Various types of galaxies are spread out throughout the Universe. The basic components of galaxies are stars, gas and dust. During their evolution these three components are interacting with each other. There are regions in the galaxies, the so-called molecular clouds, with large quantities of dust and gas (mostly atomic and molecular hydrogen) providing ideal conditions for the creation of new stars. Stars are forming, live their lives and die. During the last stages of their lives they "donate" their materials (gas and metals) to the interstellar space (via stellar winds or violent explosion events) and as a result conditions for birth of new stars are set. This cycle between star formation and enrichment of the interstellar medium is keeping the galaxies as live entities that evolve with time.

There is a special group of galaxies that emit strongly at infrared wavelengths. These galaxies are called Luminous Infrared Galaxies (LIRGs) and have infrared luminosities (LIR) higher than 10^11 Lsun (they are called ULIRGs - Ultra Luminous Infrared Galaxies - when LIR=10^12 Lsun). Over the last decade comprehensive observations from X-rays through radio wavelengths have produced a consensus picture of local LIRGs, showing that they are mergers between gas-rich galaxies, where the interaction triggers some combination of dust-enshrouded star-formation and Active Galactic Nuclei (AGN) activity.

In our study we analyzed observations of this specific group of galaxies obtained through a long-term campaign during the past ~10 years. These observations (at far-infrared and sub-millimeter wavelengths) mainly reflect the presence of the molecule of carbon monoxide (CO) through its various transition states but also other important molecules such as 13CO and HCN. Due to limitations due to Earth's atmosphere we can only observe the low transitions of this molecule from ground based facilities. We have done so by using the JCMT telescope in Hawaii and the IRAM-30m telescope in Spain. For the higher transition states we made use of the Herschel Space Observatory of the European Space Agency. This effort provided us with a unique database for exploring the properties of the gas in dense molecular clouds. It is worth mentioning that studies so far have mostly been limited to the first two transitions of CO (J=0-1 and 1-2). With our database we can, for the first time, study transitions up to J=13-12. This provides us with the tools to probe deep into the molecular clouds and determine the conditions in the densest environments of the interstellar medium in galaxies.

Moreover, submm interferometry of a local ULIRG found M(H2)~10^10 Msun of dense gas “packed” in ~100 pc-sized regions where upwards of 50 supernovae remnants (SNRs) reside (Sakamoto et al. 2008, ApJ, 684, 957). The effect of the powerful SNR mechanical feedback on the initial conditions of star formation in such extreme environments, is currently unknown. We will use JCMT, IRAM 30m, APEX and SPM to build the first molecular SLEDs of dense gas impacted by SNRs. This will help to determine the kinetic energy of the shocks imparted, while the SLEDs will beautifully quantify their effect on the dense gas, and thus on the star formation initial conditions typical in the early Universe.