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Apart from very distant, ultraluminous quasars -evolving rapidly and
associated with galaxy mergers - there is likely another population of
quasars that evolves slowly
Aeons ago, the universe was different: mergers of galaxies were common
and gigantic black holes with masses equivalent to billions of times
that of the Sun formed in their nuclei. As they captured the surrounding
gas, these black holes emitted energy. Known as quasars, these very
distant and tremendously high energy objects have local relatives with
much lower energy whose existence raises numerous questions: are there
also such “quiet” quasars at much larger distances? Are the latter dying
versions of the former or are they completely different?
Light from distant quasars takes billions of years to reach us, so when
we detect it we are actually looking at the universe as it was a long
time ago. "Astronomers have always wanted to compare past and present,
but it has been almost impossible because at great distances we can only
see the brightest objects and nearby such objects no longer exist",
says Jack W. Sulentic, astronomer at the Institute of Astrophysics of
Andalusia (IAA-CSIC), who is leading the research. “Until now we have
compared very luminous distant quasars with weaker ones closeby, which
is tantamount to comparing household light bulbs with the lights in a
football stadium”. Now we are able to detect the household light bulbs
very far away in the distant past.
The more distant, the more luminous?
Quasars appear to evolve with distance: the farther away one gets, the
brighter they are. This could indicate that quasars extinguish over time
or it could be the result of a simple observational bias masking a
different reality: that gigantic quasars evolving very quickly, most of
them already extinct, coexist with a quiet population that evolves at a
much slower rhythm but which our technological limitations do not yet
allow us to research.
To solve this riddle it was necessary to look for low luminosity
quasars at enormous distances and to compare their characteristics with
those of nearby quasars of equal luminosity, something thus far almost
impossible to do, because it requires observing objects about a hundreds
of times weaker than those we are used to studying at those distances.
The tremendous light-gathering power of the GTC telescope, has
recently enabled Sulentic and his team to obtain for the first time
spectroscopic data from distant, low luminosity quasars similar to
typical nearby ones. Data reliable enough to establish essential
parameters such as chemical composition, mass of the central black hole
or rate at which it absorbs matter.
"We have been able to confirm that, indeed, apart from the highly
energetic and rapidly evolving quasars, there is another population that
evolves slowly. This population of quasars appears to follow the
quasar main sequence discovered by Sulentic and colleagues in 2000.
There does not even seem to be a strong relation between this type of
quasars, which we see in our environment and those “monsters” that
started to glow more than ten billion years ago”, says Ascensión del
Olmo another IAA-CSIC researcher taking part in the study.
They have, nonetheless, found differences in this population of quiet
quasars. "The local quasars present a higher proportion of heavy
elements such as aluminium, iron or magnesium, than the distant
relatives, which most likely reflects enrichment by the birth and death
of successive generations of stars,” says Jack W. Sulentic (IAA-CSIC).
"This result is an excellent example of the new perspectives on the
universe which the new 10 meter-class of telescopes such as GTC are
yielding,” the researcher concludes.
J. W. Sulentic1, P. Marziani2, A. del Olmo1, D. Dultzin3, J. Perea1 & C.A. Negrete4. "GTC Spectra of z ≈ 2.3 Quasars: Comparison with Local Luminosity Analogues". Astronomy & Astrophysics. DOI: 10.1051/0004-6361/201423975
1Instituto de Astrofísica de Andalucía, IAA-CSIC (Granada, Spain); 2INAF-Osservatorio Astronomico di Padova (Italy); 3Instituto de Astronomía-UNAM (México); 4Instituto Nacional de Astrofísica, Óptica y Electrónica (Puebla, México).