NGC 4490, a galaxy bursting with star formation after a recent close encounter with a smaller galaxy. Credit:  ESA/Hubble & NASA Acknowledgements: D. Calzetti (UMass) and the LEGUS Team, J. Maund (University of Sheffield), and R. Chandar (University of To

A whirlwind romance leads to a million births

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Apr 10, 2018

A standard romcom movie plot: After a meet cute, a couple have a brief fling. Fast-forward a year or more (at least, more than nine months) and there's a knock at the door. "Meet your daughter!"

It's trite, but hey, sometimes life imitates art. Like with NGC 4490. It had a meet cute 400 million years ago, and since then there have been a lot of knocks at the door. "Meet your millions of sons and daughters!"

NGC 4490 used to be a tidy spiral galaxy. But then NGC 4485 came along, a dwarf irregular galaxy. They got close… too close. Their mutual gravity wreaked havoc on each other (I suppose you could say "hijinks ensued" if we stick to the metaphor). Gas clouds in the bigger galaxy smashed into each other, collapsed, and started forming stars at a prodigious rate.

It was a blessed event. A blessed megaevent. And now the view we have of the galaxy is simply spectacular:

NGC 4490, a galaxy bursting with star formation after a recent close encounter with a smaller galaxy. Credit: ESA/Hubble & NASA Acknowledgements: D. Calzetti (UMass) and the LEGUS Team, J. Maund (University of Sheffield), and R. Chandar (University of Tol

NGC 4490, a galaxy bursting with star formation after a recent close encounter with a smaller galaxy. Credit:
ESA/Hubble & NASA Acknowledgements: D. Calzetti (UMass) and the LEGUS Team, J. Maund (University of Sheffield), and R. Chandar (University of Toledo)

That Hubble shot shows a lot of detail because the two galaxies are about 25 million light-years from here, relatively close. The center of the bigger galaxy is still roiling, its overall shape stirred to chaos.

But what's really astonishing are those red gas clouds! Those are huge nebulae where stars are being born. There so many of them, and so many big ones! In most normal spiral galaxies you see quite a few, but giant ones like these are rare. I expected them to be cranking out lots of stars, but when I looked it up I was still shocked: It's been popping out stars at a rate of about 5 times the mass of the Sun per year. For the past several hundred million years.

Holy cow. Right now the Milky Way, our home galaxy, forms about 1 Sun's mass worth of stars per year*. So yeah, NGC 4490 is pretty fecund.

Hubble's view is pretty narrow in this shot, but happily my friend and astronomer Adam Block took a nice shot showing both galaxies:

A wide view of NGC 4490 shows the interloping NGC 4485, a smaller irregular whose gravity distorted the bigger galaxy’s shape. Credit: Michael Gariepy/Adam Block/NOAO/AURA/NSF

A wide view of NGC 4490 shows the interloping NGC 4485, a smaller irregular whose gravity distorted the bigger galaxy’s shape. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

The galaxies are about 25,000 light-years apart. That stream of matter between them is called a tidal bridge. It's made of star-forming gas that was drawn out of the galaxies by their mutual gravity. Sometimes we see such arcs flung away on the outside of the colliding galaxies, and we call those tidal tails.

Observations using X-ray telescopes show there's a lot of hot (million-degree or more) gas in NGC 4490 as well (not so much for NGC 4485, though, which may be too small with too weak gravity to hold on to any of it). That's consistent with there being a lot of supernovae in the galaxy, which also makes sense. If you make a lot of stars, you make a lot of massive stars, and massive stars explode. This pumps vast amounts of energy into the gas between stars, which then glows in X-rays.

Interestingly, they found that the X-rays are coming from parts of the galaxy where the red glow of the gas clouds is fainter, and vice versa (we say they're anticorrelated). It may be that the cooler gas we see in the Hubble image is constraining the hot gas, like a net holding fish. The hot gas is pushing against the cooler gas and may be helping it collapse to form more stars.

It's the cycle of life. Another romcom trope.

If you've ever wanted to be in a movie like that, well, you’ll get a chance. Kinda. The massive spiral Andromeda galaxy is headed straight for us. It will collide and eventually merge with the Milky Way, forming a much larger elliptical galaxy when it's all said and done.

Of course, this won't happen for another four billion years or so. Plenty of time to make popcorn for our front-row seat. Until then, gaze upon such collisions as the one that happened to NGC 4490 and 4485, and marvel at the wonderful show that the Universe puts on for us on the sky's wonderful screen.

* That's an average over the whole galaxy over time, so it's not like one star just like the Sun blinks into existence every year. It's more like: Over a hundred thousand years you find (very roughly; take this as just a f'rinstance) 85,000 new low-mass red dwarfs, a few thousand orange dwarfs, one or two thousand stars like the Sun, a few hundred like Sirius or Vega (bluer, stars with twice or mass) and just a handful of really whopping big stars.