Lecture 4: Galaxy structures

Transcription

1 Lecture 4: Galaxy structures Galactic nucleus/smbh Central bulge Disc Bars Pseudo-bulges Thin disc Thick disc Disc truncation Galactic Halo Using the MW and M31 as case studies infrared view from COBE: stars are white, dust is red Galaxies AS Our Working Galaxy Model HI GAS DISK NUCLEUS GLOBULAR CLUSTER COMPANION HALO STELLAR DISK BULGE 1

2 Each component appears to have a distinct stellar population with different metallicity and ages, this implies distinct Evolutionary phases. Lets look at each component in turn: 3 The Galactic Centre central 200 pc of the Bulge is gas-rich (10 8 M solar, 10% of the total molecular ISM) and actively forming stars bar-like in infrared images contains massive clusters Arches cluster, 3 x 10 5 M solar pc -3 2 micron map of stellar density, C. Alard, Obs. Paris Sgr B2, young stars > 10 6 L solar stellar density near central black hole is ~ 2 x 10 6 M solar pc -3 Wolf-Rayet stars of ~ 100 M solar Gal Centre WR star, P. Tuthill Galaxies AS

3 Our Galactic Centre 3

4 Evidence for SMBHs We find that stars have velocities of >110km/s within 2.5pc of the core of M31 Central Super-massive BH IF they are in circular orbits we can use the Virial theorem to calculate the mass inside r M CORE = v 2 r G = ( ) M CORE = kg = M In our Milky Way galaxy Velocities > 1000 km/s inside 0.01 pc! => 2 x 10 6 M sun SMBH 4

5 Bulge population MW bulge is best seen in the infrared due to extinction in optical radius ~ 1 kpc (small) seems taller on +l side (possibly a bulge+bar system) hence probably really a bar this side is nearer to us so by perspective appears larger Bulge contains both bulge stars AND rotating disc stars Different chemical make-up: [O/Fe]-ratios. Higher [α/fe] = older and quickly formed Galaxies AS Distinct stellar population to disc Galaxies AS

6 Galactic disks The disk is the defining stellar component of disk galaxies. It is the end product of the dissipation of most of the baryons, and contains almost all of the baryonic angular momentum Understanding its formation is the most important goal of galaxy formation theory. Ken Freeman, Terschelling, Bars and Pseudo-bulges Disk phenominae: Bars induced by resonance in stellar rotation, orbits accelerated or dragged into bar pattern Psuedo-bulges induced by epicyclic motion Galaxies AS

7 Disk stars the effective thickness of the Galactic disk of stars depends on their spectral type stars per unit volume G s and K s A s height z above disk Galaxies AS Thick and thin disks NGC a disk galaxy with a bright thick disk (Tsikoudi 1980) 14 7

8 IC 5249 also shows a very faint thick disk (Abe et al 1999) 15 The Galactic thick disk: its mass is about 10% of the thin disk s it is old (> 12 Gyr) and significantly more metal poor than the thin disk: mean [Fe/H] ~ -0.7 and a-enhanced its rotation lags the thin disk by only ~ 50 km/s thick disk thin disk higher [α/fe] more rapid formation 16 8

9 Properties of the thick disk stars Much less dense than the thin disk (minority constituent) Low metallicity => formed from primordial gas? metallicity given by Z = the mass fraction of elements heavier than H and He in practice the fraction of a heavy element relative to H, compared to solar e.g. [Fe/H] = log 10 (Z/Z solar ) assumes Fe is a representative element Very high vertical speeds (~1500 km/s) => old? Formation: possibly a remnant of the earlier thin disk that was perturbed by a passing satellite galaxy? Alternatively a relic from the initial formation phase? Galaxies AS NGC 5907 ( 2MASS JHK ) Similar rotational velocity to our Galaxy Looks like pure thin disk, but deep surface photometry shows a prominent thick disk

10 NGC 5907 thin disk + thick disk From its colors, this thick disk is not metal-poor Morrison et al But NGC 4244 (M B = ) appears to be a pure thin disk: just a single exponential component, no thick disk Fry et al

11 NGC 4565 Deeper imaging shows discs often have sharp edges: Truncated discs 21 What is the origin of this disk truncation - common and seen more easily in edge-on galaxies than in face-on galaxies Kregel et al (2001) find R max /h R = 3.6 ± 0.6 for 34 edge-on disk galaxies 22 11

12 Disk Truncation in M33 M33 Surface Brightness Profile: i-band surface photometry out to R ~ 35' TRUNCATION profile extended to R ~ 60' using star counts sharp decrease in surface brightness beyond 5 scalelengths.. V~31 mag arcsec -2 cf. van der Kruit's (1982) disk edges: ~3-5 scalelengths, then abrupt truncation (also Pohlen et al 2002) 23 Ferguson et al 2003 What causes truncation? 1. the radius where the gas density falls below a critical value required for star formation (Kennicutt 1989). 2. the radius to which the disk has grown today. The outer disk IS younger but still typically many Gyr old ( eg Bell & de Jong 2000, Ferguson et al 2003). In some galaxies (eg M83, Milky Way), star formation continues in the outer disk but there is also an underlying old component 24 12

13 Kennicutt Star-formation Law STAR-FORMATION RATE (M./yr) Kennicutt (1998)??? IR luminous galaxies (some are mergers) Nuclear region of same spirals Spiral galaxies Kennicutt (1998) determined that the surface density of star formation was very tightly correlated with the surface density of gas over a remarkably wide range of gas densities and in a wide variety of galactic states. But is there a cutoff below which star-formation cannot occur? GAS DENSITY per sq pc 25 GALEX Imaging of NGC4625 Which disc? UV disc is 3-4x optical disc HI disk is 3-4x UV disc NGC4625 UV Young stars forming rapidly out of hydrogen cloud today Optical HI Discs still growing/forming? NGC4618 Gil de Paz et al

14 NGC 300: deep r'-band counts from Gemini GMOS : exponential disk goes for at least 10 scale lengths without truncation! Bland-Hawthorn & Freeman (2005) NO TRUNCATION r-band star counts

15 Truncation classification scheme Pohlen & Trujillo (2006) 29 What causes truncation? 1. the radius where the gas density falls below a critical value required for star formation (Kennicutt 1989). 2. the radius to which the disk has grown today (I.e., truncation = growing pains?) The outer disk IS younger but still typically many Gyr old ( eg Bell & de Jong 2000, Ferguson et al 2003). In some galaxies (eg M83, Milky Way), star formation continues in the outer disk but there is also an underlying old component 30 15

16 Halo population the halo includes a few individual stars metal-poor, and ~ 1/1000 th of the number of disk stars many hot blue stars but how did they get there (migration time from disk << lifetime)? possibly formed within tidal debris or ripped out during merger of minor satellitte galaxies. globular clusters with L ~ 10 4 to 10 6 L solar metal-rich GCs lie closer to disk plane and share disk rotation metal-poor GCs ([Fe/H] ~ -0.5 to -1) are made of older stars and orbits are random oldest stars lie in GCs and are ~12 15 Gyr, so made in early Universe (t ~ 13.7 Gyr) searches underway for more tidal streams from merged galaxies Galaxies AS Sparsely populated Halo Galaxies AS

17 The Spaghetti Survey The Spaghetti Survey What the Galactic Halo might look like if the galaxy was built up from the merger of 50 dwarf galaxies Galaxies AS Perhaps DM can merge effectively without immediate catastrophic mergers Baugh et al (2006) 34 17

18 Two key points: Detailed modeling of feedback required to create discs (ie., star-formation) Mergers are likely to disrupt fragile discs without baryons merging Modeling these will require larger simulations and more baryon physics in the interim we need to also improve our observational dataset (optical --> near IR plus HI and UV) 35 Baugh et al (2006) 36 18

19 ROTATIONAL VELOCITY (km/sec) 90 % of the Galaxy s mass is in the form of dark matter Dark matter is required to explain our galaxy s rotation curve, even at the Sun s location: DISTANCE FROM GALACTIC CENTRE (kpc) 37 1% by mass 0.1% by mass NO DUST ATTENUATION 10% by mass SEVERE DUST ATTENUATION Negligible? (plasma) 38 19

20 UV OPTICAL STAR FORMATION STARS IR DUST 39 HI disk even more extended NGC 6946: the HI extends far beyond the stellar disk 40 20

21 Our expanded working galaxy model! 41 21