A Picture (or Ten) is Worth 10,000 Words: Unearthing the History of the Lenticular Galaxy NGC 3115

Title: Recovering the origins of the lenticular galaxy NGC 3115 using multi-band imaging

Authors: Maria Luisa Buzzo, Arianna Cortesi, Jose A. Hernandez-Jimenez, Lodovico Coccato, Ariel Werle, Leandro Beraldo e Silva, Marco Grossi, Marina Vika, Carlos Eduardo Barbosa, Geferson Lucatelli, Luidhy Santana-Silva, Steven Bamford, Victor P. Debattista, Duncan A. Forbes, Roderik Overzier, Aaron J. Romanowsky, Fabricio Ferrari, Jean P. Brodie, Claudia Mendes de Oliveira

First Author’s Institution: Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil

Status: Accepted for publication in MNRAS [closed access]

Galaxies come in all shapes and sizes–round, disky, smooth, spiraled, or even shaped like penguins! The two most commonly-mentioned types are spirals and ellipticals, but there also exists an intermediate between the two: the lenticular galaxy, which has a disk and a bulge like spirals, but does not have grand spiral arms. Over the years, astronomers have worked to not only classify galaxies based on their morphology, but also to understand how they form and evolve over time. While spirals and ellipticals are well studied, lenticular galaxies have one of the most enigmatic formation histories of the different galaxy types. The authors of today’s paper take on the role of galactic detectives as they try to piece together the narrative of NGC 3115, the closest lenticular galaxy to our Milky Way.

Galactic Building Blocks

The authors compiled data of this galaxy in 11 filters, stretching from the ultraviolet to the infrared. This data came from different observatories, since no single telescope exists that can image in such a broad range of wavelengths. They fed these images into a tool called GALFITM in order to do a multi-wavelength decomposition of the galaxy into different structural components. Let’s step back and talk about what that previous sentence actually means. 

Say you wanted to make a sandwich. Most sandwiches are composed of different appetizing building blocks that can be mixed and matched–bread, tomato, mayo, etc. But there are many types of sandwiches, with different ingredients and proportions. A classic BLT has white bread, lettuce, tomato, bacon, and mayonnaise, while a Reuben uses rye bread, corned beef, cheese, sauerkraut, and dressing. You can also make variations on sandwiches by changing the amount or variety of ingredients (for example. sourdough vs rye bread). Galaxies are also all composed of common galaxy-building features, but the features differ depending on their type. Some galaxies can be modeled as one disc, others could be the composition of multiple discs, a bulge, and spiral arms, each with different parameters. This is exactly what GALFITM tries to do, simultaneously using a collection of multi-wavelength images of the same galaxy to figure out what components the galaxy is made from. It takes each parameter of a component, such as its radius and Sérsic index, and fits that value into a wavelength-dependent polynomial. All the program requires from us is initial parameters to use as a launchpad to find the best model of the galaxy: how many components there are, the order of the polynomial, which tells us how much we’re letting that parameter change with wavelength, and the initial conditions (magnitudes, radii, etc.). 

The image shows three panels titled "Disc", "Disc + Bulge" and "Disc + Bulge + Bar" going from left to right, respectively. In the first panel at the top there is a rather thin oval shape with a grey color gradient, getting radially darker toward the center of the oval. This represents a disc component of a galaxy. Below this is a purple arrow pointing to the same shape, indicating that some galaxies are just composed of one disc component. As we move to panels to the right, we add on more components. The middle panel shows the same grey disc plus a small black dot at the top, which come together at the bottom of the panel. The black dot represents a bulge component and is placed directly on top and in the center of the grey disc. The rightmost panel shows the same disc and bulge components, plus a small grey sliver that is a very thin oval shape, which represents a galaxy's bar. Down below is an image showing all three components put together--the disc, in the middle and aligned with the disc being the bar, with the bulge in the center of it.
Figure 1: A simple illustration of how different components make up a galaxy. A galaxy could just be modeled as one disc (left), or be more complicated, such as having both a disc and a bulge (middle) or a disc and bulge and bar (right). Image created by Astrobite author.

Fitting galaxies is tricky because you have to achieve a balance between leaving parameters free to vary and putting constraints on some components so that your final model is not nonphysical. You can then tell which models are best by looking at a residual image (original minus model). If the model describes the galaxy perfectly, the residual should be completely smooth. After running thousands of models with different variations, the authors found that NGC 3115 is best described by three components, a bulge and two discs, which they interpret as a classical bulge, thin disc, and thick disc. They also found evidence of a faint bar and spiral arms.

This image shows an 11 by 10 grid of galaxy components. Each panel in the grid looks pretty similar in shape and color--a pretty thin grey oval with a radial gradient getting darker toward the center. The ovals change shape depending on the component they are models of--the thick disc (second to last row) is much larger and darker than the thin disk (bottom row) or bulge (third from bottom), which are both much thinner and dimmer.
Figure 2: Results from GALFITM fitting of NGC 3115. Each of the columns shows the modeling for a different filter, from far UV to infrared. The top row shows the original input image. After this are rows showing the complete model from GALFITM and the residual (the difference between input image and model) for three different galaxy compositions: a single Sérsic component (SS), a bulge + disc (BD), and a bulge + 2 discs (BDD). The last three rows show each component of the three-component model (BDD) separately: the bulge, the thick disc, and the thin disc, respectively. Figure 3 in the paper.

Too Blue to be True

Going back to our analogy, given a sandwich, it might be hard to tell how fresh the ingredients are–unless the lettuce is completely wilted or the bread is moldy, it’s usually difficult to differentiate a sandwich made from fresh ingredients to ones lying in the fridge for a few days. Just like it can be hard to tell the freshness of ingredients over a short timespan, astronomers also have the difficult task of piecing together formation histories of galaxies from a single snapshot–we can’t wait a million years to see how galaxies evolve over time. So now that they successfully built NGC 3115, the authors honed in on different parameters of its features to weave together its history. They examined the variation in color along the galaxy’s major axis and found that in the ultraviolet, the inner region of the galaxy is bluer than the outer region, with strong emission in the infrared and x-ray as well. The authors postulate that this could be due to a young stellar population in the bulge, AGN activity, or a mix of both. In order to resolve this puzzle, they use a spectral energy distribution (SED) fitting code to extract the physical properties of NGC 3115’s components. This technique aims to model how much energy the galaxy emits at different wavelengths, which can be compared to observed SEDs to extract properties of a galaxy such as its mass or star formation history (SFH). Running millions of models with different SFHs, the authors found that whether or not they included an AGN component in the bulge as part of their model, this component was always lower luminosity and subdominant to stellar emission, so they suggest that the blueness of the bulge cannot be due to AGN activity and therefore the galaxy should have undergone recent star formation.

Close Encounters of the Starburst Kind

NGC 3115 also has many globular clusters and several ultra-compact dwarf companion galaxies. To see if these affected NGC 3115’s evolution, the authors modeled one of them, KK084, and extracted its physical properties using a Python library. They found that KK084’s colors were very different from NGC 3115 itself, indicating that the two have different stellar populations. A new stellar population in the dwarf could have been caused by an encounter with NGC 3115, which would cause new star formation, but KK084 is too faint for us to prove or disprove this from modeling alone. So the authors tested this hypothesis by looking for an offset in NGC 3115’s core, which would be further evidence of a recent interaction. They found that NGC 3115’s core indeed is displaced relative to its external parts, indicating that it encountered a companion a few hundred thousand years ago, with KK084 being a good candidate. This would also help explain the faint bar and spiral arms they found when modeling the galaxy.

Hello, My Name is NGC 3115

All of the clues that the authors and previous astronomers gathered finally helped them piece together a formation history of NGC 3115, which is illustrated in Figure 3. Most likely, NGC 3115 formed from a gas-rich merger, forming the bulge and thick disc. Then through processes of accretion and feedback, a thin disc developed and an AGN was activated, which quenched star formation due to the influx of hot gas that prevented cooling. After star formation had basically stopped, the AGN had nothing to feed off of, so it too died down. But then a companion galaxy like KK084 interacted with NGC 3115, causing a rejuvenation in star formation from the influx of new gas, causing NGC 3115’s core to become displaced and spiral arms to form.

An image of how a galaxy formed. It is broken up into 6 panels, each showing a different stage in the galaxy's formation. The top left image shows two blue ovals, the center one with a black dot in the middle and the second one to the upper right and slightly tilted from the central one. The text at the top left of the panel says "At the beginning, a high-redshift disc galaxy interacts with a gas-rich object". To the right of that panel shows a large blue-gray oval (almost circular) taking up most of the panel. Inside the blue-grey oval is another oval, which is lighter blue and thinner, its major axis perpendicular to the larger oval's major axis. Inside this light blue oval is a smaller red oval whose major axis aligns with the light blue oval. From opposite ends of the red oval, 180 degrees from each other, are two darker blue shapes that look like the hands of a clock. At the top right of the panel there is text that says "This initial merger creates the halo (Peacock+15) bulge and thick disc of the galaxy". At the bottom left of the panel is the text "The interaction heats up, increases the BH mass and, from the leftover of gas, creates a thin disc". The panel to the left of this shows the same image comprised of ovals and shapes, but also adds in a dark golden bar within the red oval, a yellow donut also inside the red ellipse, whose center lines up with the black dot at the center of the image, and a few yellow ovals scattered throughout in the largest, grey-blue oval. The text at the top right says "The thin disc develops a bar, through which gas is pushed towards the BH, activating the AGN". The text at the bottom left says "The galaxy accretes dwarf companions, increasing the size of the halo and thick disc (Brodie+14, Dolfi+20)". 
Moving on to the bottom row, at the very left is the same illustration as in the previous panel, but the components that look like clock hands are now colored light red, the scattered yellow ovals are in slightly different positions, and there are 6 yellow rays (lines) coming out the donut shape in the center of the image. The text at the top right says "AGN feedback + continuing accretion of companions quench the disc" and the text at the bottom left says "An S0 galaxy!". The next panel to the right shows the same image again, but without the scattered yellow ovals, yellow donut, or yellow rays. Instead, the yellow donut and rays are replaced by a thin white circle barely larger than the size of the black dot at the center). The text at the bottom right says "Without feeding, AGN activity decreases (Almeida+18). Lastly, the final panel on the bottom right hows the same shapes as before, except the red oval at the center is now blue, and the black central dot has a small arrow pointing directly downwards. The clock-hand-like shapes have two red arcs drawn on either side. There is also a small blue oval located at the bottom left of the image, with a long red arrow stretching from the very right of the image to this blue oval. The top right text says "Recent encounter with companion KK084 creates a core displacement in NGC 3115" and the bottom text says  "The gas from the interaction causes a star formation episode in the bulge and instabilities, shaping the spiral arms".
Figure 3: An illustration of the authors’ proposed formation and evolutionary history of NGC 3115. Figure 12 in the paper.

We can uncover the building blocks of what make up the galaxy, and also learn about its physical properties that can give us hints about when it formed stars or got its shape. Combined with archival data from surveys, analyses like these will continue to aid us in understanding galaxy evolution as a whole, and enable us to recover a chapter of our universe’s history.

Astrobite edited by Wynn Jacobson-Galan

Featured image credit: X-ray: NASA/CXC/Univ. of Alabama/K. Wong et al; Optical: ESO/VLT – http://www.nasa.gov/mission_pages/chandra/multimedia/photo-H-11-248.html

About Katya Gozman

Hi! I’m a second year PhD student at the University of Michigan. I’m originally from the Northwest suburbs of Chicago and did my undergrad at the University of Chicago. There, my research primarily focused on gravitational lensing and galaxies while also dabbling in machine learning and neural networks. Nowadays I’m working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and frequently volunteer with a STEAM education non-profit in Wisconsin called Geneva Lake Astrophysics and STEAM.

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