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

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Our Working Galaxy Model HI GAS DISK

NUCLEUS

GLOBULAR CLUSTER COMPANION

HALO STELLAR DISK

BULGE

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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:

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The Galactic Centre •  central 200 pc of the Bulge is gas-rich (108 Msolar, 10% of the total molecular ISM) and actively forming stars –  bar-like in infrared images •  contains massive clusters –  Arches cluster, 3 x

105

Msolar

pc-3

2 micron map of stellar density, C. Alard, Obs. Paris

–  Sgr B2, young stars > 106 Lsolar •  stellar density near central black hole is ~ 2 x 106 Msolar pc-3 •  Wolf-Rayet stars of ~ 100 Msolar Gal Centre WR star, P. Tuthill Galaxies – AS 3011

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Our Galactic Centre

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

•  In our Milky Way galaxy –  Velocities > 1000 km/s inside 0.01 pc! € •  => 2 x 106 Msun SMBH

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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 3011

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Distinct stellar population to disc

Galaxies – AS 3011

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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

Galaxies – AS 3011

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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 3011

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Thick and thin disks

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

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

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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

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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

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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

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NGC 5907 thin disk + thick disk From its colors, this thick disk is not metal-poor

Morrison et al 1994 19

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

20 Fry et al 1999

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NGC 4565

Deeper imaging shows discs often have sharp edges: Truncated discs

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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

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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 23 truncation (also Pohlen et al 2002) 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

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STAR-FORMATION RATE (M./yr)

Kennicutt Star-formation Law 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.

Kennicutt (1998)

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IR luminous galaxies (some are mergers) Nuclear region of same spirals Spiral galaxies

But is there a cutoff below which star-formation cannot occur ? 25

GAS DENSITY per sq pc

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 hydrogen cloud today Optical

HI

Discs still growing/forming ?

NGC4618 26

Gil de Paz et al. 2005

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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

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Truncation classification scheme Pohlen & Trujillo (2006)

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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

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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 near IR plus HI and UV)

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Baugh et al (2006)

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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)

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1% by mass

0.1% by mass

NO DUST ATTENUATION

10% by mass SEVERE DUST ATTENUATION

Negligible ? (plasma) 38

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OPTICAL

UV

STAR FORMATION

STARS

IR DUST

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HI disk even more extended NGC 6946: the HI extends far beyond the stellar disk

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Our expanded working galaxy model !

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