Lecture 2: Galaxy types, spectra. Galaxies AS

Transcription

1 Lecture 2: Galaxy types, spectra Galaxies AS

2 Hubble tuning fork this is really just descriptive, but Hubble suggested galaxies evolve from left to right in this picture not unreasonable, perhaps rotating gas winds up into arms? Sloan Digital Sky Survey Galaxies AS

3 Elliptical Red, I.e., (B-V) >1 Smooth profile High Surface Brightness Egg shaped Little or no dust lane Absorption Lines only Many Globular Clusters No rotation Found in Clusters Typically: 22 < M < 18 V Galaxies AS

4 M87, giant elliptical in the Virgo cluster NGC 5866, lenticular NGC 55 - irregular Galaxies AS

5 Spirals Red bulge (B-V) >1 Bluish Arms/Disk, (B-V) ~1 Moderate Surface Brightness Dusty Emission+Absorption lines Rotating disk Numerous Globular Clusters Seen in high and low density environments Typically: 21 < M < 17 V Galaxies AS

6 M51 spiral (and satellite) M31 (Andromeda) not so easy to see the arms NGC 3351 barred spiral NGC 7793 flocculent spiral Galaxies AS

7 Blue (B-V) <0.8 Strong Emission lines Very dusty Low surface brightness Highly Assymetrical Rotating Few Globular clusters Mainly in the field Typically: 18 < M < 10 V Irregulars Galaxies AS

8 we ve also now discovered other galaxies that don t fit Hubble s scheme: disrupted galaxies low surface brightness galaxies starbursts gas-rich galaxies intergalactic gas clouds!? other galaxies Galaxies AS

9 galaxy sub-classes Spirals are classified by the tightness of the arms Sa are tight and Sd are loose capital-b means bar, e.g. SBa is Sa plus a bar lenticulars are called S0 Arms also correlate with the size of the central bulge bulge is smaller for Sd than Sa Flocculent, anemic and one-arm patterns also seen Galaxies AS

10 Galaxies AS

11 Galaxy ellipticity & inclination For ellipticals we use ellipticity: n = 10 (a-b) / a where a, b are the apparent major, minor axes sizes - so E0 s look round and E7 are the flattest we see For spirals we use Assuming the disk is thin, the dimensions define the inclination i = cos -1 (b/a) i a cos i Galaxies AS

12 Our Working Galaxy Model HI GAS DISK GLOBULAR CLUSTER COMPANION HALO STELLAR DISK BULGE Galaxies AS

13 Galaxies AS

14 Bulge disc decomposition Better physical motivation than HTF is to separate out the distinct components: Bulge = Elliptical like and pressure supported Disc = flattened rotating system Galaxies AS

15 How many stars does a galaxy contain? Absolute mag of MW =-19.6 mag Absolute mag of Sun = +5.6mag Assume MW made of solar type stars only: F MV = nf Sol M MV 2.5log(F MV ) 2.5log(nF Sol ) M Sol 2.5log(F Sol ) n =10 0.4(M MV M Sol ) = Implies a stellar mass of ~10 10 Solar masses or ~2x10 40 kg Galaxies AS

16 Optical image (Stars) Typically gas mass = stellar mass 21cm image (Gas)

17 Galaxy Spectra The combination of ~50 billion stars plus many molecular clouds and star-forming regions. The spectra tell us: The galaxy s relative velocity The star-formation rate The average age of the stellar population 3 Aspects: Continuum Absorption Lines Emission Lines

18 Continuum The combination of many Black-Body spectra spanning a range in temperatures This produces a fairly flat overall spectrum I Blue λ Red The main feature is the 4000A-break

19 Caused by: blanket absorption of high energy radiation from metals in the stellar atmospheres the lack of hot blue stars Hence: The 4000A-break Ellipticals => A strong 4000A-Break Spirals => A weak 4000A-Break Irregulars => No 4000A-Break

20 Absorption Lines Caused by Atoms/Molecules in a star s atmosphere that absorb specific wavelengths ATOMS ATMOSPHERE RADIATION CORE Can also be due to COLD gas in the interstellar medium which can EXTRACT energy from the passing radiation: Extinction in Galaxy course

21 Emission Lines Caused by gas being ionized and heated and then re-radiating at specific allowed wavelengths Stellar spectrum GAS Emission lines Stars form from gas so are often embedded Young stars ionise gas which releases radiation at a specific wavelength as it recombines

22 Absorption / Emission Lines Absorption Lines Need metals in stellar atmospheres or cold gas in the interstellar medium Implies Old stellar population = old galaxy From Ellipticals Spiral Bulges Emission Lines Need very hot gas and O and B type stars Implies Newly formed stars = star-forming/young galaxy From Spiral Disks Irregulars

23 Typical Spectral features Absorption Ca(H) = A Ca(K) = A G-band = A Mg = A Na = A Emission O[II] = A Hδ = A Hγ = A Hβ = A O[III] = A O[III] = A Hα = A S2 = A Galaxies AS

24 Example Spectrum: Elliptical Strong 4000A-Break Mg Na S2 CaH&K G-Band Strong absorption lines due to metals in the stellar atmospheres of a mostly low luminosity stellar population. No evidence of any emission lines and hence no young stars and no gas

25 Example Spectrum: Spiral Some emission and some absorption indicating both a young and old stellar population. Hα O[III] Hβ OII S2 Mg 4000A-Break CaH&K

26 Example Spectrum: Irregular A strong emission-line spectrum indicating many hot young stars heating the gas which is re-radiating at specific wavelengths which depend on the chemical composition of the gas Hα OII O[III] Hβ S2 No 4000A-Break

27 Radial Velocities Most galaxy spectra are REDSHIFTED, which means their spectral features are offset compared to those measured for gasses in the lab i.e., characteristic combinations of lines are systematically offset to longer wavelengths This is interpreted as a DOPPLER shift and implies that galaxies are moving away Positive velocities: RECEDING Negative velocities: APPROACHING Δλ λ = Δv c or λ v + OBSERVED = λ c CALIBRATION c

28 Example Radial Velocity Hα OII OII is at 4000A Hα is at 7030A v λ λcal = c( ) = c( ) λ 3727 OBS = CAL 21,974km/s 467 v = c( ) = ,500km/s GALAXY IS MOVING AWAY AT ABOUT 21,750 km/s

29 CLASS EXERCISE Work in groups of 2-3 Collect an example spectra Identify spectral features Measure the wavelengths of the spectral lines Calculate the velocity at which the galaxy is receding and its redshift (Ho=75km/s/Mpc)

30 Typical Spectral features Absorption Ca(H) = A Ca(K) = A G-band = A Mg = A Na = A Emission O[II] = A Hδ = A Hγ = A Hβ = A O[III] = A O[III] = A Hα = A S2 = A Galaxies AS