3 What is a galaxy? 2 A massive collection of stellar and interstellar matter - stars, gas, dust, isolated in space and held together by its own gravity. We reside within the Milky Way Galaxy, or just the Galaxy (note the capital G). When you look up, all the individual stars you see are those in our own Galaxy. Galaxies typically contain around 100 billion stars. There are billions of other galaxies in the universe.
4 A little history... 1610 Galileo Galilei used a telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars. Galileo Galile telescop 1785 William Herschel counted the number of stars in different regions of the visible sky and produced a diagram of the shape of the Galaxy with the Solar System close to the centre! 1920s Edwin Hubble produced photographs that resolved the outer parts of some spiral “nebulae” as collections of individual stars outside of our Milky Way. (He also discovered the expansion of the universe!).Edwin Hubblephotographs Sun takes 225 MY to orbit the Galaxy, we’re out in the suburbs! 2
5 Our home: The Milky Way 2 The Milky Way contains at least 100 billion stars and may have up to 400 billion stars. The neighboring Andromeda Galaxy contains an estimated one trillion (10 12 ) stars.Andromeda Galaxy Filling the space between the stars is a disk of gas and dust called the interstellar medium.ium. A 2010 measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7×10 11 M ☉ Mathematical models of the Milky Way suggests that the total mass of the entire Galaxy lies in the range 1-1.5×10 12 M ☉
6 Stucture of the Milky Way 2 The Milky Way system is a spiral galaxy consisting of over 400 billion stars, plus gas and dust arranged into three general componentsMilky Way The halo The nuclear bulge and Galactic Centre The disk Astronomers generally organise the interstellar medium and stars in the disk of the Milky Way in four spiral arms. All of these arms contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions and molecular clouds.H II regionsouds.
7 Stucture of the Milky Way 2 Astronomers generally organise the interstellar medium and stars in the disk of the Milky Way into spiral arms. All of these arms contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions and molecular clouds.H II regionsouds.
8 8 Our home: The Milky Way 2 30 kpc ~ 96,000 LY across
11 Orbital Motion 2 We observe a "flat rotation curve" at large radii Higher speeds at large radii mean you need to have more mass at large radii than is observed in the stars and gas alone... Therefore, galaxies must have extended dark halos.
Sagittarius A* discovered in 1974 is a radio source at the centre of our galaxy. Believed to be at the location of a 4.1 M Sun black hole 27,000 light years from Earth. Stars in orbit around Sgr A* observed to move at breakneck speeds (up to 1400 km/s). Further observational evidence for a SMBH in the Milky Way's nucleus from detection of an X-ray source at the position of Sgr A*. F SGR A*
Galaxies 1pc =3x10 13 km = 3.2 light years = 206,000AU Size ~30,000 LY. Mass ~10 Billion x Sun 70% spiral, 30% elliptical M33 - spiral galaxy M83 - Spiral galaxy M32 - Elliptical galaxy
The Closest Galaxy - Andromeda 2.5 million light years away Current measurements suggest that the Andromeda Galaxy is approaching us at 100 to 140 km/s. The Milky Way may collide with it in 3 to 4 billion years. The two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.Andromeda Galaxyin 3 tothe course of abo
Interacting galaxies Arp 148 is nicknamed Mayall’s object and is located in the constellation of Ursa Major, the Great Bear, approximately 500 million light-years away.
Milky Way, Andromeda and the Pinwheel are the 3 largest spiral galaxies, around 50 galaxies in total including dwarf galaxies. The Local Group
Clusters: Sizes 3.3-33 M LY. Mass ~ 10 14 -10 15 SM. Several 1000 galaxies. Galaxy groups & clusters Groups: < 50 galaxies. Sizes 3 to 6 MLY. Mass ~ 10 13 SM.
The Hubble Expansion 1929 Hubble found that galaxies are moving away from us and that their recessional velocity is proportional to their distance
This is the most distant optical image taken to date using the HST The galaxies you see are nearly 13 billion light yrs away These galaxies taken from 10 days of observing and are 4 billion times fainter than can be seen with the naked eye Galaxy formation
26 Why we need ground and space based observatories 26
Definition of an active galaxy Over ten billion (10,000,000,000) galaxies are visible with modern telescopes. Stars produce most of the light in these galaxies. Galaxies with luminosities 1000x greater than normal galaxies are called active galaxies. Extra light is confined to the central 1pc (~3.3 LY) (smaller distance than to our nearest star).
Taxonomy of AGN Discovered by Carl Seyfert in 1943. He found that a handful of spiral galaxies had very bright, point-like nuclei. Optical spectra had broad emission lines Called them Seyfert galaxies If brighter than M V <-23 then called a Quasar (although local Seyferts where the galaxy is visible are classified as Seyferts even if M V <-23.
The SKA project SKA scheduled for 2024. Site announced in May 2012. 4000 dishes!
Quasars – Discovery Discovered in 1950s by radio astronomers at Mullard radio astronomy lab in the UK during an all sky survey. In addition to normal radio galaxies they found some unidentified bright compact point sources. Optical identification difficult because several tens of optical counterparts in the radio position error box. In 1962, the source 3C273 was studied using occultations by the moon to an get accurate position and size of the radio emission region. Postion to 1 arc sec and showed the radio emission was doubled. Optical counterpart identified and optical spectrum taken in Dec 1962 at Palomar. observatory in 1963. Maarten Schmidt identified hydrogen emission in the broad emission lines. Distance.~950 Mpc (~3 BLY) and L=~5x10 12 L sun
Most luminous objects in the Universe! Luminosities up to 10 13 L sun Typical power of 10 40 W (~10 solar masses consumed per year). More than 300,000 known from all sky surveys. Despite being discovered at radio wavelengths, only 10% are radio loud, infact X-rays are the most efficient way to search for them! Found to be variable in brightness on scales of days, weeks and months Properties of Quasars
Standard model of Active Galactic Nuclei NLR 10pc – 1kpc BLR ~1pc
Black holes in galaxies Strongest evidence for black holes comes from observations of the centres of galaxies, including our own. Stars and gas near the centres of many galaxies are moving extremely rapidly, orbiting a very massive unseen object. Masses inferred are millions to billions times the mass of the Sun. Intense energy and short-timescale fluctuations point to massive compact objects These objects have extended radio jets reaching beyond the galaxy Explained by a huge central supermassive black hole accreting stars and gas from their surroundings
Black Holes Central engines of AGN and Quasars. Fed by infalling stellar material surrounding the black hole. Material forms an accretion disk before being sucked into the black hole
Supermassive black holes The giant elliptical galaxy NGC 4261 is one of the 12 brightest galaxies in the Virgo Cluster, located 45 million light-years away. The dark, dusty disk represents a cold outer region which extends inwards to an ultra hot accretion disk within a few hundred million miles of the suspected black hole.
Supermassive black holes The accretion disk weighs 100,000 times as much as our Sun. This object is about as large as our solar system, but weighs 1.2 billion times as much as our Sun. This means that gravity is about one million times as strong as on the Sun. Almost certainly this object is a black hole.
NGC 1316 The radio lobes span over one million light years 3C219 Radio lobes span several hundred thousand light years Jets and Lobes M87. This jet extends at least 5000 light years from the nucleus of M87
Quasar Jets The energy emitted from the jet in 3C273 probably comes from gas that falls toward a supermassive black hole at the centre of the quasar, but is redirected by strong electromagnetic fields into a collimated jet. Still not fully understood. The amount of energy deposited at knot one in the optical is the same as for a typical Seyfert galaxy! Jets can extend to several kpc, ie several 1000 LYs
Starburst galaxy M82 - IMBHs Chandra X-ray Observatory data in blue, optical data from the Hubble Space Telescope in green and orange, and infrared data from the Spitzer Space Telescope in red. First case where good evidence for more than one mid-sized black hole exists in a single galaxy. The evidence comes from how their X-ray emission varies over time and analysis of their X-ray brightness and spectra. Chandra image that shows the central region of the galaxy containing two bright X-ray sources X42.3+59 is located at a projected distance of 290 light years from the centre of M82. Mass is estimated to be between 12,000 and 43,000 times the mass of the Sun.light years X41.4+60 is located 600 light years in projection away from the centre of M82. Mass between 200 and 800 times that of the Sun, and tilted at an angle between 60 and 80 degrees, meaning that the accretion disk is viewed almost side-on.
Summary The Universe is composed of billions of galaxies, which in turn are composed of billions of stars! “If it's true that our species is alone in the universe, then I'd have to say the universe aimed rather low and settled for very little”. George Carlin