Iary Davidzon

First of all I must clarify that my name contains a couple of misspellings (something went wrong during the registry for my birth record...). It should be read 'Yari Davidsohn' (a mix of Hebrew and German).
Anyway, I got my Ph.D. at the Physics and Astronomy Dept. of the University of Bologna, working with Micol Bolzonella and Olga Cucciati on galaxy evolution and large-scale galaxy environment within the VIPERS project.
Then I moved to LAM to work with Olivier Ilbert (and C. Laigle and H. J. McCracken in Paris). We exploited the exquisite data of UltraVISTA and SPLASH to study the build-up of galaxies at 4< z < 8, i.e. in the first ~1.5 billion year of the life of the Universe.
And now I'm at Caltech, in Peter Capak's team. My goal is to investigate high-z galaxy growth and mechanisms halting star formation in the first, massive galaxies (looking forward to the launch of JWST, to move to higher redshift/smaller masses).
As a full member of the COSMOS team I am involved in many other complementary projects (and I'm also member of the Euclid Consortium).
Sometimes I tried to take a day off to finish my work on the Shapley super-cluster, which I was doing during my Master's thesis under the supervision of Antonaldo Diaferio... but invariably I use such a spare time to go bike riding or read on the couch...

VLT skyline

Research interests

The past decade has seen significant advances in the study of galaxy evolution prompted by large astronomical surveys. They have been devised to conduct a census of star-forming and quiescent galaxies at different redshifts, by sampling wide portions of the sky and going deeper and deeper to span very large intervals of cosmic time.
My work is based on such large surveys, which collect large amounts of data thus facilitating statistical studies. The galaxy stellar mass function (GSMF) is one such fundamental statistics, which I extensively use in my research to trace the history of baryonic mass assembly and its conversion into stars for various galaxy types. The large volumes probed allow me to study rare massive galaxies, in particular how they stop forming stars. Moreover, observational GSMFs are pivotal to calibrate and/or test models of galaxy formation. Some of the scientific and technical questions that lead my work are:
How does the high-mass end of the GSMF evolve?
Are there passive galxies at z>3, and what are their features?
What is the role played by active galactic nuclei?
On the technical side, I spend efforts to optimise IR photometric source extraction and SED fitting techniques, especially to correctly identify high-z galaxies, and recover their physical properties. I am also intrigued by new observational analyses like self-organizing maps and cosmic filament reconstruction.

Click here for a complete list of publications (some of them are reported below)

Davidzon et al. 2017 (A&A, 605, A70).

We obtain a view over 13 billion years of stellar mass assmebly in 10 snapshots, by deriving the stellar mass function and cosmic stellar mass density of galaxies in the COSMOS field up to z~6 (see figure below). We rely on the photometry included in the COSMOS2015 catalogue, but we provide a new estimate of photometric redhisfts at z>2.5 with an improved version of the code LePhare. Our work provides a comprehensive view of galaxy stellar mass assembly between $z=0.1$ and 6, for the first time using consistent estimates across the entire redshift range. We select a mass-complete sample (>1010.2) of quiescent galaxy candidates at 3<z<4. We investigate the link between stellar and dark matter by matching the GSMF with the mass function of their hosting haloes. Assuming a Schechter funtion profile, we find that the major limitation in this kind of studies is that the slope of the fit depends on the characterisation of the observational uncertainties, which is crucial to properly remove the Eddington bias. There is currently no consensus on the method to quantify such errors: even using state-of-the-art datasets, different recipes for error deconvolution result in different best-fit Schechter parameters.      
COSMOS2015 GSMF across 13e9 yr COSMOS2015 quiescent GSMF across 13e9 yr

Laigle, ..., Davidzon et al. 2016 (ApJS, 224, 24).

I am part of the core team realising the COSMOS2015 catalog, which contains precise photometric redshifts (see plot) and stellar masses for more than half a million objects over the 2 sq deg of the COSMOS field. Including new YJHKs images from the UltraVISTA-DR2 survey, Y-band images from Subaru/HyperSuprime-Cam, and infrared data from the Spitzer Large Area Survey with the Hyper-Suprime-Cam Spitzer legacy program, this near-infrared-selected catalog is highly optimized for the study of galaxy evolution and environments in the early universe. The COSMOS2015 catalogue is available on the ESO website      
COSMOS2015 photo-z vs spec-z

Davidzon et al. 2016 (A&A, 586, A23).

We investigate environmental effects in the evolution of galaxies between z = 0.5 and 0.9, relying on the galaxy local density field measured in Cucciati, Davidzon, et al. (2017). Several studies have already derived the GSMF as a function of different galaxy environments (e.g., clusters vs field) but this is the first one in which their number denisty is correctly normalised (from a 3-D Voronoi decomposition of the probed volume). This allows us to compare not only the GSMF shape, but also their growth as a function of redshift (see plot). We find that the evolution of low-density regions is described well by the formalism introduced by Peng et al. (2010, ApJ, 721, 193), and is consistent with the idea that galaxies become progressively passive because of internal physical processes. This mass quenching is not sufficient to describe the evolution of the mass function in the high-density regions: a significant contribution from dry mergers is required.      
GSMF(z) in under- and over-dense galaxy environments

Cucciati, Davidzon, et al. 2017 (A&A, 602, A15).

We use the final data of the VIMOS Public Extragalactic Redshift Survey (VIPERS) to investigate the effect of environment on the evolution of galaxies between z = 0.5 and 0.9. We use about 75,000 accurate spectroscopic redshift over an area of 23.5 sq deg. We also consider the photometric redshifts (and their probability distribution function) of the total sample of photometric galaxies with i<22.5 mag. We characterise local environment in terms of the density contrast (δ) smoothed over a cylindrical kernel, the scale of which is defined by the distance to the 5th nearest neighbour (see plot). We investigate the dependence of active and passive galaxy fractions as a function of δ. Comparing to the semi-analytical model of De Lucia & Blaizot we emphasize the discrepancies in the overdensities, a general result pointing out the improvments still needed in simulations to correctly describe environmental quenching mechanisms.      
W1 field: VIPERS galaxy environment

Davidzon et al. 2013 (A&A, 558, A23).

We measure the evolution of the GSMF from z = 1.3 to z = 0.5 using the first public data release of VIPERS. Thanks to the large volume and depth of the survey, Poisson noise and cosmic variance of our GSMF estimates are comparable to the statistical uncertainties of large surveys in the local universe.
We determine with unprecedented accuracy the high-mass tail of the galaxy stellar mass function, which includes a significant number of galaxies that are too rare to detect with any of the past spectroscopic surveys. We are able to separately trace the evolution of the number density of blue and red galaxies with masses above 1011.4, in a mass range barely studied in previous work (see the plot below). We detect a population of similarly massive blue galaxies, which are no longer detectable below z = 0.7. These results give initial promising indications of mass-dependent quenching of galaxies at z ≃ 1.      
comoving number density of VIPERS red/blue galaxies