Quasars are a few of the brightest objects within the Universe. The brightest ones are so luminous they outshine a trillion stars. However why? And what does their brightness inform us concerning the galaxies that host them?
To attempt to reply that query, a bunch of astronomers took one other take a look at 28 of the brightest and nearest quasars. However to know their work, we’ve to again monitor somewhat, beginning with supermassive black holes.
A supermassive black gap (SMBH) is a black gap with greater than 1,000,000 photo voltaic plenty. They are often a lot bigger than that, too; as much as billions of photo voltaic plenty. One in all these entities resides on the heart of most galaxies, excluding dwarf galaxies and the like.
They’re the results of the gravitational collapse of an enormous star, and so they occupy a spheroidal chunk of area from which nothing, not even mild, can escape.
The Milky Means has one in every of these SMBHs. It’s referred to as Sagittarius A-star (Sgr A*) and it’s about 2.6 million photo voltaic plenty. However Sgr A* is fairly sedate for a SMBH. Different SMBHs are way more lively, and so they’re referred to as lively galactic nuclei (AGN.)
In an AGN, the black gap is actively accreting matter, forming a torus of gasoline that heats up. Because it does so, the gasoline emits electromagnetic radiation, which we will see. AGNs can emit radiation all throughout the electromagnetic spectrum.
Artist’s impression of ULAS J1120+0641, a really distant quasar powered by a black gap with a mass two billion occasions that of the Solar. Quasars are essentially the most highly effective sort of lively galactic nuclei. Credit score: ESO/M. Kornmesser
There are sub-classes of AGNs, and a brand new research centered on a kind of sub-classes referred to as quasars. A quasar is essentially the most highly effective sort of AGN, and so they can shine with the sunshine of a trillion Suns. However a few of these quasars are hidden behind their very own torus, which blocks our line of sight. In research of quasars, these ones are ignored or omitted, as a result of they’re tough to see.
However that creates an issue, as a result of omitting them from the inhabitants of quasars means we is perhaps lacking one thing. It additionally signifies that one of many central questions round quasars may not be addressed correctly.
The query is basically multi-pronged: are these extraordinarily vibrant AGN powered by reasonable accretion onto extraordinarily large black holes? Or are they powered by excessive accretion onto extra reasonable mass black holes? Or possibly one thing else is occurring. Are they powered by a number galaxy transitioning from a star-forming galaxy to one thing extra sedate like an elliptical galaxy? By ignoring or omitting the quasars which might be tough to see, it makes discovering any solutions tough.
Swift’s Arduous X-ray Survey gives the primary unbiased census of lively galactic nuclei in many years. Dense clouds of mud and gasoline, illustrated right here, can obscure much less energetic radiation from an lively galaxy’s central black gap. Excessive-energy X-rays, nevertheless, simply move via. Credit score: ESA/NASA/AVO/Paolo Padovani
A group of astronomers checked out 28 AGN that have been each close by and among the many most luminous. Most of them occurred to be in elliptical galaxies. The one standards for selecting them was the extraordinary exercise of their nuclei. Their radio emissions span components of tens of 1000’s, and their plenty additionally cowl a variety. The astronomers wished to seek out out if these vibrant AGN had another distinctive qualities which might set them aside from decrease luminosity obscured AGN.
What Did They Discover?
Their are some intriguing and shocking outcomes on this research. A number of the outcomes appear to agree with different research, whereas some go in opposition to the grain.
The group doesn’t have photographs for all the host galaxies of their research, however the ones that they do have photographs for are all elliptical galaxies, or at the least bulge-dominated morphologies. That contrasts with different research of lower-luminosity quasars, and in addition with the expectation that at the least a few of the 28 host galaxies can be spirals.The host galaxies span a fairly big selection of plenty, with a focus of comparatively excessive plenty. These greater plenty, and the excessive luminosities, coincide with the transformation of lively star-forming galaxies to extra quiescent, spheroid galaxies.There’s a nice range in radio emissions within the 28 chosen AGN, which suggests there are not any “clear and strong defining traits for our sort of sources,” as they are saying of their conclusion.The vary in x-ray luminosity and black gap plenty can’t account for the huge vary of radio wave luminosity.Essentially the most luminous and obscured sources within the pattern aren’t powered by both low mass black holes with excessive accretion charges, or by massive mass black holes with decrease accretion charges.
Within the conclusion of their paper, the authors summarize their findings, and plainly for now, at the least, there isn’t a clear clarification for these most luminous of quasars that shine with the sunshine of a trillion stars.
“We discover that, as a bunch, our pattern of a few of the most luminous obscured AGN in BASS/DR1 doesn’t exhibit any distinctive properties with respect to their black gap plenty, Eddington ratios, and/or stellar plenty of their host galaxies.”
In addition they level out that the host galaxies are largely all ellipticals, a shocking discover. If this discovering might be corroborated by different researchers, “… it could lend some oblique proof in assist of the favored concept that epochs of intense SMBH progress are linked to the transformation of galaxies from (star-forming) disks to (quenched) ellipticals (i.e., via main mergers).”
There are 21 researchers behind this research, at establishments together with the Harvard and Smithsonian Middle for Astrophysics, Tel-Aviv College, Kyoto College, JPL, the Naval Observatory, the ESO, and plenty of others. The information for his or her research comes from the 70 month Swift/BAT all-sky survey, and with observations utilizing the Keck, VLT, and Palomar observatories. The research is titled “BAT AGN Spectroscopic Survey – XIII. The character of essentially the most luminous obscured AGN within the low-redshift universe.” It’s printed within the Month-to-month Notices of the Royal Astronomical Society.