lecture15

Astro 13 Galaxies & CosmologyLECTURE 15 Thurs May 17 2001 D. Koo: 

Astro 13 Galaxies & Cosmology LECTURE 15 Thurs May 17 2001 D. Koo 10m I Ask An Astronomer Period 50m II Galaxy Properties (cont.) 10m III Break 35m IV Galaxy Properties (cont. Next Class: PICKUP Graded HW#4 and answers to HW#4; Lick Trip on 24 May still uncertain Lecture notes in course description on web: http://www.ucolick.org/~koo/course.html

Galaxies: Approach to Last 4 Lectures: 

Galaxies: Approach to Last 4 Lectures This is a large and complex subject. We will approach the study of galaxies by exploring various aspects of their properties and then using a few of them to expand the discussion to related issues: 1) MAJOR PROPERTIES: include the constituents of galaxies; sizes, luminosities, colors, shapes, etc. using the Milky Way as a proto-type. 2) MASSES: this fundamental property of galaxies leads to a discussion of Dark Matter, including various evidence for its existence & possible explanations of its nature. 3) NUCLEI: of galaxies may harbor massive black holes and lead to discussion of Active Galactic Nuclei (AGN), such as Quasars, radio galaxies, Seyfert galaxies. 4) ORIGIN & EVOLUTION: of galaxies (gas, AGN, distribut- ion) and use of galaxies as probes of cosmology

Galaxies: Introduction: 

Galaxies: Introduction Galaxy: A large agglomeration of stars, gas, dust, and other stuff bound by gravity and generally recognized as a major entity of the Universe. History of Galaxies: Though proposed as early as ~1750 as an “island universe”, the Milky Way was not generally recognized as but one of countless other galaxies until the early 1900’s. Edwin Hubble in ~1930 made the discovery of an expanding Universe and established the large distances for galaxies using Cepheid variables -- this initiated galaxies as a major focus of cosmological studies.

Overall Properties of Normal Galaxies: 

Overall Properties of Normal Galaxies How bright are galaxies? Their luminosities range from ~108 suns to 10,000x brighter of ~1012 suns. In general, most of this energy is in the opticaland come from stars that make up the galaxy. There are also emanations in: Radio from hydrogen and other molecules; Infrared from dust and cool stars and enshrouded hot stars; Ultraviolet from very hot stars; X-ray from exotic stars, hot gas, supernovae, etc. Typical brightness of galaxies is ~1011 suns, with 1012 suns being very rare (1%) and many galaxies with 108 - 1010 suns. The Milky Way is a typical galaxy in terms of brightness.

Overall Properties of Normal Galaxies: 

Overall Properties of Normal Galaxies How large are galaxies? Galaxies come in a wide range of sizes. Though in general brighter galaxies are larger, some galaxies are extremely compact (only a kpc or two in size) and yet are quite bright so that the appear almost star-like in images. Yet others are very, very diffuse, i.e. large for their brightness, and are thus barely detectable in the sky. Andromeda Galaxy

Slide6: 

Typical galaxies are like the Milky Way, with optical sizes of 10’s kpc to ~100 kpc, and with a brighter center and fainter outskirts and extended cool gas best seen in the radio or hot gas in X-rays. Sizes may be different (often larger) at other wavelengths. OPTICAL IMAGE M83 RADIO IMAGE

Overall Properties of Normal Galaxies: 

Overall Properties of Normal Galaxies How many galaxies are there? By taking a census of large volumes of space, astronomers can determine the answer for intrinsically bright galaxies 109 suns or brighter. We find about 10 galaxies for every box of roughly 10 Mpc on a side (1000 Mpc3). For intrinsically dim galaxies, numbers are hard to come by since they are so difficult to see, but they may be 100x more numerous --- perhaps 1000 for every box of 10 Mpc on a side. Observable universe has 100 billion galaxies, where one has to image to very faint limits (~100 million times fainter than the limit of an unaided eye) to see these distant galaxies One Billionth Of the Sky.

Overall Properties of Normal Galaxies: 

Overall Properties of Normal Galaxies How are galaxies distributed? We already discussed this in Lecture 4 when we talked about cosmological principles and the homogeneity of the U. Galaxies are sociable. They often appear in pairs, in small groups like our own Local Group, in small clusters to larger clusters of galaxies, with even larger metropolis’s of superclusters of clusters. These groupings range in size from 10’s kpc for groups and pairs, to perhaps 10’s Mpc (1000 x larger) for superclusters. Note also that these large scale distributions may have preferred shapes similar to sheets, soap bubbles, filaments, etc. -- NOT RANDOM.

Slide9: 

Local Group to Homogeneous Universe

Overall Properties of Normal Galaxies: 

Overall Properties of Normal Galaxies What shapes/forms (morphologies) do galaxies have? There is enormous variation due to inherent differences as well viewing angle by the observer. Broad categories of galaxy morphology (known as Hubble Types) include Ellipticals, spirals, irregulars, and peculiars. Elliptical Spiral Spiral Spiral Irr Peculiar

Slide11: 

But there are many subdivisions, including presence of bars, smoothness, openess of spiral features, relative prominence of the disk and central bulge, distribution or diffuseness of the outer parts, presence of rings, etc. Most of the classification work has been done in the optical. In contrast, images taken in the radio, infrared, and X-ray range show different and complex patterns, so that shape is not well defined. OPTICAL RADIO

How do Galaxies Move?: 

How do Galaxies Move? There are two very different types of motions. First there are internal motions for individual galaxies and then there are motions of two or more galaxies relative to each other. Both kinds of motions are controlled by gravity and mass distribution. For INTERNAL MOTIONS, i.e. how do the constituent stars and gas move, one finds in general: 1) Disks rotate, with inner part faster than the outer parts (called Differential Rotation), so unlike a CD or merry-go-round. 2) Spherical structures have random (in-out/around) motions.

Slide13: 

DISK Differential Rotation Solid rotation Sphere Random Motions

Slide14: 

For relative motions, ie galaxies relative to each other, most motions are fairly random, but some exhibit organized Bulk Motions. Random Motions at few 100 km/s Cluster of Galaxies Bulk motion up to 100’s km/s

Milky Way - home : 

Milky Way - home Our home galaxy, the Milky Way, provides a number of advantages if used for studying galaxies: MW is nearby and can thus be studied in detail MW is a typical galaxy Type (spiral) MW has a luminosity of ~1011 suns MW has a typical size of ~ 30 kpc MW has a typical environment as part of Local Group MW has a typical subset of components: bulge, halo, disk Major disadvantages include: MW provides only one perspective (inside disk) MW is subject to distorted view due to dust Galaxies show a huge diversity of properties Cannot see the past history directly

Major Parts of Galaxies and MW: 

Major Parts of Galaxies and MW DISK BULGE & HALO Very flat (1:100 or 300pc:30kpc) Contains Gas & Dust Spiral structure and arms, maybe also a bar and ring Contains young to old stars Star Formation is still undeway Organized motion -- Differential rotation Lots of heavy elements (~ sun) Rough spherical shape w sizes of ~50 kpc Very little to no gas & dust No obvious substructure Virtually only very old stars No star formation for ~10Byr Random motions in 3-D 1/10 to 1/100 of solar system

Nuclei of Galaxies: 

Nuclei of Galaxies Except for irregular galaxies, most are centrally concentrated. Milky Way, e.g., has about 50,000 stars in a volume of 1pc3. This is about 1,000,000 x higher number density than near the Sun. To penetrate the obscuration by dust, astronomers exploit the infrared and radio wave telescopes to study the center of galaxies (roughly in the constellation of Sagittarius). We find: 1) high concentration of gas and stars 2) rapidly swirling gas in tiny volumes (~5 pc) 3) unusual radio pattens suggestive of energetic events 4) plausible that a 106 solar mass blackhole resides there

Slide19: 

But overall, the vast amount of radiation and energy from the Milky Way is in the optical, with some in the radio and X-rays. ===> mainly starlight What about other galaxies? Vast majority are similar to the Milky Way, but several classes of galaxies or objects have been discovered to emit enormous amounts of radiation from the nucleus or in huge globs, not only in the optical and near-IR, but also X-ray and radio. These unusual objects are cosidered to belong to a class called Active Galactic Nuclei (AGN) and depending on various properties, are further subdivided into : Radio, Seyferts, BL Lacs (Blazars), Quasars, QSO’s, etc.

Slide20: 

Radio Galaxies: Discovered by radio astronomers in the 1950’s; among nearby galaxies, but such galaxies are among very distant galaxies as well. Seyfert Galaxies: Discovered in early 1940’s by astronomer named Seyfert as a galaxy (spiral) with unusually broad emission lines from its bright cucleus. The broadness of the emission lines reflects any rapid internal motions of the gas. Quasars: Discovered in the early 1960’s as star-like (quasi- stellar) objects with strong radio emission, I.e. quasi-stellar radio sources or Quasars for short. These objects were found to be at enormous distances away, with the brightness equivalent to 1000 normal galaxies. QSO’s and BL Lacs (Blazars): By the late 1960’s, huge number of objects similar to Quasars in optical properties were found. These AGN do NOT have strong radio emission, and were thus called Quasi-stellar objects (QSO’s). BL Lacs share variability.

Images of AGN’s: 

Images of AGN’s RADIO GALAXIES Optical M87 Radio Image of Radio Source Blazar/BL Lac QSO Seyfert

Slide22: 

Together they comprise roghly a few% of all galaxies and have in common, possibility of a supermassive (100 million to 1 billion) solar mass black hole in their nucleus that generates the enormous energy observed. Photon Energy ==> Brightness AGN Radiation Normal galaxy, with Planck curve Synchrotron Radiation

Slide23: 

This radiation is not like a blackbody (Planck) that come from stars, but instead is the result of high speed particles swirling in a strong magnetic field. Synchrotron Radiation -- AGN radio radiation Magnetic lines of force Particle (electron) For comparison to our Milky Way galaxy, which is used as the reference of 1 unit in each waveband: Kind of Object X-ray Optical Radio Radio Galaxy 102 - 104 2 103-106 Seyfert Galaxy 102- 105 2 102-106 Quasar-QSO 106 250+ 107

Slide24: 

This in turn is the result of energy emitted by infalling gas and stars from an accretion disk into a huge black hole. Black holes are extremely efficient engines, being able to convert ~10% of the rest mass energy of infalling gas to outbound radiation. Thus a QSO with 100x Milky Way needs to feed 1 solar mass per year of gas to a billion solar mass blackhole. Due to radiation pressure, the infall would stop if the AGN became too luminous and the resultant relation can be used to estimate the masses of Black Holes. This luminosity maximum is called the Eddington Limit, = 3x104 (M/Mo) Lo where Mo = solar mass and Lo = solar luminosity.

Slide25: 

Hypothesis of BlackHoles with swirling gas is consistent with key properties of AGN: A) High Luminosity: Typically 10-1000x Milky Way B) Unusual Radiation -- non-stellar: Relatively large amounts of radio and X-ray. C) Rapid Internal Motions Swirling gas around HUGE blackhole now. D) Rapid and large variability of Light Typically less than a year; implies sizes < 1LY E) Common Presence of Jets emanating from the nucleus Radio jets can extend out to a million light years

What Makes an AGN?: 

What Makes an AGN? Disturbed shapes and frequent signs of recent galaxy collisions suggest interactions and mergers as probable cause. Such events may force a bite of food (gas and stars) into the waiting mouth of a massive black hole that may sit quietly for 100 Million years. The period of Activity , I.e. fed black hole, may last for a brief time, perhaps 10’s million of years. And depending on whether the host galaxy is elliptical or spiral, with large or small amounts of disk gas, and our viewpoint being obscured or aligned with the beams of energy, we may see a quasar, radio galaxy, or BL Lac. This last idea is called the Unification Model for Active Galaxies.

Slide27: 

Supermassive BH Receding Jet Approaching Jet Obs sees Blazar Obs sees a QSO Obs sees a radio galaxy AGN Class May Depend on Viewpoint

Importance of AGN: 

Importance of AGN A particularly interesting and important aspect of AGN’s is that they can be easily seen to vast distances, ie large lookback times, corresponding to the early youth of the Universe: AGN’s provide a direct Glimpse of the Past -- a billion years after the Big Bang. AGN’s show direct evidence for evolution of the U & thus support an evolving Big Bang cosmology. They may also affect the formation of all galaxies -- perhaps every galaxy has been an AGN one or more times in the past. They serve as probes of intervening gas and dark matter AGN’s may help measure the geometry of the U, Ho, and .