Chapter 1: Introduction
Stars are the building blocks of the universe. At optical wavelengths most of the light
we observe in the sky originates from stars. During their lifetime stars have profound
effects on their surroundings; heating it with radiation, sweeping up gas by
winds and outflows triggering more stars to form, and creating the heavy elements
that are the base for the formation of planets and ultimately life itself. Therefore, understanding
how stars form is one of the most fundamental topics in astronomy. In
order to study star formation, one has to study their birthplace, the cold and dense
parts of the gas that is dispersed throughout galaxies, called the interstellar medium
(ISM).
In this thesis we present a study of the ISM and how it is influenced by the formation
of stars, both in the Milky Way and in other galaxies.
Chapter 2: Modeling the infrared properties of dusty cores of starburst galaxies
Despite extensive observations over the last decades, the central questions regarding
the power source of the large IR luminosity of Ultra Luminous Infra Red
Galaxies (ULIRGs), and their evolution, are still not fully answered. In this chapter
we will focus on massive star formation as a central engine and present an evolutionary
model for these dust-enshrouded star formation regions.
An evolutionary model was created using existing star formation and radiative
transfer codes (STARBURST99, RADMC and RADICAL) as building blocks. The
results of the simulations are compared to data from two IRAS catalogs.
From the simulations it is found that the dust surrounding the starburst region
is made up from two components. There is a low optical depth (tau=0.1, which corresponds
to 0.1% of the total dust mass), hot (T~400 K) non-grey component close
to the starburst (scale size 10 pc) and a large scale, colder grey component (100 pc,
75 K) with a much larger column (tau=10). The simulations also show that starburst
galaxies can be powered by massive star formation. The parameters for this star
forming region are difficult to determine, since the IR continuum luminosity is only
sensitive to the total UV input. Therefore, there is a degeneracy between the total
starburst mass and the initial mass function (IMF) slope. A less massive star formation
with a shallower IMF will produce the same amount of OB stars and therefore
the same amount of irradiating UV flux. Assuming the stars are formed according
to a Salpeter IMF (Psi(M)~M^-2.35), the star formation region should produce 10^9Msunof stars (either in one instantaneous burst, or in a continuous process) in order to
produce enough IR radiation.
Our models confirm that massive star formation is a valid power source for
ULIRGs. In order to remove degeneracies and further determine the parameters
of the physical environment also IR spectral features and molecular emissions need
to be included.
Chapter 3: Dense gas in luminous infrared galaxies
Molecules that trace the high-density regions of the interstellar medium have been
observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate multiplemolecule
multiple-transition studies to evaluate the physical and chemical environment
of the nuclear medium and its response to the ongoing nuclear activity.
The HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0) and CN(2-1), CO(2-1), and
CS(3-2) transitions were observed in sources covering three decades of infrared luminosity
including sources with known OH megamaser activity. The data for the
molecules that trace the high-density regions have been augmented with data available
in the literature.
The integrated emissions of high-density tracer molecules show a strong relation
to the far-infrared luminosity. Ratios of integrated line luminosities have been used
for a first order diagnosis of the integrated molecular environment of the evolving
nuclear starbursts. Diagnostic diagrams display significant differentiation among
the sources that relate to the initial conditions and the radiative excitation environment.
Initial differentiation has been introduced between the FUV radiation
field in photon-dominated-regions and the X-ray field in X-ray-dominated-regions.
The galaxies displaying OH megamaser activity have line ratios typical of photondominated
regions.
Chapter 4: Mechanical feedback in the molecular ISM of luminous IR galaxies
Molecular emission lines originating in the nuclei of luminous infra-red galaxies
are used to determine the physical properties of the nuclear ISM in these systems.
A large observational database of molecular emission lines is compared with
model predictions that include heating by UV and X-ray radiation, mechanical heating,
and the effects of cosmic rays.
The observed line ratios and model predictions imply a separation of the observed
systems into three groups: XDRs, UV-dominated high-density (n>10^5 cm-3)
PDRs, and lower-density (n=10^4.5 cm-3) PDRs that are dominated by mechanical
feedback.
The division of the two types of PDRs follows naturally from the evolution of the
star formation cycle of these sources, which evolves from deeply embedded young
stars, resulting in high-density (n>10^5 cm-3) PDRs, to a stage where the gas density
has decreased (n<10^4.5 cm-3) and mechanical feedback from supernova shocks dominates
the heating budget.
Chapter 5: Multi-molecular studies of Galactic star forming regions: physical properties and comparison with an extra-galactic sample
Molecular emission line observations from Galactic star-forming regions are used
to determine the physical properties of the molecular ISM in these systems. The
results are used to benchmark similar observations from extra-galactic star-forming
regions.
A large set of observations is compared to the results predicted by models that incorporate
gas-phase chemistry and the heating by stellar radiation and non-radiative
feedback processes.
Two distinct components can be identified in the observed sources: small (~0.4 pc),
cold (40-50 K) and high-density (10^5-10^5.5 cm-3) PDRs enveloped by a more extended,
warm (~300 K) molecular component with lower density (10^4.5-10^5 cm-3), that is
heated by mechanical feedback processes. The sulphur abundance in the sources
was found to be depleted by a factor of 200 to 400 with respect to solar values.
Modeling results show that multi-molecular diagnostic studies are very effective
in determining the physical and chemical properties of star-formation regions,
where a combination of characteristic line ratios indicate specific conditions in the
source. A comparison of the molecular properties of the Galactic sample with those
of star-forming galaxies shows that the Galactic PDR components have very similar
properties as the integrated PDRs in Ultra Luminous IR Galaxies. A difference
is found in the effects of non-radiative heating on the molecular signature of the
two types of regions. Whereas in Galactic sources the heated (300K) envelope can
be clearly identied, this component is less pronounced in the extra-galactic systems
and requires lower (150-250K) temperatures. This difference results from the
increased superposition of (hot and cold) ISM components in extra-galactic systems.
Chapter 6: Time evolution of the molecular ISM in luminous IR galaxies
Molecules that trace the high-density regions of the interstellar medium may be
used to evaluate the changing physical and chemical environment during the ongoing
nuclear activity in (Ultra-)Luminous Infrared Galaxies.
The changing ratios of the HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0) and
CN(2-1), CO(2-1), and CS(3-2) transitions were compared with the HCN(1-0)/CO(1-0)
ratio which represents the progression time scale of the starburst. These
diagnostic diagrams were interpreted using the results of theoretical modeling using
a large physical and chemical network to describe the state of the nuclear ISM in the
evolving galaxies.
Systematic changes are seen in the line ratios as the sources evolve from an early
stage for the nuclear starburst (ULIRGs) to later stages. These changes result from
changing environmental conditions and particularly from the lowering of the average
density of the medium from around 10^5.5 to as low as 10^4 cm-3. A temperature
rise due to mechanical heating of the medium by feedback, from ~40K in the dense
PDRs up to ~250K in late stages, explains the lowering of the ratios at later evolutionary
stages. Infrared pumping may affect the CN and HNC line ratios during
early evolutionary stages.
Molecular transitions display a behavior that relates to changes of the environment
during an evolving nuclear starburst. Molecular properties can be used to
determine the physical properties
Chapter 7: Nederlandse samenvatting
Bijna al het licht dat we met het blote oog aan de hemel zien, is afkomstig van sterren.
Overdag zien we het licht van de zon, de dichtstbijzijnde ster. 's Nachts zien
we, behalve de sterren, de maan en de planeten die zonlicht weerkaatsen. Maar er is
meer aan de hemel dan alleen sterren. Als je naar de hemel zou kunnen kijken met
andere ogen, dan zou het er heel anders uitzien.
Figuur 7.1 geeft hier een mooi voorbeeld van. Links staat een foto van het sterrenbeeld
Orion zoals het te zien is met het blote oog. Op deze foto zie je de bekende
sterren van Orion zelf, de achtergrondsterren en, in de onderste helft van Orion, de
Orion nevel. De rechterhelft van Figuur 7.1 laat hetzelfde stuk van de hemel zien,
maar nu in infrarood-straling ("warmte-straling"). Ineens ziet alles er heel anders
uit: sommige van de sterren zijn niet meer te zien (bijvoorbeeld Rigel, rechts onderin),
en overal zijn "wolken" te zien.
Deze wolken bestaan uit stof en gas dat verspreid is in de ruimte tussen de sterren,
het zogeheten InterStellaire Medium (ISM). De eigenschappen van het ISM zijn
erg gevariëerd. Het grootste deel van de ruimte is gevuld met heel heet gas (tot wel
10 miljoen graden) dat een hele lage dichtheid heeft (minder dan een atoom per kubieke
centimeter). In dit verspreide, diffuse gas bevinden zich kleinere wolken met
een veel lagere temperatuur (ruim 200 graden onder nul) en een veel hogere dichtheid
(enkele duizenden tot miljoenen deeltjes per kubieke centimeter). Door de
hoge dichtheid en lage temperatuur in deze wolken kunnen atomen zich samenvoegen
tot moleculen. Het zijn deze moleculaire wolken die in dit proefschrift worden
bestudeerd.