Investigating the properties of dwarf galaxies with Local Group transition types

Abstract

Las galaxias enanas son las mas pequeñas y numerosas que se pueden encontrar en el Universo. Sus masas relativamente pequeñas hacen que su evolución sea susceptible a efectos externos e internos. Debido a esto, el estudio de galaxias enanas que se encuentran aisladas da la oportunidad de aprender sobre los mecanismos internos que han afectado a sus propiedades sin preocuparse sobre los efectos del entorno debido a interacciones con sistemas mucho mayores. Este trabajo se centra en una de las galaxias enanas aisladas con alto contenido en gas del Grupo Local, Aquarius, para poder determinar su cinemática interna y sus propiedades de metalicidad. El proyecto se basa en datos espectroscópicos obtenidos con FORS2/VLT durante aproximadamente 15 horas de observaciones en el modo MXU. Como objetivo, se aprenderá a reducir los datos espectroscópicos, extraer los espectros y derivar las velocidades y metalicidades para las estrellas de Aquarius. Esto se usara para determinar las propiedades básicas, como la velocidad sistémica y la dispersión de la componente estelar, así como explorar la presencia de gradientes de metalicidad. Se han encontrado velocidades heliocéntricas de entorno a −140 km/s para la mayoría de las fuentes, lo que concuerda con resultados previos sobre Aquarius. Algunas estrellas presentan velocidades distintas a lo esperado, lo que se debe a su pertenencia a la Vía Láctea y están situadas en la línea de visión de Aquarius. A partir de estas, se ha derivado la velocidad sistémica de Aquarius, −138.9±0.9 km/s, descubriendo la presencia de rotación en la componente estelar. Los resultados indican que las estrellas están rotando en sentido opuesto al gas. El estudio fotométrico de las fuentes se hizo comparando con un catálogo donde se recoge información de las estrellas hasta magnitud de aproximadamente 26 en la banda I. La mayoría de las estrellas de este trabajo tienen magnitudes en I < 21.7, como se esperaba inicialmente al seleccionar las fuentes. Para derivar la metalicidad de las estrellas se ha calculado la anchura equivalente de las líneas del triplete de calcio. Este método se ha utilizado para determinar la metalicidad en estrellas de cúmulos globulares y para estrellas gigantes rojas en sistemas más grandes y complejos, como galaxias. El triplete se localiza en torno a los 8500A y es un rasgo notable en los espectros de las gigantes rojas. Es posible obtener una aproximación precisa al valor de la metalicidad utilizando espectros de resolución baja-intermedia con este método. La metalicidad obtenida de la galaxia es [F e/H] = −1.56 dex con σF e/H = 0.19 dex. Simulaciones teóricas predicen que la rotación en una galaxia provocaría que la metalicidad de las estrellas fuese parecida a todos los radios. Tras estudiar esta variación de [F e/H] con el radio galáctico se ha encontrado que no hay gradiente de metalicidad, lo que supone una prueba observacional para comprobar la veracidad de las simulaciones.

Dwarf galaxies are the smallest and most numerous galaxies found in the Universe. Their relatively small masses make their evolution susceptible both to internal and external effects. In this respect, the study of dwarf galaxies found in isolation gives the opportunity to learn about the internal mechanisms that have affected the properties of the galaxies, without concerns about environmental effects due to interactions with much larger systems. This project focuses on one of the isolated gas-rich Local Group dwarf galaxies, Aquarius, in order to determine its internal kinematics and metallicity properties. The project is based on 15h of VLT/FORS2 spectroscopic data in MXU mode. The objective will be learning how to reduce the spectroscopic data, extract spectra and derive velocities and metallicities for the individual stars. These will be used to determine basic properties such as the systemic velocity and dispersion of the stellar component of the galaxy and explore the presence of metallicity gradients. The total number of stellar sources in this project is 55, distributed along the galaxy. Most of them correspond to red giant stars whose spectral features, such as the near-infrared calcium triplet lines, are a good indicator of the metallicity. The reduction of the data was performed using software of IRAF, Python and IDL. This project is an improvement respect to previous spectroscopic studies of Aquarius, as we have approximately twice more targets to study. We have found heliocentric velocities of about −140 km/s for most of the observed stellar sources, which is in agreement with previous results obtained for this galaxy. We have found several stars with different velocities from what was expected. This is due to the presence of stars belonging to the Milky Way in the line of sight of Aquarius. Several shifts have been applied to the spectra in order to extract these heliocentric velocities. They were due to several effects, as putting the spectra in the heliocentric system, cross-correlate the spectra with a sky spectra to associate the wavelength accordingly and correct the difference in the position of the center of the slit with respect to the position of the center of the star. This have led to some quality checks, which have been done to the various shifts in the determination of the individual velocities to see if all the corrections were done properly. The photometric study of the sources was done by making a comparison with a previous catalogue with photometric information of the stars of the galaxy down to magnitude 26 in the I band. The catalogue used contained information about the magnitudes both in the V and I bands. The magnitudes found for most of the stars in the I band < 21.7, which was expected when initially the sources were selected for the observations. To derive the systemic velocity for Aquarius, some criteria have been introduced in order to select the stars of the sample which where indeed from the galaxy. This criteria was based on the values of the individual velocities of the stars, its colors and its magnitudes. The initial 55 stars were finally reduced to 44 possible members of the galaxy by applying an iterative method based on the median absolute deviation of the distribution of heliocentric velocities. The systemic velocity of the galaxy based on these 44 stars is −138.9 ± 0.9 km/s. It has been also noticed the presence of rotation in the stellar component. Previous results showed the presence of rotation of the gas around the minor axis of the galaxy, but none of them have seen rotation of the stars. According to our results, the stars are counter-rotating respect to the gas, which may be possibly due to the fact that the stars in this study are relatively old giant branch stars. Younger stars are still associated to the gas of the galaxy because they have been formed recently there. However older stars are been studied here, which are decoupled from the gas as they have been formed many years ago. If any perturbation to the gas has occurred in a galaxy like this with really low velocity dispersion (approximately 10 km/s), older stars would not be in principle highly affected. This result will require of deeper and further investigation. To derive the metallicity of the stars of Aquarius, we have determined the equivalent width of the calcium triplet lines and using relations between the calcium triplet equivalent width and the [Fe/H] abundance, determined in the literature. The lines are located around 8500A and are clear ˚ features of the spectra of red giant stars. Due to this fact, it is possible to obtain an accurate approximation of the value of the metallicity with low-intermediate resolution spectra, saving the observational time that would be required for obtaining high resolution spectra. The value obtained for the metallicity of Aquarius galaxy is [F e/H] = −1.56 dex with σF e/H = 0.19 dex. This result is compatible with previous results and also with the expected relation metallicity-luminosity for a galaxy like this in the Local Group. As Aquarius presents rotation both in gas and stars, the gas should be more or less uniformly distributed along the galaxy. Theoretical simulations predict that this would produce that the metallicity of the stars should be roughly equal at all radius. We have studied the variation of the metallicity of the stars along the radius of the galaxy to see if there was a visible variation. No metallicity gradient has been found, which gives some observational support to these theoretical predictions. Nevertheless, this simulations are still to be totally confirmed with the study of metallicity gradients in others dwarf galaxies in the Local Group.