What do enormous, fabulous slugs have to do with giant galaxy formation?

Title: A Multiwavelength Study of ELAN Environments (AMUSE2):

Mass budget, satellites spin alignment and gas infall in a massive z ∼ 3 quasar host halo (Paper 1)

Ubiquitous dusty star-forming galaxies associated with enormous Lyα nebulae on megaparsec scales (Paper 2)

Detection of a dusty star-forming galaxy within the enormous Lyman α nebula at z = 2.3 sheds light on its origin (Paper 3)

Authors: (order varies per paper, first authors in bold) Fabrizio Arrigoni Battaia, Chian-Chou Chen, Marta Nowotka, Hau-Yu Baobab Liu, Carlos De Breuck, Maud Galametz, Michele Fumagalli, Yujin Yang, Anita Zanella, Allison Man, Aura Obreja, J. Xavier Prochaska, Eduardo Bañados, Joseph F. Hennawi, Emanuele P. Farina, Martin A. Zwaan, Roberto Decarli, and Elisabeta Lusso

Status: Submitted to ApJ

In today’s bite, we cover three papers that take a closer look at Enormous Lyman-alpha Nebulae (ELANe, sometimes called Lyman-alpha blobs) — big blobs of bright glowing gas that emit Lyman alpha photons excited by extreme sources. These powering sources are often starburst galaxies that host intense star formation activity, quasars that feature bright emission and jets powered by their central black holes, and other galaxies that host active central black holes called active galactic nuclei (AGN). The papers each draw from the AMUSE2 project: A MUltiwavelength Study of ELAN Environments. The primary goal of this program is to study the content and environments of quasars that host ELANe, focusing on observations in the far-infrared and submillimeter to target heated dust and gas in these systems. 

ELAN are large reservoirs of cool, dense gas, and point to rare overdensity peaks in the early Universe. Likely the sites of forming protoclusters (unsettled progenitors of massive galaxy clusters today), they host active star formation and major mergers of galaxies, and take in lots of infalling gas to fuel this activity. Studying these extreme, extended, luminous objects helps us gain understanding of how massive galaxies form and evolve into the giant elliptical galaxies we see today.

Paper 1: A Fabulous Case Study

In the first paper, the team focuses on one source, dubbed the Fabulous ELAN. With observations spanning wavelength and type, they aim to investigate the astrophysical processes powering this extreme system and forecast the evolution of the source. In particular, they study the gas, dust, and stellar content of four objects associated with the Fabulous ELAN: 3 active galactic nuclei (AGN, in galaxies with activity around their central black hole), and 1 Lyman-alpha emitter (LAE, a star-forming galaxy). These sources overplotted with the extended Lyman-alpha emission are shown in Figure 1.

Figure showing a larger blob of Lyman alpha emission and the identified sources over plotted on the blob, showing they are all within the blob (ELAN) structure
Figure 1. Map of the Fabulous ELAN field showing the extended Lyman-alpha emission in the color scale (the blob in blue) and the detected sources the authors study marked with their names. The main quasars powering the ELAN are noted as “QSO”, the galaxy with an active galactic nuclei is referred to as “AGN”, and the more normal, star-forming galaxies are Lyman-alpha emitters or “LAEs”. (Adapted from Figure 1 in the paper)

The authors estimate the total mass contained within the entire system by computing the mass of stars, gas, and dust, as well as deriving the dynamical mass based on kinematic information. Based on their observations tracing the gas content and dynamics, the authors show that the extreme, extended luminosity of the ELAN is powered by a complex combination of a range of sources of radiation, including an interplay between AGN and star formation driven ionization. The four sources they study all feature high rates of star formation, with gas and dust masses consistent with extreme, powerful galaxies at this point in the Universe. 

The authors predict the fate of the system by estimating how long it would take to convert all the available gas into stars while accounting for any additional infalling gas. Given their analysis, they present a possible scenario regarding the evolution of the system: for their estimated total system mass, they claim it would evolve into a massive 10^14 Msun halo at the present day, consistent with a giant massive elliptical galaxy. They explain with this gas content and rate of converting this available fuel into stars, it should remain starbursting until redshift z~2, as long as there is minimal gas infall or major merger triggering additional star formation. They also point to the fact that it may be part of a large-scale galaxy protocluster.

Paper 2: Connection to Dusty Galaxies

The next paper in the series is motivated by studying physical connections between Enormous Lyman-alpha Nebulae and dusty star-forming galaxies (DSFGs, see Astrobites on these galaxies here and here), massive starbursting galaxies that are obscured by dust. DSFGs are often signposts for intense sites of star formation in the Universe, and our current understanding of these galaxies is that they’re an extreme starbursting phase in the lifetime of massive galaxies. More specifically, DSFGs are proposed to evolve into massive elliptical galaxies today with lots of phases in between: including quasars, non-star-forming quiescent galaxies (as featured in this Astrobite), and major mergers.

To study DSFGs in ELAN environments, the authors conduct a submillimeter survey of four ELAN and the surrounding environments. Their basic analysis is to compare number counts (counting sources above a certain brightness in some region, see Figure 2) of detected sources to look for an overdensity of sources compared to existing models of number counts. Averaged across the four ELAN environments, they find 3.6x more DSFGs overall compared to the expected counts in blank fields, and find star formation at a rate 300x more than the average in the Universe.

Four panel figure showing the dusty source detections around the central object for each ELAN field (Fabulous, Slug, MAMMOTH-1, and Jackpot). Each panel shows a clear over density of sources with about 10 sources detected
Figure 2. Submillimeter maps of the four ELAN fields (Fabulous, Jackpot, MAMMOTH-1, and Slug). The central ELAN objects are noted as yellow stars, and sources with strong detections are highlighted with cyan circles. The white dashed line marks the map edge and corresponds to a region 150 times larger than the size of the Milky Way Galaxy. (Figure 1 in the paper)

Given these results, the authors suggest a scenario where DSFGs coevolve with quasars in ELAN fields. As a region of intense star formation and galaxy formation, the two types of sources likely sit in similarly dense environments where such intense activity is more naturally triggered. Still, number counts are a two-dimensional measurement (on the plane of the sky), and understanding the overdensity in three dimensions using emission line measurements will be a crucial next step for characterizing overdensities of dusty sources in ELAN fields.

Paper 3: Dusty Slug

The Slug ELAN, one of the four Enormous Lyman-alpha Nebulae covered in Paper 2, is one of the most luminous and large blobs of bright Lyman-alpha emission known today. To investigate the role of dusty starbursting galaxies and how they are connected to quasars in dense environments, the authors use submillimeter observations to search for dust-obscured sources within the Slug ELAN field.

In this data, the authors recover the two previously known quasars in Slug: UM287, considered the main power source for the ELAN, and its companion quasar, called compQSO. In addition, they discover a new dust-obscured galaxy, which they dub the Slug-DSFG. By mapping out the submillimeter emission in three dimensions, they investigate the positions and kinematics of the extended emission originating for these sources, and propose an evolutionary scenario based on these results.

Making some simple assumptions about the interplay between the three galaxies based on positions and dynamical information, the authors propose that the extended Lyman-alpha tail streaming out of the Slug-DSFG is being caused by stripped gas as the DSFG moves through the dense ELAN environment (see Figure 3). This gas then contributes to the dense cold gas reservoir of the ELAN that allows it to emit so much Lyman-alpha radiation. As the DSFG continues to be stripped of gas while simultaneously converting any gas it holds onto into stars, eventually the DSFG will run out of gas and become quenched or quiescent (non-star-forming). 

Two panel schematic showing a larger pink halo representing the system, two quasars (UM287 and compQSO), and the DSFG in between them stripping gas to supply the system
Figure 3. Schematic showing the authors’ proposed evolutionary scenario for the system, focused on the DSFG. On the left, the Slug-DSFG has entered the halo of the ELAN (pink) and its gas starts to get stripped. The two quasars, UM287 and its companion compQSO, are assumed to not move relative to each other. At a later time (right panel), based on their analysis of the position and kinematics of the DSFG, its gas gets stripped and enters the reservoir of gas hosted by the halo. (Figure 7 in the paper.)

This interpretation supports a picture of DSFGs in the early universe undergoing intense starbursts and black hole accretion, transitioning into a quasar phase, experiencing major mergers, and eventually settling into a quenched state and becoming one of the giant elliptical galaxies we see today.

Astrobite edited by Huei Sears

Featured image credit: adapted from imaged provided by user Hemingway To Go! via The Something Awful Forums

About Olivia Cooper

I'm a second year grad student at UT Austin studying the obscured early universe, specifically the formation and evolution of dusty star-forming galaxies. In undergrad at Smith College, I studied astrophysics and climate change communication. Besides doing science with pretty pictures of distant galaxies, I also like driving to the middle of nowhere to take pretty pictures of our own galaxy!

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