Summary of the General Assembly of the International Astronomical Union in Bejing

The International Astronomical Union XXVIII general assembly took place in Beijing, China during the last two weeks of August. The IAU General Assembly is perhaps one the largest astronomical meetings, gathering hundreds to thousands of astronomers working in many different topics: from stars and planets to galaxies, the interstellar medium, or cosmology. It only happens once every three years (can't wait for Hawaii 2015) and it holds roughly 12 different simultaneous sessions every day ranging from small joint discussions and special sessions to the large symposia. Here I'll try to present a brief (and extremely biased) summary of my experience there. Other astronomers with different perspectives are encouraged to complement my side of the story!

Banner for the IAU General Assembly Meeting in Beijing, China

The most interesting meeting for me was the "The intriguing life of massive galaxies" which included both an observational and theoretical perspective on the formation and evolution of the most massive galaxies. This symposium took place only during the second week, but there were other very interesting discussions during the first week such as special sessions on UV-emission in galaxies, cosmic evolution in galaxy clusters, and secular evolution. The latter featured very interesting discussions on the processes driving morphological transformations of disk galaxies and also quenching mechanisms related with these transformations. I specially enjoyed John Kormendy's summary on bulges (classical ellipticals), pseudo-bulges (bulges within spiral galaxies) and the processes involved in their formation, namely dissipational processes for the bulges and secular processes (bars, rings) for the latter, and their kinematic properties: pseudo-bulges are fast-rotators whereas bulges are mainly dispersion dominated (check his last paper and the 2004 review for more details). There were also observational results from the SAURON and ATLAS3D teams showing the motion of these structures for galaxies in the local universe. A quick note from the UV-emission special session, G. Bruzual presented some results from the recently updated version of their stellar population synthesis models (commonly used for SED fitting). Among the upgrades they will include improved libraries in the UV region of the spectra and a revised treatment of the thermally pulsating AGB phase to reduce its contribution to the NIR emission, bringing the values closer to their 2003 models.

Let's go back to the symposium on massive galaxies, where I spent most of my time. The session started with some talks about recent observational results on z~7-10 galaxies. Bouwens and Stark talked about the weak evolution of the specific star formation rate (sSFR, the star formation rate divided by the stellar mass) with redshift, and their efforts to improve their results including new data from deepest IRAC surveys. We certainly missed our CANDELS experts here because that was all the mention of very high redshift galaxies for the remainder of the week. On the cosmic-noon side, S. Wuyts, I. Trujillo and E. Daddi presented summaries on our current knowledge of the structural, kinematic, and star formation properties of galaxies at z~2 and the evolution of these down to z~0. Let's see if I can summarize some of the highlights mentioned in their talks (and many others) without making a horribly long post!

The most popular figure of the meeting was, without discussion, the stellar mass-size relation. Although it was intended to depict the sizes of all massive galaxies, it was almost exclusively used to describe the remarkable size evolution of non-star forming galaxies. Since the notion that the small sizes of these galaxies are robustly measured is now widely accepted, most of the discussion revolved around the possible growing mechanism, with minor merging being the most frequently advocated. Nonetheless, several speakers acknowledged that current measurements of the merger ratio seem to be insufficient to explain the observed size growth from z~2 to 1. It was also highlighted in the closing summary that the size evolution of the massive non-star forming population does not involve a single population, but a mixture of older and recently quenched (star formation has recently been halted) galaxies that are being continuously added, therefore making the interpretation even more complicated.

The second most popular figure (or referred term) was the so-called main-sequence that refers to the correlation between stellar mass and SFR. This relation has been observed up to z~3 with little change in the slope and an evolving normalization towards higher SFRs at early times. The importance of this relation was highlighted by both observers and theorists. The first showed increasing observational evidence of a secular, steady growth, phase for galaxies in the main-sequence co-existing with a starburst phase for galaxies with enhanced star-formation efficiencies (forming more stars per volume of gas), probably indicative of differences in the interstellar medium. Several talks highlighted the importance of the slope of the main-sequence and the relative fraction of galaxies in the starburst phase to characterize the evolution of the stellar mass function at both the most and least massive ends. A slope of unity implies that the galaxy mass function (the number of galaxies with a given stellar mass in a given volume of the Universe) simply shifts to higher masses without changing its shape, typically characterized by a Schechter function. Recent CANDELS results from Wuyts, Targett, and Kartaltepe were mentioned in the context of studying the evolutionary mechanisms that move galaxies from the main-sequence to the quiescent red-sequence after perhaps a brief transit through the starburst phase.  

Regarding talks on galaxy structure and kinematics, we saw really impressive results from SINS and the more recent MASSIV surveys using the NIR/IFU SINFONI to make velocity maps of star-forming massive galaxies up to z~2.  These surveys provide evidence of rotating disk structures in most of these galaxies, although the increased velocity dispersions suggest a more turbulent star-formation (along with short lived clumps) different from the more ordered disks observed at z<1. On the quiescent side, CANDELS results from Van der Wel, Bruce, and Mozena seem to identify a population of passive disks co-existing with the compact spheroids. This could place strong constraints in the quenching mechanisms and transformation processes, as we know that the majority of passive galaxies will become large ellipticals. Different velocity dispersion measurements for small samples of quiescent galaxies at z > 1 confirm that in fact these are 2 to 5 times denser than their local analogs as suggested by their small sizes. Van de Sande presented some new results from their XSHOOTER survey including a galaxy with a velocity dispersion above 400km/s! 

Before making this post too long, I'd like to finish this brief overview with a quick note on the theory side and the models for galaxy formation. Talks from Oser, Naab, and Johansson discussed what they call a two-phase formation scenario, which consists of the in-situ formation of a massive compact galaxy at z>~2 followed by a continuous growth in mass and size as a result of mergers and satellite accretion. Gabor and also Peng and Lilly focused on a more general process controlling galaxy growth and quenching through the evolution of the dark matter halo (its mass and temperature) and the galaxy environment. In this regard, and to conclude on the observational side, Duc showed some extremely deep amazing images from the Virgo NGVS survey reaching down to 29 mag/arcsec. With this kind of data it is possible to study galaxy-galaxy interactions with a high level of detail: enormous tidal features, minor merging, and even satellites of satellites around massive galaxies!

All in all, the IAU general assembly provides a wonderful venue to catch up on the most recent updates for your favourite topic, and also a fantastic opportunity to discover interesting results in other related topics. The large number of attendants ensures that you'll meet a lot of different people and make new connections, so I strongly recommend the experience. Don't miss the next one in Hawai'i 2015!