Postdoctoral fellow at the Infrared Processing and Analysis Center (IPAC/Caltech)
Keith Spalding bldg, 1200 E. California Blvd., 91125 Pasadena, CA (USA)
+1 626 395 1279
also associated to INAF Astronomical Observatory of Bologna
Hi! 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...
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.
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 ℳ⊙) 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.
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>
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.
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.
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 ℳ⊙,
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.