ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney. Class 22 November 26, 2018 Ellip/cal Galaxies: Kinema/cs & Mergers

Transcription

1 ASTRO 310: Galac/c & Extragalac/c Astronomy Prof. Jeff Kenney Class 22 November 26, 2018 Ellip/cal Galaxies: Kinema/cs & Mergers

2 Key points on Ellip/cal galaxies Largest (most massive) galaxies in universe Oldest galaxies in universe most of their stars formed early in universe; the galaxy may have grown or changed since early universe Appear simple but are complex kinema/cs of stars some/mes reveal surprises 2

3 Ellip/cal galaxies 3

4 Key points on Ellip/cal galaxies Largest (most massive) galaxies in universe Oldest galaxies in universe most of their stars formed early in universe; the galaxy may have grown or changed since early universe Appear simple but are complex kinema/cs of stars some/mes reveal surprises 4

5 spectrum of ellip/cal (earth s atmosphere) UV blue red near- IR 5

6 Spectra of main sequence stars Cool + red low mass star long- lived like Sun Warm + yellow Hot + blue high mass star short- lived 6

7 model spectra of galaxies (stellar popula/ons) at different /mes aber burst of star forma/on simple stellar popula9on: bunch of stars of different masses, all with same age during burst lots of gas emission lines (like extreme Spiral) long a6er burst absorp/on lines from stars, no gas emission lines (like Ellip/cal) /me in Gyr aber start of burst ver$cal offset arbitrary! in reality the spectra for 5-18 Gyr are nearly on top of one another 7

8 model spectra of galaxies (stellar popula/ons) at different /mes aber burst of star forma/on Sparke &Gallagher Fig 6.18 similar plot as previous one BUT: a) ver/cal axis represents true flux vs /me b) this one also shows younger stellar popula/on c) this one smaller wavelength range 8

9 Cool + red spectra of main sequence stars Warm + yellow Hot + blue spectrum of ellip/cal 9

10 spectrum of ellip/cal 10

11 Compare the spectra of K giant star & S0 galaxy star (K giant) S0 galaxy Q: How are these spectra different? 11

12 Compare the spectra of K giant star & S0 galaxy star (K giant) S0 galaxy 2 differences: 1. galaxy spectrum is redshibed wrt MW star (expansion of universe) 2. lines broader in galaxy due to velocity smearing. 12

13 The observed profile of a stellar absorp/on line from a galaxy is generally complex because: 1. The distribu/on of stellar veloci/es at any point in a stellar system may be complex. (Stars in galaxies form a collisionless system, so the stars which pass through any point in a galaxy can have very different orbits.)(this is not true for gas, which is collisional, and generally has simpler kinema/cs) 13

14 The observed profile of a stellar absorp/on line from a galaxy is generally complex because: y x z 2. The observed lines are a weighted sum of stars within a cylindrical 3D volume: all stars along the line- of- sight (z direc/on) and within the spa/al resolu/on element (x+y direc/ons) 14

15 Within each resolu/on element extended in x & y direc/ons and at each depth z there are y stars on various orbits x Slice at depth z only the LOS (z) component of velocity is observed via doppler shib z direc/on = Line of sight (LOS) GALAXY 15

16 Introduc/on to kinema/cs for Ellip/cals Ordered mo9ons: v: mean velocity v = v rot + v noncirc measured by peak or mean of line Disordered mo9ons: σ: velocity dispersion, measured by linewidth The ra/o v/σ is used to compare the rela/ve importance of ordered and random mo/ons 16

17 small σ large σ 17

18 How many stars at each velocity along line- of- sight? the Line- of- sight Velocity Distribu/on (LOSVD) LOSVD Velocity dispersion σ - - fit LOSVD with gaussian (even if distribu/on is not gaussian!) LOS veloci/es of stars 18

19 Line- of- sight Velocity Distribu/on (LOSVD) Observed spectrum Spectrum of 1 star LOSVD Velocity dispersion σ - - fit LOSVD with gaussian (even if distribu/on is not gaussian!) 19

20 Line of sight velocity distribu/on (LOSVD) for spiral disk measure spectrum in small area of spiral km/s 0 km/s +200 km/s V los Small area of outer spiral disk Simple case Fast rota/on mo/on V los = 200 km/s Small random mo/ons σ los = 20 km/s V los /σ los >>1 FWHM V los V los = mean los velocity σ los = los velocity dispersion = FWHM/2.35 (if gaussian) 20

21 τ<100 Myr (O,B stars) ~100 pc random mo/ons of stars in disk of Milky Way is different in different direc/ons!! more random mo/on in radial direc/on than azimuthal and ver/cal direc/ons!

22 Stellar velocity dispersion in general is anisotropic (different in different direc/ons) σ R, σ φ, σ z for stars need not be equal this can happens for stars since they are collisionless par/cles that experience many collisions end up with equal random mo/ons in all direc/ons 22

23 LOSVD for ellip/cal km/s 20 km/s 0 km/s +200 km/s V los FWHM Small area of ellip/cal V Simple case los slow rota/on mo/on V los ~ 20 km/s large random mo/ons σ los ~ 200 km/s V los /σ los <<1 measure spectrum in small area of E galaxy V los = mean los velocity σ los = los velocity dispersion = FWHM/2.35 (if gaussian) 23

24 E galaxy rota/on vs. dispersion v<<σ NGC 1399 cd ellip/cal Center of Formax Cluster measure spectra at many posi/ons across E galaxy V rot ~30 km/s σ~250 km/s à v/σ ~0.1 is low!! Most mo/on in random direc/ons! v and σ ~ constant over much of galaxy (outside center) - > oben reasonable to characterize E galaxy with single value of v and σ 24

25 Complex case of LOSVD km/s 30 km/s 0 km/s +200 km/s V los FWHM Few stars with large counter- rota/onal mo/ons and small random mo/ons (accreted small galaxy?) V los Few stars with large Main component of line profile rota/onal mo/ons not symmetric and small random mo/ons not gaussian (disk component?) but V los < σ los For most stars random mo/ons dominate 25

26 Rota/ng both ways at once! the amazing galaxy NGC 4550 NGC 4550 Image looks featureless no dust and no ongoing star forma/on an Ellip/cal E7 (flavest known E) But the mo/ons of the stars reveal bizarre behavior WIYN BRI

27 Measuring Spectra in NGC 4550 WIYN Sparsepak fiber array spectrograph Obtain a spectrum at each of 90 posi/ons Measure mo/ons of stars from the Doppler shibs of their spectral lines Fiber posi/ons on image of NGC 4550

28 Spectra showing mo/ons of stars in NGC 4550 Each spectral line shows the mo/ons of stars, via the doppler shib Lines show 2 peaks one bunch of stars rota/ng toward us (blueshibed), the other bunch of stars rota/ng away from us (redshibed)

29

30 How did NGC 4550 form? Merger of gas- rich galaxy with pre- exis/ng stellar disk Counterrota/ng gas sevles to plane of rota/ng stellar disk Gas undergoes star forma/on once it sevles to disk plane, forming a counterrota/ng stellar disk! 30

31 Which Ellip/cal has odd core kinema/cs? A B C D Franx, Illingworth & Heckman 1989

32 Which Ellip/cal has odd core kinema/cs? A B C D Franx, Illingworth & Heckman 1989

33 Which Ellip/cal has odd core kinema/cs? Franx+1989 ATLAS 3D Krajnovic+2011 In NGC 4406 (M86), the inner and outer galaxy rotate about different axes! They differ by 90 deg. - > Evidence of merger Stellar velocity map of central +/- 30 in M86

34 Kinema/cally decoupled cores light intensity (~ surface density of stars) stellar velocity field (mean mo/ons of stars at each posi/on) gas driven to center in merger sevles to disk in plane different from outer galaxy undergoes star forma/on to form central stellar disk NGC 4365 SDSS image 34

35 Kinema/cally decoupled core in ellip/cal NGC 4365 NGC 4365 SDSS image Davies etal 2001 Inner 7 rotates orthogonally to main body of galaxy Inner 4 disky, outer part boxy Inner part v/σ =1.3 (disklike) 2% of total mass in central decoupled disk Shallow central cusp Overall galaxy is triaxial No shells or other morphological peculiari/es No sign of dust Age of stellar popula/on is ~14 Gyr in both decoupled disk and main body Center more metal rich Formed through old merger(s)

36 Are Ellip/cals Oblate Spheroids whose shape is governed by rota/on?.. 36

37 Are Ellip/cals Oblate Spheroids whose shape is governed by rota/on? Doved line: oblate spheroid flavened by rota/on 2 oblate spheroids with same σ : small v rot large v rot slightly ellip/cal, only a bit flavened by small v rot highly ellip/cal, flavened by large v rot 37

38 Are Ellip/cals Oblate Spheroids whose shape is governed by rota/on? Open circles: mid- sized E s (lower luminosity, M B >19.5) Filled circles: larger, luminous E s (M B <19.5) Doved line: oblate spheroid flavened by rota/on Projec/on effects: both V los and ε vary by appx sin(i) for inclined system 38

39 E galaxy rota/on vs. dispersion v<<σ NGC 1399 cd ellip/cal Center of Formax Cluster measure spectra at many posi/ons across E galaxy V rot ~30 km/s σ~250 km/s à v/σ ~0.1 is low!! Most mo/on in random direc/ons! v and σ ~ constant over much of galaxy (outside center) - > oben reasonable to characterize E galaxy with single value of v and σ 39

40 Are Ellip/cals Oblate Spheroids whose shape is governed by rota/on? Open circles: mid- sized E s (lower luminosity, M B >19.5) Filled circles: luminous (M B <19.5) E s Doved line: oblate spheroid flavened by rota/on (O.S.F.B.R.) Projec/on effects: both V los and ε vary by appx sin(i) for inclined system Most mid- sized E s (& bulges) consistent with O.S.F.B.R. Many luminous E s NOT O.S.F.B.R. but must instead be triaxial bodies flavened by anisotropic distribu/on of random veloci/es. Systems not fully relaxed by violent relaxa/on process, since fully relaxed system WOULD BE O.S.F.B.R. Orbits s/ll contain informa/on from /me of forma/on. 40

41 Triaxial galaxies: flavened by anisotropic velocity distribu/on Random mo/ons dominate ordered mo/ons (e.g. rota/on). But random mo/ons can have different amplitudes in different direc/ons (x,y,z). 41

42 Disky & boxy isophotes in Ellip/cals Schema/c diagrams of disky isophotes with a 4 /a=0.1 and boxy isophotes with a 4 /a=- 0.1 a a 4 Bender etal

43 V/σ vs. Disky- Boxy 1.0 (v/σ * ) 0.1 Open circles: mid- sized E s (lower lum., M B >19.5) Filled circles: luminous (M B <19.5) E s (v/σ * ) = (v max /σ) / (v/σ iso ) SG Fig 6.15 (Bender) Disky E s have high v/σ These galaxies contain significant disks (o.s.f.b.r.) in addi$on to dynamically hoier component Most boxy E s have low v/σ dynamically hot formed (at least partly) by mergers 43

44 V/σ vs. Luminosity (~stellar mass) Open circles: mid- sized E s (lower lum., M B >19.5) Filled circles: luminous (M B <19.5) E s (v/σ * ) = (v max /σ) / (v/σ iso ) low mass high mass SG Fig 6.15 (Bender) In low mass E s rota/on important In many high mass E s random mo/ons most important 44

45 V/σ vs. Luminosity and Disky- Boxy Open circles: mid- sized E s (lower lum., M B >19.5) Filled circles: luminous (M B <19.5) E s (v/σ * ) = (v max /σ) / (v/σ iso ) SG Fig 6.15 (Bender) Many high luminosity E s are triaxial bodies with low v/σ and boxy isophotes - > Could form through mergers of gas- poor galaxies (less dissipa/on during forma/on) Many low luminosity E s are oblate spheroids with high v/σ and disky isophotes à Could form through mergers of gas- rich galaxies (since gas sevles to a rota/ng disk before it forms many stars)(more dissipa/on during forma/on) Disky E s contain embedded stellar disks, may be part of con/nuous sequence with S0 s. 45

46 Disky ellip/cals are a natural extension of the sequence of galaxies with bulges & disks but livle cold gas or star forma/on. Disky E s have higher B/D than S0 s. Their bulges are rota/ng. Boxy ellip/cals are different. No disks. Not much rota/on. 46

47 Ellip/cal galaxies cd L>>L* (L~2-10L*) (most massive galaxies, in centers of clusters) generally triaxial ellipsoids & boxy luminous L~L* (L~0.5-2L*) generally triaxial ellipsoids & boxy midsized L<L* (L~ L*) generally oblate spheroids & disky No sharp cutoffs L* = 2x10 10 L 47 sun L MW = knee (break) in galaxy luminosity func/on

48 Disky- Boxy vs. Radio luminosity Open circles: mid- sized (lower luminosity, M B >19.5) E s Filled circles: luminous (M B <19.5) E s Boxy E s generally have the strongest radio AGN sources 48

49 Radio & other emission (synchrotron) Radio emission from AGN jet at galaxy nucleus Model of supermassive black hole With accre/on disk & jet 49

50 Radio (& other) emission from AGN in Centaurus A X- Ray jet from AGN (synchrotron) X- Ray: hot shocked gas from jet interac/on (thermal/shocked) Radio jet & lobe from AGN (synchrotron) X- Ray: hot gas (thermal) Colour composite image of Centaurus A, revealing the lobes and jets emana/ng from the ac/ve galaxy s central black hole. This is a composite of images obtained with three instruments, opera/ng at very different wavelengths. The 870- micron submillimetre data, from LABOCA on APEX, are shown in orange. X- ray data from the Chandra X- ray Observatory are shown in blue. Visible light data from the Wide Field Imager (WFI) on the MPG/ESO 2.2 m telescope located at La Silla, Chile, show the stars and the galaxy s characteris/c dust lane in close to "true colour. Credit: ESO/WFI (Op/cal); MPIfR/ESO/APEX/ A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Krab et al. (X- ray) 50

51 Disky- Boxy vs. X- Ray luminosity Open circles: mid- sized (lower luminosity, M B >19.5) E s Filled circles: luminous (M B <19.5) E s Boxy E s generally have most luminous X- Ray emission (the most hot ISM) 51

52 X- ray emission from hot gas (10 7 K) in ellip/cals X- Ray: hot gas Op/cal: stars In most disky ellip/cals much of the gas is cold. In most boxy ellip/cals much of the gas is hot (gas heated by stellar mo/ons or AGN). 52

53 Disky- Boxy vs. Radio & X- Ray luminosity Boxy E s generally have the strongest radio AGN sources and the most luminous X- Ray emission (the most hot ISM) not well understood 53

54 Evidence for accre/on & mergers in E s Kinema/cally dis/nct cores Shells & ripples & outer /dal features in stellar distribu/ons Major merger remnants resemble E s Disturbed dust & gas

55 Ellip/cal with shells Spiral NGC 470 Ellip/cal NGC 474 Regular image Image processed to highlight substructure 55

56 Merger remnant NGC 7252 (Schweizer 1981) & Merger simula/on equal mass disks (Hernquist 1992) Images of NGC 7252 at different depths (Schweizer 1981) Radial light profile resembles r 1/4 law in both merger remnant and simula/ons N7252 sim Sim Hernquist 1992

57 HI distribu/ons in Ellip/cals E s NGC 5903 & NGC 5898 guess the HI distribu/on in these 2 ellip/cals Van Gorkom etal 1986

58 Peculiar HI distribu/ons in Ellip/cals accreted gas? NGC 1052 E s NGC 5903 & NGC 5898 Van Gorkom etal 1986 Van Gorkom etal ellip/cals: one with no HI, the other with a highly disturbed HI distribu/on

59 Mass ra/os of mergers Equal mass (1:1): and major (<3:1) original stellar disks ~totally disrupted - > forms E* Intermediate- mass ra/o (3:1 10:1) original stellar disk of larger galaxy may (partly) survive, - > forms spiral or S0 with larger bulge* Minor (>10:1) stellar disk heated, not heavily disturbed (these are the most common) * If galaxies have gas, much of gas may sevle to new disk, undergo SF, form new stellar disk

60 Large frac/on (>50%) of E s show evidence for merger or accre/on more in boxy than disky, but also many disky à E s con/nue to increase their mass with age Structural complexity of E s is hidden within fundamental plane. Despite local irregulari/es, E s s/ll a.) obey virial theorem globally b.) have similar mass distribu/ons