NGC 5474 | Ray Garner

M101 Group: Galaxy Evolution in a Nearby Laboratory

Sun, 09 Aug 2026 00:00:00 +0000

I have focused on the M101 Group since graduate school. Because it is only a few megaparsecs away, this small, isolated galaxy group gives us a detailed look at how spiral galaxies form and change. We can study star formation in individual H II regions, map chemical abundances across a whole galactic disk, use stellar populations to trace spiral structure and interactions, and examine neighboring galaxies as part of the same dynamic system.

I started studying M101 by asking: how much star formation happens beyond the bright optical disks of galaxies? This question led me to explore how M101 has grown, changed chemically, responded to its surroundings, and interacted with its satellites, especially the unusual galaxy NGC 5474.

In this research, I have used deep, wide-field imaging, long-slit and integral field spectroscopy, nebular abundance measurements, stellar population modeling, and multiwavelength analysis. Each finding led to new questions. The M101 Group has been both a laboratory and a foundation for shaping how I study galaxy evolution.

Why M101?

M101, or the Pinwheel Galaxy, is a large, nearby spiral galaxy seen face-on. Its disk is noticeably asymmetric, featuring a “big wonky arm.” The outer spiral structure looks disturbed, and its largest satellite, NGC 5474, is also unusual, with a bright central bulge that is clearly offset from the center of its underlying disk.

These features have suggested for some time that M101 and NGC 5474 recently interacted. However, the system is puzzling: its shape looks disturbed, but other signs of a strong interaction are unexpectedly subtle.

These questions have motivated much of my research. What effects does a weak tidal interaction have on a large spiral galaxy? Where should we look for its signs: in current star formation, chemical abundances, stellar ages, spiral structure, or satellite dynamics? And how long do these signs last?

The M101 Group provides a nearby laboratory for studying spiral structure, satellite interactions, chemical evolution, and star formation. Image credit: Doublas Rollain - Astrobin

Searching for Star Formation Beyond the Optical Disk

For my first major M101 project, I used deep, wide-field narrowband imaging from the Burrell Schmidt telescope to search about 6 square degrees around the group for very faint areas of ongoing star formation.

We focused on Hα, Hβ, and [O III] emission lines and created a method to distinguish real emission-line sources apart from foreground stars and background galaxies. Our goal was to find isolated H II regions or new star-forming dwarf galaxies that could mark tidal material from an encounter between M101 and its companions.

The surprising result was largely a null detection. Nearly all of our candidates were associated with the extreme outer disk of M101 rather than a widespread intragroup population. Only two unusual objects were associated with other systems: an outlying H II region near NGC 5474 and a star-forming object belonging to a background galaxy NGC 5486.

A zoomed-in view of the detected regions near NGC 5474 (top row) and NGC 5486 (bottom row). Left: the continuum-subtracted Hα image. Right: the broadband B image, stretched to show the faint structure of the outer disks. The red circle represents the size of a galaxy mask. The orange circle is an object detected in all three narrowband images, while the green circle is an object detected in the Hα and [O III] image.Figure 9 of Garner et al. (2021)

Instead of being disappointing, this result changed my perspective on the group. If M101’s obvious asymmetries came from a recent encounter, it seems that event did not spread much star-forming gas into the surrounding area. Or, if it did, the Hα signals we looked for had already faded.

What This Work Demonstrated

Through this project, I gained experience with deep optical imaging, careful continuum subtraction, detecting low surface brightness sources, emission-line photometry, building catalogs, rejecting contaminants, following up with spectroscopy, and interpreting null results.

It also taught me an important lesson that has guided my research: not finding an expected signal can still tell us something physical. The lack of intragroup star formation helped narrow down possible interaction histories for the system and encouraged me to search for more subtle signs within the galaxies.

Chemical Evolution Across M101

If the interaction had not produced large numbers of isolated star-forming regions, perhaps its signature could instead be found in the chemical structure of M101’s disk.

Using an expanded Burrell Schmidt dataset including Hα, Hβ, [O III], and [O II], I measured strong-line oxygen abundances for 720 H II regions across M101, NGC 5474, and NGC 5477.

M101 shows the usual drop in metallicity as you move farther from the center, but the full two-dimensional data revealed a more complex situation. The gradient flattens in the outer disk, and H II regions there show clear azimuthal abundance patterns. These differences from a simple radial gradient mostly appear in parts of the disk that may have been shaped by M101’s interactions with NGC 5474.

The Kobulnicky & Kewley (2004) oxygen abundances of M101 fitted with a simple exponential (dashed black line) and a broken exponential (solid black line). Regions in the turn-around region (orange circles), upper branch (blue triangles), and lower branch (green inverted triangles) are distinguished. Outside 1.3R₂₃, we display both the upper- and lower-branch solutions connected by a dotted line for each individual H II region.Figure 10 of Garner et al. (2022)

What This Work Demonstrated

This project expanded my expertise in nebular astrophysics and chemical evolution, including reddening corrections, emission-line ratios, strong-line abundance diagnostics, metallicity gradients, uncertainty propagation, and spatial statistics.

More broadly, it taught me to connect local and global views of galaxies. Individual H II regions show local conditions, but hundreds of these measurements can reveal patterns in chemical enrichment and mixing across kiloparsec scales. Linking the physics of single nebulae to the evolution of their host galaxies became a key theme in my later work.

Spiral Dynamics Through Stellar Populations

The same narrowband dataset also held information I had not planned to use. Besides nebular emission, some filters captured age-sensitive stellar absorption features, like Balmer absorption and changes in the continuum near the Balmer and 4000 Å breaks. By comparing these measurements to stellar population models, I developed a way to estimate stellar ages across M101’s disk.

The resulting maps showed the expected trend: younger stars are found farther from the center, which fits with inside-out growth. More interestingly, the stellar ages change in a regular way across spiral arms. In the mostly undisturbed inner disk, these age patterns match what we expect from large-scale spiral shocks. Surprisingly, similar patterns also show up farther out, beyond the expected corotation radius of the main inner spiral.

I saw these results as evidence that M101’s seemingly organized structure may come from several dynamically linked spiral patterns, not just one rigidly rotating spiral density wave.

The equivalent width through the Hβ filters (W_Hβ) and [O II] filters (W₃₇₄₆) for all pixels in the main disk of M101 distinguished by environment: the inner disk in red, spiral arms in blue, and interarm regions in green.Figure 9 of Garner et al. (2024)

What This Work Demonstrated

For this project, I had to use an observational dataset for a new question it was not meant to answer. I developed techniques for modeling stellar populations, linked narrowband observations to synthetic spectra, made spatial age maps, and interpreted these results in terms of spiral dynamics.

This work expanded my research from studying present-day ionized gas to looking at the stellar fossil record. Gas emission shows what a galaxy is doing now, while stellar populations hold clues about its past. By combining both, we get a much clearer picture of galaxy evolution than by using either one alone.

NGC 5474: A Hidden Galaxy in Plain Sight

My dissertation led me to another question: if an encounter with NGC 5474 helped reshape M101, what happened to NGC 5474 itself?

NGC 5474 has a strange shape. Its bright central part, usually called its ‘bulge,’ is about a kiloparsec away from the center of its star-forming disk. Some have suggested that this part is not a bulge, but actually a separate dwarf galaxy seen in front of NGC 5474.

With PINGS PPak integral field spectroscopy, I measured the velocity of the stars in the supposed bulge and compared it to the ionized gas velocity in the surrounding disk. The two are offset by about 24 km/s, which gives kinematic evidence that they are separate systems.

An RGB color image of NGC 5474.Figure 1 of Garner et al. (2025)

This idea was supported by work I supervised with undergraduate researcher Liv Moody, who used the narrowband data to study stellar populations across NGC 5474. The offset bulge is made up mostly of old stars and is different from the surrounding disk. The galaxy’s outer stellar populations also show extra spatial structure that may relate to its interaction history.

This part of the project also marked a change in my role as a researcher. I moved from doing the analysis myself to designing research questions, developing methods, and mentoring another researcher through an independent project.

Returning to M101 with HETDEX

My current work brings me back to M101, but with a very different dataset. The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) observed M101 using the VIRUS integral field spectrograph, giving us complete spectroscopic coverage of M101’s disk for the first time.

I am using these spectra along with multiwavelength photometry to map the galaxy’s stellar ages, metallicities, star formation history, and both stellar and gas kinematics. This work brings my M101 research full circle. My first projects used individual H II regions to study the galaxy’s current state. Now, HETDEX lets me compare those signs to the history recorded in the overall stellar population.

The Broad Picture

Taken together, these projects show that galaxy interactions do not always leave just one clear observational sign.

In the M101 Group, signs of dynamical evolution look different depending on which tracer you study:

The surrounding group contains little widespread intragroup star formation;
The disk of M101 contains chemical structure beyond a simple radial gradient;
Its stellar populations encode both inside-out growth and complex spiral dynamics;
NGC 5474 contains a dynamically distinct central component and unusual stellar populations;
The stellar fossil record provides a way to connect these present-day signatures to longer evolutionary timescales.

This is one of the main lessons I have learned from these projects. Galaxy evolution is complex and happens on many scales and timescales. Features like shape, ionized gas, chemical abundances, stellar populations, and kinematics do not all react to an interaction in the same way or keep its signature for the same length of time. The M101 Group is especially valuable because its closeness lets us study all these tracers together.

Methods and Expertise

My work on the M101 Group has developed and demonstrated expertise across several areas of observational astronomy:

Observational Techniques: Deep wide-field imaging · Narrowband imaging · Long-slit spectroscopy · Integral field spectroscopy · Multiwavelength photometry

Galaxy Evolution: Star formation · Galaxy interactions · Spiral structure · Stellar populations · Chemical evolution

Nebular Astrophysics: H II region identification · Emission-line photometry · Reddening corrections · Strong-line abundances · Metallicity gradients

Stellar Populations: Synthetic stellar population modeling · Age-sensitive spectral features · Full spectral fitting · Star formation histories · Stellar metallicities

Data Analysis: Python · Astropy · CIGALE · pPXF · Spectral fitting · Image analysis · Catalog construction · Spatially resolved mapping · Statistical modeling

Selected Publications

First Author Work

Garner et al. 2026:
Spatially resolved stellar ages and metallicities from HETDEX spectroscopy.
Garner et al. 2025:
Kinematic evidence that the offset bulge of NGC 5474 is a distance stellar system.
Garner et al. 2024:
Using age-sensitive narrowband measurements to trace disk growth and spiral dynamics.
Garner et al. 2023:
Spectroscopic confirmation and interpretation of unusual objects discovered in the original narrowband survey.
Garner et al. 2022:
Spatially resolved chemical abundances across M101 and its major satellites.
Garner et al. 2021:
Deep search for star formation and low-luminosity galaxies throughout the M101 Group.

Collaborative & Mentored Work

Moody & Garner 2025:
Undergraduate-led project extending my stellar population methodology to NGC 5474. I mentored the project and contributed to the analysis and physical interpretation.
Watkins et al. 2024:
Collaborative investigation of the origin of diffuse ionized gas using the same deep imaging dataset together with ultraviolet observations and photoionization modeling.

Dissertation

My PhD dissertation brought together the originaly outlying star-formation, chemical abundances, and stellar population studies into a unified investigation of M101 and its group environment.

Where This Work Is Going

The M101 Group is still a key part of my research because it lets me connect physical processes measured on many different scales. The methods I developed here can be used beyond M101. Studying nearby galaxies allows us to link detailed nebular physics, stellar populations, chemical abundances, and galactic dynamics in ways we cannot with more distant galaxies. These local relationships can then help us understand galaxies across cosmic time.

Peculiar Stellar Ages of the Peculiar Dwarf Galaxy NGC 5474

Fri, 01 Aug 2025 00:00:00 +0000

When Is a Bulge Not a Bulge? Revealing the Satellite Nature of NGC 5474's Bulge

Tue, 01 Apr 2025 00:00:00 +0000

The Peculiar "Bulge" of NGC 5474

Mon, 03 Mar 2025 00:00:00 +0000

The Mystery

Almost two years ago when I was writing up , I needed to write up a “Conclusions and Future Work” chapter. The conclusions were straightforward, but coming up with an idea of things I can do in the far off future was hard. I think any scientist would tell you that doing the science is usually the easy part; coming up with the idea in the first place is the hard part!

But throughout my thesis which focused primarily on the nearby giant spiral galaxy M101, known as the Pinwheel Galaxy, I kept turning my attention to its companion dwarf galaxies. The biggest of these doesn’t have a cute name, but is intead known by its catalog number, NGC 5474. And just like M101, NGC 5474 is peculiar! Optically, it looks like a typical dwarf spiral galaxy with loose spiral arms and numerous sites of star formation. That is, except for its central bulge being offset to the north!

This galaxy really caught my attention, but sadly, I didn’t have time for it in my thesis to do a deep dive on it as much as I had M101. This was a perfect candidate for some of that “future work,” to figure out what’s going on with that offset bulge. Recently, a performed some photometry of the bulge to look at resolve stellar populations, basically seeing what types of stars were in the bulge and the surrounding disk of NGC 5474. Based on that data, they proposed that the bulge of NGC 5474 could actually be a dwarf elliptical galaxy orbiting around the disk of NGC 5474 instead of being a bulge in the plane of the disk as is the case for normal galaxies.

How Can We Figure Out the Puzzle?

How can you tell that for sure? Well, you’d need spectroscopy. Spectroscopy is a tool that allows you to see how much light of each individual wavelength there is in an object called a spectrum. Astronomers (and physicists and chemists) have put a lot of work into understanding spectra and what sorts of objects produce the different wavelengths of light. For example, star-forming regions that are full of gas ionized by hot, young stars produce “emission lines”, things like ionized hydrogen, oxygen, nitrogen, sulfur, etc. Meanwhile, stars have elements in their stellar atmospheres that absorb light before it can leave the star, so stars have “absorption lines”, like hydrogen, magnesium, iron, calcium, etc.

So for our case of NGC 5474, if we can take the spectrum of the bulge area, we’ll see a spectrum likely composed of both emission lines and absorption lines. Thankfully, we’re not the first people to do this in general, so we know that the disks of galaxies are where the star-forming regions lie that produce the emission lines. Meanwhile, elliptical galaxies which don’t tend to have any ionized gas should produce mostly the absorption lines produced by populations of stars.

One more thing to consider. If an object is moving away from us, its light will be at wavelengths shifted from what we measure here on Earth in our labs. This is the well-known that you probably have lots of experience when an emergency vehicle passes you on the road. As it approaches you, the sounds waves coming from its sirens are squished together and you hear a higher pitch. As it races away from you, the sound waves are spread apart and you hear a lower pitch. Light waves do this too and we call this being blueshifted and redshifted, respectively.

What does that mean for our spectroscopy of the bulge and disk of NGC 5474? Well, if there is a wavelength offset between the absorption lines in the bulge and the emission lines in the disk, then that indicates a velocity offset as well. A velocity offset would be a strong indication that the bulge and disk are two separate objects!

All of this was proposed by that group of Italian astronomers in . Sadly, I did not have any spectroscopy of NGC 5474. There was one collaboration called ¹ that would have covered NGC 5474 in their spectroscopic survey, but they hadn’t released the data for this galaxy yet. Oh well. I’d have to go out and collect this spectroscopy at some far off date. Who knows when that’ll be? I wrote it up this idea and got to work defending my thesis.

The PINGS Data

Barely a year after my thesis, and after I had picked up everything to start my new postdoc position at Texas A&M, I figured I could try and answer this question. So I worked up the courage to cold email the head of the PINGS collaboration and see if they were planning on doing anything with their spectroscopic data of NGC 5474. Thankfully, the head, Fabián Rosales-Ortega was more than happy to share the data with me! Yes! I could now try and figure out what the nature of this peculiar offset bulge is.

What did this data look like? Well it’s a special kind of spectroscopic dataset called “integral field spectroscopy” or IFS. Basically, instead of throwing down a thin slit on a galaxy and being able to only collect the light in that slit, IFS is like taking a larger picture of a galaxy at each wavelength of light. The result is a “data cube”, a 2D picture of the galaxy with a spectrum in each pixel. That means that not only do we have spectral information, we could also see how the spectra vary across the galaxy!

Or at least, that’s the idea in theory. In practice, things can go wrong. The PINGS survey collected IFS data on many galaxies, but out of all of them NGC 5474 was the only one who’s data collection was compromised. This galaxy’s data was collected not by Fabián, but by the folks who manage the observatory. Due to no fault of their own, this meant that the accuracy of the data was not up to par across the entire galaxy. But! The region around the bulge was of good quality and that’s all we needed for this project.

Circular Apertures

Now, how do we actually extract the spectra and measure their (potentially offset) wavelengths? There were two basic methods that we tried. The first is to just sum up all of the light in some circular apertures centered on the bulge. This has the benefit of increasing the signal-to-noise ratio of the data. The second method is to use the entire 2D dataset to produce velocity maps, so we can more easily see where velocities are different and where they’re the same.

I’m not going to go into all of the details of how we actually do this (that’s what is for!), but I’ll show you some interesting results.

First up are the circular apertures. We did this in a series of circular apertures ranging from relatively small that encompassed only a fraction of the bulge up to a large aperture that gets the entire bulge. We did this because the smaller aperture minimizes “contamination” from the underlying disk while the largest aperture maximizes the signal-to-noise at the expense of containing more of the underlying disk. Below, you can see the spectra for each aperture in varying shades of blue and green.

The spectra of circular apertures centered on the bulge of NGC 5474. The top panel shows the spectra for the four apertures used with their radii indicated in the legend. The middle panel shows an example of the absorption spectrum overlayed in black for one of the apertures. The bottom panel shows the emission line spectrum produced after subtracting the absorption line spectrum.

In each spectra, you can see both absorption and emission lines. If we focus on the largest aperture which is shown by itself in the middle panel, you can more clearly see these two features. The black line is a fit to the stellar population and its absorption lines, those parts that have lower fluxes at a particular wavelength. You can also see the emission lines sitting on top of the absorption line spectrum. Subtracting away the absorption line spectrum gives you just the emission lines in the bottom panel. These lines are very familiar to astronomers who study star-forming regions. We see lines of ionized hydrogen, oxygen, nitrogen, sulfur, and even ionized neon!

Now we just need to estimate the velocity offset for the emission and absorption lines separately. Remember, the absorption lines are likely being produced by stars in the bulge, while the emission lines are being produced by star-forming regions in the disk. When we estimate the velocity of the absorption lines, we get approximately 230 km/s. Doing the same for the emission lines gives a velocity of approximately 255 km/s. That’s a velocity difference of 25 km/s!

Spatially Resolved Velocities

Alright, but that’s just for some circular apertures. If we use each pixel in the data, each containing its own spectrum, what do we get for the velocities across the entire datacube? I won’t bore you with the details, but we can produce velocity maps that show how the velocities in the absorption lines and emission lines change across the entire dataset.

From left to right: the stellar velocities, gas velocities, and velocity differences across NGC 5474. The top row of panels shows the velocities, while the bottom row shows their associated uncertainties. The red circle outlines the position of the bulge in each map.

In the above figure, we’ll focus on the top row (the bottom row contains the uncertainties on the velocity measurements). In all of the plots, the red circle is the position and size of the bulge. The left plot is the velocity of the stars, i.e., measured from the absorption lines. You can see that the bulge is pretty coherent, all around the same velocity of about 230 km/s. The middle plot shows the velocity of the ionized gas, i.e., measured from the emission lines. There’s a much larger area that is of the same velocity, but importantly this area contains the bulge at a velocity of about 250 km/s. Finally, the right plot shows the velocity difference and the bulge stands out clearly! Spatially resolved, the bulge has a velocity difference between 20 and 40 km/s, very similar to the result from the circular apertures!

Is This The Only Answer?

Now, just because we’ve measured a velocity difference of about 25 km/s, doesn’t mean these two objects are separate. We need to put this measurement in the context of similar measurements and see if any other explanation is plausible.

First, the gas and stars in normal galaxies don’t have a large velocity difference. For example, in of 30 isolated spiral galaxies, a velocity difference of only 2 km/s was found, ten times lower than what we’ve measured in NGC 5474.

The Cigar Galaxy, M82. Credits: NASA/ESA/Hubble Heritage Team/J. Gallagher/M. Mountain/P. Puxley

Maybe we’re measuring some sort of gaseous outflow from the disk? If you look at galaxies that have intense rates of star formation, like the Cigar Galaxy (M82), you see that gas is being pushed out perpendicular to the plane of the disk by rapid star formation and all those strong stellar winds. Since NGC 5474 is face-on and not edge-on like M82, maybe we’re just looking straight down the barrel of this funnel of gas. Well, NGC 5474 is not a starbursting galaxy. It has a meager star formation rate, and it doesn’t have an active supermassive black hole at the center that could push gas out either.

It does look like this velocity difference between the bulge (absorption lines) and disk (emission lines) is real and isn’t caused by some of the more common explanations. This galaxy, which has been seen as just one peculiar galaxy, looks like it is instead a superposition of two galaxies! The fact that we don’t see any debris from an interaction (no long streams of stars, no huge burst of star formation, no wildly distorted velocity maps) likely says that these two objects are in the process of weakly interacting with each other.

The larger question remains: how does this all connect to M101? I’ve buried the lede, but approximately 300 million years ago, . NGC 5474 swung by M101, close enough to distort M101 and create that “big wonky arm” as I have grown to call it. We thought that the interaction also might have shifted the bulge of NGC 5474 north, but our new results seem to indicate otherwise. Clearly that close passage with M101 didn’t unbind the bulge-disk pair of NGC 5474, or maybe the bulge is a more recent addition? Honestly, we don’t know for sure. All I know is that someone needs to do some dynamical modeling of this three-body interaction, something that is famously difficult.

That’s all I’ve got for you! I am very happy that I got to put some words down about this project and even happier that I got to investigate this peculiar little galaxy. This only goes to show that even the smallest galaxies are curiosities unto themselves!

PINGS stands for the PPak IFS Nearby Galaxies Survey, an acronym that contains two acronyms in it! ↩︎
Yep, I really did include this line as the opening quote to the final chapter of my thesis! ↩︎

Investigating the Galactic Evolution of M101 with Deep Narrowband Imaging

Sat, 01 Jul 2023 00:00:00 +0000

NGC 5474 | Ray Garner

M101 Group: Galaxy Evolution in a Nearby Laboratory

Why M101?

Searching for Star Formation Beyond the Optical Disk

What This Work Demonstrated

Related Publications

Chemical Evolution Across M101

What This Work Demonstrated

Related Publications

Spiral Dynamics Through Stellar Populations

What This Work Demonstrated

Related Publications

NGC 5474: A Hidden Galaxy in Plain Sight

Related Publications

Returning to M101 with HETDEX

Related Publications

The Broad Picture

Methods and Expertise

Selected Publications

First Author Work

Collaborative & Mentored Work

Dissertation

Where This Work Is Going

Peculiar Stellar Ages of the Peculiar Dwarf Galaxy NGC 5474

When Is a Bulge Not a Bulge? Revealing the Satellite Nature of NGC 5474's Bulge

The Peculiar "Bulge" of NGC 5474

The Mystery

How Can We Figure Out the Puzzle?

The PINGS Data

Circular Apertures

Spatially Resolved Velocities

Is This The Only Answer?

Investigating the Galactic Evolution of M101 with Deep Narrowband Imaging