Stellar Populations: Resolved vs. unresolved

Transcription

1

2 Outline

3 Stellar Populations: Resolved vs. unresolved Individual stars can be analyzed Applicable for Milky Way star clusters and the most nearby galaxies Integrated spectroscopy / photometry only The most common case in extragalactic astronomy

4 Stellar Evolution

5 For resolved stellar populations: Colour-magnitude diagram Colour-magnitude diagram of two open clusters

6

7 Intermission: What do the colour indicies mean? U B V R I (Y) J H K

8 The Stellar Luminosity Function Quantifies the luminosity distribution of stars in a stellar population i.e. stars per luminosity bin within. a star cluster, a galaxy or a subcomponent of a galaxy (e.g. the disk)

9 The Stellar Initial Mass Function (IMF) If you know the lifetimes of stars of different masses, you can use the the observed stellar luminosity function to say something about the IMF. The IMF is often expressed in power-law form: dn M dm dn is the number of stars per mass interval dm. = 2.35 represents the slope of the Salpeter (1955) IMF. This classical IMF is usually assumed to be a reasonable fit to stars of mass M> M in the local Universe.

10 The Stellar Initial Mass Function (IMF) II Mass range of stars: M Popular choices for the local IMF: Kroupa (2001) and Chabrier (2003) both predict far fewer M<1 M stars than the Salpeter IMF

11 Star Formation Rate (SFR) Elliptical galaxy SFR Spirals Globular cluster Time

12 Spectral synthesis: Putting it all together

13 Spectral synthesis II Example of model prediction: Colours as a function of age

14 Intermission: What do these galaxy spectra tell you?

15 Star formation

16 Cosmic star formation history Star formation rate (SFR) per comoving volume The cosmic SFR has dropped since z 2, i.e. for about 10 Gyr

17 Indications of star formation I

18 Recombination emission lines n= n=4 n=3 n=2 Ionization Lyman lines Balmer lines H H n=1 UV Optical Ground state

19 Recombination emission lines Flux Wavelength But beware: Other processes (shocks, black hole accretion etc.) can also contribute to emission line fluxes

20 Emission-line equivalent width How strong are the lines relative to the continuum? High equivalent width Low equivalent width Flux Flux Wavelength Wavelength High equivalent width (EW) in hydrogen recombination lines indicates presence of high-mass stars (M>10-20 M ) with lifetimes < 20 Myr For instance, high EW(Hα) young or actively star-forming system

21 Recombination emission lines 42 SFR( M /yr) = L solar Hα (erg/s)

22 UV continuum Young, massive stars are hot High UVluminosity L UV can (in analogy with L H ) be related to SFR Hot star Flux Cool star UV Wavelength

23 IR Thermal Continuum Hot, young star UV radiation Dust grains Black-body reradiation (20-40 K) In IR High L IR /L B indicates high star formation

24 Radio continuum emission Recall: Dust extinction is not an issue for radio observations

25 CO from Molecular Clouds

26 Intermission: What wavelength range? Andromeda at four different wavelengths

27 The Interstellar medium

28 Dust extinction Reddening of the spectrum Before contact with dust Flux After contact with dust Wavelength

29 Dust extinction II

30 Star Formation Made Simple Gas cloud Collapse due to self-gravity Temperature rises Thermonuclear reactions kick in A star is born

31 When Does Star Formation Occur? Internal pressure Gravity wins when Length > Jeans length, J : J = v G Gravity Or equivalently, when mass > Jeans mass, M J : Gas cloud M M J = 6 3 j m

32 When Does Star Formation Occur? M<M J ensures stability on small scales On larger scales, regions of size D are prevented from collapse by disk rotation if: D D critical = 2G 3 2 Surface Density Angular velocity Low-surface brightness disks fulfil this criterion!

33 Star formation triggers

34 Negative Feedback from Star Formation A Wolf-Rayet star (high-mass star with huge ionizing flux and mass loss due to winds)

35 Star Formation Efficiency Typically less than 10% of the available gas is converted into stars before feedback prevents further star formation Star formation rate (assumed constant during star formation Star formation episode) efficiency Duration of star formation episode SFR = M 0.1 H 2

36 Starburst Galaxies M81 & M82 Starburst Galaxy M82

37 Intermission: What are you witnessing here?

38 Starburst Galaxies M82 in X-rays

39 Recommended Definitions of Starbursts Starbursts are transient phenomena unless new gas is added!

40 Starburst galaxies Lots of research in Uppsala in past 30 years on these t gas = M gas SFR

41 Starburst Galaxies

42 Galaxy Interactions & Mergers

43 Signs of interaction: Shells

44 Signs of Interactions: Warps

45 Signs of interaction: Tidal Tails

46 Intermission: What do you think is happening here?

47 Metallicity (number of A atoms / number B atoms) object A/ B = log 10 (number of A atoms / number of B atoms) sun of

48 Metallicity log R 23 = log L L OII 3727 OIII L Hβ 4959,5007

49 Dwarf Galaxies

50 Dwarf Galaxies

51 Dwarf Spheroidals (dsph)

52 The Fornax Dwarf Spheroidal galaxy

53 Dwarf Ellipticals (de) & Compact Ellipticals M32

54 Dwarf Irregulars

55 Dwarf Irregulars

56 Intermission: What type of dwarf?

57 Chemical evolution A star is born......consumes its fuel......possibly explodes......then fades away Stellar evolution made simple

58 The Closed-Box Model

59 The Closed-Box Model Z( t) = Z(0) p ln M M gas gas (0) ( t) However, dsph are gas-poor and metal-poor

60 Relaxation of the Closed-Box Assumption

61 Chemical Evolution of Individual Elements [O/Fe] 1 Gyr 5 Gyr 10 Gyr [Fe/H]