Звездообразование в галактиках. Интервью с итальянским астрономом

[Music] Astronomy. The first question: your work has been published in a very important magazine. It's well—usually it places really important works. So could you explain why it is so important? Because, as I understand, you observed the situation when star formation was driven not by winds from a black hole in the center of the galaxy but with the merger, and a huge number of interstellar gas was ejected from the galaxy. The galaxy is going, it's [Music].

Thanks for the question. So there are two reasons I would say why this discovery is important, maybe three. So the first reason is that, as you are saying, we are observing a galaxy in which star formation is going to be shut down soon because a large quantity of gas has been expelled outside of the galaxy.

This means that the galaxy will not have material for forming new stars in the future. This is important because this is the first time that this process has been observed in a massive galaxy. Theorists and observers have been asking for a long time what causes massive galaxies like the one we observed to stop forming stars. So by observing this phenomenon, we are saying that one of the ways in which massive galaxies can stop forming stars is by ejecting gas from galaxies.

It was previously theorized, but now we are observing it. It's indirectly... Yes. The other two reasons why this is important are that we were able to study the statistics of these events. So we're trying to understand if they are enough to produce the number of dead galaxies in the universe. Our studies suggest that these events are rare, yes, but are also frequent enough that they can explain the formation of passive galaxies. And when I say passive, I mean galaxies that are not forming stars.

Could I interrupt you for a moment? When you speak about the statistics, how can you estimate the statistics using just one example of the galaxy? That's another very good question. The reason why we can estimate statistics is because this is a single event that has been observed from a larger data set. We took the ALMA telescope and we observed more than 100 galaxies at the same cosmic epoch, but we observed this event, this ejection, only in one of those objects.

So this fact that we observed just one ejection over a hundred objects allowed us to understand how frequent these events are.

And the third reason—so the third reason—you're promising! Yes, yes, yes. So the third reason is that the signature through which we discovered this event is actually extremely similar to the signature of other phenomena, which I think you also mentioned: are those winds from AGN. So what we are showing here is that these two phenomena can be very, very similar: winds and these ejections from mergers.

So this perhaps is telling us that maybe in other cases these phenomena were confused in the past. This is normal, and this is how science goes: that sometimes we proceed by different trials and errors.

Yeah, yeah! Perfect explanation, thank you.

Catalyst: Now the next question: the galaxy is rather far distant, yes. And the scale is very small. The Hubble image has about one second, it's very tiny. How is it possible to estimate the star formation rate and the structure of star formation within the galaxy with such a small scale of resolution?

So, for the star formation rate, we just need integrated measurements. We don't need resolved measurements. You definitely cannot calculate individual stars at this distance, of course. No, absolutely not. This is only possible for the closest galaxies. This galaxy is so far away that we cannot resolve individual stars.

So we can compute the global star formation rate of the object in several ways. In a specific study, we took observations in the far infrared from the Herschel satellite, and we can convert the measurements of the falling through luminosity into a star formation rate.

How exactly this estimate is—you mean you can transform your measurements into infrared to star formation rate? As a result, you got a huge deformation rate of about 500 stellar masses per year in this galaxy, which is very high, right?

Yes, because the question is: what precision, what accuracy is in this transformation of the infrared measurements with this deformation rate? So in this case, it's quite accurate. The measurement—it's—so the galaxy has a surface measure rate of about 550 solar masses per year, and the error is of the order of 10 solar masses per year.

Just 10 seconds, very, very small! Yes, but this is also because we have a very good coverage of the galaxy's spectral energy distribution, so the galaxy's emission at different wavelengths.

With our models, we can produce very accurately the shape of the galaxy's spectral energy distribution. Then this is why it’s also very important to have a large data set of different wavelengths, because this allows the model to be very well constrained.

And what about this tidal bridge or this—you mean that kind of a tail which is expelled from the galaxy? Again, the resolution was insufficient to be able to resolve this tail. Everything we can look at—the pictures from your paper, you mean—these isoforces? Yes, asymmetry of the image of the galaxy. That's what is made by ALMA, right?

So yes, let me explain, because this is not exactly an image. We observe the galaxy with ALMA; the resolution of the ALMA images is not very high—about one arc second. The motor corresponds to about eight kiloparsecs. If you want to have an idea of that scale, this is slightly smaller than the radius of the Milky Way.

The ALMA observations are not images like the ones that you can see here from the HST telescope. This is because ALMA is an interferometer; it's a submillimeter telescope, so it works slightly different than the way we are used to think about telescopes.

What you're looking at here is a reconstructed image—not exactly what the telescope sees. There is also quite some noise, so the structures that you see in the images are not really real structures. This is not a resolved image, more than the pattern of the noise.

The shapes that you see are not real, it's not real; it's not the shape of the broad component. The way we can say that the broad component has been expelled is by looking at the previous image, which is the spectrum.

Yeah, this one, especially the top left. This is very important because this gives us an indication of how the flux is distributed in velocity. It gives us an idea of the velocity of the gas, the reaction.

Exactly, as you can see, the broad component is displaced with respect to the galaxy emission, which is the normal component. And in this way, we can tell that the gas is being ejected.

Maybe the last question—and it doesn't refer to the paper itself—but you see, I'm quite interested in the ways students from Russia can find some opportunities to find their thesis in the West, to use Western telescopes for their job and everything.

It's a very acute question now in Russia, because people try to find some opportunities to get to new instruments, huge telescopes, and everything. You know that Russia now has a few large telescopes. About 50 years ago, we had the largest telescope in the world—this was the six-meter optical telescope in the Northern Caucasus, and it was a real giant for that time.

But now that time has passed, and now we have BLT and ALMA and ELT, which is being built now. So that's a very, very serious question for students to be able to get to these kinds of opportunities.

I noticed that in your co-authors, people from London, Lyon, Munich, Spain, Copenhagen—you’re Italian, so you started in Italy? Yes. Now you went to France, and then to Bristol University. Now, yes?

What makes it possible to change so easily from country to country, from instrument to instrument, to have some more and more places to work in, places to study?

To one extent, this is a little bit the life of a scientist, I would say. First of all, during the PhD, you might have chances to travel, if you wish to do so. This is something I really wanted. So when I was a PhD, I went to ISO, and that was an amazing opportunity because that allowed me to know a lot of people from [Music] all over the world, I would say.

And then, moving on with my postdoc career, it's a little bit straightforward, I would say, to just apply in the world. So in my case, I went to Paris for my first postdoc, and then now I’m in Durham for my second postdoc. But this is also a little bit how astronomy works, and it's actually one of the things that I really like about astronomy: that there is a lot of collaboration and interaction in the community.

One of the ways of connecting with the community would be—I think one of the nice things that we have now, during Coronavirus, is that a lot of conferences have been moved online; some of them are even for free. So one good way would be to look at those. And for people who want to start PhDs, there might be opportunities in Europe.

Thank you, Anna, very much. That was a very nice talk. Thank you! Thank you! Hope to see you again and your favorites. Thanks! Bye-bye. [Music]