Phil James

Liverpool John Moores UniversityAstrophysics Research Institute

22nd August 2007

STFC PhD Summer School, Durham

The Structure of Galaxies

Talk overview

• Look at the diversity of galaxy Structures seen in the local Universe

• Link structural properties with Content of galaxies (gas, stars, dust, dark matter, black holes) and Processes connecting these

• Identify open questions; galaxies are far from being fully understood

I’m not going to mention this…

Diagram courtesy Space Telescope Science Institute

…but we need to understand this

Baldry et al. 200466846 SDSS galaxies0.004<z<0.080

Red and blue sequence galaxies in the Virgo Cluster

Image: CFHT

STRUCTURES: Field galaxies

Image: A. Block

Structures: Disks

NGC 2683 Image: D. Matthews & A. Block

Structures: Disks and Bulges

NGC 4565 Image: Hugo, Gaul & Black (KPNO)

Structures: Disks and Bulges

M 104 Image: HST

Structures: Bars

STRUCTURES: Elliptical galaxy

Galaxy Contents

• Gas (atomic and molecular)

• Stars

• Dust

• Black holes

• Dark matter

Gas in galaxies

• This is a ‘dissipative’ component – if 2 gas clouds collide, they can shock and radiate energy, so collisions are highly inelastic

• If there is any initial angular momentum, this naturally leads to the formation of a disk:

Superthin galaxies

These galaxies have little or no bulge: pure disk systems.

Gas in galaxies

• This is a ‘dissipative’ component – if 2 gas clouds collide, they can shock and radiate energy, so collisions are highly inelastic

• If there is any initial angular momentum, this naturally leads to the formation of a disk

• The gaseous disk then forms stars, once gas density is sufficiently large

Measuring star formation

• Hα from gas ionized by hot young stars:

Red light spectrum of a galaxy

Measuring star formation

• Hα from gas ionized by hot young stars

• Mid/Far-infrared emission from hot dust around star formation regions (IRAS, Spitzer)

NGC 1410 Image Bill Keel

Dust in galaxies

Spitzer IR Space Telescope

M81 observed by Spitzer

Measuring star formation

• Hα from gas ionized by hot young stars

• Mid/Far-infrared emission from hot dust around star formation regions (IRAS, Spitzer)

• UV emission from young stars (GALEX)

• Radio emission from star formation regions, or from supernova remnants

Star Formation (‘Schmidt-Kennicutt’) Law – SFR α (Gas density)1.4

Kennicutt 1998

←Starburst nuclei

←Normal disks

Conversion of gas to stars

• Star formation law works well, with wide applicability (normal galaxies and starbursts)

• It is largely empirical, however – no physical basis for power law index

• Does it apply to star formation in densest regions (globular clusters and nuclear clusters) or is there another mode of star formation for these?

Globular Cluster M3

Image K Teuwen

T. Böker et al. 2002 HST images of compact nuclear clusters

Some personal opinions (not all would agree…)

• Gravitational collapse of gas clouds naturally leads to disks in undisturbed systems

• Such disks will always start forming stars when a critical density is reached (note that there are ~no gas-rich, quiescent galaxies)

• This star formation is continuous, at a broadly constant rate, in the absence of outside influences, and as long as the gas supply holds up

Star formation timescaleR-luminosity dependent extinction correction

Bulge-dominated

Star formation timescaleR-luminosity dependent extinction correction

Bulge + disk

Star formation timescaleR-luminosity dependent extinction correction

Bulge-free

UGC 8508, Im UGC 9240, Im

Mean R profile

Mean Hα profile

Difference, Hα-R

Sm Im

But many galaxies are not disks…

• Q: Where do elliptical galaxies and spiral galaxy bulges come from?

• A: This seems to require the presence of stars (a non-dissipative component, unlike the gas), and something to stir them up

• Internal processes (bars, spiral arms) seem too weak – large bulges and elliptical galaxies probably need outside interference:

Simulation: J. Dubinski, U. Toronto

Some real interactions and mergers

Atlas of peculiar galaxies, H. Arp

The Antennae, NGC 4038/4039. Colour Image: HST, B. Whitmore & F. Schweizer

Tadpole galaxy, Image:HST

Galaxy mergers – results from simulations

• Colliding disc galaxies form long tidal tails and arms

• After a close approach, they are likely to spiral together and merge

• Gas becomes centrally concentrated, → nuclear starburst

• Merger remnant density profiles resemble elliptical galaxies or bulges

• Characteristic ‘relaxation’ timescales quite short – few x 108 years

• Summary: undisturbed galaxies stay as thin discs, collisions make bulges or ellipticals

Under currently-favoured hierarchical cosmologies, mergers are common – most bright galaxies will have experienced at least one merger since their formation. ‘Minor mergers’ with dwarf galaxies may just build bulges or thicken disks; ‘major mergers’ of two large galaxies can make disks directly into ellipticals.

Bulge star-formation histories

• Colours, population synthesis analyses show that most bulges are dominated by old stars, ~10 Gyr old

• Bulges and ellipticals have little cold gas

• Full understanding of this involves feedback processes

• Feedback can come from stars (stellar winds and supernovae):

Galactic superwindin starburst M82

Bulges and feedback processes (contd.)

The last decade has shown that bulges are closely linked to even more energetic phenomena than starbursts…

M31

Image: R. Gendler

Kormendy 1988a

Magorrian et al. 1998

MOST or ALL galaxies with bulges have central supermassive black holes!

Magorrian et al. 1998(Also Ferrarese & Merritt 2000, Gebhardt et al. 2000, Tremaine et al. 2002…)

All galaxies with bulges went through a quasar phase

Quasar Images: HST

• Quasar phase early in evolution of most galaxies

• Often linked to disturbance/mergers in HST imaging

• Results in ejection of gas from central regions

• Leaves a gas-free bulge with no further star formation

• Enriched gas from bulge can enhance metallicity of disk – possible solution of the ‘G-dwarf problem’ (very few low-metallicity disk stars in our galaxy)

Summary

• Initial collapse of gas cloud → rotating disk• Gaseous disk → star formation (S-K law)• If left alone, SF continues at ~ const. rate• Bulges result from mergers, after a stellar

component has formed• Subsequent SF history shaped by

feedback processes• Bulge formation linked to supermassive

nuclear black holes

Summary cont’d

• AGN feedback → ejection of gas from bulges/ellipticals, transition from blue → red sequence if feedback is strong enough

• Disk (re-)establishes itself around bulge, with gas enriched in heavy elements

• AGN becomes a quiescent BH when gas supply exhausted

• SF continues in disk, at rate and for a time dependent on gas supply

Plenty of open questions with this story…

Some people who know a lot about galaxies would say that most of this is WRONG. They hold that bulges and ellipticals can form from the initial collapse phase of a galaxy, with no need for mergers – Monolithic Collapse.

(Everyone should read the paper by Eggen, Lynden-Bell and Sandage on the evidence for this from our Galaxy)

NGC 4449, HST Image

Why are low-mass galaxies irregular, ratherthan disks, given their short relaxationtimescales? (Gas fraction, DM, SN feedback?)

ESO 510-G13 Image: HST

If mergers are as common as hierarchical theories imply, how do so many disks survive?

How do globular clusters form?How come the densest stellar systems form in the lowest density environment? Do all the stars form before the first burst of SNe? Why no angularmomentum?

M3 Image K Teuwen

Why do ellipticals have more globularclusters per unit mass than spirals?Why do many galaxies have two setsof clusters, red and blue?

M3 Image K Teuwen

Can bulges form without outside interference?

• Some authorities (e.g. Combes) claim that bulges can develop through processes internal to disk galaxies (‘Secular evolution’)

• Principal mechanism is bar instability in disks

NGC 1300 Image: HST

Can bulges form without outside interference?

• Some authorities (e.g. Combes) claim that bulges can develop through processes internal to disk galaxies (‘Secular evolution’)

• Principal mechanism is bar instability in disks• Bars efficiently funnel gas into central regions,

fuelling star formation and potentially building bulges

NGC 1365 Image: VLT

Can bulges form without outside interference?

• Some authorities (e.g. Combes) claim that bulges can develop through processes internal to disk galaxies (‘Secular evolution’)

• Principal mechanism is bar instability in disks• Bars efficiently funnel gas into central regions,

fuelling star formation and potentially building bulges

• However, the resulting structures are flat ‘lenses’ – ongoing debate about whether ‘bulges’ have to bulge

What physical mechanism causes the mass scaling between bulges and nuclear black holes? (Not even clear to me in which direction any causal link should act.)

How do the black holes form at all? Are they linked to the dense nuclear clusters seen in the bulge-free galaxies?

The 90% we have ignored so far…

• What is the dark matter?

• How is it distributed around galaxies?

• What effect does it have on bars?

• Do all galaxies have DM haloes?

NGC 3379 PN velocities: Romanovsky et al. 2003

Final conclusions/ annoying rant

• Always have a science question in mind, whatever you are working on.

• Always be willing to at least consider the answer you don’t want or don’t expect.