Lecture Five:

Mo, vd Bosch & White, chapters 9, 10

literature Tuesday 1st March

Star Formation in Galaxies…!

Physical Process Star formation defines the visible properties of galaxies and

this means that any theory of galaxy formation needs to include a theory of how stars form…

A full understanding of star formation in a cosmological

framework is challenging – the typical mass of gas in a

galaxy is ~1011M!

with a density of ~10-24 gcm-3, and for a typical star this is ~1M

! and ~1gcm-3.

What does Galactic star formation tell us?!

•! Star formation occurs in molecular

clouds

•! More precisely, it occurs within the

dense parts of molecular clouds

This raises the question of whether star formation occurs

because the gas is dense or because it is molecular or

both

Why molecular gas?!

only molecular gas can cool below 100 K and

therefore collapse under its own gravity:

Jeans criteria, Bonnor-Ebert

atomic gas does not emit efficiently at these

temperatures but molecular gas does, mostly with

roto-vibrational transitions. the more its emits the

more it cools, the more its density increases (so gravity increases). at some point gravity will

exceeds the pressure, the equilibrium is broken

and the collapse starts.

Star Formation in molecular clouds!

Molecular gas in the plane of the

Milky Way

Molecular core

Simplified view of IMF!•! Field Star IMF is within errors same as that

inferred for Orion Nebula Cluster and other nearby star forming regions

•! It has a power law (Salpeter) down to about 0.5-1 M

! with most mass in solar mass stars

but most luminosity at high M

•! Evidence for deviations from standard IMF in some Gal. Center clusters

Bastian et al. 2010 ARAA

One possible explanation of the IMF!

•! It reflects the mass distribution of the

cloud fragments or cores in the

molecular cloud

•! The “typical” mass of around 1 M! then

reflects the Jeans Mass (very T

dependent)

M(JEANS) ~ T3/2 n-1/2

The origin of the Initial Mass

Function

(see also Testi & Sargent 1998; Motte et al. 2001) !

Submm continuum surveys of nearby protoclusters suggest that the mass !

distribution of pre-stellar condensations mimics the form of the stellar IMF

NGC2068 protocluster at 850 µm!

Motte et al. 2001!

Condensations mass spectrum in ! Oph!

"! The IMF is at least partly determined by fragmentation at the pre-stellar

stage.!

Consequences for extragalactic SF!

•! If fragmentation is fundamental in determining the

IMF, the Jeans Mass and hence the temperature

may determine the critical turn-over mass

•! This could cause the IMF in galactic nuclei to be

more biased towards high mass ???

•! Temperatures in Galactic Center clouds are high

Galactic timescale for Star

Formation tSF

•! One might naturally think it was the free-fall time at the mean density of molecular clouds

•! But as pointed out in the 70s by Zuckerman and Evans, real galactic SF Rate is lower (tSF=109 yr) than from free fall time (tff roughly 106 years)

•! This has given rise to two classes of theories: –! “slow”: including “ambipolar diffusion” modulated theories.

–! “inefficient”: turbulence, HII regions and winds

Conclusions

•! Extragalactic star formation may well be just

galactic writ large

•! But we do not understand what determines

the efficiencies and timescales

•! Of course the IMF might be playing tricks

Stars form in spiral arms!M33 Spitzer Image Physical Process We assume that for understanding large scale effects on

galaxy evolution we don’t need to go into the specific small scale details of star formation.

GLOBAL PROPERTIES

Need to understand how the GLOBAL properties of star

formation averaged over a large volume of gas, depend on the GLOBAL properties of the gas, such as mass, density,

temperature and chemical composition.

Empirical Star Formation “Laws”

SFR, !, in terms of mass in stars formed per unit area per unit time

Gas consumption time,

Since the most obvious requirement for star formation is the presence of gas, it is only

logical to look at the relation between SFR and surface density of gas:

Schmidt (1959)

Kennicutt-Schmidt law!

Kennicutt (1998) ApJ, 498, 541

The study of star formation in normal

spiral galaxies and also starbursts

have shown Schmidt is a surprisingly

good description of global SFRs

(averaged over entire SF disc) –

Interpretation!

Kennicutt (1998) ApJ, 498, 541

SFR is controlled by the self-gravity of the gas? This would mean that the rate of

star formation will be proportional to the gas mass divided by the time scale for

gravitational collapse (free-fall time).

" is the free-fall time of the gas divided by gas

consumption time, or star formation efficiency

If all galaxies have approx. same scale height, this implies:

in good agreement with empirical law

However, this interpretation implies

Which suggests that self gravity isn’t the only important process.

Perhaps only a small fraction of gas participates in star formation, or the star formation

time scale is #/". In either case – additional physics required to explain empirical law.

Dynamical time scale!

Kennicutt (1998) ApJ, 498, 541

In addition to the Schmidt law – there

is an equally strong correlation

between star formation rate and the

gas surface density divided by the

dynamical time

defined as the orbital time at the outer

radius R of the relevant star forming

region.

$ is the circular frequency

This implies 10% of available gas forms stars per orbital time

The HI Nearby Galaxy Survey: (THINGS) HI maps

CO maps of Nearby Galaxies H2 maps

GALEX far-UV maps current SFR

SPITZER IR nearby galaxy survey (SINGS) past SFR

Walter et al. (2008)

Gil Paz et al. (2007)

Kennicutt et al. (2003)

Helfer et al. (2003); Leroy et al. (2008)

Nearby Galaxy Surveys!

This combination yields sensitive, spatially resolved measurements of kinematics, gas

surface density, stellar surface density, and SFR surface density across the entire

optical disks of 23 spiral and irregular galaxies.

The observation that HI in disk galaxies typically extends well beyond the optical disk

suggests that star formation is somehow suppressed in the outer disk: truncated

Small Scale Star Formation Measurements!

Bigiel et al. (2008) AJ, 136, 2846

Molecular Schmidt law HI saturates at ~9 M!

pc-2

gas in excess is only

molecular

No universal relation between

total gas density and sfr

!SFR is found to drop whenever the surface densities of cold gas drop below 10M!

pc-2.

Leroy et al. (2008) AJ, 136, 2782

Formation of cold phase: where T drops

to ~500K, the molecular fraction reaches

10-3 and Qgas ~ 1. Good indicators that cold HI is common and H2 formation is

efficient.

Schaye (2004)

Shear, limit the formation of

molecular clouds

Hunter et al. (1998)

Instability of gas disk in presence of

stars:

Rafikov (2001)

Gravitational instability:

Martin & Kennicutt (2001)

Star Formation Thesholds!

Local Star Formation Laws!

Leroy et al. (2008) AJ, 136, 2782

Global star formation laws averaged over whole disk – really need to understand the

importance of different physical parameters (gas density, orbital time scales) on smaller

spatial scales.

This relation valid from galaxy to galaxy but not within a galaxy, meaning that the orbital

time seems to have no impact on LOCAL star-formation efficiency.

on the other hand does change within a galaxy.

It can be seen that looking at the relations with atomic and molecular gas separately,

and molecular gas correlated much better with SFR.

A single power law is a poor fit, as there is a break at low gas surface densities. This

corresponds to an abrupt truncation in the SFR.

Schmidt law for molecular gas

Two laws: 1. for transformation from atomic into molecular gas 2. formation of stars from

molecular gas

Bigiel et al. (2008) AJ, 136, 2846

Estimating stellar masses By fitting a set of model predictions to

observational data (magnitudes,

spectral features), finding the scaling

factor required to reproduce the

observed fluxes we can estimate the

stellar mass of a galaxy (and it is usually

well constrained - e.g. Bell & de Jong 2001).

This has led to stellar mass

having become the most

important independent

variable in galaxy evolution

studies.

Star Formation Tracers!

Predicted spectra of coeval stellar population: 1, 10, 100, 400 Myr, 1, 4, 13 Gyr

Solar metallicity & Salpeter IMF

Lyman break D4000 break (depends strongly on Z)

Star formation tracers!

Steller Masses & SFHs!We see how we can determine the SED of a galaxy from a SFH. The inverse – can

we determine physical properties of galaxies (e.g., stellar masses and SFHs) from quantities that are directly observed (e.g., luminosity, spectrum). These observed

quantities are convolutions of the SFH, IMF, dust extinction, etc.

Exponential SFHs of the form:

characteristic SF time scale

Bell & de Jong (2001) ApJ, 550, 212

Sloan/2MASS colours to M/L!

Bell et al (2003) ApJS, 149, 289

Star formation diagnostics! Kennicutt (1998) ARAA, 36, 189

UV Continuum (1250-2500A) :

Number of massive stars in a galaxy is directly proportional to the current SFR, as long

as it is not absorbed on the way. Only possible from the ground for z=1-5. For z<1 need

space telescope. Assuming time scale ~108 yr, or longer:

Nebular Emission Lines:

The ISM around young, massive stars is ionised by Lyman continuum photons produced

by these stars, giving rise to HII regions. The recombination of this ionised gas produces

H emission lines (e.g., H% but also H&, P%, P&, Br%, Br'), which can be used as SFR

diagnostic, because their flux is proportional to the Lyman continuum flux from young

(<2x107yr) massive (>10M!

) stars.

Forbidden lines:

For galaxies with z>0.5, H% emission is redshifted out of optical. The strongest feature in

the blue is [OII](3727 forbidden line doublet. Unfortunately luminosities do not depends

only on the local radiation field, but also the ionization state and metallicity of ISM. It has

been successfully empirically calibrated, and can be used out to z=1.6 (in the optical).

Star formation diagnostics! Kennicutt (1998) ARAA, 36, 189

FIR Continuum (8-1000µm):

Typically the ISM associated with star forming regions can be quite dusty, so a significant

fraction of the UV photons produced by massive stars is absorbed. This heats the dust

and is subsequently re-emitted in the FIR. This does depend on opacity of dust, if it is not

optically think, need to specify the escape fraction. There is a also a contribution due to

older stars. Works well for short duration intense star formation, ie., starbursts

(10-100Myr old).

Spectral Indicators!

Kauffmann et al. 2003 MNRAS, 341, 33

An essential tool As we shall see, stellar population synthesis has become an

indispensable tool for most studies of the galaxy population.

Star formation histories.

Stellar masses

Stellar ages.

The history of chemical enrichment

The assembly of mass in the Universe

Dust content of distant galaxies

And this will continue!

But in reverse, it can also be informed by galaxy observations -

learning about complex, rare & important stages of stellar

evolution.

Properties by mass

z-band M/L as a function of K-corrected magnitude. The line indicates the M/L of a galaxy which has

been forming stars at a constant rate for a Hubble time. Lower, more current SF; Upper, more in the

past

more SF in past

more SF recently

Kauffmann et al. 2003 MNRAS, 341, 33

bright, massive galaxies faint, low mass galaxies

Break occurs at a stellar mass, M~3x1010 M!

Kauffmann et al. 2003 MNRAS, 341, 33

Full Spectrum Analysis!

Heavens et al. 2000 MNRAS, 317, 965

Full spectrum fitting Fitting a grid of models to the full F(!) to recover SFH(t), metallicity &

stellar mass, or simply to do continuum subtraction. Also useful to

estimate velocity dispersions in a consistent manner.

STECKMAP!STARLIGHT!

MOPED!

VESPA!

SEDFIT!

ULySS!

k-correct!platefit!

NBURSTS!

Tojeiro et al (2009)

GASPEX!

Mass with redshift

Panter, Heavens & Jimenz 2004 MNRAS, 355, 764

Using MOPED algorithm " SFH, ZFH + dust " mass

Estimating stellar mass assembly

When you know the stellar mass of each galaxy in

your sample, you can add it all up to calculate the

mass density in stars at that redshift.

Either way you can calculate the history of

stellar mass assembly in galaxies using

population synthesis modelling

You can also look back: Calculate SFHs for all

nearby galaxies and add them up to give you a

history of mass assembly.

Finally you can fit models to the integrated light at

various redshifts and then get an integrated mass

assembly.

Build-up of Stellar Mass

Panter et al. 2007 MNRAS, 378, 1550

The SFH of the Universe

Panter et al. 2007 MNRAS, 378, 1550

Downsizing

Panter et al. 2007 MNRAS, 378, 1550

Emission-lines!

Tremonti et al. 2004 ApJ, 613, 989 Tremonti et al. 2004 ApJ, 613, 989

mass-metallicity relation

Tremonti et al. 2004 ApJ, 613, 989

Relation between stellar mass, in units

of solar masses, and gas-phase

oxygen abundance for 53,400 star-forming galaxies in the SDSS

The Mass-metallicity relation & the transition mass.!

Tremonti et al. 2004 ApJ, 613, 989; Brinchmann et al. 2008 A&A, 485, 657

Kauffmann et al. 2003 MNRAS, 341, 54

data ! models ! physical parameters

Brinchmann 2009 arXiv:0910.1533v1