Lecture 4: Galaxy Structures Our Working Galaxy Model

Lecture 4: Galaxy structures

• Galactic nucleus/SMBH Using the MW and M31 as case studies • Central bulge • Disc – Bars – Pseudo-bulges

– Thin disc infrared view from COBE: stars are white, dust is red – Thick disc – Disc truncation • Galactic Halo

Galaxies – AS 3011 1

Our Working Galaxy Model GLOBULAR CLUSTER HI GAS DISK NUCLEUS COMPANION

HALO

STELLAR DISK BULGE

1 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:

The Galactic Centre

• central 200 pc of the Bulge is gas-rich 8 (10 Msolar, 10% of the total molecular ISM) and actively forming stars – bar-like in infrared images • contains massive clusters 2 micron map of stellar density, 5 -3 C. Alard, Obs. Paris – Arches cluster, 3 x 10 Msolar pc 6 – Sgr B2, young stars > 10 Lsolar • stellar density near central black hole 6 -3 is ~ 2 x 10 Msolar pc

• Wolf-Rayet stars of ~ 100 Msolar

Gal Centre WR star, P. Tuthill Galaxies – AS 3011 4

2 Our Galactic Centre

3 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

v 2r (110 ×103)2 × 2.5 × 3×1016 M = = CORE G 6.67 ×10−11 37 6 MCORE =1.4 ×10 kg = 6.8 ×10 M⊗

• In our Milky Way galaxy – Velocities > 1000 km/s inside 0.01 pc! € 6 • => 2 x 10 Msun SMBH

4 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

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Distinct stellar population to disc

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5 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, 2005 11

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

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6 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

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Thick and thin disks

NGC 4762 - a disk galaxy with a bright thick disk (Tsikoudi 1980)14

7 IC 5249 also shows a very faint thick disk (Abe et al 1999)

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

8 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] = log10 (Z/Zsolar) 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 3011 17

NGC 5907

( 2MASS JHK )

Similar rotational velocity to our Galaxy

Looks like pure thin disk, but deep surface photometry shows a prominent thick disk 18 45

9 NGC 5907

thin disk + thick disk

From its colors, this thick disk is not metal-poor

Morrison et al 1994

But NGC 4244 (MB = - 18.4) appears to be a pure thin disk: just a single exponential component, no thick disk

Fry et al 199920

10 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 Rmax /hR = 3.6 ± 0.6 for 34 edge-on disk galaxies

11 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

12 Kennicutt Star-formation Law

Kennicutt (1998) determined

Kennicutt (1998) 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.

IR luminous galaxies (some are mergers) But is there a cutoff below Nuclear region of

STAR-FORMATION RATE (M./yr) RATE STAR-FORMATION same spirals which star-formation cannot Spiral galaxies occur ?

GAS DENSITY per sq pc 25

GALEX Imaging of NGC4625

NGC4625 Which disc ?

UV disc is 3-4x optical disc HI disk is 3-4x UV disc UV

Young stars forming rapidly out of HI hydrogen cloud today Optical

Discs still growing/forming ?

NGC4618

26 Gil de Paz et al. 2005

13 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 27

14 Truncation classification scheme Pohlen & Trujillo (2006)

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 Halo population

• the halo includes a few individual stars – metal-poor, and ~ 1/1000th 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. 4 6 • globular clusters with L ~ 10 to 10 Lsolar – 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

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Sparsely populated Halo

Galaxies – AS 3011 32

16 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 3011 33

Perhaps DM can merge effectively without immediate catastrophic mergers

Baugh et al (2006)

17 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)

18 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: ROTATIONAL VELOCITY (km/sec) VELOCITY ROTATIONAL

DISTANCE FROM GALACTIC CENTRE (kpc)

1% by mass

0.1% by mass

NO DUST ATTENUATION 10% by mass SEVERE DUST ATTENUATION

Negligible ? (plasma)

19 UV OPTICAL

STAR STARS FORMATION

IR DUST

HI disk even more extended

NGC 6946: the HI extends far beyond the stellar disk

20 Our expanded working galaxy model !