IMPROVING OUR UNDERSTANDING OF LOW SURFACE BRIGHTNESS GALAXIES

Abstract

IMPROVING OUR UNDERSTANDING OF LOW SURFACE BRIGHTNESS GALAXIES

Low Surface Brightness Galaxies (LSBs) studied in this project are Spiral Galaxies whose discs have a central surface brightness weaker than 22 mag/arcsec 2. Such value is determined by the historical value defined by Freeman (1970), which basically indicates the low stellar density of these galaxies’ discs. In the last decades, it was discovered that LSBs also have enigmatic properties: in spite of having a very low Stellar Formation Rate (less than 1 solar mass per year) they contain the whole array of stellar populations found in other Spiral Galaxies. As a consequence of the low SFR, they have a low metallicity and therefore small amounts of dust. Given these three properties, LSBs are an interesting subject of study.

In this project, the researcher intended to achieve a better understanding of physical processes involved in formation and evolution of LSB galaxies, considering various observable phenomena. Age and metallicity of LSB Galaxies and their function with structural parameters such as nucleus size and brightness and disc scale parameters, were obtained from optical/near-infrared colors and optical spectra obtained in Chile and from other data bases.

Most relevant results include proving that LSBs’ scale relations tend to satisfy a secular evolution model; that is, nuclei are formed from the slow evolution of the disk (Galaz et al., 2002, 2006). On High Surface Brightness Spiral Galaxies, on the contrary, the disk seems to be formed after monolithic formation of the nucleus.

Likewise, size of nuclei tends to be strongly correlated to environment metallicity of LSBs. This result had not been previously observed. It was also possible to study the relation between surface brightness and distribution of galaxy groups through models, and properties of the luminosity function in galaxies that are part of a group in comparison to isolated galaxies (Gonzales et al. 2005).

At the final stages of this project, it was also possible to study more intimate properties of LSB Galaxies’ interstellar environment, which is a fundamental subject for understanding, for example, why is LSBs’ SFR so low.

For the first time, a group of LSB galaxies was studied in sub-mm using APEX and it was possible to detect 12CO(J=3-2) emission in two galaxies located at the Freeman limit (ų0(B)=22.0 mag/arsec-2). Nature and magnitude of emission in these galaxies suggest that LSBs' interstellar environment has a higher temperature than that of High Surface Brightness Spiral Galaxies (Galaz et al., 2008). This higher temperature would be the key factor that inhibits condensation of the interstellar environment at higher densities and lower temperatures, which is suitable for the consequent stellar formation. This mechanism would explain the low SFR. Low SFR, in turn, involves low metallicity and reduced amount of dust. This amount, in turn, strongly influences a lack of “shielding” to UV radiations, and heats the environment.

The nature of such energy source has yet to be studied. It must be noted that no LSB included in this study has an active nucleus.

This project helped 4 students obtain their Bachelor Degree in Astronomy, and allowed results to be presented in 8 national and international conferences. As a result, 5 refereed publications directly related to the subject of this study have been published, including a Letter and other similar publications.

Comisión Nacional de Investigación Científica y Tecnológica - CONICYT