Ministerio de Economía y Competitividad Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

Astrophysics Research Projects

The Local Universe

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Kinematic, Structural and Composition Studies of the Interstellar and Intergalactic Media (P/308603)

J E BECKMAN

Alejandro Serrano Borlaff, Joan Font Serra, Fatemeh Tabatabaei

Colaboradores del IAC: Emilio Casuso Romate, Johan H. Knapen, Begoña García Lorenzo, Martín López Corredoira, Inmaculada Martínez Valpuesta

L. Gutiérrez Albores, M. Rosado, A. Rodríguez, P. Velázquez (UNAM); P. Erwin, D. Willman (MPIE); M. Querejeta, G. van de Ven (MPIA, Heidelberg); P. Amram (Obs. Marsella); S. Comerón, S. Díaz (Univ. Oulu)

Introduction

The basic objective of the project is to investigate the evolution of galaxies by deepening our understanding of the interaction between the insterstellar medium and the stars. The main technique which we use is the two-dimensional kinematic study of whole galaxies observed using our instrument: GHaFaS, a Fabry-Perot interferometer on the William Herschel Telescope of the ORM.

We combine the data from GHaFaS with our own photometric imaging using ACAM on the same telescope, with a variety of sources of images in the infrared and ultraviolet, and with emission line maps in the radio and millimetre range to explore the effects, and the physical parameters which the determine the star formation rate and the mass function of the stars depending on the sites of their formation in galaxies.

In the present phase of the project the dynamical aspects which we are exploring are basically two: the resonant structure of discs and bars, and the effects of the zones of massive star formation on their surroundings via stellar winds and supernovae. In addition, in preparation for the exploration of galaxy formation in earlier epochs, we are studying how the interactions between galaxies stimulate and condition star formation within them before they merge. In the future these studies will be extended, methodically, in two directions: outwards to galaxies at increasing redshift, and inwards by applying our methods to the study of interstellar processes in the zones of massive star formation in the Local Group galaxy M33.

We combine our research into galaxy kinematics with evolutionary aspects of galaxy discs based on precise observations of their surface brightness profiles, both in local galaxies and in galaxies at intermediate redshift.

Milestones

1. The observation of a relation between the amplitude of the non-turbulent magnetic field of a galaxy on large scales and the mass of the galaxy.  This was based on the possibility of separating the turbulent and the aligned fields using radio continuum observations. This relation is simple (the B field depends on the ~0.3rd power of the mass) but requires an explanation which cannot be that of the simple dynamo model.

 

2. The discovery of two distinct populations of massive interstellar clouds, those with high mass, and those with very high mass (the separation between them is at some 3 million solar masses) in the discos of galaxies. These clouds start life as molecular clouds, but in those which harbour massive star formation they become HII regions. We have shown that the very high mass clouds are bound by their own internal gravity, while the others are bound by the pressure of the gas column outside them.  In the clouds of very high mass the star formation rate per unit gas mass is enhanced, and this probably leads to a different initial stellar mass function from that within the lower mass clouds. There are highly significant implications for stellar populations in galaxy discs.

 

3. It was shown that if the disc of a galaxy shows radial flaring in the distribution of its stars perpendicular to the galactic plane, if it is observed edge-on it will give a strong impression of a truncation. This modelling exercise explains why abrupt truncations have been observed in edge-on galaxies, while in face-on discs the fall-off in the radial surface brightness distribution towards the galaxy edge is much more gradual. There are important implications for models of disc evolution.

 

4. The new method of measuring corrotation radii for galaxy bars, developed within our group, (the "Font-Beckman" method) has been used to determine the ratio between the corrotation radius and the bar length for the main bars of some 70 galaxies. The bar length is found from infrared images obtained by the Spitzer satellite in the S4G project. This sample has more than doubled the number of galaxies where this ratio has been measured, and with greater precision than with previous methods. The results have been used to show that many of the bars rotate slowly, due to the braking effect of the dark matter halos of the galaxies, a result which contradicts those of the majority of previous studies in the field. Using simulations it was shown that this result is explained because the bars lengthen by acquisition of stars at the same time as they slow down, an effect which had not previously been taken into account. This result supports the presence of dark matter halos around the galaxies observed.

 

5. The field of the Tycho supernova remnant has been observed using the high velocity resolution and wide field of GHaFaS, to distinguish between the components of its emission lines produce by the different phases within its structure. It was possible to distinguish for the first time between the gas which has not yet been affected by the expansion shock and the post-shock gas, and it has also been inferred that the pre-shocked lines are broadened by heating due to the cosmic rays produced within the remnant. It will soon be possible to combine these observations with others obtained with OSIRIS on the GTC to carry out a complete set of diagnostics on the conditions within this highly energetic cloud.-

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