique, the BzK break technique or the BzK break galaxies also exist . In this process, attention is paid to the 4000 Ångström break and a continuous UV spectrum , i.e. filters in the B, Z and K band are used , which gives the technology its name.
Furthermore, one can also speak of the Balmer break technique or Balmer break galaxies , if the drop in the spectrum of the Balmer series at 3648 Angstroms is taken as a filter criterion through appropriate filters .
See also
swell
- ↑ Archived copy ( memento of the original from October 1, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Archived copy ( Memento of the original from June 10, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Archived copy ( Memento of the original from June 10, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Johan Fynbo: Lyman-break Galaxies. (No longer available online.) Archived from the original on April 8, 2009 ; accessed on May 10, 2009 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Charles C. Steidel : Mapping the Distant Universe - Color Technique for Finding High Redshift Galaxies. Retrieved May 10, 2009 .
