STARS AND GAS IN DOUBLE-BARRED GALAXIES
Stellar bars are elongated stellar structures present in the majority of disc galaxies, including our own Milky Way! In 1975, de Vaucouleurs presented the case of a galaxy hosting not only one, but two stellar bars: NGC1291 thus became the first known double-barred galaxy in a list that has now reached around 50 individuals.
A double-barred system is composed of an outer bar, pretty much like the single bars we see in most galaxies, and an inner bar, rather smaller and apparently rounder than the outer bar. While few galaxies with just one small bar, as small as some inner bars within double-barred systems, exist, such single small-barred galaxies are very rare. Inner and outer bars show random orientations between them, indicating they both rotate with different pattern speeds. Numerical simulations developed in the 1990s indeed predicted that inner bars rotate faster than outer bars, but let me emphasise that reproducing the formation and evolution of double-barred galaxies was a challenging goal at that epoch, further hampered by the little amount of observational information available. Double-barred galaxies are complex objects to observe and analyse due to their richness in structure. The advent of integral-field spectrographs made this task easier, although still tough.
I am very interested in unveiling the secrets hidden by double-barred galaxies through integral-field spectroscopic observations. I aim at characterising the stellar and gas kinematics, as well as the stellar populations of such beautiful objects, thus providing new observational properties for constraining theoretical models and probing the formation of double bars through numerical simulations.
You can get more information about my work on the formation and spectroscopic properties of double-barred galaxies in:
- «Clocking the assembly of double-barred galaxies with the MUSE TIMER project» de Lorenzo-Cáceres, Sánchez-Blázquez, Méndez-Abreu, et al. 2019, MNRAS, 484, 5296
- «Distinct stellar populations in the inner bars of double-barred galaxies» de Lorenzo-Cáceres, Falcón-Barroso, and Vazdekis 2013, MNRAS, 431, 2397
- «Constraining the formation of inner bars: photometry, kinematics and stellar populations in NGC357» de Lorenzo-Cáceres, Vazdekis, Aguerri, Corsini, and Debattista 2012, MNRAS, 420, 1092
- «Stellar kinematics in double-barred galaxies: The σ-hollows» de Lorenzo-Cáceres, Falcón-Barroso, Vazdekis, and Martínez-Valpuesta 2008, ApJ letters, 684, 83
GALAXIES ARE MADE UP OF STELLAR STRUCTURES
Indeed they are! Discs, bulges, bars, rings, spiral arms… All these stellar structures shape the galaxies so they appear as we see them, but their presence cause opposite effects. On one hand, the more structures coexist in the galaxy, the more complicated its observation and analysis are… on the other hand, the stellar structures hold valuable information about the history of the galaxy: about the dynamical processes that have taken place in it and about the star formation and merging events it has undergone. So if we overcome the difficulties inherent to the study of late-type galaxies with multiple structures, we will certainly decipher the biggest enigma: the complicated lives of the most complicated galaxies.
To disentangling the puzzle that has been set by structurally complex galaxies, I use all strategies at hand:
Photometric decompositions. We only need to look at them! Images of distant galaxies already tell us they are shaped by many different components. The fanciest technique currently available for extracting information about the many overlapping stellar structures by simply analysing images is to make two-dimensional models of every component, add them all… and fit the image. This way we can retrieve all photometric details about every isolated structure: bulges, discs, bars… even double bars! But, why in two dimensions? From Earth (when observing galaxies, even space telescopes are effectively «on Earth») we see galaxies as projected onto the plane of the sky: two dimensions. We are missing all information about the third dimension, along our line of sight. We are however saved by maths: statistical deprojections allow us to recover information about that third dimension for many galaxies. Three dimensional multi-component photometric decompositions of galaxy structures (bulges, discs, bars… even double bars!) are now possible.
Stellar kinematics. In the galaxy, all structures coexist and even share the same space: inner bars or inner discs might be embedded within bulges or large bars, and the main galaxy disc is supposed to underlie every other component. But still, each one keeps a certain degree of independence and they sure can hold a different dynamics. That is the reason why some structures produce distinct kinematic features that can even be measurable depending on their position or the position of the galaxy with respect to ourselves. This is the case of the σ-hollows that I mentioned above: they are signatures of the presence of inner bars within double-barred galaxies. Similarly, box/peanut structures leave an imprint in the stellar kinematics of the galaxy. Box/peanuts are central stellar structures first detected in edge-on galaxies; once thought to be isolated structures, today we know they are just the central parts of stellar bars that have undergone the so-called buckling phase, puffing up their innermost regions. We have our own peanut: it indeed looks like the stellar bar of the Milky Way has already buckled… and guess what! Before finding that the observable properties of the stars in our galaxy most likely correspond to a box/peanut, researchers even considered the possibility of the Milky Way being a double-barred galaxy. Cool!
Spectro-photometric decompositions. The picture is clear: first, photometric decompositions allow separating the structural components in a galaxy, but only morphologically, i.e., shape and amount of light; second, spectra contain valuable information for studying, for example, the ages of the stars and the amount of gas enclosed in the galaxy. Wouldn’t it be the best to be able to combine both options? Wouldn’t it be great to separate the spectra from each structure, so we can study the ages of the stars in the bulge or the amount of gas forming stars in the bar? Having it all, i.e., performing spectro-photometric decompositions of galaxies, is of course the wish of every researcher, but only very recently such analysis has been possible. In 2019 we presented C2D, a novel spectro-photometric decomposition code that we have already applied to bulge+disc galaxies and that we will soon apply to barred systems as well… stay tuned!
Read more about the various approaches I use to study stellar structures:
- «Deconstructing double-barred galaxies in 2D and 3D – II. Two distinct groups of inner bars» de Lorenzo-Cáceres, Méndez-Abreu, Thorne, and Costantin 2020, MNRAS, 494, 1826
- «Deconstructing double-barred galaxies in 2D and 3D – I. Classical nature of the dominant bulges» de Lorenzo-Cáceres, Méndez-Abreu, Thorne, and Costantin 2019, MNRAS, 484, 665
- «Inner bars also buckle. The MUSE TIMER view of the double-barred galaxy NGC 1291» Méndez-Abreu, de Lorenzo-Cáceres, Gadotti, et al. 2019, MNRAS Letters, 482, 118
- «Spectro-photometric decompositions of galaxy structural components» Méndez-Abreu, Sánchez, and de Lorenzo-Cáceres 2019, MNRAS, 484, 4298
- «Star formation in CALIFA early-type galaxies: a matter of discs» Méndez-Abreu, Sánchez, and de Lorenzo-Cáceres 2019, MNRAS Letters, 488, 80
THE POWER OF STELLAR POPULATIONS IN GALAXIES
Apologies! This webpage is always under development. I will come back soon to tell you a little bit about my recent research on how to exploit the study of stellar populations in galaxies to unveil different aspects of their formation and evolution.
★My involvement in international collaborations exploiting the best integral-field spectroscopic facilities at the time is one of the greatest assets of my research. I want to emphasise my role in TIMER (MUSE spectrograph at 8-metre VLT telescope, Cerro Paranal Observatory in Chile), CALIFA (PMAS spectrograph at 3.5-metre telescope in Calar Alto Observatory, Spain), and, more recently, BEARD (MEGARA spectrograph at 10-metre GTC plus observations with three other telescopes, Roque de los Muchachos Observatory, Spain). Data from these surveys have made possible some of the latest publications highlighted throughout this webpage.