Astronomy & Astrophysics
using multiwavelength data
Rainbow over the Anglo-Australian Telescope in Siding Spring Observatory, NSW, Australia © Á.R.L-S.
Massive stars, nebulae and star-forming galaxies
My astrophysical research is mainly focused in the analysis of star formation phenomena in both our own Milky Way galaxy and external galaxies, especially galaxy groups and in the so-called starburst galaxies, but using a multiwavelength approach. Hence, I'm combining optical, radio, infrared, ultraviolet and X-ray data in order to get a better understanding of the physical and chemical processes of astronomical objects. Indeed, it is a huge field and there still a lot of things to do. In particular, I'm interested in the following topics:
Despite of their relatively low number and short lifetime in terms of evolutionary timescales, massive stars have a fundamental influence over the interstellar medium and galactic evolution: they are responsible for the ionization of the surrounding gas; they deposit mechanical energy, first via strong stellar winds and later as supernovae; finally, they enrich the interstellar medium, not only by returning nuclear processed material during their whole lifetime but also in supernova explosions.
Although it is not my main research topic, the understanding of the basic parameters of the massive stars is fundamental since they are of prior importance for the study of star-forming galaxies. Indeed, the second chapter of my PhD is focused in massive stars and their general characteristics (luminosity, ionizing flux) and the importance of their stellar winds. In particular, Wolf-Rayet stars, evolved descents of the most massive, extremely hot and very luminous O stars, that are interpreted as central He-burning objects that have lost the main part of their H-rich envelope via strong winds and in consequence show products of different burning stages.
H II regions or diffuse nebulae are portions of molecular clouds where new star formation has taken place within the last few million years, hosting massive stars that are still in their hot main-sequence stage. The study of the spectra of the ionized gas in H II regions let estimate their chemical abundances. This is one of the three main methods to calculate the metallicity of an object in the Universe, apart from the stellar spectroscopy and the abundances measured in our Solar System. But the analysis of the ionized gas has the advantage of its high luminosity, high surface brightness and emission line spectrum, permitting its direct observation in far objects if the ionized cluster is massive and bright. This possibility lets to get observational constrains in different epochs and places to the chemical composition of the medium that is observed.
Indeed, that is one of my main topics because the chemical analysis of the ionized gas in starburst galaxies is essentially the same that the analysis of Galactic nebulae. Chapter 3 of my PhD is focused in this aspect. I analyze the physical parameters (electron density and temperature, extinction, ionization degree, kinematics... ) and the chemical parameters (chemical abundances of metallic elements: O, N, S, Ne, Ar, Fe, Cl, ...) of the ionized gas in H II regions. I'm also working in the comparison of different so-called empirical methods using the strongest collisional excited lines with the results given by the direct method. Finally, I'm also interested in the ionic abundances derived using the weak recombination lines of metallic elements (O++ and C++), the discrepance problem when their results are compared with those obtained using the collisional excited lines and the temperature fluctuations needed to explain such discrepancy.
Spiral galaxy NGC 247,
member of the Sculptor group of galaxies.
Center of the starburst galaxy NGC 5253,
combining data from the HST.
Starburst galaxy NGC 5253,
member of the M 83 group of galaxies.
Image combining H I data from LVHIS project (blue), deep R image (green) and Hα (red).
© A.R.L-S., LVHIS Team.
Spiral galaxy ESO 274-G001 in H I.
© B. Koribalski, J. van Eymeren & A.R.L-S.,
Multiwavelength image (UV + Optical + HI) of the Galaxy Pair NGC 1512/1510. See paper Gas Dynamics and Star Formation in the Galaxy Pair NGC 1512/1510. Koribalski, S. & López-Sánchez, Á.R. 2009, MNRAS.
Galaxies display a large variety of observational and physical properties. Some of them, such as luminosity and size, correlate well with galaxy mass. Others, such as colour, emission-line strength, amount of neutral gas and far-infrared luminosity are well correlated with morphological appearance such as tightness of the spiral arms the bulge-to-disk ratio, that are consequence of the amount of star-formation, gas and dust present in the galaxies. Understanding relationships between galaxy properties in detail is paramount to constructing a consistent picture of galaxy evolution. Star formation in galaxies is indeed my main research field. See Chapter 4 of my PhD Thesis for more details.
Following the definition given by Leitherer (2000), a starburst is a system which have a star formation rate (SFR) high enough that a statistically significant number of stars form which produce UV radiation. Such stars have masses between 10 and 100 \Mo. Equivalently, galaxies require SFRs of at least one order of magnitude above the SFR in the field outside the starburst region. The strength of the star formation in these objects is so high that the material available for the production of stars would be exhausted in a very short time compared to the age of the Universe. Their spectra remembered those of giant H II regions, so they are also named isolated extragalactic H II regions or H II galaxies.
I´m particullary interested in the so-called Wolf-Rayet (WR) galaxies. They are a subset of emission-line and H II galaxies whose integrated spectra show broad emission features attributed to the presence of WR stars, indicating the presence of a substantial population of this sort of massive stars in the ionized cluster(s) of the star-formation bursts. Indeed, my PhD Thesis is titled Massive star formation in dwarf Wolf-Rayet galaxies. I also investigate the Blue Compact Dwarf Galaxies (BCDGs). I analyze the chemical and the physical properties of the ionized gas in these objects using the tools developed to the study of both massive stars and H II regions. The analysis of the neutral gas is performed via H I radio observations. I complete the multiwavelenth analysis of Wolf-Rayet galaxies and BCDGs compiling all X-ray, ultraviolet and infrared data available in previous studies. This study led to understand the trigger mechanism of the strong star formation activity (the majority of the cases, it is consequence of interactions with dwarf or very diffuse objects), estimate the star formation rate, the chemical evolution, the stellar populations and the dynamical properties of the galaxies.
There is increasing evidence that the environment in which galaxies reside plays a key role in both their evolution and final fate. Most galaxies are found in groups of no more than a few dozen members, as hierarchical formation models predict. Interactions and merging processes are common in groups of galaxies: they induce profound morphological and kinema-tical galaxy transformations, trigger strong star formation activity, eject material into galactic medium originating Tidal Dwarf Galaxies (TDGs) and drive the final destiny of their members. Therefore, groups of galaxies are fundamental targets to get clues about the formation and evolution of galaxies.
I'm performing multiwavelength observations of nearby starburst galaxies in group of galaxies to perform a comprehensive analysis of each system in order to understand the general properties of galaxies, their nature, environment and star formation history and the importance of the interactions and mergers between galaxies in their evolution.
The Local Volume HI Survey (LVHIS) is a project comprising deep H I line and 20-cm radio continuum observations for all nearby, gas-rich galaxies. The initial sample consists of all galaxies with D < 10 Mpc that are detected in the HI Parkes All-Sky Survey (HIPASS).
The PI of the LVHIS project is Bärbel Koribalski (CSIRO Astronomy and Space Science / ATNF). The LVHIS team is listed here. My main contribution to this project is to perform a multiwavelength comparison of the LVHIS radio data with the results obtained in other frequencies (UV, optical/NIR and MIR). New exciting results are coming soon!