Active Galactic Nuclei (AGN) Active Galactic Nuclei
◼ Many galaxies have active nuclei, with jets, X-ray and gamma-ray emission. ◼ Spectral lines show large Doppler shifts, indicating velocities up to 10 - 20% of the speed of light.
Centaurus A Layover: Cosmic Dust
◼ Cosmic dust is made of particles of various sizes, from a few molecules to sand-like grains.
▪ Dust particles have random shapes. ▪ Most of the dust is very cold: 10 – 50K (-440 – -370oF). Dust Properties
▪ Chemically, these particles belong to 2 distinct types: ▪ Silicates: sand (Si,O,H) ▪ Graphites: soot (C,O,H) ▪ Light absorption:
Heavy
None Why Is Plume Red?
◼ A: Because redwood is being burned.
◼ B: Because fire is red, it paints the smoke red too.
◼ C: Because soot absorbs blue light more than red.
◼ D: Because it is a sunset, and the Sun is red at sunset.
◼ E: Because the smoke is hot, it glows red. Centaurus A
Visible Near infrared Active Galactic Nuclei
◼ There are many types of AGN: DRAGNs, Seyfert galaxies, LINERs, quasars, blazars. ◼ LINERs and Seyfert galaxies are the weakest of all AGN – their nuclei are dimmer than their host galaxies. ◼ DRAGNs (Double Radio-source AGN) are the next class of AGN – their nuclei are brighter than their host galaxies. ◼ Quasars and blazars are the brightest of them all – the AGN outshines its host galaxy completely. Active Galactic Nuclei
◼ AGN are distinguished by strong and wide emission lines in their spectra. Reminder: Spectral Lines Width Of Spectral Lines
◼ Sometimes spectral lines are narrow, sometimes they are wide. Why? Width Of Spectral Lines
◼ Doppler blue- and red-shifts of individual atoms set the width of the spectral line. ◼ The faster the atoms move, the wider the line. ◼ It does not matter why atoms move – because the gas is hot and atoms move randomly relative to each other or because the gas rotates really fast in the disk. ◼ Conclusion: ❖ Narrow lines: cold gas and slow rotation ❖ Broad lines: hot gas and/or fast rotation. Width Of Spectral Lines
◼ Conceptually:
◼ A bit more precisely:
◼ Bottom line: from one number (line width) we cannot determine 2 numbers (T and vrot). Seyfert Galaxies
◼ Carl Seyfert (1911-1960) was one of the first astronomers who studied AGN. He died in an auto accident. ◼ The most common class of AGN – Seyfert galaxies, is named after him. ◼ There are two types of Seyfert galaxies: I and II. Why? That’s a topic for a later story.
NGC 1068 DRAGNs (Double Radio-source AGNs)
◼ DRAGNs are defined by their strong radio emission (Cen A is a DRAGN poster child). ◼ Radio emission comes from two “radio lobes” well outside the host galaxy.
Centaurus A Story Of Quasars
◼ First quasars were discovered in the radio – powerful radio sources with no “optical counterpart” (no star or galaxy at this location). ◼ As optical telescopes got better, two most bright quasars (3C48 and 3C273) were identified with faint blue “stars” whose spectra made no sense with only unknown spectral lines. ◼ Maarten Schmidt was able to identify them as lines of normal hydrogen, but redshifted by 16%. AGN Story Of Quasars
◼ Quasars were unresolved – they appeared as stars, not galaxies. ◼ They also varied on time-scales of years and even less, so have to be small. ◼ Two possible explanations: A. A faint star nearby, but then how to explain a large redshift. B. If redshift is due to the Hubble expansion, then quasars must be very far away and hence extraordinary luminous – in fact, brighter than any galaxy!
AGN Story Of Quasars
◼ Because they look like stars, they were initially called “quasi-stellar objects” or QSO. The long name was later shortened to a “quasar”. ◼ Quasars are often found in colliding (aka “merging”) galaxies.
AGN Story of Quasars
◼ Galaxies “under the quasars” have indeed been observed with the Hubble Space Telescope. Story of Quasars
◼ Galaxies “under the quasars” have indeed been observed with the Hubble Space Telescope. What Powers AGN?
◼ In order to outshine the whole galaxy completely, the nucleus must have a darn powerful source of energy. ◼ A typical large spiral galaxy (like the Milky Way) contains 100 billion stars, the average star is about ½ solar luminosity, so a typical 10 galaxy shines as 50 billion suns (5x10 L8). ◼ Burning HgHe releases 0.7% of rest energy. Stars only do it in their cores (inner 10% by mass). ◼ Hence, stars as a whole convert about 0.07% of their rest energy into light. What Powers AGN?
◼ The nucleus cannot contain more mass than the galaxy as a whole. Hence, whatever produces the energy in the center must convert rest energy into light more efficiently than 0.07%. ◼ How much more? Measuring Masses
■ In a rotating system, centrifugal force balances gravity:
■ This is simply the third law of Kepler! Thank You, Kepler!
◼ Cancel one M:
◼ Re-arrange:
◼ A miracle! We got the mass of something we cannot put on a scale or even reach! Rotation Curve
◼ Rotation curve plots orbital velocities of rotating bodies versus their distance from the center.
◼ A rotation curve for the Solar system has a definite shape – Keplerian rotation curve. Masses of AGN Nuclei
◼ AGN nuclei masses are measured by the same Kepler’s law. Masses of AGN Nuclei
◼ AGN nuclei masses vary, but on average are about 0.5%-1% of the mass of the galaxy.
◼ Whatever powers the AGN must be 10 times more efficient than nuclear reactions. A Case Study: Our Own Milky Way Karl Jansky (1905 – 1950)
◼ U of Wisconsin undergrad.
◼ Built a radio antenna on a rotating platform – “Jansky's merry-go-round”.
◼ In 1933 he discovered radio emission from the center of the Milky Way galaxy – he called it Sagittarius A object.
◼ He died at the age of 44 – probably, missing the Nobel Prize by only a few years. Reber Radio Telescope
◼ First radio telescope in the world – build by Grote Reber (1911 – 2002) in IL from Karl Jansky’s blueprints.
◼ An astronomical unit of radio brightness – Jansky is named after Karl. Center of the Milky Way in Radio
◼ Sagittarius A (Sgr A) object actually consists of 3 different things: an old hyper-nova remnant (Sgr A East), a cloud of gas (Sgr A West), and the true center of our Galaxy: Sagittarius A*.
Next image Center of the Milky Way in Radio Center of the Milky Way in X- rays
◼ There is a lot of X- ray activity at the 20 pc very center of the Milky Way.
◼ It appears as if Sgr A* is blowing hot gas away from the Galactic plane. Stellar Orbits Around Sgr A*
◼ Since 1996, astronomers were tracking orbits of individual stars around Sgr A*.
◼ Stars move very fast near it – up to 5,000 km/s. Question
◼ From the sizes and periods of stellar orbits around Sgr A*, we can determine:
◼ A: masses of orbiting stars.
◼ B: mass of Sgr A*.
◼ C: mass of the Milky Way galaxy.
◼ D: density of gas at the Milky Way center.
◼ E: that astronomers have nothing better to do. Mass of Sagittarius A*
◼ From stellar orbits, in particularly S2, we can measure the mass of Sgr A* - recall Kepler’s third law:
6 ◼ Result: MSgr A* = (3.3±0.7)×10 M8 (3.3 million suns). Rather large for an object less than 0.37AU in radius! ◼ Gas is blowing away
◼ In the midst of Milky Way.
◼ Stars whiz by like specs of light,
◼ Hurled by colossal might.
◼ What can take on such a role?
◼ Make a guess – it’s a …black hole! Hypervelocity Stars
SS433