Galaxy morphology changes once you go to a different color. You are more sensitive to different stellar populations. Blue filters pick up young, massive stars for preference and redder filters the older population of galaxies, one is more sensitive to star-formation, and the other overall stellar mass (something the S4G survey used to great effect).
This brings me to the narrow-band magic. Narrow filters are only sensitive to a short wavelength range. But if an emission line happens to lie in that range, the contrast for those images will be fantastic.
This is the idea behind the Merian Survey and what this week’s paper is all about:
A Nonparametric Morphological Analysis of Hα Emission in Bright Dwarfs Using the Merian Survey
The two filters used, each capturing either Halpha or Hbeta+OII. So instead of a morphology estimate that is dominated by stellar populations, the morphology of your images is almost exclusive the emission line. These emission lines, especially Halpha, is powered by new star-formation. So this survey maps new star-formation in nearby galaxies and where it occurs. They combine their observations with a local estimate of the stellar continuum from z-band.
The optical image, narrow-band image, the continuum contribution and the line emission image of galaxies of this survey. This is a neat way to map lots of galaxies fast!
This allows for a clean segmentation of the image. The issue with Halpha imaging is often that it is very fractured. Individual HII regions are not inter-connected. So it is hard to define the part of the image over which to compute…drumroll please…morphometrics!
some of the segmantations of the images. Continuum defines the area, and then the morphometrics can be calculated over the Halpha flux.
One can then start exploring the HII morphology space and its dependence on inferred galaxy properties; stellar mass and overall star-formation rate or combine these into specific star-formation rate (SSFR).
The Gini-M20 plane with Asymmetry color-coded to show where Halpha and sdss-r contunuum light morphology lie for the sample. Note that the continuum lies in the disk galaxy space, but Halpha shows a much greater range.
This is the Gini-M20 space that Lotz+ has used to identify disks, spheroids and interacting galaxies. The divisions look to be very different in Halpha morphology though!
The distributions of morphometrics as a function of stellar mass. These seem to not change much with mass.The morphology in Halpha does seem to change a lot with specific star-formation. No surprise since Halpha is driven by star-formation. If there relatively a lot of it, the morphology changes. Even if the underlying disk is not very perturbed.
The potential weakness is that the view of Halpha is skewed by dust. The authors address this and correct for this some. But to correct the morphology completely for that, commensurate hot dust (e.g. 20 micron imaging) would be necessary. Peter Kamphuis used something like that on…you guessed it NGC 891.
The correlations with SSFR is a first good exploration. I am curious to see what the survey team is going to be working on next!
Spiral galaxies come in all sizes. One of the more impressive things about them is that over 6 orders of magnitude they are self-similar. Meaning that unless you had an inkling of the distance, all disk galaxies are exponential declining light profiles, rotate with similar (enough) rotation curves and basically look the same.
But up to a point right? There is such a thing as too big a disk galaxy. Extremes can be very informative in an observational science so what are the biggest disk galaxies at a particular time in galaxy evolution?
Locally the biggest disk galaxy title is held by “Rubin’s Galaxy” (UGC 2885), named after Vera Rubin. This is one I studied together with my collaborators extensively (some of the papers were going to come out in 2020…). The amazing thing about this galaxy is that despite that it is 10x the mass and 5x the size of the Milky Way, it neatly lies on all the scaling relations for disk galaxies! It’s an Sc galaxy…just…really big.
The Hubble mosaic of Rubin’s Galaxy, UGC 2885. Rubin originally pointed out in her other 1980 paper that this was an unusually large disk galaxy. It shares the title of biggest disk galaxy with Malin 1, an low surface brightness disk. How such a disk galaxy grows over time and importantly, stays a disk is a key point my collaborators and I hope to learn more about.
And there is a class of big spiral galaxies called “super-spirals” of about the same size and mass (log(M*) ~11.5). Those are often quite perturbed looking and are assumed to be the result of a merger where the disk miraculously survived.
This brings me to the paper this week:
A Giant Disk Galaxy Two Billion Years After The Big Bang
A big disk like that at high redshift is a prime target to see if disks rotate still the same in this much earlier epoch. So the authors targeted this disk with three slit observations for rotation. And indeed there it is; rotating pretty much like a local galaxy.
Kinematic long-slit observations. These may or may not capture the turnover point for a rotation curve. Pretty convincing curve though.
With distances, morphology, and now kinematics from HII regions (just as V. Rubin found her rotation curves in the 1970’s), this galaxy can directly be compared to the scaling relations in the local Universe.
Scaling relations for z=3 galaxies and Big Wheel. The galaxies is more extended (top panesl) than one expects from trends. The star-formation rate is where one would expect it to be at z=3. The kinematics however resemble more a settled z=0 disk, a super-spiral, than z=3 disk galaxies observed so far.
And while it lies in the same mass-range as Rubin’s Galaxy or the super-spirals from Ogle+ 2015,2019 and Di Teodoro, it looks like a normal disk galaxy in most measures for a z=3 galaxy (star-formation rate) but it is much more extended for its mass than z=3 galaxies.
I wonder how Big Wheel (Tori Amos fan, the authors?) will evolve. Will it become an LSB giant? Something like Rubin’s Galaxy? Or will a disk not survive the 10 Billion years of further evolution?
To become Rubin’s galaxy, star-formation would have to crash, a drop of about 2 orders of magnitude. Other than that, it has a similar rotation velocity and stellar mass! And frankly, a very similar morphology (4 arms, Sc galaxy. Even our view of it (inclined) is similar.
How much stellar mass is there in a galaxy? We see a certain amount of light from galaxies and that implies a certain number of stars. It depends on the mix of stars and the redder, the easier the conversion is (less dust, or over-weighting blue, short-lived stars).
And honestly, the problem of weighting the stellar mass of galaxies had shifted into the “solved problem” category. Not perhaps something to get complacent about but no longer estimates what were orders of magnitude off. I quite clearly recall a talk where the uncertainty was set at 0.2dex (less than a factor 2) for any survey.
But that was for big galaxies, that have been around a long time. Think Milky Way and bigger. We studied those the most, we understand those the best. We are entering the era of the Dwarf (insert obligatory Tolkien or Rings or Power reference here). We will observe many more dwarf galaxies in the near future and understanding dwarf galaxies is critical in our understanding how galaxies form, reionize the Universe, and coalesce into bigger galaxies.
And stellar mass estimates for dwarf galaxies…well…
This was a problem that was neatly show at the NASA Galaxies Science Interest Seminar by Professor Mia de los Reyes: YouTube.
Which brings me to this week’s paper and one I was looking forward to:
Stellar Mass Calibrations for Local Low-Mass Galaxies
Which addresses how well each stellar mass conversion works for dwarf galaxies that we have made in simulations. This has a few benefits: first you know the ground truth (the galaxy is in your computer) and secondly, it allows you to sidestep any issues with photometry between different surveys. Gotta keep track of all the systematics and here is one specific one you want to tackle.
The traditional photometry to stellar mass estimators. I can think of at least one more but this neatly shows how a luminosity and a single color is converted into a stellar mass. Works okay for massive galaxies, starts to introduce significant biases and error in the lower mass regime.
The first is straight-up conversion from one filter and a color into a mass. As you can see, in the dwarf regime, the deviation from true is dramatic, often quite dire. This will work okay for your survey of big galaxies or even Milky Cloud Galaxies (Dr de los Reyes is a proponent of calling these not after Magellan anymore, see here).
Correcting the bias can be worked out but a noisy relation remains. Perhaps that is sufficient for your purposes. It may not be.
But knowing this, one could, conceivable, calibrate this conversion (this mass-to-light ratio) and make it mass-dependent. As the plots above show, it can be fixed. Some.
But we rely on full Spectral Energy Distribution modeling these days to get stellar mass, dust mass, star-formation rates and increasingly, star-formation history and metallicities as well. How do these do in the Dwarf regime? Below are a few different star-formation histories. Each a different function of the age of the galaxies and how many stars it produced at that time.
Parameterized star-formation histories going into SED fits. These are simple functions like a constant, an exponential decline or an exponential decline with a single spike. These are very significant assumptions to make of the shape of a SFH (e.g. it must be declining) for the sum of the history is the mass we are after.A more flexible approach is the non-parametric (a misnomer I feel, there are parameters, just more of them?) to allow for all kinds of shapes for the SFH. Hopefully constraining better the kind of total integrated mass from these histories.
There are even less parameterized functions to account for the star-formation history. There are some in the above plot.
The point is that there is still an offset. Not massive in terms of big galaxies, but critical if you want to understand smaller galaxies. It those are suddenly all under-estimated, your whole study can be critically biased.
I think a systematic approach such as this one, is present-day extra-galactic science at its best. A methodical study of the inherent noise and bias in our estimates. I have always been a bit nervous about the fact that so many of our SED codes are effectively calibrated on NGC 891 and maybe the Sombrero and Arp 220. This is getting better but one worries about the effect of the survivor bias in what was studied in detail.
Disk galaxies often have a “bar” at their center. A rectangular shaped, often yellow-ish structure. We understand this is a bunch of the older stars in the disk moved from circular orbits to highly elliptical ones.
NGC1300 the original barred galaxy. There are many like it.
Now this leaves us with a few questions: how often does this happen? That is something the GalaxyZoo project can help you with. That would give you visual classifications and those can be quite good. The other way to find out is to fit isophotes (lines of equal amount of light) to the galaxy. The Bar stands out as highly elliptical isophotes and the position angle of those ellipses radically changes at the edge of the bar. It leaves a clear signal. Like in the galaxy below.
The isophotal method of bar detection. The ellipticity drops when outside the bar and the position angle goes all over the place (see at 10⁰)
This brings me to this week’s paper:
The Abundance and Properties of Barred Galaxies out to z∼ 4 Using JWSTCEERS Data
Where the CEERS team looked at galaxies far away, as seen with the JWST. The benefit here is that we can do this exact analaysis to much greater distances and thus lookback times. This brings me to the other question: how long have galaxies had bars? That can be trickier to answer and there was quite a bit of disagreement with the early Hubble studies. I was impressed with these, cleverly using the above type of analysis on Hubble fields and calibrating the counts with local galaxy images to see how many bars had been missed. But Hubble has its limitations and beyond redshift z~1 would be too challenging for Hubble’s cameras and their wavelength range.
To recap, one can find bars visually or with the isophotal technique. There have been many studies using Hubble and now a few with JWST to expand the range to much higher redshifts. The observational situation is as follows:
The fraction of bars of a population of galaxies observed with Hubble (small symbols, many different authors) and the few JWST studies and the two done by Gao+ in this paper. The visual and isophotal techniques agree pretty well and if JWST is to be believed, bars were already present in substantial numbers at z~1. This definitely pushed the appearance of bars up.
Bars in the present day is ~40% of all disk galaxies. But as we look back over the last 10 Gyr, the fraction drops to 10% or so. What is amazing is that there seem to be bars already at z=3.5. Some have been reported even further! That would make bars something that could form much earlier and perhaps stick around for a much longer time than we thought?
We are still not done though. Are these bars the same bars we see locally? Do they spin around the disk at similar speeds? Do bars from slowly at first and then a lot in the past 5 Gyr? Are bars just a phase or a longer, sustainable pattern in a disk? What is remarkable is what we can also see: a galaxy 1 Gyr after the Big Bang can have a bar in a disk. That pattern establishes itself early.
In a previous blog post I gushed about the wonderful new JWST data on NGC 891, an edge-on galaxy with an ultraviolet halo. This week I’ll talk about this paper:
Illuminating the Incidence of Extraplanar Dust Using Ultraviolet Reflection Nebulae with GALEX
The authors use archival GALEX data to look for ultraviolet light from well above and below the plane. One of the authors has already done so with H-alpha, the emission line of ionized hydrogen. The idea is to look of blue light tracing star-formation can be seen well away from the disk of the galaxy seen edge-on.
A figure from Seon+ 2014 on NGC 891 (yes that one) showing the UV coming from more than just the thin disk.
This is important because we think there is no star-formation there. So if we see the blue or ultraviolet light there, it must have reflected off something: high latitude dust clouds. Also called cirrus or galactic cirrus. And apart from a lot of paper on NgC 891 (including a few I’m involved in) there isn’t a lot of evidence of galactic cirrus in other galaxies.
This is why this is a nice and clever paper. They collected all the edge-on galaxies for which there was GALEX data, examined them all individually and found 7% have this extra-planar UV halo. Earlier authors [] also found similar incidence rates. But GALEX is not very high resolution data so it’s hard to tell. In this paper, they also stacked all the data to look for a UV halo. The authors conclude the scatter off cirrus is much more common than 7% and there is UV light coming from well above and below disks.
They relate this to mass and star-formation rate of the galaxies here:
Figure 15 from the paper showing the vertical profile stacks for different star-formation rates.
This figure above shows the absolute star-formation rate, I would have asked for a relative (specific) star-formation rate as well. Do smaller galaxies with relatively more star-formation produce more cirrus?
The authors link the mass, star-formation and the amount of gas extra-planar in this cirrus.
The normalized luminosity with respect to the Milky Way on the x-axis and the amount of extra-planar gas implied on the y-axis. On the left and middle are the stacked results — either by type or star-formation rate — and on the right are the directly detected cirrus UV reflections. The lines are assuming 5,10 or 15% of all the disk gas is in the halo. There is quite a bit of variety with disk luminosity!
Unfortunately, this will be it until we get more ultraviolet imaging. Maybe with SWIFT (not the singer) or a new NASA mission UVEX. The latter is set to observe the whole sky, deeper and in high resolution. That will give us the kind of data one can use to study the galactic cirrus in much more detail.
Ok so why do we care again about galactic cirrus? because this hints at a self-sustaining refueling mechanism in disk galaxies. The cirrus material is blown out, the metals and dust in that material helps cool hot halo gas and it all rains back down on the disk of the galaxy, triggering new star-formation!
And it is apparently pretty common. Maybe this is how disk galaxies get (much? Most) of their refueling!
Some galaxies get all the attention. This is often simply because it was the easiest to observe to begin with, nice and close, fit in the telescope’s viewer or told us something interesting thanks to our perspective.
In the case of NGC 891, it is all of the above. Easily seen from the Northern Hemisphere, it was the subject of a bunch of studies on the thickness of disk galaxies. It is perfectly edge-on and pretty close.
The vertical structure of galaxies is a subject unto its own. Why are disk galaxies so flat? What can we learn from the interstellar medium in these disks by studying from the side? My PhD advisor Piet van der Kruit used this galaxy to formulate how to describe the light distribution of galaxies for the first time back in 1981.
And it has been a favorite of astronomers (and myself) ever since. We have looked at this galaxy with every telescope that can reach it and any that was sent to space! Herschel, Spitzer, Hubble (Subaru is the one above). When I organized a conference on how to model dust in disk galaxies, we kept a tally how many people used this galaxy in their talk. It was nearly everyone! (good thing it wasn’t a drinking game).
There is gas sticking out of the plane of this galaxy (Oosterloo+ 2007) and early-on those dust filaments or fingers sticking out of the disk drew attention (Howk+ 1998/1999). Is that typical? We have often used the light from this galaxy to calibrate and benchmark the models that describe all galaxies (see Popescu+ 2000, Bianchi+ 2011). So what if this galaxy is not that representative? Back in 2007 for example, Peter Kamphuis and I worked out that we could see how much dust was out of the plane of the galaxy using the left/right symmetry above the disk in both infrared and H-alpha emission. Both originate from star-formation, so they should be similarly symmetric. Except H-alpha is more affected by dust. There was evidence for a dust component well above and below the plane (up to 2 kpc). I think that was the fastest paper I have ever been part of (from concept to acceptance in something like 3 months. It was fast!).
A figure from Kamphuis+ 2007: the blue light on the left does not go through as much dust and muck as the blue light from the HII regions on the right, so the H-alpha image was skewed towards more blue light from the left. Meanwhile the 24 micron image, also from the same HII regions, did not suffer from the dust and was mostly symmetrical left/right. From the difference, we could infer the vertical dust component of NGC 891.
So it is professionally very exciting to see a paper on NGC 891 with the latest and greatest telescope, the James Webb Space Telescope. The paper came out this week:
JWST MIRI and NIRCam observations of NGC 891 and its circumgalactic medium
The James Webb Space Telescope observations of NGC 891.JWST can observe in amazing detail but the field is small: you have to put observations next to each other and still not get the whole galaxy. About 1/3 is observed with one camera NIRCam and a second camera MIRI is observing at the same time in the red squares. A false color image of NGC 891 using the NIRCam images and existing Hubble Space Telescope images for the blue channel.
Like a lot of great studies, this uses all the space-based information there is, including HST data for the V-band (blue). You can see the blue light peeking around all the dust in the central plane of the galaxy!
The vertical profile of NGC 891.
The vertical structure is very differnt in the infrared compared to the optical (BRI). The F150W is 1.5 micron and pretty close to the optical. You can still see the divot in there close to the centerline of the disk where dust absorbs light even at 1.5 micron!
This is why back in 1981, Piet van der Kruit “softened” the vertical profile some from exponential to something that had a smooth transition in the center (sech²) because of that dust in the plane.
There are two components to this study though: there is also the new MIRI observations at 7.7 micron, where big molecules called Polycyclic Aromatic Hydrocarbons emit light.
The vertical profile of the MIRI observations. This is well above the disk of NGC 891.
One of the open questions is “is there and how much of a thick disk?” meaning to explain the light, does one need one or two exponential disks to fit the vertical and radial profile. To give you an idea how much debate there was on these, here is a title from Comeron+ 2018: “The reports of thick discs’ deaths are greatly exaggerated Thick discs are NOT artefacts caused by diffuse scattered light”
And here with JWST, the best resolution you can get, there is clear evidence for a second component, also in 7.7 micron with MIRI.
But where we really start to learn new things is when we look at some of the detail the MIRI observations show:
Details of the MIRI image. You can see the arcs of material that are the edges of bubbles blown out by star-formation in NGC 891’s disk.
There are clear arcs of material coming out of the disk that have PAHs in them. This is the effect of star-formation blowing bubbles in the surrounding medium (a galaxy is called to be“evervescent” which litterally means “bubbly”). We see these bubbles now clearly with JWST/MIRI and well out of the main disk of NGC 891, out to 4 kpc!
This interplay between disk, its star-formation and how much material gets blown out is important in understanding how galaxies work, specifically how they get fresh gas to support all this star-formation. The material outside the disk mixes with the hot gas in the halo, the metals in the dust allow that gas to radiate away some of its energy and cool. Cooler gas can flow to the galaxy and start fueling star-formation. These bubbles can help sustain star-formation, without drawing gas from much further away!
This paper also makes some first inroads into connecting the star-forming clusters responsible to the bubbles we can identify. I think many more studies on NGC 891 with this exquisite data are in our future (drink!).
There are two ways to explore how galaxies have changed over time with telescopes: first is to look further and further back into earlier epochs. Thanks to the speed of light, the images of galaxies far far away is also from long long ago. That is what Hubble and now James Webb have excelled at.
The other is Galactic Archeology, identifying generations of stars in our own or very nearby galaxies and mapping out where they are within that galaxy. Our own Milky Way is both easier (we are in it) and harder (we are IN it) to do. Nearby galaxies, with deep and sharp enough images, can offer an opportunity to map where different stars ended up. How many metals were available at the time they were formed? Are they in the disk or further out? Especially the halo of stars around galaxies is believed to be made up of some of the oldest components, left over from when the first proto-galaxies collided into what is now the main galaxy. The long dynamic times (it takes foreeeever to cross that much space) means that these leftovers havent complete dissapated yet and can be identified as coherent groups of stars (unlike gas, the stars behave mostly as ideal particles in a gravitational simulation too, if you know where they are and where they’re going, you know where they’ve been).
Which brings me to this week’s paper:
A Timeline of the M81 Group: Properties of the Extended Structures of M82 and NGC 3077
The M81/M82 group of galaxies with Red Giant Branch stars distribution on the left and their densities overlaid over the gas distribution on the right.
A prime example is the M81 group. A trio of galaxies with a lot of material in between, both stars and gas (see above). With Subaru’s camera at this distance, one can identify individual stars in this group, especially well outside the galaxies themselves.
The color-magnitude diagram of stars in M81/M82. There are Red Giant Branch (RGB) and the Tip of the Red Giant Branch (TRGB) and above that, the brigh Asymptotic Giant Branch (AGB) stars.
The individual stars can be identified and so one has a brightness and a color if there is more than one image. In the above figure, you can see how different groupings of stars are visible: Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars. To the left (bluer) is the Main Sequence (so named because it is the main feature but not a sequence) of all the bright stars. The RGB is especially useful. Red Giant Branch stars track the total stellar mass pretty well. The top of it is well defined so it is a great distance indicator. The mix of AGB and RGB is a good indication of overall age and the slope of the RGB branch is an indicator of metallicity. So distance, mass, metallicity and rough age. That is a lot of information of any section!
So you can define areas in the group to correspond to clean sections of halo, streams of stars from accretion and interaction etc.
The sections of the image corresponding to halo and two streams.
Generate the color-magnitude diagrams, compare those to models and trace the components back into time.
Then we get to the neatest image of this paper: a history of this group with a future prediction. I love timelines and this is such a neat visual way to show how the ages fit together in the evolution of this compact group of galaxies:
The timeline for the M81/M82 group. With the future final merger predicted!
And there we have it. M81’s halo formed first, then M82’s, followed by two tidal events resulting in streams which eventually will be that all of these galaxies will coalesce in some two more Billion years. A bit long to wait around but the finl stretch in terms of the ages in this group.
Neat result by very neat people I collaborated with as part of the GHOSTS collaboration (doing the stellar population thing but with Hubble). It will be really interesting to see what all we can learn from Euclid and Rubin stellar populations of nearby haloes!
