Ignacio Trujillo: Astronomical Research

My field of research is focused on the following topics of the Extragalactic Astronomy:

On the structural properties of nearby galaxies

On morphology and environment

On the evolution of galaxies

On the effect of cosmological large scale structure on the orientation of galaxies

On the structural properties of nearby galaxies: elliptical galaxies

The availability of high-resolution imaging with the Hubble Space Telescope has permited to identify two distinct classes of galaxy centers: "power-law" galaxies, where the central surface brightness increases into the limit of resolution with something like a steep power-law profile; and "core" galaxies, where the luminosity profile turns over at a fairly sharp "break radius" into a shallower power-law. We have found a way to link this classification to the global structure of the galaxies. We demostrate that most, if not all, so-called power-law galaxies are better described as "Sérsic galaxies" - i.e. their global light distribution from the center to the outer region is well modeled by the three-parameter Sérsic model - and that "core" galaxies are best understood as consisting of an outer Sérsic profile with an inner power-law cusp, which is a downward deviation from the inward extrapolation of the Sérsic profile (Trujillo et al. 2004).

The centers of the galaxies host  enormous black holes with masses  ranging from one million to several thousand million solar masses. Surprisingly, the mass of these objects is closely related to the central velocity dispersion of the stars of the host galaxies (the sigma-black hole mass relationship). Our  group has found another strong connection between the black hole mass and the shape of the galaxy (the galaxy light concentration-black hole  mass relation). There is not yet a complete and satisfactory theoretical explanation of the observed relations.

                                       Elliptical Galaxy M87
Galaxia Espiral NGC4414

Left: Elliptical galaxy M87 (Photo by David Malin). Right: Spiral Galaxy NGC4414 (Photo by the Hubble Space Telescope from NASA)

On the structural properties of nearby galaxies: spiral galaxies

A detailed understanding of how the galaxies evolve in size is currently missing. We still do not know how and where the stars are created and how they are assembled into the galaxies. A key element to understand this process is located in the outskirst of galactic discs.  At the galaxy periphery the imprints of galaxy formation survive for longer than in the inner regions where the own self-gravity erase very fastly the information about the process of formation. Our group has find three different types of surface brightness distribution in the fainter regions of the spiral galaxies.

                                              Different spiral types

These three different types of shapes are related to the global morphology of the galaxies, consequently, whatever the responsible mechanisms for placing (or creating) new stars in the galactic outskirts are strongly linked to the process involving in the growing and shaping of galaxies.

On morphology and environment

The structure of the galaxies depends on the enviroment in which they exist.  Elliptical galaxies are preferentially located at the center of the galaxy clusters (groups of galaxies with hundreds or even thousands of members that are gravitationally bound). The spiral galaxies are more numerous at larger distances from the cluster center. 

In the clusters, the galaxies are being affected by several physical processes which modifies their  structure. Of these processes  tidal effects are important. The tidal forces destroy the less massive galaxies (the dwarf galaxies) and shorten the size of the discs of the spiral galaxies.

Using nearby galaxy clusters (Virgo, Coma and Abell 2443) we have found different examples of these enviroment-induced effects . One is the relationship between the galaxy light concentration of the ellipticals and their position in the cluster of the galaxies (galaxy light concentration-density relationship): ellipticals with a larger concentration of light at their centers are also located in the densest areas of the cluster.

Another interesting effect is the shortening of the size of the spiral galaxy discs in comparison with their field counterparts.  At a given luminosity and bulge-to-disc ratio, the size of the discs is 30%  smaller in the cluster enviroment.

                                                                                                      Central region of the Coma cluster of galaxies

Central region of the Coma cluster of galaxies. The photo was obtained using the Wide Field Camera at the INT telescope in the Observatorio Astrofísico del Roque de los Muchachos in La Palma (Spain). Photo by David Martinez-Delgado, Antonio Marin-Franch and Antonio Aparicio.

On the evolution of galaxies

Galaxy formation and evolution is a complex combination of hierarchical clustering, gas dissipation, merging and secular evolution. While gravity drives the bottom-up assembly of cosmic structures, gas cools at the centres of dark matter halos forming a disc that acquires angular momentum through tidal torques from nearby structures (Peebles 1969; White 1984). Gas eventually fragments and forms stars. The mass and the angular momentum that settle into the disc are assumed to be fixed fractions of the mass and the angular momentum of the halo respectively (Mo, Mau & White 1998). Since the mass and the size of the halos are tightly linked to the density of the Universe at the time the halos were formed, disc galaxies are expected to grow with cosmic time.

The more distant galaxies are  the  youngest objects of the Universe. Using one of the  deepest infrared images ever taken (from the FIRES project) and one of the largest infrared survey (POWIR) we have analyzed the size evolution of the galaxies up to distances where the Universe only had one third of its present age. We have found that the size evolution of the galaxies is  mass dependent: galaxies with intermediate stellar masses have  similar sizes to their local counterpart, whereas more massive galaxies have evolve dramatically since z~2.

Although the size evolution of the galaxies is becaming to be well established, we currently lack a detailed knowledge of how this evolutionary process works. To explore directly the size evolution of the galaxies one can follow the evolution of characteristic features of the galaxy surface brightness like the radial position of the breaks These breaks are associated with the density threshold for star formation, and consequently the radial evolution of the position of the break is a perfect tool to explore the inside-out growth of the spiral galaxies.

                                                      Break evolution

On the effect of cosmological large scale structure on the orientation of galaxies


Galaxies are expected not to be randomly oriented in the space but aligned following a characteristic pattern dictated by the large-scale structure of the invisible dark matter that surrounds them. We have found the first observational evidence that galaxies these is the case (Trujillo, Carretero & Patiri 2006)

                                                       Void based-methodProbability density distribution
Left figure shows a schematic representation of the technique used to measure the angle between the spin axis of a galaxy and the orientation vector of the surrounding material. Large voids are detected in the cosmic web. We search for edge-on galaxies within the shells surrounding the voids. Right figure is the probability density distribution of the previous angle. The dotted line represents the null hypothesis. The solid line corresponds to Lee's analytic prediction within the framework of the tidal torque theory.
Press release
Sky & Telescope Article
Nota de prensa

Other topics

a) We have also worked on the effects of seeing (the atmospheric blurring) on the surface brightness distribution of  galaxies. If you want to know more details see Trujillo et al. (2001).

b) Elliptical galaxies are known to rest in a plane called  the Fundamental Plane. This plane is understood to be  a consequence of the Virial Theorem. However, the theoretical expectation and the observed plane are tilted with respect to each other. We have investigated the origin of this tilt and found that it can be completely explained by a combination of  the observed structural non-homology (3/4) and the observed change in the stellar populations (1/4) in the family of elliptical galaxies.

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