Star Formation in High Redshift Galaxy Mergers

Star Formation & Galaxy Mergers

In this essay, we will describe the processes leading to the formation of stars in the bottom of dark matter halo potential wells and how the interaction and merging between galaxies can further induce and enhance this star formation. We do this by partially following Mo et al. (2010) for a detailed and extensive in-depth treatment of the mechanisms involved in star formation mentioned here.

Having isolated (for now) a dark matter halo, the gas eventually makes up the observable galaxy. Hence, our primary source of information will start condensing while sinking toward the bottom of the dark matter potential well. In its simplest form, condensation is the process of becoming more dense and cooling. From a chemical point of view, a condensation reaction is the formation of molecules from smaller reactants. This is precisely what is thought to happen when gas condensates to form galaxies and stars.

The internal energy of the gas reservoirs in galaxies decreases mainly by radioactive cooling. The gas is under the gravitational influence of the dark matter halo and, to some extent, its own self-gravitation and, therefore, contracts and increases its density as it cools. The gas mainly loses its energy via bremsstrahlung, where electrons are slowed down by interaction with atomic nuclei in the gas and hence loses energy and cools. When the gas has cooled and contracted significantly for actual interactions to happen, cooling via collisional ionization (atoms ionized by colliding electrons), recombination (an ionized atom captures an electron), and collisional excitation (and the atom is excited from a non-ground state and afterward decays to the ground state) also sets in. Complimentary to the radiative cooling, Compton scattering, where electrons and photons exchange energy, also contribute.

Allowing the gas to cool radiatively, of course, assumes that the gas can stream freely from the cloud, i.e., it is assumed that the galaxy is not opaque at the emitted wavelengths. The cooling, and thereby the contraction of the gas, is counteracted by the produced radiation that, instead of streaming freely, re-reacts with the gas resulting in photoionization heating. Hence, the cooling of the gas clouds in the galaxy is a balance between the self-gravity of the gas particles, the cooling radiation, and the radiation pressure.

And the Jeans formalism, therefore, also applies in this case. Thus, what determines whether the gas cloud continues to collapse or comes to a hold is the size of the initial perturbation of the particular region in the gas cloud, expressed via the Jeans length, λJ, The mass within a sphere of diameter λJ is called the Jeans mass, MJ. If the amount of gas within the Jeans sphere is larger than the Jeans mass, i.e., if Mgas > MJ, the considered the counteracting pressure does not stop gas parcel and will continue to collapse.

Such a free-falling gas contraction is referred to as the ‘cold mode’ of gas accretion. This is in contrast to the ‘hot mode’ of gas accretion, where it is not only the radiation pressure of the gas that counteracts the collapse. In the hot mode, the gas is heated to the virial temperature by shocks happening during the condensation. Cold mode accretion usually happens in less massive dark matter halos, whereas hot mode accretion predominantly happens in massive halos.

Eventually, whether the gas is condensing via cold or hot mode accretion, the gas will reach an equilibrium where the temperature, i.e., the kinetic energy, balances the cooling mechanisms. Thus, a thermal equilibrium is reached between the density of the gas, ρgas, and the gas temperature, Tgas,. At this stage, instabilities can be obtained if the temper[1]ature or the density of the equilibrated system is perturbed, either by thermal instabilities, i.e., by further internal cooling or heating, or by hydrodynamic instabilities as, for instance, turbulence where parcels of gas start moving around within the condensed gas cloud.

All of the above applies under the assumption that only the energy of the radiating gas contributes to the balances and equilibria. However, if stars have already formed, stellar winds or stellar explosions add new terms to the equation. Furthermore, suppose an AGN has been ignited at the center of the galaxy. In that case, this radiation will also counteract further collapse and needs to be included, which complicates the matter even further.

If all the radiative processes are nevertheless overcome by the gravitational contraction and the radiative and collisional cooling, the most dense regions of the gas cloud will eventually become so dense that they can start transforming1 into so-called giant molecular clouds. Giant molecular clouds mostly consist of H2, but depending on the enrichment of the gas, i.e., how many times the gas has been recycled via stellar winds and explosions, it might also contain, e.g., CO, H2O, and HCN. If the recombination of hydrogen to form H2 is more efficient than the photodissociation of H2, these clouds become stable. It is generally accepted that a galaxy’s overall star formation rate is determined by its ability to form these dense molecular clouds.

Assuming that the galaxy efficiently forms giant molecular clouds, the next challenge to overcome is to create an over-density of gas and molecules within this cloud, so dense that it starts burning hydrogen in its core, i.e., so dense that a star is born. The efficiency of forming stars from a giant molecular cloud has been estimated to be SFR ∼ 0.002, i.e., only of the order 0.2% of the available molecular gas is turned into stars . This last step of turning the densest regions of the molecular cloud into stars is a complicated matter and has not yet been fully understood. The reason for the very low star formation efficiency in molecular clouds is believed to be due to a combination of magnetic fields, winds from already formed stars dispersing the surrounding molecular material to prevent further star formation, and/or (maybe most importantly) turbulence in the molecular cloud itself. New stars are born assuming that all of the above conditions are in place and satisfied.

Thus, it is clear that without large enough gas reservoirs to form giant molecular clouds, stars will not ignite. Hence, it is not surprising that one of the more well-established relations, describing the star formation in galaxies.

As the Schmidt-Kennicut law relates the global star formation rate to the gas surface density of galaxies, it cannot tell us much about the actual star formation in individual clouds within galaxies but only provide information about the formation and evolution of galaxies as a whole. It has been shown that a clear relation between Σgas and ΣSFR is lacking within galaxies. More localized observational empirical relations between star formation rate and atomic and molecular gas have been established, enabling a more detailed study of the star formation within single galaxies rather than the global star formation in populations of galaxies Equation . Apart from the self-regulated star formation described above, another way to obtain the dense gas and giant molecular clouds needed to ignite stars is by compressing the material via external forces. An example of such an event is external interaction between another galaxy and its halo, or galaxy merging in the more extreme case. The gravitational interaction of close encounters can produce tidal tails. Such an enhancement in the density and kinetic energy of the gas and molecular clouds can trigger bursts of star formation by kick-starting the chain of processes outlined above.

The Star Formation & Galaxy Mergers 101 galaxy interactions and mergers produce density waves and tidal tails and enhance the gas’s angular momentum loss. This is an important step in funneling gas into the central regions of galaxies .If the loss is big enough, this can feed a possible AGN engine. More modest angular momentum loss can lead to enhanced density and temperature in the inner regions and trigger nuclear starbursts. Hence, the interaction and merging of galaxies can enhance and induce star formation and is, therefore, an important player in the evolution of galaxies.

FAQs about Star Formation and mergers

What is star formation?The process through which dense areas inside interstellar clouds of gas and dust collapse under gravity, resulting in the creation of new stars, is known as star formation. These areas are known as “stellar nurseries.”

What triggers star formation?

Star creation can be initiated by a variety of reasons, including the compression of gas clouds caused by shockwaves from nearby supernovae, the collision of galaxies or gas clouds, or the gravitational interaction of surrounding stars.

How long does it take for a star to form?

The timescale for star formation varies depending on the mass of the star and the conditions in the molecular cloud. Generally, it can take anywhere from a few hundred thousand to several million years for a star to form.

What are the stages of star formation?

The stages of star formation include the collapse of a molecular cloud, the formation of a protostar, the development of a pre-main-sequence star, and eventually reaching the main sequence, where the star spends most of its life.

What determines a star’s final mass?

A star’s final mass is determined by the amount of material present in the initial molecular cloud and the efficiency of the star formation process. Larger clouds with more material result in more massive stars.

Do all stars go through the same stages of formation?

While most stars follow a similar formation process, the details can vary based on the initial conditions of the molecular cloud, such as its density, temperature, and chemical composition.

What is a galaxy merger?

A galaxy merger is a cosmic event where two or more galaxies collide and eventually coalesce due to gravitational interactions.

How common are galaxy mergers?

Galaxy mergers are relatively common in the universe, especially in the early universe. Over time, smaller galaxies merge to form larger ones, contributing to the growth of galaxies.

What happens when galaxies merge?

When galaxies merge, their gravitational interaction causes stars, gas, and dust to interact, leading to significant disturbances. The combined gravitational forces also trigger intense star formation and sometimes the formation of supermassive black holes at the galactic centers.


Do galaxies always merge peacefully?

Galaxy mergers are often violent and chaotic events, especially when the galaxies have different sizes. The tidal forces can distort the shapes of the galaxies, and in some cases, the central black holes can merge, releasing enormous amounts of energy.

Can galaxy mergers create new stars?

Yes, galaxy mergers can trigger star formation. The gravitational interactions can compress gas and dust in the colliding galaxies, leading to the formation of new stars in what are known as “starburst” regions.

What happens to the supermassive black holes during a merger?

During a merger, the supermassive black holes at the centers of the merging galaxies can be brought closer together. They eventually spiral inwards due to gravitational waves and merge to form an even larger black hole.



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