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Galaxy Properties Globular Cluster Evolution Galaxies in Color and Star Formation List of Properties of Ellipticals and Spirals Surface Brightness Profiles Dark Matter Winding Dilemma Differences in Kinematics Stellar Populations Hubble Classification Scheme Revisited References • Kutner – Chapter 17 in substantial detail!! • Chaisson – Chapter 23 second half Chapter 24 first half • Read 2 articles on Dark Matter Observational Cosmology • Original aim: use galaxies as probes to study cosmology • Solve for Ho and qo and Λ • Problem – galaxies do evolve Æ Stars inside a galaxy do evolve ÆOverall luminosity and colors of galaxies change ÆNeed to understand contents of galaxies first Galaxy contains • • • • • • Stars – several populations Gas (ionized, atomic, and molecular) Dust (cold dust, and warm dust) Stellar remnants Non Baryonic Dark Matter AGN -- sometimes Question: How do you study these components – at what wavelengths do you see what? How do galaxy appearances change as galaxies age? Step I: (today) Understand Galaxies nearby Step II: (next week) Look back in time Questions M87 & M84 Color is roughly right – reddish tint Elliptical Galaxies are rather different from Spirals: Elliptical Galaxies • What can you say about the stellar populations of Elliptical Galaxies? Integrated galaxy luminosity and galaxy colors Spirals in Color Dominated by the colors of the most luminous stars Spiral galaxies display ongoing star formation in the spiral arms. Many stars in the arms are young stars. The most luminous stars dominate the overall color of those regions. These are the young hot (blue!!) stars. Thus the spiral arms tend to have a bluish hue. Young Cluster: Blue stars are still on main sequence Old Cluster: The most luminous stars, i.e., the blue, hot, and young stars have evolved off the main sequence and have terminated their lives. Æ Age of Galaxy correlates to the time since the last major star formation epoch. Galaxy Colors Dominated by the colors of the most luminous stars Young Cluster: Old Cluster: Blue stars are still on main sequence The most luminous stars, i.e., the blue, hot, and young stars have evolved off the main sequence and have terminated their lives. Æ Age of Galaxy correlates to the time since the last major star formation epoch. Aging galaxy clusters Dust in Spirals Galaxy Colors Galaxy type Elliptical Sa Sb Sc Irregular B-V color 0.95 0.65 0.55 0.4 0.3 red bluish Question: What do these colors tell us about the last star formation epoch? The Light from Regions where Intervening Dust blocks our view are REDDENED (like the Sun at sunset) Details showing HII regions and some Reflection Clouds. The bright regions in the arms are star forming regions. The massive, hot, young and luminous stars dominate the overall light, thus giving it a bluish hue. The bright yellowish looking regions, mostly the bulge, also have some young stars, however, since there is relatively more dust in the bulge, we see only the longer wavelength light that can penetrate through the dust. M 83 M 101 Careful – always look at relative colors The central regions are somewhat redder, while the spiral arms are bluer. The Spiral Arms get fainter and fainter and extend FAR out. Reddening – caused by Dust NGC 891 Ground based Observation together with HST overlay Whirl Pool Galaxy M51, series of 3 pictures Whirl Pool Galaxy – M 51 – series of 3 pictures Drawn before films existed Whirl Pool Galaxy – M 51 – Nucleus NGC 1365 – Spiral Arm Detail NGC 253 Pretty Details NGC 4314 – Nuclear Ring NGC 7742 – Ring Galaxy Irregular and peculiar galaxies • Early studies in the 1980’s • Term: Starburst Galaxy • Mixture of bluer colors due to how young stars and interstellar reddening due to higher fractions of interstellar dust… • Wider spreads of galaxy colors and luminosities Integrated Colors • • • Use broad band filters Look at UV color and at a red color U-V & V-K U-V blue See relative amounts of massive stars See relative amounts of low mass red stars And a few red supergiants V-K red The Antenna Antenna - contiuned NGC 1365 – barred Spiral WFPC/Nicmos Observations Infrared Galaxy Gallery Notice that the Spiral structure goes all the way to the center of the Galaxy. The Bar also goes to the center. The Spiral Structure • What causes the spiral structure? • First look at some characteristics of the spiral arms • Then determine if spiral arms can “wind up” A Galaxy at 21 cm – Atomic Hydrogen Notice that the Visual and the 21-cm almost anti-correlate A Galaxy in the Visual The Rotation of Galaxies and the Spiral Structure Constant velocity beyond 5 to 10 kpc What causes the Spiral Structure? The Winding Dilemna One rotation of the Galaxy takes roughly 200 million years (2 x 108 years) The age of the Galaxy is roughly ~20 billion years (2 x 1010 years) How many revolutions would you have? Is this observed? What is the age of the galaxy? How often has it rotated around its axis during its life-time? # of revolutions = age of universe time of one orbit # of revolutions = 2 ⋅1010 yrs = 100 2 ⋅108 yrs What would you expect the galaxy to look like after 100 revolutions? The Density Wave The Spiral Density Wave M87 & M84 Color is roughly right – reddish tint Elliptical Galaxies – but there is something a little strange Elliptical Galaxies are rather different from Spirals: • They do Not have a Disk; • They are tri-axial (comparable to a football); • They do not have dust; • They do NOT have hot young stars; most stars have formed a long time ago; • Their colors are red not because of reddening by dust, but because mostly old red stars are left. • Ellipticals are believed to be “old”, i.e. “evolved” galaxies. Elliptical Galaxies Challenges • • • • Problems with Orientation Resolution (Distance) Edge on Spirals – classify by size of bulge Lenticulars versus Ellipticals – hard to distinguish • Ellipticals (Orientation!!) b EN = 101 − a Minor axis Major axis Understanding Galaxy Formation and Evolution Global Properties – Ellipticals ~20 to 25 % First Step: Classify Galaxies according to some scheme that makes sense. • • • • • • • • • Second Step: Design a more meaningful scheme as we learn more… Study galaxy contend in detail and look for connections: • Amounts and Distributions of Stars, Gas, Dust, etc • Star Formation Histories as related to Stars, Gas, Dust • Astrophysical Processes responsible for observed colors, spectra, etc • Presence of a particular type of “Active Nucleus” # of stars 105 to 1013 Masses up to 1013 solar Masses Magnitudes -9 to -24 Sizes 1 to 200 kpc Dust < 1% Hot gas in some ellipticals Stars all population II Colors B-V ~ 0.95 Æ “red” Metallicity > 2%; hight ~ 3 times solar Surface Brightness Profile Studying Ellipticals Light distribution falls off exponentially Ellipticals can be described by deVaucouleurs r1/4 law. I (r ) = I (ro )e − r ro 1 4 Surface Brightness Profiles How does this profile change, if: • There is a black hole in the center? • It is a cD galaxy? I (r ) = I (ro )e − r ro 1 4 Giant elliptical Galaxy in the center of a cluster that has swallowed up others Global Properties – Spirals ~ 60 to 70% • • • • • • • • • # of stars 104 to 1012 Masses up to 1012 solar Masses Magnitdes -7 to -22 Sizes 5 to 50 kpc Dust ~ 10% Warm gas Stars all population II in halo; Population I otherwise Colors B-V ~ 0.3 to 0.9 Æ bluer than ellipticals Metallicity varies though roughly solar The Rotation of Galaxies and the Spiral Structure Surface Brightness Profiles for Spirals Fit by an exponential disk and a separate bulge component Constant velocity beyond 5 to 10 kpc This is based on empirical models Mass distribution falls off much more slowly Can calculate density of dark matter Kutner section 17.4 Æ M/L ratio increases with distance Disk Galaxies 2 GM (r ) v (r ) = for circular orbit r2 r dM = 4πr 2 ρ (r ) density distribution dr v (r ) = vo const velocity further out vo2 ρ (r ) = 4πGr 2 • • • • Bulge Disk Halo Dark Matter HW and Reading Mass to Light Ratio • Density of halo falls off as 1/r2 • Light profile falls off exponentially Æ there is Dark Matter Mass-to-Light ratio in center of spiral galaxies: 1:3 Mass-to-Light ratio at edge of visible disk: 1:20 Mass-to-Light ratio farthest point measured: 1:100 What is this dark Matter? Question: What happens to spirals if they are stripped of their gas? • A) cols Æ redder • B) arms disappear Æ SO Global Properties Irregulars and Peculiar galaxies ~ 3% • • • • • • • • • # of stars 104 to 1010 Masses up to 1010 solar Masses Magnitudes -7 to -18 Sizes 1 to 10 kpc Dust > 10% Warm gas Stars population I – active star formation Colors B-V ~ 0.3 to 0.7 Metallicity varies though roughly solar, often less • “Does Dark Matter Exist?” By Milgrom – Nice overview of evidence for DM – Some wacky ideas about MOND • “Dark Matter in the Universe” by Rubin – Article by one of the leaders in the field – More traditional view • Post your Opinion about DM by Tuesday • Articulate a question you have Global Properties – Lenticulars ~ 10% • • • • • • • • Spiral galaxies without spiral arms but a disk and a bulge # of stars 104 to 1012 Masses up to 1012 solar Masses Magnitdes -7 to -22 Sizes 5 to 50 kpc Dust ~ 10% Warm gas Stars all population II in halo & Bulge; Population I otherwise • Colors B-V ~ mostly 0.9 Æ “red” • Metallicity varies though roughly solar to 3 times solar Dwarf Galaxies • Do not fit classification scheme • Not always sure if disk or spheriodal galaxies • Sometimes irregular • Low surface brightness • • • Disk Galaxies Kinematics – Spirals versus Ellipticals Kinematics Flatening due to Rotation Bulge Disk Halo • Disk in Spirals – flattened due to Rotation • • • • Ellipticals – some rotate far too slowly Shape of Ellipticals is kept up by velocity dispersions Some Triaxial Some are “Boxy” • Have prolate and oblate elliticals NGC 4650A Polar Ring Galaxy with dust lane Star Formation Histories in Ellipticals and in Spirals Major differences between Ellipticals and Spirals Shapes: Ellipticals – biaxial footballs (some may be triaxial) Spirals: Have a disk with spiral arms, a bulge and a halo. Stellar populations & Ages: Very efficient Star Formation in Ellipticals Spirals have Gas and Dust and Young Stars – continual star formation Ellipicals are devoid of gas and dust and can no longer form young stars Ellipicals have old stars – but they have high metallicity Kinematics: Ellipticals: Velocities of stars in ellipticals are more or less random Velocity dispersions are responsible for the overall shape of galaxies. Oblate and Prolate Ellipticals – how that? Spiral: Velocities of stars in spirals are more ordered. Stars rotate around the galactic center in a disk surrounding it – Halo is random. Spiral galaxies are flattened by rotation (ellipticals are not). Modelling galaxy interactions Hubble’s Original Proposal: There is an evolutionary connection Could this be correct? Arguments for & against… Discussion Movie 1: GALINT Movie 2: Galcollision1 Movie 3: Merger2 Transforming Galaxies Nature versus Nurture? Nature versus Nurture • One interpretation: Elliptical galaxies just from as ellipticals Details of how clouds collapse into ellipticals is different from that of spirals (remember in E’s star formation is efficient) • Other Interpretation Dynamical evolution of Spirals into ellipticals Did Ellipticals form though mergers? • Pro Simulations Observe bridges and Tails Enhanced star formation in interacting pairs Rotation not correlated to flattening Some ellipticals have dust lanes (how did that get there?) But cD’s multiple nuclei & extended Halos (cannibalism) Nature: Initial conditions Nurture: Mechanisms • Merging • Galactic Cannibalism • Harassment • Slow Starvation • Galaxy – Intercluster Medium Interactions • Effects by the Gravitational Cluster Potential as a Whole • Galaxy Infall Did Ellipticals form though mergers? • Contra Star formation during merger must have been very efficient Metallicity gradient E’s in core of dense clusters – velocities too high for interaction Ellipticals have color luminosity relationship # of globular clusters – E’s have 3x that found in Spirals How did field ellipticals form E’s should be systematically larger than S Dwarf E’s how did they form?