Archivi tag: Doppler effect

Exploring the properties of the Universe by Doppler lensing

Sappiamo che l’Universo contiene centinaia di miliardi di galassie, basti guardare le spettacolari immagini che ci ha fornito il telescopio spaziale Hubble. Ce ne sono tante di diverse forme e dimensioni, ma quali sono in definitiva quelle più grandi? E poi, quali sono quelle più vicine alla nostra galassia che sembrano apparentemente più grandi delle altre? Naturalmente, non è possibile rispondere a queste domande analizzando semplicemente le immagini astronomiche poichè, di fatto, è necessario conoscere le distanze a cui si trovano le galassie in modo tale da ricavare una stima delle loro dimensioni reali.

Astronomers have their ways to measure a distance to a galaxy which allows them to solve this conundrum. One of the most popular methods, and in most cases, the only method that can be used to measure a distance to a remote galaxy, is to analyse its electromagnetic spectrum which includes the visible light that enables us to see it. Since the Universe is expanding, all distant galaxies are moving away from us. Because of this motion the spectrum of a galaxy is shifting towards its red part, the redshift as it is known to astronomers. The redshift phenomenon is a manifestation of the Doppler effect, the faster the motion, the larger the shift of the frequency. Therefore, the larger the redshift, the greater the distance to the observed galaxy. The exact relation between the redshift and distance follows from the cosmological model of the Universe. So if astronomers can measure a distance in some other way, then by comparing the observed distance and redshift with a prediction, they can measure the properties of our Universe such as for example the amount of dark matter and dark energy. There is, however, one problem here.

If a galaxy is moving on the top of the global expansion of the Universe, then this motion, via the Doppler effect, contributes to the observed redshift. And galaxies move all the time, just as molecules of the air, or bees within a swarm. The contribution from this local motion is not big if compared to a motion that follows from the expansion of the Universe. Still this additional redshift introduces noise to our measurements. This noise then distorts our estimation of the distance, and therefore our estimation of the real size of the observed galaxy. This is what is called the Doppler lensing, “Doppler” because of the Doppler effect involved, and “lensing” because this effect distorts the inferred size, just as the observed size of an object is distorted when observed through an optical lens. How then can we tell what is the real size of a galaxy? If all galaxies are moving and if their motion distorts our measurements then that sounds like a real mess. However, this “mess” or to be precise the amount of “messiness” can give us a very good insight into what our Universe is made of. Astronomers are now in a situation similar to radar operators who during World War II complained about “noise” in returned echoes due to rain, snow, and sleet. Back then it was a nuisance, now we actually look for this “noise” in order to predict weather. Similarly, if astronomers could measure apparent sizes of a very large number of galaxies, and correlations between them, then they could estimate an average amplitude of the “noise”. Using the technique based on the Doppler lensing effect, they can measure properties of our Universe and estimate how much dark matter and dark energy it contains.

With large galaxy surveys such as Dark Energy Survey (DES) and the contribution from the Australian OzDES we will be able to measure this effect. Further, much larger surveys will follow after completion of the Square Kilometre Array (SKA) telescope, currently being built partly in Western Australia and partly in South Africa, and utilise the Doppler lensing effect to get a better insight into properties and mysteries of our Universe. The calculations and the method itself were recently developed by a group of astronomers from Australia, South Africa, and United Kingdom. The method shows how by measuring correlations in the distortion of sizes of galaxies we can learn about the properties of our Universe (such as amount of dark matter and dark energy). This method and predictions that follow from this method will be presented today at the 8th Workshop of the Australian National Institute for Theoretical Astrophysics (ANITA) hosted by the Sydney Institute for Astronomy (SIfA) at the University of Sydney.

The Conversation: The measure of the universe through doppler lensing

arXiv: Cosmology with Doppler Lensing

Discovered a new type of black hole quasar-like object

E’ noto che ogni galassia ospita nel suo nucleo un buco nero supermassiccio spesso circondato da un disco di accrescimento super brillante composto di gas a temperature elevate che dà luogo alla fenomenologia tipica dei quasar. Ora, un gruppo di ricercatori della Penn State University hanno individuato con grande sorpresa una nuova classe di quasar distanti la cui esistenza non è prevista dagli attuali modelli che descrivono le proprietà e la fenomenologia dei nuclei galattici attivi.

The gas in this new type of quasar is moving in two directions: some is moving toward Earth but most of it is moving at high velocities away from us, possibly toward the quasar’s black hole“, said study co-author Niel Brandt, Distinguished Professor of Astronomy and Astrophysics at Penn State. “Just as you can use the Doppler shift for sound to tell if an airplane is moving away from you or toward you, we used the Doppler shift for light to tell whether the gas in these quasars is moving away from Earth or toward these distant black holes, which have a mass from millions to billions of times that of the Sun“. Matter around these black holes forms a quasar disc that is bigger than Earth’s orbit around the Sun and hotter than the surface of the Sun. These quasars generate enough light to be seen across the observable Universe. The international research team, led by Patrick Hall of York University in Toronto, discovered the unusual quasars with data from a large sky survey, the Sloan Digital Sky Survey (SDSS-III). “Matter falling into black holes may not sound surprising“, said Hall, “but what we found is, in fact, quite mysterious and was not predicted by current theories“. Such gas is found in only about 1 out of 10,000 quasars, and only 17 cases now are known. “The gas in the disc must eventually fall into the black hole to power the quasar, but what is often seen instead is gas blown away from the black hole by the heat and light of the quasar, heading toward us at velocities up to 20 per cent of the speed of light“, Hall said. “If the gas is falling into the black hole, then we don’t understand why it’s so rare to see infalling gas. There’s nothing else unusual about these quasars. If gas can be seen falling into them, why not in other quasars?” Hall noted there is one other possible explanation for these objects. “It could be that the gas moving away from us is not falling into the black hole but is orbiting around it, just above the disc of hot gas and is very gradually being pushed away from the black hole“, he said. “A wind like that will show gas moving both toward us and away from us. To make an analogy, imagine an ant on a spinning merry-go-round, crawling from the center to the edge. You will see the ant moving toward you about half the time and away from you about half the time. The same idea could apply to the gas in these quasars. In either case, the gas in these quasars is moving in an unusual fashion“. Models of quasars and their winds will have to be revised to account for these objects. To help understand what revision is needed, the research team is observing these quasars further using Canadian and American access to the Gemini-North telescope in Hawaii.

PSU : Researchers discover new type of black hole quasar


arXiv: Broad Absorption Line Quasars with Redshifted Troughs: High-Velocity Infall or Rotationally Dominated Outflows?