Abstract

How galaxies form and evolve across cosmic times is one of the fundamental questions in modern astronomy. Over the past decade, modeling the panchromatic emission of galaxies has become one of the key tools in measuring their properties. As new and next-generation facilities progressively open a new era in astronomy, we face new and specific challenges in this endeavor: LSST and SKA will provide us with an avalanche of data, the advent of e-ROSITA and the preparation for Athena makes it ever more pressing to include X-ray emission into the standard UV to radio panchromatic models, JWST will observe the first galaxies with extreme stellar populations, and in the meantime ALMA is already starting to provide us with remarkable dust and metal observations at high redshift. The aim of this meeting is to gather theoreticians, modelers, and observers to present and discuss the current frontier in the panchromatic modeling of galaxies and establish where we need to push these frontiers forward to ensure that we will be able to fully exploit the exquisite datasets at our disposal in the 2020s.

The meeting will take place from 12 to 16 November 2018 at Osaka University conference hall in Japan. Osaka, is a large historic city of Japan with a rich intellectual life. It is tightly connected to the rest of the world with direct flights from Asia, Europe, the Middle East, North America, and Oceania. It it connected with Tōkyō through a fast train, offering even broader connections to the world.

 

Rationale

How galaxies form and evolve across cosmic times is one of the fundamental questions in modern astronomy. Over the past decade, modeling the panchromatic emission of galaxies has become one of the key tools to measure their properties. As new and next-generation facilities progressively open a new era in astronomy, we face new and specific challenges in this endeavor: LSST and SKA will provide us with an avalanche of data, the advent of e-ROSITA and the preparation for Athena makes it ever more pressing to include X-ray emission into the standard UV-to-radio panchromatic models, JWST will observe the first galaxies with extreme stellar populations, and in the meantime ALMA is already starting to provide us with remarkable dust and metal observations at high redshift. The aim of this meeting is to gather theoreticians, modelers, and observers to present and discuss the current frontier in the panchromatic modeling of galaxies and establish where we need to pushed these frontiers forward to ensure that we will be able to fully exploit the exquisite datasets at our disposal in the 2020s.

Following the experience of the highly successful Modelling galaxies through cosmic times workshop in Cambridge in 2015 (co-chairs Boquien & De Looze), this meeting is built on two fully integrated parts. First we will delineate what is the current frontier in the panchromatic modeling of galaxies in terms of building blocks, modeling techniques, and
the integration with simulations. Then we will establish what are the challenges we must overcome to model the panchromatic emission of galaxies in the “new era” of astronomy in the 2020s. In particular, we will discuss the latest progress to address the major challenges of the upcoming decade: modeling of the first galaxies, integration of X-ray in panchromatic models, and modeling of overwhelmingly large datasets.

The current frontier of panchromatic modeling of galaxies: what we have and what we need

1. The current frontier: fundamental building blocks of panchromatic models

The emission spectrum of a galaxy is the result of a complex interplay between its different physical components: stellar populations of all ages; dust that absorbs short wavelength photons and radiates the energy at longer wavelengths; gas ionized by massive stars that emits various emission lines from UV to radio wavelengths as well as continuum; an active nucleus consisting of a massive black hole surrounded by an accretion disk which emits continuum from X-ray to radio; etc. The determination of the physical properties relies in a critical way on the availability of reliable models. It is therefore of utmost importance that we understand the current state-of-the-art for these fundamental building blocks. Over the past years, evidence has been accumulating that they have their own shortcomings: low-metallicity stars and rotation can strongly affect the short wavelength spectrum with direct consequences on (i) UV colors, (ii) the production of Lyman continuum photons, and (iii) the recombination lines, all of which can strongly affect broadband fluxes, especially at high redshift; there are systematic discrepancies between dust mass estimates derived from infrared and optical observations; smooth and clumpy AGN models have significant differences that we cannot distinguish from broadband data; etc. Each and everyone of these issues can and will have consequences for next generation facilities as we increasingly probe extreme and unfamiliar objects for which we have little to no reference anchor in the nearby universe. In preparation for next-generation facilities, it is becoming urgent that we address such problems to fully exploit the treasure trove of data that will be flowing to us in the 2020s. The first step in this journey is to gather researchers building these fundamental blocks to present the state-of-the-art and discuss the current issues in light of the requirements to exploit next-generation facilities.

2. The current frontier: methods to model the panchromatic emission of galaxies

The large number of modeling codes publicly available shows the diversity of approaches to understanding the emission of galaxies across redshifts: spectra or broadband SED, energy balance or radiative transfer, parametric or non-parametric, grid-based or MCMC, maximum-likelihood or Bayesian, etc. This diversity hints at the difficulty of modeling galaxies and the many open questions: how can we model the emission of galaxies from the local universe to the very first galaxies, what are the common properties/physical processes and what are the specific ones needed to best reproduce galaxies at very different redshifts and in very different environments along their evolutionary path, what do galaxy SEDs tell us depending on their spectral resolution, wavelength range, physical processes probed, etc., how can we incorporate and account for radiative transfer effects in SED models, can we link dust properties in extinction and in emission, how can we combine spectroscopic and broadband data to measure more reliably the physical properties of galaxies, what observations are required to better constrain our current physical models, how can we disentangle the different physical components, what do we need to overcome degeneracies, etc. With the breadth and quality of data provided by next-generation facilities, answering these questions is becoming ever more
critical. This meeting will allow us to learn about the most advanced modeling techniques and how they will be applied to understanding galaxy evolution in the upcoming decade. 

3. The current frontier: “observing” theoretical galaxies with simulations & models

Cosmological models with numerical hydrodynamics and/or semi-analytical recipes are of increasing importance to understanding galaxy evolution. Yet, confronting simulations with observations is not always straightforward. In this endeavor, we now increasingly have the capability of combining hydrodynamical simulations with radiative transfer codes (such as SKIRT or Sunrise), allowing for a direct comparison of synthetic images of simulated galaxies with actual observations (non-parametric morphologies, colours, dust obscurations, etc.) on a more realistic basis. This interface between models and observations is a powerful tool to interpret observational results within a cosmological context and to improve the modeling of complex physical systems. Moreover, they are playing an increasingly important role and are becoming essential in the preparation of upcoming facilities, from JWST to LSST and as such are a stepping stone to the “new era” of astronomy in the 2020s. Such simulations can serve as a reference to create mock observations from next-generation facilities and test the data processing pipelines and the methods to measure the physical properties of galaxies. As such the combination of simulations with panchromatic models is a key component for both the preparation and the exploitation of next-generation facilities. In this spirit, in this meeting we will gather experts on galaxy modeling and cosmological simulations to build the bridge between simulations and models on one side and observations in the era of next-generation facilities on the other side.

Pushing the frontiers: major challenges of panchromatic modeling in the “new era” of astronomy

4. Pushing the redshift frontier: modeling the first galaxies observed with ALMA & JWST

The first galaxies in the universe are still largely terra incognita. How did the first stellar generations form? What are the properties of population III stars and their effect on their environment? How are their evolution and spectra affected by rotation and binarity? What can we tell about their physical conditions (gas density, temperature, metallicity, etc.) from the recombination line spectra? How did the dust form in such extreme objects? Since its inception, ALMA has been providing excellent opportunities for probing dust to high redshift ( z~7 or higher). Surprisingly, it revealed a large variety in dust content already established at high redshift: some galaxies are rich in dust but others are extremely poor. This suggests that the early stages of galaxy evolution present a large variety of physical conditions, driving galaxies along different paths. The variety should also be related to the stellar populations that we will observe with JWST, since metals, of which dust grains are composed, originate from stars. Therefore, in order to comprehend the evolution of the first galaxies, we need to undertake a large panchromatic effort to fully understand the stellar, the gas, and the dust content in high-redshift galaxies, rather than develop the models separately as is often the case. This conference is suitable for these aims because of the attendance of observers working at various wavelengths and theoreticians working on the most advanced models of dust evolution and SEDs.


5. Pushing the wavelength frontier: extending models towards X for e-ROSITA & Athena

The new X-ray missions e-ROSITA and Athena will bring far-reaching changes to our understanding of black hole physics and AGN evolution over cosmic time. Their synergy with other world-class facilities will provide us with unprecedented samples of at least several hundred thousands quasars with multi-wavelength information. With these samples it will be possible to open a new branch of observational cosmology, to build the most detailed X-ray and bolometric AGN luminosity function, and to investigate AGN clustering as a function of the Eddington ratio, λ Edd =L bol /L Edd , the black-hole mass, and the AGN host-galaxy properties (such as stellar mass, morphology, and star formation rate), just to provide a few examples. However, one of the main challenges in order to tackle all these points will be to determine nuclear and host galaxy properties of quasars through accurate SED modeling. Historically, panchromatic models have only covered the electromagnetic spectrum from the ultraviolet to the radio, or a more restricted wavelength range. The loss of information in foregoing X-rays wavelengths is damaging to measure reliably the respective properties of host galaxies and AGN, as nuclear activity and star formation can be difficult to disentangle reliably. This conference will allow us to discuss state-of-the-art X-ray modeling of galaxies and the prospects of combining them with panchromatic models.

6. Pushing the Big Data frontier: extragalactic astronomy in the era of LSST, Euclid, WFIRST

LSST, Euclid and WFIRST will produce imaging and spectroscopic surveys with observations of billions of galaxies, observed multiple times over periods in excess of a decade; dwarfing even the largest extragalactic surveys to date. For example, LSST alone will discover through photometry and variability ~20 billions galaxies and ~1 billion AGN, with efficiency and completeness analogous to those of spectroscopic surveys. The size and dimensionality of these
data, combined with their limited wavelength coverage, creates the need to rethink our panchromatic modeling methods. This will require the community to focus its efforts on developing new methods and techniques that utilize the volume and diversity of the data rather than high signal-to-noise measurements of small numbers of galaxies. In order to maximize the information that can be extracted from these imaging surveys, experts in the panchromatic modeling of galaxies with need to engage with statisticians and observers. This meeting will be ideal to identify the first steps being undertaken in this direction and to develop a path towards the exploitation of LSST, Euclid, and WFIRST data.

Having covered the frontiers we face, we will have a clear view of current models and observational data, and on what should be done for the coming era. As a consequence, we will have a pathway for galaxy evolution studies in the 2020s, which will eventually lead to a panchromatic and panoramic understanding of galaxy formation and evolution over the entire cosmic history and a large volume of the Universe.

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