Archive by Author | R Smethurst

#GZoo10 Day 3

We’re ready for the final day of the Galaxy Zoo 10 workshop at St Catherine’s College in Oxford; it’s been great to have so many people following along on the Livestream – yesterday’s talks are still up, and today’s schedule is:

09:30 Alice Sheppard (Forum Moderator 2007-2012)
10:00 Brooke Simmons (UCSD)
10:20 Nic Bonne (Portsmouth)
10:40 Coffee
11:00 Coleman Krawczyk (Portsmouth)
11:20 Mike Walmsley (Edinburgh)
11:40 Carie Cardamone (Wheelock)
12:00 Karen Masters: Summary (Portsmouth)

We’ll be blogging these talks as they happen here but you can also keep an eye on the twitter hashtag for updates too!


Our first speaker this morning is Alice Sheppard who was here from the beginning as a forum moderator on the original Galaxy Zoo site. She’s talking about the past 10 years and how she got involved with Galaxy Zoo site. She was very keen to get involved in the project and help classify galaxy images. After finding images that weren’t easily classifiable, users started to email members of the science team to ask them what to do. After this happened many times, the team realised that a place where users could talk together and interact with the team about classifications would be really useful. So the Galaxy Zoo forum was born! Alice was one of the first people to sign up and was asked to moderate the forum – she (along with other moderators) even started welcoming each new user who signed up with a friendly “Welcome to the Zoo!”

Alice is now talking us through some of the findings made by the users. These discoveries including the Green Peas, which when first spotted by the users they immediately started investigating what they were using the links to the science survey site. In the original Galaxy Zoo there was also no button for an irregular galaxy, so users started collating their own collection of irregular galaxies! But what makes users keep coming back to the Galaxy Zoo forum time and time again? One success story was the Object of the Day – the moderators even crowd sourced the users to find good images!

Alice has discussed some suggestions for future engagement with users online – always give people room to chat; whether it’s about astronomy or not at all! Remember: as good as you think you citizen science system, tools, tutorials etc are – the volunteers will teach you how to do it better!

Next up this morning is Brooke Simmons talking about what she’s calling probative outliers. The things that tend to break the mould and challenge our world (or Universe!) view. She starts with the bulgeless galaxies – those that look like pure disks – but are hosting growing super massive black holes in their centers. This is weird because the most accepted theory is that super massive black holes grow in mergers of galaxies BUT mergers also grow bulges – so how did these bulgeless things grow their black holes? Brooke is showing us some beautiful follow up observations of Galaxy Zoo SDSS images taken with the Hubble Space Telescope that will help to try and figure this out.

Thing is, Brooke only has about 100 of these galaxies – but not for lack of trying! They just seem to be really rare. If we could actually cover the southern sky in the same way that SDSS covered the northern sky, Brooke would be very grateful! Could we also use trained machines to pick out the weird outliers as well? In which case, Brooke thinks we need to adapt the next iteration of Galaxy Zoo to be both machine as well as user friendly. This will mean leaving things behind but let’s not be afraid to make changes!

Next up, we have Nic Bonne from Portsmouth who’s going to tell us all about making luminosity functions using the data from Galaxy Zoo. So what’s a luminosity function? It’s basically a count of the number of galaxies at different luminosities (or brightness) which can give us clues about how the Universe formed and evolved.

Luminosity functions are especially interesting if you start making them for different morphologies or colours of galaxies. This is what Nic has been doing using the Galaxy Zoo 2 classifications. He’s found something a bit weird though – that the galaxies classified as smooth seem to be more numerous than those classified as featured at the low luminosity (low mass) end. Bringing the colour of the galaxies into this picture as well shows you how similar red featured and red smooth luminosity functions are. Nic says there’s a lot more to do be done with this work though, including using Ross’s new debiased classifications to improve the sample completeness and investigating how the luminosity functions change shape for different kinematic morphologies.

Next up is Coleman Krawczyk who’ll be showing us some of the initial results from the Galaxy Zoo 3D project. This project asked users to draw around the features of a galaxy on an image so that researchers could pick out the spectrum of that particular feature using MaNGA data. Users were asked to either mark the centre of the galaxy or draw around any bars or spiral arm features.

Users could also choose which classification task they would prefer to help out with. This meant that the easy classification task of marking the centre of the galaxy was finished within a couple of days – whereas the spiral drawing task took 6 weeks for classifications to finish. Coleman has now reduced these classifications and has made “maps” for every galaxy marking which pixels are in which features. He’s now started making diagnostic plots to map the star formation rate in the different features of classified galaxies. Turns out we’re going to need more classifications in order to do the science we want to, so this project could have new data coming soon!

Next up is Mike Walmsley, who’ll be joining the research team as a PhD student in October.

He’s not yet done any work with Galaxy Zoo but as part of his Masters research he looked at doing automatic classification of tidal features in galaxies. His goal was to write a code that trained a machine, using a neural network, to detect these tidal features. He also figured out that masking the main galaxy light in the image makes it easier for the machine to spot tidal features. So does this method actually work? It identifies tidal features with ~80% accuracy – which is actually a much higher quality than other automated methods! He’s hoping to apply these methods to new and bigger surveys during his PhD.

Next up is Carie Cardamone from Wheelock College talking about her work building on the the discovery of the Green Peas by Galaxy Zoo volunteers.

So what are the Green Peas? First up their name describes them pretty well because they’re small, round and green. Carie originally wanted to study them because she thought they might be growing super massive black holes, but it turned out instead they have extremely high star formation rate for their relatively small mass. There has been many further studies on these objects so we now know a lot more about them, but one thing we still don’t really know is what galaxy environment they live in. Carie is trying to quantify this but the first problem was that she didn’t have enough Peas! There’s only 80 in the original GZ2 sample but now with the better analysis tools Carie has been able to select 479 candidate Peas. Analysing this sample and comparing it to a sample of well studied luminous red galaxies, the results suggest that peas are less clustered. i.e. the Peas have fewer galaxy neighbours.

We’re now coming towards the end of the meeting (sad times guys) and to remind us all why we’re here and what we talked about, Karen Masters is going to give us a summary of the past couple of days. She’s first pointing out how great we are as a team and the impact the research has had on the galaxy evolution community. Fitting for the 10 year anniversary is that we have 10 published papers with over 100 citations!

Karen has noticed a couple of themes from the past few days that she’s summarised for us. The first is that we have to keep engaging with the Galaxy Zoo community on Talk. The second is that we shouldn’t be afraid of change – let’s not get hung up on how it’s always been done and think about how best to do it now. The third is that galaxy’s are messy and we need to think carefully how we use the classifications. The fourth is that the users will always give you what you ask for – so be careful what you ask! But sometimes you get more than you asked for and end up with a wonderfully collaborative research team!

New paper on active black holes affecting star formation rates!

Good news everyone, another Galaxy Zoo paper was published today! This work was led by yours truly (Hi!) and looks at the impact that the central active black holes (active galactic nuclei; AGN) can have on the shape and star formation of their galaxy. It’s available here on astro-ph: http://arxiv.org/abs/1609.00023 and will soon be published in MNRAS.

Turns out, despite the fact that these supermassive black holes are TINY in comparison to their galaxy (300 light years across as opposed to 100,000 light years!) we see that within a population of these AGN galaxies the star formation rates have been recently and rapidly decreased. In a control sample of galaxies that don’t currently have an AGN in their centre, we don’t see the same thing happening. This phenomenon has been seen before in individual galaxies and predicted by simulations but this is the first time its been statistically shown to be happening within a large population. It’s tempting to say then that it’s the AGN that is directly causing this drop in the star formation rate (maybe because the energy thrown out by the active black hole blasts out or heats the gas needed to fuel star formation) but with the data we have we can’t say for definite if the AGN are the cause. It could be that this drop in star formation is being caused by another means entirely, which also coincidentally turns on an AGN in a galaxy.

fig1

A random sample of galaxies which host a central active black hole used in this work. The disc vote fraction classification from Galaxy Zoo 2 is shown for each image. Images from SDSS.

These galaxies were also all classified by our wonderful volunteers in Galaxy Zoo 2 which meant that we could also look whether this drop in the star formation rate was dependent on the morphology of the galaxy; turns out not so much! If the drop in the star formation rate is being caused directly by the AGN (and remember we still can’t say for sure!) then the central black hole of a galaxy doesn’t care what shape galaxy it’s in. An AGN will affect all galaxies, regardless of morphology, just the same.

 

 

A perfect woollen gift for the SEVENTH anniversary

In the UK on a seventh anniversary the traditional gift is one made of wool. So considering it’s the SEVENTH anniversary of Galaxy Zoo TODAY (July 11th) our very own, super talented Karen Masters has knitted us the Galaxy Zoo logo!

knittedlogo

If you feel like getting your astronomical knit on for this momentous occasion, here’s a few inspirational photo’s.

Karen also knitted our favourite Penguin Galaxy as a birthday present for the lovely Alice :

 

knittedpenguin

And check out the skills of Jen Greaves and the NAM Knitters who knitted a WHOLE GALAXY CLUSTER at this year’s National Astronomy Meeting in Portsmouth:

knittedcluster

 

For those of you itching to stretch your creative muscles but haven’t been struck by that bolt of inspiration yet, here’s a KNITTING PATTERN (a first I believe for the Galaxy Zoo blog) for the Galaxy Zoo logo to keep you busy…

KnittedGalaxyZooLogo

Our SEVEN Favourites

To honour the SEVENTH anniversary of Galaxy Zoo (July 11th) we’ve put together a gallery of the Science Team’s favourite images from the site (and why) for your visual pleasure…

Chris’s favourite: I love the flocculent spiral galaxies. The ones you can stare at and still have no idea how many spiral arms they have.

flocc

Karen’s favourite: I’m a sucker for merging galaxies, despite the fact that I work on barred galaxies mainly! This one which looks like the yin-yang symbol (or maybe a heart) is a particular favourite. It’s amazing that the Universe can be so vast that we can find galaxies in so many different shapes. 

heart

 

Kevin’s favourite: The penguin galaxy shows the power of human pattern recognition – and a crucial stage in galaxy evolution!

Penguin

Brooke’s favourite: when the latest Galaxy Zoo launched, the volunteers made a find almost right away that turned out to be a very rare kind of object called a gravitational lens. I love this image because it shows not just the variety of things that are out there in the Universe — in this case the very distant universe — but also the rare place that Galaxy Zoo itself occupies. It’s a diverse community and diverse images like this are part of the reason why.

lens_brooke

Kyle’s favourite: did you know that we can spell Galaxy Zoo out of galaxies? The users originally started collecting a list of galaxies that look like letters and now we have writing.galaxyzoo.org thanks to Steven. Since it’s the anniversary, here’s my favourite letter G.BestG

Bill’s favourite: Hanny’s Voørwerp really started something – the blue stuff in the image – other teams are now finding similar objects at smaller and larger distances too. 

hanny

Becky’s favourite: this amazing image has SO much going on in it – mergers, interactions, spirals, bars, ellipticals, grand designs, foreground stars etc. It feels like a visual representation of thoughts in my head at times, which is clearly why I love it.merger

BONUS:

The Hubble Ultra Deep Field!!!! Just to remind us all why we’re all here. Every single thing you can see in this image is a galaxy – even the most minuscule of dots. And the size of the image on the sky is about 1/20 the size of the Full Moon… Let’s just all take a minute to let that sink in as we stare and wonder...

hudf

 

The SEVEN wonders of Galaxy Zoo

Friday 11th July 2014 is the SEVENTH anniversary of Galaxy Zoo! So to celebrate this momentous achievement, we’ve put together a list of seven of the greatest Galaxy Zoo discoveries (so far!); all thanks to YOU, the classifiers…

1. Chirality  of Spiral Galaxies

clock

A clockwise spiral

An anticlockwise spiral

An anticlockwise spiral

One of the first major results from Galaxy Zoo wasn’t even Astronomical. It was Psychological. One of the questions in the original Galaxy Zoo asked whether spiral galaxy arms rotated clockwise or anti-clockwise; we wanted to check whether they were evenly distributed or whether there was some intrinsic property of the Universe that caused galaxies to rotate one way or the other. When the Science team came to analyse the results they found an excess of anti-clockwise spinning spiral galaxies. But when the team double checked this bias by asking people to classify the same image that had been flipped there was still  an excess of anti-clockwise classifications; so it’s not an astronomical phenomenon. Turns out that the human brain has real difficultly discerning between something rotating clockwise or anti clockwise; check out this video if you don’t believe me – you can watch the dancer rotate both ways! Once we’d measured this effect we could adjust for it, and we went on to establish that spirals which were near each other tending to rotate in the same direction.

2. Blue Ellipticals

The enigmatic blue ellipticals in many ways started the Galaxy Zoo. Galaxies largely divide into two: spiral galaxies like our Milky Way shining with the blue light of young stars being constantly born, and the “rugby ball-shaped” elliptical galaxies who no longer make new stars and thus glow in the warm, red light of old stars. Clearly, when galaxies stop making new stars, they also change their shape from spiral to elliptical. But how exactly does this happen? And what happens first? Do galaxies stop forming stars, and then change their shape, or the other way round? Answering that question is the first step in understanding the physics of transforming galaxies. With the Galaxy Zoo, we found a whole population of blue ellipticals: galaxies which have changed their shape, but still have young stars in them. With their help, we’ve been making a lot of progress in galaxy evolution. It looks like a galaxy merger, a giant cosmic collision, changes the shape of galaxies from spiral to elliptical and then somehow – and very rapidly! – star formation stops. We don’t know quite why yet, but we think active black holes are involved. This is hugely relevant for us as in a few short billion years, the Milky Way will crash into our neighbour, the spiral Andromeda galaxy. And for a short time, the Milky Way and Andromeda will be a blue elliptical before star formation in the newly-formed Milky-Dromeda ceases. For ever.

blue_red_sp_ell

A normal red elliptical and normal blue spiral on the top row. Unusual discoveries of blue ellipticals and red spirals on the bottom row.

3. Red Spirals

Ellipticals are red, Spirals are blue, Or so at least we thought, until Galaxy Zoo…. Think of your typical spiral galaxy and you’ll probably picture it looking rather blueish. Thats’s what astronomers used to think as well – suggest a red spiral to Edwin Hubble and he probably would’ve told you not to be so ridiculous. Before Galaxy Zoo if astronomers saw something looking red they generally tended to think it was elliptical; however to the untrained eye, the colour does not bias any classifications, which means that you all found lots of red spirals and discs which were hiding in plain sight. This put the cat amongst the pigeons for our galaxy evolution theories because, as said earlier, we thought that when galaxies stop making new stars, they also change their shape from spiral to elliptical. The red spirals mean that we now have a different evolutionary path for a spiral galaxy where it can stop making new stars and yet not change its shape. We now think that those spiral galaxies which are isolated in space and don’t interact with any neighbours are the ones that make it to the red spiral stage.

4. Green Peas

greenpea

The Green Peas, discovered by Citizen Scientists due to their peculiar bright green colour and small size, are a local window into processes at work in the early Universe.  Although, they were in the data for many years, it took humans looking at them to recognise them as a class of objects worth investigating.  First noticed in some of the earliest posts of the Galaxy Zoo Forum in 2007, a group of dedicated citizen scientists organised a focused hunt for these objects finding hundreds of them by the summer of 2008, when the Galaxy Zoo science team began a closer look at the sample. The Peas are very compact galaxies, without much mass, who turn out new stars at incredible rates (up to several times more than our entire Milky Way Galaxy!).  These extreme episodes of star formation are more common to galaxies in the early Universe, which can only be directly observed very far away at high redshifts.  In contrast to the distant galaxies, the Peas provide accessible laboratories that can be observed in much greater detail, allowing for new studies of star formation processes.   Since their initial discovery, the Peas have been studied at many wavelengths, including Radio, Infrared, Optical, UV and X-ray observations and detailed spectroscopic studies of their stellar content.  These galaxies provide a unique probe of a short and extreme phase of evolution that is fundamental to our understanding of the formation of the galaxies that exist today.

  5. The Voørwerp

hannyProbably the most famous new discovery of Galaxy Zoo has been Hanny’s Voørwerp. Hanny van Arkel called attention to it within the first few weeks of the Galaxy Zoo forum with the innocent question ”What’s the blue stuff?”, pointing to the SDSS image of the spiral galaxy IC 2497. The Sloan data alone could indicate a gas cloud in our own Galaxy, a distant star-forming region, or even a young galaxy in the early Universe seen 10 billion light-years away. After a chase to obtain new data, above all measurements of the cloud’s spectrum, with telescopes worldwide, an unexpected answer emerged – this galaxy-sized cloud was something unprecedented – an ionization echo. The core of the galaxy hosted a brilliant quasar recently on cosmic scales, one which essentially turned off right before our view of it (so we see the gas, up to 100,000 light-years away from it, shining due to ultraviolet light form the quasar before it faded). This had never before been observed, and provides a new way to study the history of mass surrounding giant black holes. Further observations involved the Hubble Space Telescope and Chandra X-ray Observatory (among others), filling in this historic view.  The cloud itself is part of an enormous stream of hydrogen, stretching nearly 300,000 light-years, probably the remnant of a merging collision with another galaxy. As it began to fade, the quasar started to blow out streams of energetic particles, triggering formation of stars in one region and blowing a gaseous bubble within the galaxy. In keeping with the nature of Galaxy Zoo, the science team deliberately had much of this unveiling play out in full view, with blog entries detailing how ideas were being confronted with new data and finding themselves supported,  discarded, or revised. The name Hanny’s Voørwerp (which has now entered the astronomical lexicon) originated when an English-speaking Zoo participant looked up “object” in a Dutch dictionary and used the result “Voørwerp” in a message back to Hanny van Arkel. Following this discovery, many Zoo volunteers participated in a focused search for more (the “Voørwerpjes”, a diminutive form of the word) – as a result we now know 20 such clouds, eight of which indicate fading nuclei. Other teams have found similar objects at smaller and larger distances; Hanny’s Voørwerp really started something!

6. Bars make galaxies redder

redbarA galactic bar is a straight feature across a spiral galaxy. It’s the orbital motions of many millions of stars in the galaxy which line up to make these bars, and in computer simulations almost all galaxies will form bars really quickly. In the real world it’s been known for a long time that really strong (obvious) bars are found in about 30% of galaxies, while about 30% more have subtle (weak) bars. One of the big surprises about the Galaxy Zoo red spirals was just how many of them had bars. In fact we found that almost all of them had bars and this got the science team really curious. We followed this up with a full study of which kinds of spiral galaxies host bars using the first classifications from Galaxy Zoo 2. In this work we discovered a strong link between the colour of disc galaxies and how likely they are to have a bar – with redder discs much more likely to host bars. We now have half a dozen papers which study galactic bars found using Galaxy Zoo classifications. Put together these works are revealing the impact galactic bars have on the galaxy they live in. We have found evidence that bars may accelerate the processes which turn disc galaxies red, by driving material into the central regions to build up bulges, and clearing the disc of the fuel for future star formation.

7.  Bulgeless galaxies with black holes

bulgelesssSupermassive black holes are the elusive anchors in the centres of nearly every galaxy. Though they may be supermassive, they are quite difficult to spot, except when they are actively growing — in which case they can be some of the most luminous objects in the entire Universe. But how exactly they grow, and why there seems to be a fixed mass ratio between galaxies and their central black holes, are puzzles we haven’t solved yet. We used to think that violent collisions between galaxies were The Way you needed to grow both a black hole and a galaxy so that you’d end up with the mass ratio that we observe. And violent collisions leave their signatures on galaxy shapes too. Namely: they destroy big, beautiful, ordered disks, re-arranging their stars into bulges or forming elliptical galaxies. So when we went looking for pure disk galaxies with no bulges and yet with growing central black holes we weren’t sure we would find any. But thanks to the volunteers’ classifications, we did. These galaxies with no history of violent interactions yet with large central supermassive black holes are helping us test fundamental theories of how galaxies form and evolve. And we are still looking for more of them!

So here’s to SEVEN more years – keep classifying!

Bars as Drivers of Galactic Evolution

Hello everyone – my name is Becky Smethurst and I’m the latest addition to the Galaxy Zoo team as a graduate student at the University of Oxford. This is my first post (hopefully of many more) on the Galaxy Zoo blog – enjoy!

So far there have been over 100 scientific research papers published which make use of your classifications, some of which have been written by the select few Galaxy Zoo PhD students (most of us are also previous Zooites). The most recently accepted article was written by Edmund, who wrote a blog post earlier this year on how bars affect the evolution of galaxies. As part of the astrobites website, which is a reader’s digest of research papers for undergraduate students, I wrote an article summarising his latest paper. Since the Zooniverse team know how amazing the Galaxy Zoo Citizen Scientists are, we thought we’d repost it here for you lot to read and understand too. You can see the original article on astrobites here, or read on below.  

Title: Galaxy Zoo: Observing Secular Evolution Through Bars
Authors: Cheung, E., Athanassoula, E., Masters, K. L., Nichol, R. C., Bosma, A., Bell, E. F., Faber, S. M., Koo, D. C., Lintott, C., Melvin, T., Schawinski, K., Skibba, A., Willett, K.
Affiliation: Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064

Galactic bars are a phenomenon that were first catalogued by Edwin Hubble in his galaxy classification scheme and are now known to exist in at least two-thirds of disc galaxies in the local Universe (see Figure 1 for an example galaxy).

Throughout the literature, bars have been associated with the existence of spiral arms, rings, pseudobulges, star formation and even Active Galactic Nuclei.

bar
Figure 1: An example of one of the galaxies inspected in the study by Cheung et al., showing the bar likelihood p_{bar} and the scaled bar length L_{sbar}.

Bars are a key factor in our understanding of galactic evolution as they are capable of redistributing the angular momentum of the baryons (visible matter: stars, gas, dust etc.) and dark matter in a galaxy. This redistribution allows bars to drive stars and gas into the central regions of galaxies (they act as a funnel, down which material flows to the centre) causing an increase in star formation. All of these processes are commonly known as secular evolution.

Our understanding of the processes by which bars form and how they consequently affect their host galaxies however, is still limited. In order to tackle this problem, the authors study the behaviour of bars in visually classified disc galaxies by looking at the specific star formation rate (SSFR; the star formation rate as a fraction of the total mass of the galaxy) and the properties of their inner structure. The authors make use of the catalogued data from the Galaxy Zoo 2 project which asks Citizen Scientists to classify galaxies according to their shape and visual properties (more commonly known as morphology). They particularly make use of the parameter p_{bar} from the Galaxy Zoo 2 data release, which gives the fraction of volunteers who classified a given galaxy as having a bar. It can be thought of as the likelihood of the galaxy having a bar (i.e. if 7 people out of 10 classified the galaxy as having a bar, then the likelihood is p_{bar} = 0.7).

They first plot this bar likelihood using coloured contoured bins, as shown in Figure 2 (Figure 3 in this paper), for the specific star formation rate (SSFR) against the mass of the galaxy, the Sérsic index (a measure of how disc or classical bulge dominated a galaxy is) and the central mass density of a galaxy (how concentrated the bulge of a galaxy is). At first glance, no trend is apparent in Figure 2, however the authors argue that when split into two samples: star forming (log SSFR >  -11 yr^{-1}) and quiescent (aka “red and dead” galaxies with log  SSFR < -11 yr^{-1}) galaxies, two separate trends appear. For the star forming population, the bar likelihood increases for galaxies which have a higher mass and are more classically bulge dominated with a higher central mass density; whereas for the quiescent population the bar likelihood increases for lower mass galaxies, which are disc dominated with a lower central mass density.

figure3
Figure 2: The average bar likelihood shown with coloured contoured bins for the specific SFR (SFR with respect to the the total mass of the galaxy) against (i) the mass of the galaxy, (ii) the Sérsic index (a measure of whether a galaxy is disc (log n > 0.4) or bulge (log n < 0.4) dominated and (iii) the central surface stellar mass density. This shows an anti-correlation of p_{bar} with the specific star formation rate. The SSFR can be taken as a proxy for the amount of gas available for star formation so the underlying relationship that this plot suggests, is that bar likelihood will increase for decreasing gas fraction.

Bars become longer over time as they transfer angular momentum from the bar to the outer disc or bulge. In order to determine whether the trends seen in Figure 2 are due to the evolution of the bars or the likelihood of bar formation in a galaxy, the authors also considered how the properties studied above were affected by the length of the bar in a galaxy. They calculate this by defining a property L_{sbar}, a scaled bar length as the measured length of the bar divided by a measure of disc size. This is plotted in Figure 3 (Figure 4 in this paper) against the total mass, the Sérsic index and the central mass density of the galaxies. with the population once again split into star forming (log SSFR >-11 yr^{-1}) and quiescent (log SSFR < -11 yr^{-1}) galaxies.

Lbar
Figure 3: The average (in bins of ~ 100 galaxies) length of a galaxy bar (L_{sbar}) against (i) the mass of the galaxy, (ii) the Sérsic index and (iii) the central surface stellar mass density for both the star forming and quiescent population of galaxies.

As before, Figure 3 shows that the trend in the star forming galaxies is for an increase in L_{sbar} for massive galaxies which are more bulge dominated with a higher central mass density. However, for the quiescent population of galaxies, L_{sbar} decreases for increasing galactic mass, increases up to certain values for log n and log \ \Sigma_{1 kpc}^{*} and after which the trend reverses.

The authors argue that this correlation between p_{bar} and L_{sbar} within the inner galactic structure of star forming galaxies is evidence not only for the existence of secular evolution but also for the role of ongoing secular processes in the evolution of disc galaxies. Furthermore, they argue since the highest values of p_{bar} are found amongst the quiescent galaxies (with log n ~ 0.4) that bars must play a role in turning these galaxies quiescent; in other words, that a bar is quenching star formation in these galaxies (rather than an increase in star formation which has been argued previously). They suggest that this process could occur if the bar funneled most of the gas within a galaxy, which is available for star formation, into the central regions, causing a brief burst of star formation whilst starving the majority of the outer regions. This evidence becomes another piece of the puzzle that is our current understanding of the processes driving galactic evolution.