Star formation across cosmic time
Astronomical observations show that ten billion years ago,
galaxies formed their stars much more rapidly than now. As
stars are formed from cold molecular gas, this implies a
significant gas supply and galaxies near the peak epoch of
star formation are indeed much more gas-rich: the
winding-down of star formation seems to be mostly due to the
diminishing cold gas reservoirs. But the star formation processes
could also have been different, and potentially more
efficient, earlier in the history of the Universe...
To investigate star formation at different epochs, I am part
of the PHIBSS and PHIBSS2 teams revolving around IRAM
Plateau de Bure & NOEMA molecular gas observations within normal
star-forming galaxies. I am also involved in observations at
the Atacama Large Millimetre Array (ALMA).
For more details:
||Genzel et al. (2015): Combined CO & Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star Formation Rate and Stellar Mass
||Freundlich et al. (2013): Towards a resolved Kennicutt-Schmidt law at high redshift
||Genzel et al. (2013): Phibss:
Molecular Gas, Extinction, Star Formation, and Kinematics in
the z = 1.5 Star-forming Galaxy EGS13011166
The cosmic star formation history with its peak ten
billion years ago (Madau & Dickinson 2014)
The Kennicutt-Schmidt relation between the gas and star
formation rate surface densities characterizes the star
formation efficiency and suggests similar star formation
processes at low and high redshift (Bigiel et al. 2008, Freundlich et
al. 2013, Genzel et al. 2013).
Galaxies at the peak epoch of star formation
Galaxies near the peak epoch of star formation are not as
regular as nearby galaxies: their rotating gas-rich disks are fragmented in a
few star-forming clumps, are particularly turbulent and host
violent gravitational instabilities which could contribute to
their high star formation rates. Cycles of compaction, depletion
and replenishment of the gas could maintain a relatively tight
relation between their star formation rate and their stellar mass,
until star formation eventually quenches. This quenching might be
due to a combination of factors including gas removal by
supernovae or active galactic nuclei winds, the shutting down of gas
accretion onto the galaxy, a sudden drop in the gas cooling, a
change in morphology and environmental effects.
Together with high resolution observations such as those obtained by
ALMA, numerical simulations help better understand the evolution of
galaxies and the quenching of their star formation.
EGS13011166, a clumpy
star-forming galaxy at z=1.5 as seen by the Hubble Space
Telescope (Genzel et al. 2013).
How do galaxies get their gas?
To sustain high levels of star formation, high-redshift
galaxies require a significant gas supply, which can either be
brought through major mergers of galaxies or through relatively
smooth and steady accretion. Cold gas can notably penetrate deep inside
galactic haloes along dense streams stemming from the filaments of
the cosmic web. But these streams are prone to Kelvin-Helmholtz
instabilities and could also fragment gravitationally: do they
reach the galaxies' centers?
||Dekel et al. (2009): Cold streams in early massive hot haloes as the main mode of galaxy formation
Gas streams from the cosmic web
feeding a galaxy from the MareNostrum simulation (Dekel et al. 2009).
The influence of baryons on dark matter haloes
In the standard cold dark matter paradigm, each galaxy is
assumed to be embedded in a diffuse dark matter halo. But while
dark matter cosmological simulations predict steep 'cuspy' inner
density profiles for these halos, observations favor shallower
'cores'. Feedback mechanisms from stars and active galactic nuclei (AGN)
seem essential to resolve this discrepancy. Repeated gravitational
potential fluctuations induced by stellar winds, supernova
explosions and AGN could dynamically heat the dark matter halo and
lead to the formation of a core.
We propose a theoretical model in which the potential
fluctuations leading to core formation arise from feedback-induced
stochastic density variations in the gas distribution and their
dynamical effects are model as a diffusion process. This model
provides a relatively simple parametrization of the cusp core
transformation but should be further tested with hydrodynamical
simulations and compared with other models.
For more details:
Artist's view of a galaxy
surrounded by its dark matter halo (A. Evans, adapted by
J. Freundlich & F. Ducouret).
Stochastic density fluctuations
in the gas distribution of an initially cuspy halo leads to the formation of a
core (El-Zant et al. 2016).
Star formation in the interstellar medium
The giant molecular clouds in which star are formed are not
smooth, regular features: they are highly structured at
smaller scales and host complex networks of overdense
filamentary structures driven by
turbulence. Most pre-stellar cores lie within these
filaments. Does their cylindrical geometry affect core and
To investigate the growth of gravitational instabilities
within filamentary structures, we consider idealized
self-gravitating filaments and studied the
dispersion relation arising from small perturbations within
them. Such calculations might also be relevent for the
filaments of the cosmic web.
For more details:
Herschel reveals filamentary structures in the
Aquila star-forming complex (Herschel "Gould Belt survey" Key
Programme / P. André & D. Arzoumanian).
Fragmentation of an idealized
self-gravitating filament as beads on a string (Freundlich 2015).