Episode 32 - Ancient star clusters and growing black holes

Well hello Nicole. Hello Michelle. Happy Friday for us, but a different day for whoever's

listening to the star card. Oh, that'll be a Monday, right?

Well, because yeah, everybody obviously listens immediately as we really see.

Of course.

They've got an alarm set, notifications turned on, they're subscribed on various channels

to the star card, so yeah, they'll be getting it Monday, yeah.

And if not, then you're late. Yes, but do enjoy. We've got some great papers for you

today, so it's me and Nicole on deck, me being Michelle and me being Nicole.

There you go. Yeah, we do sound similar actually, but we do.

And to pile as well, so it can be confusing. We appreciate that, but that's just because

we all have excellent sounding voices. But anyway, I digress. We're here to talk today

to talk about some new and exciting papers from the world of AstroPH.

So Nicole, do you want to go first, second?

I'll go first. Brilliant.

I'll get us started.

Yeah, so today I've gone for a bit of a theme. I'm going to be talking a lot about globular

clusters because they are very, very cool.

I do enjoy a globular cluster, definitely, definitely.

And so the first paper I'll be talking about is not all nitrogen rich field stars originate

from globular clusters. I'm already excited when I saw you voted for this because I do

feel like this is a debate that has gone on, on and off throughout the years. Are these

all globular cluster stars? No, they're not. Yes, they are. No, they're not. So where are

we with this whole saga?

I think that this paper offers quite an intriguing insight that I hadn't quite considered kind

of before, which I won't spoil it now. Yeah, thank you. Let's start from the beginning.

So the authors of this paper is Ellen Lightinger et al from the University of Degglie, Stuydi,

DiBelonia in Italy, and this was published in A and A. And so, yes, like, why does this

paper matter? Why do we care about globular clusters? So they contain stars that have,

I'd been identified to have these very distinctive chemical abundance patterns that you don't

typically see in field stars. So this is things such as enhanced nitrogen, enhanced sodium,

depleted carbon. And these signatures that we find, they commonly associate them with

the second generation globular cluster stars. The idea being that they formed from gas that

was enriched by earlier generations within this, like, you know, the very dense cluster.

This is the second generation of stars that is happening not too long after the first

generation, right, because globular clusters are almost single, single age populations.

Yes, so it's old when we still don't know how that happens. But yes, the second generation,

OK. Yes. And so because of this, when, you know, we astronomer's we look out and we find

these nitrogen rich stars in the galactic field, we think, ooh, you know, were these originally

born in a globular cluster and then have escaped because the globular cluster has either been

tidally stripped or disrupted. And so this kind of assumption has been widely used in galactic

archaeology to estimate like how many globular clusters have been destroyed, how much of the

Milky Way halo formed from dissolved clusters, like so all of these different things. And so what

this paper tries to do is to really test this underlying key assumption of our nitrogen rich

field stars, always former globular cluster stars. And the authors conclude from this that many

are likely not. Oh, yeah. So this is the angle that we're going in. I like it. And so for the data,

they use stars in the Kepler fields, they use a combination of spectroscopic data from

Apogee DR 17. So they use this to get their nitrogen carbon and other chemical elements.

For the astrocyzed mology, they use Kepler astrocyzed muc data. So they use this to measure the

oscillations in the stars. And what this can allow you to do is to get precise estimates of your

stellar masses. And therefore your stellar ages. And this is very crucial because, you know,

we look at all of the the chemical compositions of these stars. But that alone can't really determine

a star's origin. But as soon as we start including the age and the mass, we can start kind of

teasing out these different formation scenarios. And so the sample that they're working with,

they have 133 stars that had spectroscopic measurements. Then they had 92 stars that had

reliable astrocyzmic elements. 20 of these had nitrogen enrichment. And so all in all kind of

condensing this down, they end up with 13 and nitrogen enriched stars with reliable astrocyzmic

ages. Very cool. I guess I'm always reminded of like how astrocyzed mology is quite kind of this

rare thing. And so that sample size very quickly dropped down. So they have this final subset of

13 stars. And essentially what they find from this is that many of these nitrogen rich stars

are too young to come from globular clusters. Right. So obviously, as we said, you know, they're

typically these very old systems like older than 10 giga years. And so stars that originate from

globular clusters should equally be old. But with these 13 stars, they found that only three of them

are older than eight giga years. So that kind of makes them a bit, you know, incompatible with

a globular cluster origin like the remainder of these 10 stars as they appear significantly younger.

And so they kind of went into why this could be. And I think it's like the thing that a lot of

people are considering now is that binary stars. I was going to ask about this. Is there a mass

transfer involved somewhere? That is exactly the point that they're getting at. And so yeah,

the idea is that these young ages, because of how we use astrocyzed mology to compute these ages

based on the mass. Obviously, if you're having mass transfer between binary stars or kind of even

just stellar merges, you're going to have a very skewed like because the stars are going to be

more massive, they're going to appear to be younger, even if they're not. Yes, I seem to remember,

there's every now and then a paper comes up like this, is that we found something younger than

we expected. And they're like, oh, but binary. This seems to be that our binary. I love binaries,

but yeah, yeah, consider that. They just like to trip us up, you know, it's a keep us. They could

always be playing a role. That's the key message, never underestimate a binary. Yes, definitely.

And I mean, not only just in terms of the mass, but they also talk about how the binary actions

can also mimic the chemistry, like this peculiar chemistry you're seeing in globular cluster stars.

So obviously with the binary mass transfer, you're altering the star's surface composition.

So the star can accrete material from a companion that's undergone all this nuclear processing,

and it transfers all of this nitrogen rich material to the surface. But, you know, this will also

become apparent in other elements like barium, for instance. But essentially, what they say is that

this can end up producing chemical patterns very similar to what we see in globular clusters.

So when we identify these stars in the field, they could just be kind of masquerading as,

you know, globular cluster stars. Well, this brings me to a whole nother set of questions.

This must be a stupid question that somebody's already said, no, don't be so stupid too.

But if binaries can do this to make stars look like they're in second generation globular cluster stars,

could that be how you make second generation globular cluster stars?

That is a very good question. I think the argument against it is no.

I think it must be, right? Because otherwise they would say that in this paper.

So, yes. And I assume the answer is no.

I think I believe it's something to do with AGB stars. And perhaps maybe they're not so

common in globular clusters. I'm not entirely certain, actually.

No, I'm sure they've talked about this. And I just don't remember.

But I just immediately are like, well, then could it, we don't know how second generation stars

form, but binaries can do this. Is this, but yeah, I must, the answer must be no.

And binaries, I guess, are destroyed quite easily in very dense systems, maybe.

Or then, yeah, that might make sense. I also, I'm just making things up now, part.

Yeah, pile the coal. Sorry, piles not here.

I'm just making things up now. So, why don't we focus on the paper that there's, there's, yeah.

Yeah, of course. So, they kind of, yeah, they interpret this information that they've got.

So, obviously, channel one is that, yes, you know, they determined that three of the stars

were old enough. So, they could be globular cluster escapees given by their old ages and

their chemical abundance patterns. But also, yes, they could just be the kind of products of binary

evolution. And so, what this kind of then leads down on the rabbit hole is, you know, maybe our

estimates of disrupted globular clusters are actually too high. If this is what we're basing it on.

Also, perhaps the point that nitrogen rich stars might not be a reliable tracer of globular

cluster debris. And also, just kind of talking about how kind of doing more astrosized

mology, getting more ages for stars, I guess having a much larger sample size as well would

really, really help. And especially if you're able to kind of resolve whether or not they are

binaries as well, just all this additional information where this will help us be able to tease it apart.

But, yeah, no, that, that was the paper. And I just really liked it because it just kind of

threw that out at you and, you know, make a question a few things. And yeah, no, I like to do it very

cool. I agree. Right, what is your first paper, Michelle? Well, I've also gone for a theme today.

My theme is black holes, effectively. But specifically, how do black holes grow and accrete stuff

and evolve? And can we get to the supermassive black hole seeds that we need to explain so many

things in the universe? So that, in my first paper, is called small hosts, big appetites.

I love it, colon. Unveiling, rapid and early, low mass black hole growth in a cosmological zoom

inseminations of dwarf galaxies. And this is led by Julia or Tamé Idol from the Institute of

Astronomy in Cambridge and is submitted to MNRAS. And I'd pick this one because here at the

star guy we obviously love black holes in all shapes and sizes from the small to the most massive

and we know that most, the massive galaxies in our universe, including our own, all host these

supermassive black holes at their very center. And we sort of don't fully understand how they form.

They require fairly large seeds. How do those seeds grow over time? What's really seeding these

massive black holes? Now, this is even more important or problematic at the current day,

because sometimes, as we're seeing with data versus T, we're forming a lot of supermassive

black holes that are much more massive than we would expect. When we look into the local universe,

you find there are strong correlations between things like the mass of the supermassive black hole

at the center of a galaxy, and the stellar mass of its bold, or the velocity dispersion of its

bold, so you get these beautiful scaling relations. But the work from J.L.A.R.S.T. is showing that

a high ratio of two plus you're finding a sort of non-trivial population of galaxies hosting

black holes that lie well above these relations. So they're... How rude of it. Exactly, overly supermassive.

And given that we were already struggling to figure out what seeds these supermassive black holes

are, it's just making that problem a bit more chronic. People have been working on trying to

understand this across multiple different fronts, and we've talked about a number of papers that

are trying to understand how we make these very, very massive black holes on the star hive.

And now this paper is focusing on studying black hole growth and ADN in low mass galaxies,

using high resolution simulations of a dwarf galaxy. So kind of looking for that, if we're thinking

about the seeds, can we get something like an intermediate mass black hole that then rapidly

grows to sort of really massive seed size in the center of a dwarf galaxy? Because these are

the kind of hosts that should have, if they have central black holes, should be in the I.M.

intermediate mass or I.M.B.H. range. And so can we then grow them? And after they have like ideal

conditions for this? So the reason we're looking at dwarf galaxies, they're interesting,

we seem to focus on as one, their low masses mean that they are often uniquely sensitive

to feedback processes. And this could be particularly useful for trying to understand

the activity through ADN that you might need in order to grow a really massive black hole in

the center of these things, because if you have too much feedback, you really alter the properties

of the galaxies, because they're so low mass. They're also the regime where we think based on

scaling relations that the elusive intermediate mass black holes will form. And recent work is also

shown that nuclear star clusters, which are common in dwarfs, may efficiently funnel gas onto central

black holes in these systems, which could possibly allow rapid growth of a black hole.

Now the caveat to that is when people have tried to model this in the past, you often find as well,

because these are quite low mass hosts. The black hole can easily sort of be displaced from

the center during this accretion process and sort of moved away from where the gas is densest,

which means you don't grow as effectively as you could. So how then do you, and then they kind of

just stop accreting material and they don't grow as massive as you would like them to.

So this paper wants to sort of address this and basically study black hole growth in the dwarf

galaxies that they're simulating. And to do that, they're employing a novel method for black

hole accretion models, which is a sync particle method. So whereas most papers tend to use something

called the Bondi accretion method, which I think is sort of a prescriptive formula where you kind of

work out how accretion is happening over time and it's sort of based on masses and things like that.

My knowledge of the Bondi model is low, but it has used a lot to sort of measure to sort of

study accretion onto black holes in the centers of galaxies. So here instead they want to use a

sync particle method where they see the single black hole particle on the fly in their

simulated galaxy. So here they're using the fable suite of cosmological simulations, tweaking

some of the fiducial feedback in particular strong star formation feedback, and then using these

to place single black hole sync particles into them on the fly. So in this particular case,

they identify the cell with the highest gas density at the redshift they're interested in

seeding in the most massive galaxy halo at that time. And then they play around with exactly when

they seed the black hole and how massive the black hole seed is. So they have a low redshift

seed at redshift four, which has a mass of about 10 to the four solar masses for the black hole,

and then a high redshift seed at redshift five when they use 10 to the three solar masses for

the black hole mass. And then they model the accretion through their sync particle method and

compare it with what happens with the bond diocretion method to see if anything changes.

Now the sync particle method in particular doesn't penalize black hole growth in low

mass galaxies in much in the way that bond diocretion does, which is about as much as I understood

from that because I'm still not entirely clear on how all of these methods work. And this is the

wiggle from the center. Well, I think it's the accretion onto the black hole itself. So it's how

you model the accretion of the black hole, the gas around the black hole onto the black hole.

The bond di method and the sync particle method, but using eye there may end up changing how the

black hole behaves, I think. I see. So there's some limitations to both things. So they do this

and compare with who the two and they change a couple of things about feedback and the

duty cycle of the AGM, which can affect how effective or not feedback is at preventing

further accretion and things like that. And then they compare the two to see how easy it is to grow

these black hole seeds that they placed in to understand black hole accretion and seed black

holes. So the key results that are that the nuclear regions of dwarf galaxies have plenty of gas,

which could fuel efficient black hole accretion. So those nuclear star clusters are really great

for that, which is consistent with other papers. But bond di models still have lower accretion

rates, which lead to negligible impact of AGM on the interstellar medium. So there it

seems that you just can't, with bond di methods, you typically aren't very efficient at growing

these black holes and are creating a lot onto them and then having fun AGM that does stuff to

the galaxy. Now, depending on the efficiency of AGM feedback, they find that in some cases,

this can quench dwarf galaxies, which is really interesting because a lot of people have argued

that AGM are not that important for quenching in dwarf galaxies, especially star formation.

And they also find that when you play around with a duty cycle of AGM in their simulations

and give it a duty cycle of about 25 mega years, it allows more efficient black hole accretion

earlier times, as feedback is delayed, which can allow more growth in the black hole population.

They find that traditional dynamical mass estimators don't perform as well at measuring

the masses of these systems at high rate shift, which could be important for some of the JWST

objects. So the masses that they're comparing for the scaling relations might be off as well.

Oh, I see. And they find that using their sync particle method and efficient AGMs,

the simulations can produce more massive sort of black holes.

So what are they? The masses of these black holes, kind of roughly.

Oh, I can't remember now. So they start to tend to the three and tend to the four solar masses

in the simulation. And I think they grow by an order of magnitude or two possibly.

Oh, okay.

But it's kind of variations depending on which method they use and how the feedback works.

And so I got, I can't remember all that now. I should have looked at the paper a bit more

recent and I did. I find it very interesting. So I'm not entirely glued up on black holes,

but just the fact that these can be seeded from dwarf galaxies, I find incredibly interesting,

just because, you know, they're small. Yes, yeah, but, you know, if they're massive,

especially the more massive dwarf galaxies, right, they're still, if you just place them on

the scaling relation, you can see what kind of mass black holes they should have. And it could be

of order 10 to the four solar masses, I think, that you might expect or 10 to the three certainly

as well. But no one's ever really found things in that sweet spot. So there are intermediate-ish

mass black holes at both the sort of higher ends, so 10 to the five-ish and the lower ends of 10

to the two-ish. But that really, you want to find them in the middle there. And dwarf galaxies,

if they just follow the scaling relations, should host black holes of that kind of mass.

I see, but they're hard to now. Yeah, it's really hard to measure them because if they're not

really active, you just can't necessarily constrain them. So we find ADN in some dwarf galaxies.

Yeah. So they must have black holes, but I think back in the math, those ones tend to be the more

mass, on the more massive side of things as well. So not quite as low as we would like them to be.

Oh, super interesting. Yeah. So what's your second paper?

Yeah, so we're cutting back to globular clusters. Yeah. And so the second paper is called a tale

of two origins. In situ, verses accreted in nitrogen-rich field stars, back with the nitrogen.

Back with the nitrogen. And the authors of this was Yechow et al from Sunyet Sen University in China.

And I think this was submitted to AppJ. Yeah, so I already discussed from the previous paper

about, you know, the key issue is whether or not the nitrogen-rich stars in the field are

actually originating from globular clusters. And from that, it suggested that it may not because

of binary evolution. But what this paper does is it looks at it from a slightly different perspective.

And instead of kind of focusing on how this nitrogen-richment formed, it's more of a where did the

parent systems of these stars kind of originate from in a Milky Way? And so specifically, they want

to know whether the globular clusters that produced these stars formed inside of the Milky Way,

or whether they formed inside dwarf galaxies or other systems that were then later accreted

through mergers. And so to investigate this, they combine chemical abundances with kinematics. So

this is the camera dynamics. And for this, they study 33 nitrogen-rich giant stars in the

Galactic field. So many of these were originally identified in the Lamost survey. And this was,

I kind of was looking at how do they identify these nitrogen-rich stars? And so it's somewhat

through the characteristic molecular features. So they'll have like a strong CN band or weak CH

bands. And these are kind of your indicators of enhanced nitrogen and depleted carbon. And of

these 33 stars that they studied, 18 were analyzed with these detailed chemical abundances for the

first time. Kind of in this work. And so to study these compositions in detail, they used high

resolution spectroscopy. And from this, they managed to determine abundances for up to 25 elements,

which always makes me excited. So that's anything from kind of the light elements of your carbon

and nitrogen to your adze, odd z elements, your alpha elements, of course, because we love alpha

elements, IMP elements, and also neutron capture elements. Okay. And these will be quite interesting

later on kind of coming up back to the binary side of things. And what they kind of gather is

that from the abundance patterns that they see in these stars, they do really strongly resemble

the kind of multiple population peculiar chemical abundances that you find in globular clusters,

which is great, because they're the target of the work. And so what they do is that they decide

that with chemistry alone, and this has been kind of a thing of galactic archaeology, it can be

had just from chemistry to determine the origin of these stars. So this is where the

orbital motions, they use gyroestrometry to kind of calculate these full three-dimensional

galactic orbits. And so from this, they use the orbital energy, the angular momentum, the

centricity, and kind of other properties to determine whether or not they are in situ or

recreated. And the key result from this is they end up with somewhat four kind of populations

from this. So they take these nitrogen rich stars and they're able to divide them into two distinct

orbital groups. So you have a high energy and a low energy population. And out of this high-end

low energy, they also split it up between metal rich and metal poor for somewhat both of them.

I won't go too much into detail on that, but they do do quite a bit of chemical analysis kind

of across those two. But kind of broadly looking at the low energy population, they tend to exhibit

kind of higher metalisties as well as alpha abundances. And so kind of more typical of stars

associated with in situ Milky Way stars. And so as a result, they say these likely escaped from

globular clusters that formed within the Milky Way. But coming to the high energy population,

these have kind of the more extreme halo-like orbits, somewhat kind of lower metalisties and

lower alpha abundances as you would expect from these accreted populations. And therefore,

they propose that the globular clusters that formed in dwarf galaxies and were later

accreted by the Milky Way, that's where these stars came from. Nice. And I think even

associate that some of these stars might be associated with the Gaia-Sosage Enceladus merger,

which is a fun one and has been brought up many times on the podcast. Yes, it always seems

like it comes back to Gaia-Sosage Enceladus in the end. It does, a good thing it's got a fun

of name. But what was really interesting, especially about this high energy population? So as I

said beforehand, they looked at the neutron capture elements. And what they found is that it

behaves like normal halo stars, like for this region. And I guess why this is important

is if you were going to see any binary transfer, you would expect to see more S-process enrichment.

But what they're actually finding is that when you compare the R to S-process enrichment,

they're ending up with very high values. So that means very low S-process. And so this kind of

argues against, at least for these stars, this kind of binary. So it's not binaries.

Well, yeah, I think this is where it really shows that kind of having the full collection of the

chemistry and the ages and the dynamics, you can really start to kind of tease these apart.

But what they did find is they found a star whose orbit shows a very close dynamical encounter

with a known globular cluster, a NGC 6235. And so they kind of run this orbital integration.

And they suggest that this star, this nitrogen rich star that they found may have been

ejected from this cluster in the past. So I like that. That's neat. Yeah. And so you know,

there's a great, because it's kind of direct dynamical evidence, which connects this nitrogen-rich

field star to a globular cluster. So once again, we're in the remit of, well, it could be. But it could

not be. It could be more than one thing. And another thing that they mention in the paper is this

kind of intriguing possibility involving nitrogen emitter galaxies, which are observed at high

reaches. Now I didn't really know about these at all. But these galaxies display kind of this very

unusually strong nitrogen emission lines, which could potentially indicate like a rapid or very

like unusual chemical enrichment in the early universe. And so what they note in this paper is that

some of these metal poor nitrogen rich stars in the Milky Way could share similarities with

stellar populations that produced these like very distant nitrogen emitters. And so it's very

speculative the connection there, but they do mention it. And so the idea is is that by looking at

these nitrogen rich stars, it could end up like providing clues about star formation and chemical

enrichment in the early universe from like these weird systems. And yeah, I think overall it just

goes to show how, especially of these nitrogen rich stars, that they could be very interesting,

like fossil records of the Milky Way's formation history, which I find very, very intriguing,

because I've always like, when I've done my kind of chemical tagging stuff before, I always

avoid nitrogen because it's one of the ones that's always in the mixing, like in the, so it's very

interesting seeing, I guess, in the context of globular clusters, it being used as an indicator like

this. Yeah, yeah, it's very cool. Yeah. What was your final paper, Michelle? Well, as I said,

I've gone on black holes and how they grow. So moving on from where we were placing high

mass seeds into dwarf galaxies to learn about how they might grow, we're going to now learn about

the in-situ growth of cellomass light seed black holes in nuclear star clusters. So this is by

Jan Longxi and Norman Murray at CETA in Toronto and submitted to APJ. And in the same vein as my

last paper, this is another one about growing massive black holes. So I do like a theme and this

one is again considering these dense, gas rich nuclear star clusters, which were a feature in the

last paper as little black hole nurseries. So given that you have all this gas, it's quite dense

environment. Are they just a great place to grow black holes? However, in this case, they are

considering less massive black hole seeds starting from cellomass black holes, which are the

product of massive star deaths effectively. Yeah. So can we use that go from there all the way up

to a massive black hole seed mass, which is something that has been discussed in the past that

it's tricky to do because typically you need long periods of super-eddington accretion to get

from a cellomass black hole all the way up to something that has, you know, 10 to the many.

So in this paper, they're using Magneto hydrodynamic simulations of giant molecular clouds

with masses between 10 to the five and 10 to the nine cellomasses, which they evolve for

between 10 and 30 mega years, which result in compact nuclear star cluster like objects by the end

of the simulation. But it's only a short period of time that they evolve in four, just I think because

of computational limitations on what they're trying to do. And incidentally, they are using Bondi

accretion models here. So not the sync particle method, which is not a combination. Yes, but very

much of the normal state of the artist. So in this scenario, to become an intermediate mass black

hole, these cellomass seeds will need to sustain accretion at super-eddington rates, which is tricky

to do because, you know, things like feedback from star formation within these systems can turn off

your gas supply. Doing it, like it's being sub-eddington for a long time, it's not really something

that we tend to see in the real universe. So typically, it's found been found that it's not very

efficient or important at these cellomass black holes in supermassive black seed formation.

So, you know, it's felt that for cellomass black holes in these sort of environments, only like

sort of 0.1 to 1 percent will be efficient at accreting stuff and growing to some mass scale that

may become important, so it's quite like mass. However, results like this tend to start from

pre-existing black hole seed models where you just put the seed in and then let it evolve in an

environment. And this paper wants to take a step back and consider how the seed originated. So,

you have a massive star evolving in this cluster like environment, but then goes and turns into a

black hole and then what does it do? So kind of tracking the whole lifetime of the massive star

through to the black hole seed formation. And this is interesting because while cellophied

back could prevent rapid growth, the dense gas could be captured by massive stars and then be

there and available once you go black hole to become an accretion disk. So I think they're kind

of trying to see does that have an impact on how how the accretion goes after was rather than

just starting from injecting a black hole. So to investigate this, they need simulations that

self-consistently form massive and very massive stars, so that very massive stars are typically

greater than 100 solar masses. They need to evolve these stars so they can understand their

evolution, they need to capture the feedback effects from these kind of stars in their environments,

they then need to form the black hole remnant and then study its subsequent accretion to kind of

see the whole journey. Now to do this, they use a modified version of the 5.3 framework to

simulate star forming cloud complexes for greater than 10 mega years, which is longer than the

lifetime of a very massive star and they can combine this with sort of a gizmo architecture to do

their simulations. Now simulations like fire typically don't track individual stars, they use

simple stellar populations, so where you have like one particle is a population of stars,

but in this case they use a sub hybrid method where they track individual massive stars, so

objects with masses between sort of greater than 50 to 100 solar masses, so they're treated as

individual particles and then they treat any lower mass stars in a single stellar population kind

of approach. It's just too expensive I think to do all star by star, but you do obviously need to

track the individual stars for this particular study for the very massive ones. So then run a

suite of simulations where they isolate the impact of key physics that may affect the growth of

these stellar mass black holes, so the properties of the giant molecular clouds themselves,

the metallicity that they are forming in, the initial mass function and any natal kicks that may

then throw the star or the black hole off center or work from over its formed into some other area.

And so what do we learn? Firstly they find that by using the fiducial feedback regulated black

hole accretion, no heavy seeds form as dense gas is efficiently removed by stellar feedback. So

very consistent with previous results that it's just really hard to grow a stellar mass black hole.

Notable black hole accretion only occurs in GMCs where the free four time is greater than three

mega years and with high surface densities which still retain gas at the time of the black hole

formation. And typically this is only happening in the most massive clouds, so right at that end

that sort of 10 to the 9 upper envelope of what they were studying. They find that in massive and

low-metallicity clouds a small subset of the black holes grow from less than 300 solar masses to

about 400 to 500 solar masses. So increasing in mass but you know by sort of less than a

factor of two or about a factor of two and they do experience some periods of super editing

accretion but these aren't sustained for long enough to really make these supermassive seeds.

They found that actually top heavy IMFs don't really increase heavy seeds, only light seeds

and this is because they also suppress star formation because of feedback which then kind of

has so you have this sort of dual approach of you've got more things that could be high mass

seeds but they just don't grow effectively. I see. And overall there's a limited accretion of

in-situ gas onto stellar mass black holes in their simulations. They do caveat that this is not

a cosmological simulation. So one thing for example that they are missing is that you can't replenish

gas which you may be able to do if you are doing this as a cosmological, fully cosmological

simulation. So you may then get more gas supply coming in and you also they are only stimulated

for about 10 mega years. So what happens after that perhaps then you get some kind of periods of

sustained accretion after that. So those are the caveats for the study but from what they saw

it's still clear that stellar mass black holes perhaps aren't growing as fast as we need them to.

How interesting. I do love the the idea of tracking it kind of from that beginning with the

stellar mass black hole and then going all the way along. Yeah I like that too I thought it was neat

and you can imagine that it might change things and it seems here that maybe in some cases it can

that it seems to depend more on the properties of the giant molecular cloud and actually the stars

themselves. Yeah very interesting. Yeah but that's all I got for this week. Yes the me too.

And so I think we're all done but thank you all for joining us but you can find out more about

the podcast as usual on StarCive.com and also subscribe wherever you get your podcast. I think that's

all from us. So bye everyone. Goodbye.