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Lecture 6
Star Formation
The formation of stars
 gas clouds collapse
Initial Mass Functions
 parameterizations
Observational indicators of Star Formation (SF)
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recombination lines
UV continuum
FIR from dust emission
radio continuum
CO from molecular clouds
Lyman Break Galaxies
Star Burst (SB) galaxies
The SF along the cosmic time
Depto. de Astronomía (UGto)
Astronomía Extragaláctica y Cosmología Observacional
 The formation of stars
 star formation process is not well established due to: non hydrostatic equilibrium state of the
proto-star entities, the complexity of ISM and the unknown internal dynamics of clouds
(that prevents us of choosing which initital condition to use), importance of the
magnetic fields, interactions of borning stars with their neighbours in clusters, etc
 young star clusters (OC) are invariably associated with with dense interestellar clouds and
with spiral arms
 The formation of stars
Gas clouds collapse:
 gas in Giant Molecular Clouds (molecular and atomic gas, with some dust, and 106-107 M)
may be eventually compressed by shock fronts
 fragmentation and gravitational collapse take place, against some combination of
resisting turbulent and magnetic energies (as the ionization level of material, governed
by high energy radiation and cosmic ray particles, becomes small, the coupling of magnetic
field weakens and it dissipates)
 as the cloud fragment free-falls, gravitational energy is released as radiation (luminosity
rises rapdly but temperature remains quite low, ~ 10-20K, but growing in the center)
 the dense core (proto-star) gradually becomes opaque, trapping the radiation and heating up
 The formation of stars
 at about 2000 K, H2 molecules dissociates into HI, sinking energy and accelerating the
collapse of the core
 temperature continues rising and H becomes ionized (HII)
 free e- pressure starts to balance the collapse that eventually halts in the core
 convective proto-star reachs the Hayashi limit in the CM diagram and enter the hydrostatic
equilibrium regime (becoming a T-Tauri star)
 the core starts to burn D and Li and becomes radiative, the envelope continue contracting
at nearly constant luminosity, the effective temperature continues rising, violent surface
activity and strong protostellar winds occur (cleaning the remaining envelope)
 ultimately the core temperature rises to the point at which the thermonuclear fusion of H
into He becomes possible, and the star settles on to the ZAMS
 The formation of stars
 The formation of stars
Associations:
 chain reaction
 Initial Mass Functions
 distribution of masses of a freshly formed (just after a burst of SF) stellar population
dN = N0 Ж(M) dM
where N0 is the normalizing constant – the number of solar masses contained in the burst,
and Ж(M) is the IMF
 we have no a priori reason to suppose that Ж is a universal function that applies to all SB,
but observations indicate consistency at least for M  M
 determinations of the IMF are more difficult and unreliable for low mass stars, because these
stars are slow to settle on the MS and their spectra deviate significantly from that of a
black-body
 Initial Mass Functions
Parametrizations of the IMF:
 E. Salpeter [1955, ApJ 121, 161] proposed a simple power law for the IMF:
Ж(M)  M –2.35
 More recently, J. Scalo [1986, Fundam. Cosmic Physics 11, 1] presented new data on IMF,
which can be adequately fitted by three power-law segments:
Ж(M)  M –α
Salpeter
Scalo
α=
2.45 for 10 < M < 100M
3.27 for 1 < M < 10M
1.83 for 0.2 < M < 1M
 Kroupa, Tout & Gilmore [1993, MNRAS 262, 545]
also advocated a three power law form for Ж,
but one that falls off more steeply at the
high mass end:
α=
2.7
2.2
1.3
for 1 < M < 100M
for 0.5 < M < 1M
for 0.08 < M < 0.5M
Minimum mass (for H burning) → 0.08 M
Maximum mass (for equilibrium) → 100 M
 Initial Mass Functions
[Salpeter 1955, ApJ 121, 161]
[Miller & Scalo 1979, ApJS 41, 513]
[Scalo 1986, Fundam. Cosmic
Physics 11, 1]
[Kroupa, Tout & Gilmore 1993,
MNRAS 262, 545]
 Initial Mass Functions
Parametrizations of the IMF:
 Kennicutt, Tamblyn & Congdon [1994, ApJ 435, 22] argued, from the position of their galaxies
in the [B-V, W(Hα)] plane, that the IMF in these systems has to be about as rich in massive
stars as the Salpeter IMF predicts
[Kennicutt 1983, AJ 88, 1094] – IMF close to Salpeter
at high masses
[Kennicutt, Tamblyn & Congdom 1994, ApJ 435, 22]
 Observational indicators of Star Formation
Past SF:
 colors
Present SF:
 recombination emission lines
 UV continuum from hot stars
 thermal far infra-red (FIR) from dust
 radio continuum
 CO emission from molecular clouds
Future SF:
 amount of gas available
 Observational indicators of Star Formation
 Observational indicators of current Star Formation
Recombination emission lines:
 line emission is characteristic of HII regions (zones of ionezed gas, around young star clusters)
Mechanism:  H is ionized by absorption of Lyman continuum photons (λ < 912 Å, energy
above 13.6 eV) produced by the hot OB stars
 the line radiation we detect arises from the recombination of the e– so released with
another p+, and a cascade toward the ground state
M33
 Observational indicators of current Star Formation
Recombination emission lines:
 predominantly Balmer lines, especially Hα (the
strongest and easiest to deal with)
 narrow band imaging at Hα is usually used to find
HII regions
 SF rates may be calculated from the intensity of Hα
lines (and by measuring equivalent widths)
SFR [M/year] = 8.93  1042 L(Hα) [erg/s]
SFR [M/year] = 1.4  1041 L(OII3727) [erg/s]
NGC 5427 (Hα)
[Kennicutt 1983, ApJ 272, 54]
H
OII
Orion Nebula
OIII
OIII
H
NII
NII SII
 Observational indicators of current Star Formation
Recombination emission lines:
 not surprinsingly, the SFR per unit starlight climbs to later Hubble types
[Kennicutt & Kent 1983, AJ 88, 1094] 
 Observational indicators of current Star Formation
Ultraviolet continuum:
 stars not massive or hot enough to produce HII regions, but also young (less than 109 years
for early A stars, p.e.) can be traced by their brightness in UV
Mechanism:  they produce a UV continuum in SF galaxies at wavelengths longer than the
Lyman limit (λ = 912Å)
 this continuum is remarkbly flat, as can be modelled by spectral synthesis codes (first noted
by Lilly & Cowie [1987, Infrared Astronomy w/ Arrays] and Cowie [1988, ApJL 332, L29]); this
is due to the fact that, although these luminous stars have short lifetimes, they are constantly
being replaced by new stars
 the intensity of the flat part of the UVcontinuum is directly proportional to the rate of
formation of heavy elements, since these stars are the ones that produce supernovaes
 reddening (extinction) is a strong effect and correspondly serious uncertainty
SFR [M/year] = 1.7  1028 L(UV1250-2500) [erg s1 Hz1]
[White 1989, The Epoch of Galaxy Formation]
 Observational indicators of current Star Formation
Thermal far infra-red from dust emission:
 SF galaxies are also strong emitters in the FIR waveband because of the presence of dust
in the SF regions (on average, SF galaxies are stronger emitters in FIR than in the UV), as
was first shown by IRAS survey
Mechanism:  dust grains are heated by absorption of starlight, which operates most efficiently
in the blue and UV (as the waveband comes closer to the characteristic grain size)
 dust cools again by (approximately) black-body emission, with peak about 20-40 K
 although the mechanism is not well understood, the total UV-optical energy from young stars,
removed by dust absorption, must emerge in FIR (the luminosity from 10-300 μm may
equals that emitted originally from 912-3000 Å)m
 since there is an empirical tight relation between total FIR emission and Hα, this radiation
may be strongly coupled to current SFR
SFR [M/year] = 1.3  1029 L(FIR60m) [erg s1 Hz1]
 Observational indicators of current Star Formation
M82
Radio continuum:
 SF galaxies also emit in cm radio waveband, much of which must be connected, directly or
no, with SF
Mechanism:  emission is nonthermal, from synchrotron process (accelerated particles, perhaps
in SNe remnants, radiating while spiralling through large-scale magnetic fields)
 radio emission is a valuable tracer in heavily obscured regions: VLA mapping proved to be
a useful tool in identifying IR-loud galaxies at faint optical magnitudes since its positions
are much more accurate than IRAS centroids.
SFR [M/year] = 5.9  1029 L(HI1.42GHz) [erg s1 Hz1]
 Observational indicators of current Star Formation
CO emission lines from molecular clouds:
 since Giant Molecular Clouds are the immediate precursors of SF, their mapping is also an
indicator of SF
 CO molecules, the most abundant after H2, detected
at 1.3 and 2.6 mm (230 and 115 GHz) are usually
the tracers (H2 cannot be observed in radio domain
because it is symmetric and does not possess an
eletric dipole)
Antenae
 Lyman Break Galaxies
Photometric search of high z
SF galaxies:
 a color-selection technique for identifying
high redshift galaxies (with flat rest UV
continuum, like SF galaxies, and small
extinction) was first used by Steidel &
Hamilton [1992, AJ 104, 941], by Lilly et
al. [1995, ApJ 455, 108] for CFRS data
and Madau et al. [1996, MNRAS 283,
1388] for HDF data, the last ones to
measure SF at earlier times.
 since Lyman break is observed at
912(1+z) Å, for a z ≥ 2.5 it is found
in the optical – galaxies with z ~ 3
will be seen in B, V and R filters,
with similar magnitudes, but will not
be seen in U filter. These are called
Lyman break galaxies
[Steidel 1999, PNAS USA 96, 4232]
 Star Burst (SB) galaxies
Mark 357
 some galaxies show evidence of a
recent and transient increase in SFR
by as much as a factor of 50
(hundreds of M/year)
 since the galaxy gas is rapdly
consumed in the SF, exhaution
timescales are of order 107-108
years
 the burst is often confined to a few hundred pc near the nucleus, although disc-wide bursts are
also common
 SB are usually found in interacting galaxies, merging systems
and bursting dwarves
LMC (IRAS)
 global (or super) winds are also found, powered by energy of
starlight, stellar winds and supernovae
 associated very luminous compact star clusters (up to 108 L)
frequently occur (if these objects have a normal IMF, and
remain gravitationally bound after the mass loss from massive
members is complete, they will eventually become GC)
 it is possible that all galaxies may pass anytime a SB phase
30 Dor
 Star Burst (SB) galaxies
Observational characteristics of SB:
 large Balmer lines luminosity and equivalent width
 high ratio LFIR/LB
 unusual strong radio continuum emission
 optical spectra resembles those of HII regions
Possible SB mechanisms:
 cloud collisions in a perturbed disc
 collisions between clouds originally belonging
to different galaxies
 channelling of gas through bars towards the center
 tidally induced density waves
 disk instabilities produced by perturbations in the
gravitational potential
 physical transfer of gas during encounter (over a
critical limit)
 direct impact of gas rich dwarf satellites into disks
[Poggianti et al. 1999, ApJ 518, 576]
 Star Burst (SB) galaxies
M82 – X-rays, Chandra
M82 – Hα + SII, WIYN
M82 – UV
M82 – IR, SAO
M82 – radio, VLA
M82 – opt, HST
 SF along the cosmic time
The “Madau-Lilly” plot:
 the greatest rates of global SF occurred at z ~ 1-2!
[Gallego et al. 1995, ApJL 455, L1]
[Lilly et al. 1995, ApJ 455, 108]
[Connally et al. 1997, ApJL 486, L11]
[Madau et al. 1996, MNRAS 283, 1388 and
1998, ApJ 498, 106]
[Steidel 1999, PNAS USA 96, 4232]
 Star Burst (SB) galaxies
SB  AGN:
 for very luminous galaxies, which are dusty enough that most of their power emerges in the
FIR (once known as IRAS galaxies, now sharing such acronyms as LIRGs, ULIRGs, PIGs
or ELFs) it is difficult to determine whether the dominant energy source is a SB or a AGN
 many galaxies exhibit both nuclear activity and considerable SF activity, specially
interacting and merging systems
SB
• energy supplied by OB stars
• more diffuse radio emission
• lack of high ionization species
• strong PAH features (6.2 μm, p.e.)
• flat UV continuum
AGN
• energy comes from continuum produced by
central accretion disk
• compact, flat radio spectrum
• high ionization species
• PAH features are destroyed by the intense
hard radiation
• UV continuum inclined