An Introduction to Galaxies and Cosmology. Jun 29, 2005 Chap.2.1~2.3

Transcription

1 An Introduction to Galaxies and Cosmology Jun 29, 2005 Chap.2.1~2.3

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3 2.1 Introduction external galaxies normal galaxies - majority active galaxies - 2% high luminosity (non-stellar origin) variability First, we will see the normal galaxies.

4 Hubble Classification Hubble classes of galaxy elliptical lenticular spiral irregular Fig 2.1 Edwin.P.Hubble

5 Hubble Classification

6 Elliptical galaxies E + b a E0, E1,..., E7 M89 (type E0) NGC 4552 f : a : b : a b f= 10 a flattening factor semimajor axis semiminor axis (E8,E9 : unstable) M49 (type E4) NGC4472 M59 (type E5) NGC 4621

7 Spiral galaxies characterized by a circular disc a nuclear bulge subclass bar existence openness the arm and the relative size of the central bulge Fig. 2.5 M 31 (Sb) Let's see each of these type galaxy

8 Spiral galaxies NGC7217 (Sab) NGC4650 (SBa) M77 (Sb) M91 (SBb) M99 (Sc) NGC 1073 (SBc)

9 Lenticular galaxies (Lens-shaped galaxies) have a disc, bulge no arm quite large bulge (on average) NGC 524 (S0) NGC 936 (SB0)

10 Irregular galaxies Fig. 2.7(a) IC5152 Fig. 2.8 SMC Fig. 2.7(b) LMC

11 Hubble Classification difficulties different observers may classify the same object differently. the appearance of a galaxy may be affected by such things as exposure time. short-exposure elliptical longer exposure overall shape

12 dwarf elliptical and others ~ 106 solar mass Over 60% are elliptical. fewer than 30% are spiral. fewer than 15% are irregular. Fig. 2.8 Leo I In any survey it is important to account for bias. 60% of spiral and lenticulars are barred.

13 The physical characteristics of the Hubble classes EVOLUTION? FALSE! Fig. Hubble's Tuning Fork

14 The physical characteristics of the Hubble classes The angular momentum per unit mass elliptical and irregular low spiral and lenticular high The ratio of the gas(form atomic or molecular) mass to the stars' mass Milky Way 10% spiral Sa ~ Sc (SBa ~ SBc) 5-15% irregulars elliptical and lenticular ~1% They contain hot gas.

15 The physical characteristics of the Hubble classes The range of variation of the properties (mass, luminosity, diameter) spiral galaxies narrow range of mass 109~1012Mo elliptical galaxies wide range of mass 105~1013Mo irregular galaxies intermediate range 107~1010Mo

16 Ellipsoid oblate (a = b > c) prolate (a > b = c) triaxial (a > b > c) Fig. 2.9 An ellipsoid Recent study indicates that some elliptical galaxies' flattening are not due to their rotation.

17 Peculiar galaxy M87 jet short-time exposure M 87 is a type E0p. p means a peculiar. medium exposure M 87 is an active galaxy. (detailed in Chap.3)

18 Gravitational interacting peculiar galaxies Fig. NGC 2207 and IC 2163

19 Gravitational interacting peculiar galaxies Fig. Stephan's Quintet

20 Gravitational interacting peculiar galaxies NGC 6872 and IC 4970 NGC 4676

21 cd galaxies Fig NGC 4874 'c' indicates a super giant system. 'D' indicates that the galaxy has a large diffuse envelope.

22 15-25% ~1% Population II MO low Population I and II MO high MO

23 2.3 The determination of the properties of galaxies galaxy's property : luminosity, diameter, mass how such information was obtained? apparent surface brightness Fig unit : W m-2 arcsec-2 usually made in a specified waveband, such as the widely used V-band.

24 2.3 The determination of the properties of galaxies isophote no edge of galaxy log contour edge is noise level Fig isophotes of the giant elliptical galaxy NGC 4278 Fig Apparent surface brightness an averaged face-on Sc galaxy

25 2.3 The determination of the properties of galaxies determining the luminosity flux not straightforward luminosity Example spiral galaxy not radiate uniformly in all directions dust effect depends on orientation but, we can estimate the effects of orientation We can estimate the luminosity. (many uncertainties and assumptions) for comparison use

26 2.3 The determination of the properties of galaxies determining the mass three methods of determining galactic masses Rotation curves for spiral galaxies Velocity dispersions for elliptical galaxies X-ray halos of ellipticals their results are not fully consistent since they certainly differ in their sensitivity to dark matter. (and may also have other shortcoming)

27 2.3 The determination of the properties of galaxies Rotation curve for spiral galaxies observation model lower limit - mass within radius r To obtain the greatest radius r Doppler shift of 21cm radiation by HI gas

28 2.3 The determination of the properties of galaxies red blue - away from us towards to us NASA Jet Propulsion Laboratory (NASA-JPL) angular resolution Fig M 81 by 21cm image Fig rotation curves for some nearby galaxies.

29 2.3 The determination of the properties of galaxies distant galaxy - cannot resolve - line broadening determine the rotation curve (Lewis et al., ApJS 1985) maximum rotation speed (from line broadning width) We can estimate the mass of the distant galaxy.

30 2.3 The determination of the properties of galaxies THE VIRIAL THEOREM mutual gravitational interaction system - galaxies, globular clusters, clusters of galaxeis a galaxy exists in a stable state 1 E k = E g 2 Ek : kinematic energy Eg : potential energy Virialized state

31 2.3 The determination of the properties of galaxies Velocity dispersions for elliptical galaxies M 12 v R from Virial Theorem v velocity dispersion M mass of the elliptical galaxy R scale length related to its size R : known v : estimated from Doppler shift measurements M ( a lower limit )

32 2.3 The determination of the properties of galaxies X-ray halos of ellipticals X-ray observation extent, temperature, density (hot gas) mass Fig NGC 3923 widely used for giant ellipticals and clusters of galaxies

33 2.3 The determination of the properties of galaxies Composition of galaxies This section aims to answer the questions How many stars are there in a galaxy? What kind of stars predominate? How much of the galaxy's mass is due to gas?

34 2.3 The determination of the properties of galaxies How much of the galaxy's mass is due to gas? MH M MH : mass of atomic hydrogen M : total mass M - from rotation curve or velocity dispersion MH - from 21cm radiation certain galaxies are dominated by H2. atomic hydrogen ionized hydrogen elliptical giant a little less massive similar to the proportion of mass of gas in atomic or molecular form in spiral galaxy lower than giant elliptical spiral a few per cent - irregular 15-20% -

35 2.3 The determination of the properties of galaxies How many stars are there in a galaxy? What kind of stars are there in a galaxy? main method - population synthesis observed best fit relative numbers of stars in each category their contribution to the total mass and total luminosity integrated spectrum Fig NGC 1427 Population synthesis of the spectrum

36 2.3 The determination of the properties of galaxies Table 2.2 Population model of M 31's nuclear bulge