Galaxy Formation and Evolution in the Era of the Nancy Grace Roman Telescope

Conference summary from the “Galaxy Formation and Evolution in the Era of The Nancy Grace Roman Telescope” conference.

Astrobite Authors: Lukas Zalesky, Gourav Khullar, & John Weaver

Talks were live-streamed on Facebook, and the complete (official) conference proceedings will be available at Zenodo.org shortly.

What is the Roman Space Telescope?

You might know it by its original name: the Wide-Field Infrared Space Telescope (WFIRST).  Earlier this year the space-based facility was formally named after Nancy Grace Roman, a pioneer in space-based astronomy in an era when women in leadership positions were virtually non-existent. Nancy Grace is known as ‘the mother of Hubble’ (the telescope) and worked tirelessly advocating and organizing the Hubble Space Telescope, which has forever changed our view of the universe. 

Roman was selected as a result of the US Decadal Survey, which highlighted the need for a wide-field infrared space mission to perform survey work in the era post-Hubble and post-Spitzer. It has several distinct science goals all of which are immediately served by the combination of wide-field, infrared imaging and multi-object spectroscopy uniquely enabled by Roman. With a view 100x greater than Hubble in a single snapshot (see Figure 1), broad-band filters, and a slitless grism, along with a groundbreaking coronagraph to directly image exoplanets, Roman is slated to be a key contributor in the next decade of astronomy.

The event was held as a part of an ongoing series of science meetings to discuss the potential yields of Roman hosted by the Space Telescope Science Institute, attracting nearly 300 participants on the internal event feed and many more on the live stream. This particular meeting was aimed at bringing together the galaxy evolution community to look forward to the era of Roman

The first session was focused on outlining the capabilities of Roman, as well as matching those capabilities to the specific science objectives in galaxy evolution that they will enable. The capabilities of Hubble to see deep into the universe with excellent spatial resolution as well as multi-object spectroscopy have transformed our understanding of galaxies, and so it makes sense to see them again in Roman. However, the ‘pencil-beam’ surveys of Hubble, although extremely deep, have been hampered by their size for the simple reason that galaxies obey gravity and form clusters (i.e. the cosmic web). We require a much larger, but still deep, field of galaxies in order to advance our understanding of galaxy evolution, which Roman will do owing to its extremely large field of view.

Figure 1: Roman’s incredible field-of-view with Hubble-resolution will immediately dwarf the ultra-deep field which transformed galaxy evolution. Surely Roman will do the same.

The downside and the upside to all of this extremely deep and incredibly wide imaging are the same: we will have an enormous number of galaxies to study and the amount of data will be insane, on the scale of petabytes. Speakers highlighted the necessity (not recommendation) to begin thinking seriously about how we will interact with such datasets. You won’t be able to download all the images on your home computer, so toolkits and interfaces must be developed to enable the wider community to access the data in meaningful, productive ways. This will be one aspect that will set aside the next decade of astronomy from anything we have previously dealt with.

In addition to the 10 sessions we briefly summarize below, there were an enormous number of contributions in the form of posters and pre-recorded talks which significantly benefited the overall science discussion of the event. The authors of this bite wish to sincerely thank everyone who attended for making the week so enjoyable and interesting.

Session 2: Connecting the Near-Field and Deep Field View of Galaxy Formation 

Bullock, Williams, McQuinn, GuhaThakurta, Pearson

This session focused on investigating local galaxies in spectacular resolution, and how we can use lessons learned in the nearby universe to study the more distant cosmos. Again, we see Romans unique wide field of view take center stage, this time to map out galaxy stellar populations and other properties rapidly in a way that would be far too expensive and time-consuming Hubble. 

Additionally, the need was raised to utilize Roman to explore ultra-faint and diffuse galaxies which until recently were extremely difficult to image owing to their low surface brightness. Enabled by Roman’s wide field of view and extremely sensitive detectors, this kind of science should become commonplace, and will allow us to better understand the role of dark matter as it’s most unexplored domain is in these faint, small galaxies. On a similar note, exploring other faint features of nearby galaxies such as tidal streams and merger debris will be deeply empowered by Roman for these same reasons. Lastly, repeated long-baseline observations can further science in the extremely near-field — our own galaxy — by synergy with Gaia to constrain the proper motion of stars.  

Session 3: High-Redshift Galaxies 

Bowler, Bagley, Wold, Koekemoer, Khullar, Rhoads

The deep + wide approach enabled by Roman will enable us to locate the first galaxies in the universe, which are extremely faint and so they are incredibly difficult to find. With a greater area of the sky surveyed, we stand a much better chance of finding these lifelines in our understanding of galaxy formation and assembly within the first billion years. 

This session focused on how to locate these first galaxies, or at least the easiest ones to find in the first place: bright, massive galaxies. As it turns out, these kinds of galaxies, although ‘easy to find’ (think Hot Jupiters in exoplanet science), are the exact kinds of galaxies that help to constrain our models of how galaxies grow. The case is simple: it’s easy to grow a small-ish galaxy quickly, but to build a massive galaxy within a billion years is quite a big challenge to simulations and theorists! 

Specifically, the speakers highlighted the need to survey large numbers of these early giants to understand the distribution of their properties and how they contributed to re-ionizing the neutral hydrogen that pervaded the inter-galactic space following the big bang. Helpful probes, such as gravitational lensing (as discussed in-depth by an author of this bite!) and supernovae, can aid us in gaining even more information about these first galaxies. 

Session 4: AGN and Blackholes

Walsh, Woods, Wang, Petric, Wingyee Lau

Session 4 detailed the need to understand many of the deeply mysterious aspects of supermassive black holes which still elude us. Despite being awarded as the topic of a Nobel Prize earlier this month, our understanding of these cosmic giants has only just begun.

Speakers discussed the need to connect the observed properties and behaviors of supermassive black holes and the galaxies which host them (as seen in their quasar-mode), requiring physics to bridge several orders of magnitude in relative size from that of the entire galaxy down to a region the size of our solar system dominated by the supermassive black hole. As Roman is equipped with incredible sensitivity and a wide field, it makes sense to try and find some of the earliest, most massive quasars in the universe, which to date have eluded more narrow searches.

In particular, the case was made for linking the birth of quasars to the difference in observed properties we see, namely the level of dust obscuration local to the black hole to make a ‘red’ quasar. What causes the build-up of this dust, and how does a galaxy undergo a ‘blow out’ episode to become an unobscured, blue quasar, if ever? Roman will provide new means of finding the dimmest of these dusty quasars in the hopes of better understanding why they exist in the first place, and what fraction of the true quasar/AGN population they represent. Others want to go a step further to ask how did supermassive black holes form in the early universe? Building up that much mass takes a lot of time — more than the universe had at that point!

Session 5: Data Science for Astrophysics in the Roman Era

Gawiser, Lower, Gilda, Zanisi, Huertas-Company

SED fitting and Machine Learning were the main themes for this section. Each speaker (in one way or the other) advocated for using the latest techniques in making forecasts for galaxy morphologies, photometric redshifts, galaxy stellar masses and star formation histories, metallicity evolution, and dust characterization. Moreover, ground-truthing these methods in both real and simulated observations was a theme shared by many advocates in this session. 

Some of the ideas from this session include phasing out functional forms of star formation histories due to biases, using fast and efficient machine/deep learning methods to quantify stellar masses and morphologies in thousands of future-discovered galaxies, and paying attention to systematic uncertainties in both model and data when predicting physical properties of distant galaxies that effectively look like point sources.

Session 6: Relating the Dark Matter Density Field to Galaxy Properties

Papovich, Kubo, Huang, Samuel, Chapman

This session spoke to participants who were interested in making the connection between dark matter and baryonic matter in the Universe, on scales from satellite galaxies to proto-galaxy clusters! Speakers advocated for studying low surface brightness galaxies that highlight the connection of the dark matter halo to mass assembly (and even mass loss), characterizing the nascent predecessors of galaxy clusters with deep surveys by connecting their infrared properties (Roman) to sub-millimeter and radio properties (South Pole Telescope, ALMA). Moreover, we heard about even answering some questions close to home — what is the distribution of satellite galaxies around the Milky Way?

Session 7: Dust & Star Formation in Galaxies 

Calzetti, Battisti, Faucher-Giguere, Landt, Alavi

One of the most fundamental aspects of inferring galaxy properties across cosmic time is figuring out how much of the radiation emission of galaxies has been obscured by dust attenuation. This session was a reminder that dust attenuation models are often assumptions (e.g. a blanket screen vs a spherical dust cloud in the Milky Way), which require strict testing. Speakers in this session spoke of the geometry and chemistry of dust in a diverse set of galaxies and matched those to physical phenomena like star-formation cessation (i.e. quenching) and galactic winds captured in simulations. In synergy with other telescopes, Roman has the capacity to break several logjams surrounding dust observations and their correlations with properties like stellar mass, star formation rate, and metallicity in galaxies.

Session 8: Challenges for Theory, Modeling, and Synthetic Observations 

Wechsler, Drakos, Weaver, Garg, Ghosh

Thursday’s second session discussed the needs of the theory, modeling, and simulation communities. Firstly, Rise Wechsler gave one of the only purely theoretical presentations during the conference where she outlined the case for Roman to bring forth a new level of constraints for cosmological-volume galaxy simulations, urging observers to utilize more sophisticated analyses to better match those being provided by simulation teams. Other talks varied in their content greatly but focused mainly on extracting galaxy properties from simulations (including a talk by Prerak Garg) and what we can look forward to with Roman

A talk on mock catalogs by Nicole Drakos introduced us to exactly the kind of view we expect with Roman and provides proof of the challenge that will await for photometry techniques when the first real data becomes available. Significant innovation will be required, with a step above what is already the frontier of galaxy photometry highlighted by a talk by John Weaver (also an author of this bite!). This session also saw a foray into machine learning by Aritra Ghosh, which together with mentions in other talks underscores the need for and highlights the interest in machine learning approaches to data analysis in the era of Roman.

Session 9: Synergies with Wavelengths and Facilities 

Chary, Banerji, Pozzetti, Zalesky, Muzzin, Robertson

This series of talks began with a discussion of the existing mysteries within galaxy evolution, particularly within the epoch of reionization (z > 6), and the wavelengths at which we can hope to address them. Observations with Roman will contribute enormously to existing and future datasets by providing ultradeep NIR photometry and slitless spectroscopy over enormous regions of the sky. In particular, the combination of Roman’s High Latitude Survey, which will cover some 2,000 square degrees on the sky down to 27th mag, and Rubin providing depth-matched photometry in the optical regime, will provide one of the most exquisite datasets for extragalactic astronomy ever produced. Prior to the launch of Roman, ESA hopes to launch the Euclid satellite, which will survey over 15,000 square degrees to 24th mag, mapping the sky in NIR and laying much of the foundation for deeper observations with Roman. Collectively, these datasets will enable a premier census of galaxy properties across space and time, sampling a range of environments and extragalactic scales.

The Roman telescope will not launch for another 5 years. Euclid is roughly 2 years from launching, and Rubin‘s primary survey (LSST) will require 10 years after its first light to achieve its final depths. However, some of the exciting science promised by the combination of these surveys can begin sooner, thanks to surveys like the Hawaii Two-0 Twenty Square Degree survey. The Hawaii Two-0 survey will cover 2 of the 3 Euclid Deep Fields with deep optical imaging with Subaru Hyper Suprime-Cam and also benefits from deep Spitzer mid-infrared imaging (both processed by authors of this bite!). Euclid will cover these fields immediately upon its launch, and together with Hawaii Two-0, we will get a glimpse into the future science promised by ultradeep optical+NIR surveys over large cosmic volumes. 

Session 10: Galaxy Formation across Decades of Physical Scales

Treu, Marchesini, Bell, Rodriguez, Zhang

As the first speaker of this session made clear in his opening statements, the Roman telescope “will make even the rarest of astronomical objects common.” Rare objects in space tend to reveal unique information about the universe, and the first topic focused on the rare class of objects of strong gravitational lenses. Using strong lenses, astronomers can gain unique insights into the shape of the mass profiles of galaxies, and also learn about the interplay between their dark and baryonic components. In this respect, the Roman telescope will prove enormously fruitful, as it will discover and enable precision characteristics of over 20,000 strong lenses (where 2,000 of these are expected to be spiral galaxies, which make up only ~10% of the known strong-lensing systems). 

Here we also learned of a few other areas where Roman will shine, which include measuring the properties of galaxies across a range of scales. With Roman’s HLS covering over 2,000 square degrees down a K-band magnitude limit of ~27th mag, we will be able to precisely measure the properties of galaxies as a function of galaxy type, morphology, environment, and stellar mass, obtaining high-quality statistics in even the more rare regimes like in high-mass systems. With Roman’s resolution and depth, we will also be able to trace fainter galaxies within groups, ordinarily difficult to detect, to better understand the role of environment in driving galaxy evolution. 

Edited by: Mitchell Cavanaugh

Frontpage Image Credit: NASA

About John Weaver

I am a second year PhD student at the Cosmic Dawn Center at the University of Copenhagen, where I study the formation and evolution of galaxies across cosmic time with incredibly deep observations in the optical and infrared. I got my start at a little planetarium, and I've been doing lots of public outreach and citizen science ever since.

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