Gli astronomi di solito riescono a sentire le galassie lontane attraverso radiazioni ad alta energia come i raggi X. È estremamente raro ricevere radiazioni UV di energia relativamente bassa da galassie antiche come AUD01. I fotoni a bassa energia vengono solitamente assorbiti lungo il percorso o dall'atmosfera terrestre. Hubble Space Telescope (HST) has been very helpful in avoiding effects of earth’s atmosphere but even HST could not detect signal from this galassia probably due to noise.
Ora, l'imaging ultravioletto telescopi on Indian satellite AstroSat has detected extreme UV light for the first time from the galassia AUDFs01 situated 9.3 billion light-years away from Earth which is remarkable1.
Oggi siamo in grado di esaminare il universo e vedere stelle ed galassie si è formato miliardi di anni fa perché il mezzo intergalattico è trasparente alla luce. Non fu così per diverse centinaia di milioni di anni dopo il Big Bang. Il periodo chiamato dagli astronomi Secolo Buio Cosmico era il periodo in cui il mezzo intergalattico era pieno di gas neutro che assorbiva fotoni ad alta energia e creava il universo opaque to light waves. It was the period starting from the time when cosmic microwave background radiation emitted up to the time when first stelle ed galassia were formed. The universo then entered into what is called the Epoch of Reionization, when the dark matter started to collapse due to its own gravity and eventually started forming the stelle and the galaxies.
I cosmologi si riferiscono allo spostamento verso il rosso z per designare un'epoca cosmica. Il tempo presente è indicato con z=0 e più alto è il valore di z ci si avvicina al Big Bang. Ad esempio, z=9 denota un momento in cui universo was 500 million-year-old and z=19 when it was only 200 million-year-old, near Dark Age. At higher z values (z ≥ 10) it becomes extremely difficult to detect any object (star or galassia) due to sharp decline in inter galactic medium transmission. Scientists have been able to observe quasars and the galaxies up to z approximately equal to 6.5. Theories suggest that the stelle and the galaxies could have been formed much earlier at say higher z values and with advancement in technology we should be able to detect fainter objects at higher z values also [2]. However, most of the detection of galaxies are limited to approximately z=3.5 and are detected in the X-rays range. It is extremely difficult to detect the stars and galaxies in the extreme ultraviolet as it is heavily absorbed in the atmosphere.
Group of scientists led by Saha at Inter-University Centre for Astronomy and Astrophysics (IUCAA) were able to achieve this unique feat using Ultraviolet Imaging Telescope (UVIT) aboard the Indian satellite AstroSat . They observed the galassia AUDFs01 located in the Hubble Extreme Deep field using extreme-UV light from the galassia. It could be possible because the background noise in the UVIT detector was much less than the ones on HST. The discovery is important as it opens up a new domain for detection of far off galaxies in the EUV range.
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Riferimenti:
- Saha, K., Tandon, S.N., Simmonds, C., Verhamme, A., Paswan A., et al. 2020. AstroSat detection of Lyman continuum emission from a z = 1.42 galassia. Nat Astron (2020). DOI: https://doi.org/10.1038/s41550-020-1173-5
- Miralda-Escudé, J., 2003. L'età oscura dell'universo. Scienze, 300(5627), pp.1904-1909. DOI: https://doi.org/10.1126/science.1085325
