Galaxy Zoo Talk

Wednesday, 3rd July, 2013: What Are Galaxies?

  • JeanTate by JeanTate

    That's AGZ0005fyo, as displayed in Galaxy Zoo Examine. It's also known as SDSS J232309.71+221239.6 (DR8 ObjId 1237679478003466473). NED tells us that it's also known as 2MASX J23230971+2212397, a 2MASS infrared source.

    The SDSS objects look better in Galaxy Zoo Examine than they do in SDSS DR8, in my opinion anyway; see for yourself:

    Why make this rather ordinary-looking yellow splotch elliptical galaxy the Object of today? Why not something like AGZ00035ql?

    That's NGC 5908, UGC 09805, PGC 054522, and many other names besides. It's a glorious (nearly) edge-on spiral, with a very prominent dust-lane, and a beautiful, large, faint golden bulge. And it was recently nominated by one of this forum's most recently active members, fatha731 😃. It's included in several zooites' GZ4 Collections, including the aptly named beautiful, by wwg1775.

    Well, because zooite efcmaster, who has posted just eight comments here, wrote this in her (his?) comment on 2MASX J23230971+2212397: "what are galaxys".

    And I thought, "Hmm, you know, all us oldbie zooites, who've been classifying galaxies since like it seems forever, we know what galaxies are. But if you're a schoolkid, and your class has suddenly been pointed to [url=https://www.zooniverse.org/project/hubble]Galaxy Zoo[/url], that's actually a very good question!" So I thought I'd have a go; off the top of my head, and in just a few hundred words, how would I answer?

    Here's what I wrote (~550 words):

    They're systems of stars, gas, and dust, and something mysterious called 'cold dark matter'.
    The stars - more accurately, star systems - are like our own Sun, and brighter all the way to over a million times brighter, bigger, and ~a hundred times more massive; all the way down to a million times dimmer, as small as a city, and ~a hundred times less massive. Some have systems of planets that are far more complex (and interesting) than our own solar system; some have no planets. Check out Planet Hunters if you're interested in finding some of these exo-planets (planets beyond our solar system). The light you see from galaxies comes mostly from the stars in it.

    The gas - which includes molecular gas (like CO, carbon monoxide, but mostly just H2), atomic gas (like helium, but mostly just hydrogen), and ionized gas (sometimes called plasma, atoms - and some molecules - which have lost one or more electrons, making them ions) - is mostly so rarified that it's a 'harder' vacuum than any we can create here in our labs, but can be as dense as the upper parts of our own atmosphere (but that's still a pretty darn good vacuum). Some of the light from 'blue' spiral (and irregular) galaxies comes from gas that is being zapped by hot, bright stars (or shocked by supernovae). In some special galaxies, e.g. the Green Peas discovered by zooites a few years' ago, the light from this gas dominates what we see.

    The dust is tiny, smaller than the smallest smoke particles or the diesel pollution that so worries people who live in Beijing. It is mostly carbon or silicon ('soot' and 'grit'), often with a coating of water (or other) ice. Some is like really tiny iron shavings (also often coated). We see dust mostly because it makes a galaxy look redder; the more dust, the redder. This is particularly noticeable in edge-on spiral galaxies, which often have a dustlane running down the center.

    The 'cold dark matter' is matter of a form we have never seen here on Earth. It is completely invisible, and in total about five times the mass of everything else in a galaxy. We think it is mostly distributed as a dented sphere, kinda like a fishbowl, with the visible galaxy as a small fish in its exact center.

    In the very center of almost all galaxies is a super-massive black hole (SMBH), which is exactly what its name is (in mass they range from ~a million times that of our Sun to more than tens of billions). When they are surrounded by (ordinary) matter in a disk (called an accretion disk), they can be incredibly bright (the accretion disk, not the SMBH), even outshining the rest of the galaxy. We see these as AGN (active galactic nuclei), the brightest of which are called quasars.

    The stars, gas, and dust in spiral galaxies are arranged in two distinct systems, a disk (a more-or-less flat thing, like ... wait for it ... a disk) and a bulge, which is a more-or-less spherical blob at the center. Within a disk, there may be arms, rings, pseudo-rings, bars, ... In elliptical galaxies there's usually no dust, and often little gas. These are yellow(-ish) blobs, mostly round but sometimes almost cigar-shaped.

    What do you think? Need a PhD to understand that? tl;dr?

    How would you answer newbie efcmaster's question?

    (Copied from Wednesday, 3rd July, 2013: What Are Galaxies? in the Galaxy Zoo forum)

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  • fatha731 by fatha731

    Experimenting again? 😛

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  • JeanTate by JeanTate in response to fatha731's comment.

    Yes 😉

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