Profile of a Lonely Galaxy

Article written: 2 Apr , 2011
Updated: 24 Dec , 2015
by

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The vast majority of galaxies exist in clusters. These clusters are joined on larger scales by filaments and sheets of galaxies, between which, gigantic galactic voids are nearly entirely free of galaxies. These voids are often hundreds of million of light years across. Only rarely does a lonely galaxy break the emptiness. Our own Milky Way rests in one of these large sheets which borders the Local Void which is nearly 200 million light years across. In that emptiness, there have been tentative identifications of up to sixteen galaxies, but only one has been confirmed to actually be at a distance that places it within the void.

This dwarf galaxy is ESO 461-36 and has been the target of recent study. As expected of galaxies within the void, ESO 461-36 is exceptionally isolated with no galaxies discovered within 10 million light years.

What is surprising for such a lonely galaxy is that when astronomers compared the stellar disc of the galaxy with a mapping of hydrogen gas, the gas disc was tilted by as much as 55°. The team proposes that this may be due to a bar within the galaxy acting as a funnel along which gas could accrete onto the main disc. Another option is that this galaxy was recently involved in a small scale merger. The tidal pull of even a small satellite could potentially draw the gas into a different orbit.

This disc of gas is also unusually extended, being several times as large as the visual portion of the galaxy. While intergalactic space is an excellent vacuum, compared to the space within voids it is a relatively dense environment. This extreme under-density may contribute to the puffing up of the gaseous disc, but with the rarity of void galaxies, there is precious little to which astronomers can compare.

Compared with other dwarf galaxies, ESO 461-36 is also exceptionally dim. To measure brightness, astronomers generally use a measure known as the mass to light ratio in which the mass of the galaxy, in solar masses, is divided by the total luminosity, again using the Sun as a baseline. Typical galaxies have mass to light ratios between 2 and 10. Common dwarf galaxies can have ratios into the 30’s. But ESO 461-36 has a ratio of 89, making it among the dimmest galaxies known.

Eventually, astronomers seek to discover more void galaxies. Not only do such galaxies serve as interesting test beds for the understanding of galactic evolution in secular environments, but they also serve as tests for cosmological models. In particular the ΛCDM model predicts that there should be far more galaxies scattered in the voids than are observed. Future observations could help to resolve such discrepancies.

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23 Responses

  1. Question says

    any estimates on the age of the poor little guy?

    • Hon. Salacious B. Crumb says

      Distance is about 6.4±0.5 based on the motions in respect of the local group. The best redshift to date is 0.001425 +/- 0.000020. It is likely a difficult object to measure distance because of its relatively close proximity; where its own motion is significant value compared to just the Hubble expansion.
      It size measured is about 1.2×0.6 , and the magnitude is given as 17.08.

    • Hon. Salacious B. Crumb says

      I could find no thing about the age of this object from the information available. We can only presume that it is as ancient as every other galaxy or globular star cluster, but it certainly has many old stars in it.

      • Question says

        thanks for responding.

        i wasn’t thinking when i asked that poorly worded question. i appreciate that you humored me.

  2. Hon. Salacious B. Crumb says

    Jon
    Just a minor point. You should attribute the image of this galaxy to the ALADIN CDS software program, whose copyright is to the UDS/CNRS and distributed under GPL v3 licence (They request you acknowledge the source of the images.)
    Whilst technically you probably won’t be bailed up on it, it is a common practice to acknowledge the source of the image/ data.
    The Centre de Données astronomiques de Strasbourg (CDS) is quite kind enough to freely let anyone use their system for astronomical information. In reciprocation, we should kindly in return acknowledge their useful contributions.
    Remerciements tellement.

    • Member
      Jon Voisey says

      Thanks for that. I’d pulled the image up on Aladin, but wasn’t sure where it was getting the images from. It simply said DSS, without explanation. I looked through their help page, but it only seemed to have information on citing Aladin itself.

  3. RickJ says

    Redshift is rather unreliable this close. A better measurement is using the Tip of the Red Giant Branch. It is thought such stars are a “reasonable” standard candle. Using that method, the distance is about 7.83 mega parsecs or about 25.5 million light years. It was fully resolved by the Hubble Space Telescope in 2004. The raw images are available at the Hubble Legacy Archive.

    Rick

    • Hon. Salacious B. Crumb says

      A detailed discussion on this problem is in a paper by R.B. Tully et.al.; “Our Peculiar Motion Away from the Local Void”; A.J., 676, 184 (2008)
      The text is available at; http://iopscience.iop.org/0004-637X/676/1/184/71947.text.html

      Tully has done extensive studies on the dwarf galaxy. However, I could not find and Red Giant Branch papers of such observations made specifically of this galaxy. (I know of Kirby, E.M., et.al. “Deep Near-Infrared Surface Photometry of 57 Galaxies in the Local Sphere of Influence“; AJ., 136, 1866 (2008), but only does the photometry on the whole galaxy not the colour magnitude diagram of individual stars. Another similar one applies to; Sharina, M.E., et,al “Photometric properties of the Local Volume dwarf galaxies“; MNRAS, 384, 1544 (2008))

      Thanks for clueing us all in on the HST observations, though.

  4. Hon. Salacious B. Crumb says

    “As expected of galaxies within the void, ESO 461-36 is exceptionally isolated with no galaxies discovered within 75 million light years.”

    This is contrary to RickJ 25.5 million light year and the 6.4±0.5 Mpc. referred by me.

    The statement you have made seems unlikely to be true. Any ideas of the reason for the numbers?

    • Hon. Salacious B. Crumb says

      Should this distance be 7.5 million light years instead of the given 75??

      • Member
        Jon Voisey says

        According to the paper, that should have actually been 3 Mpc = 9.7 ly which I’ve rounded to 10. I believe I must have mashed the keyboard when putting in the 3 Mpc since 23 Mpc = 75 million ly. Thanks for noticing.

        I’m not sure where Rick is getting his figure from. SIMBAD didn’t list any other publications that looked like they’d include other values. Most were simply lists of nearby dwarf galaxies.

      • Hon. Salacious B. Crumb says

        I suggest you use the NED (NASA/IPAC Extragalactic Database) at http://ned.ipac.caltech.edu/

        Just input the name of the galaxy in “Object> By Name” field, and you get all the information on that galaxy and references, current images, etc, to it. You can also search for specific data too. [I’ve used the Batch Job a couple of times in producing list.] Quite handy.
        Cheers

      • HeadAroundU says

        I’m a bit confused. Nothing is within 10 MLY. How far away is it from earth?

      • Hon. Salacious B. Crumb says

        Compared to objects in the Local group, it is 6.4±0.5 Mpc or 20.9 million light years from Earth. it is assumed that in the void around it, there are no galaxies within 10 million light years of it.

  5. “ratios into the 30?s”

    Really no need for that apostrophe.

    • Hon. Salacious B. Crumb says

      Typically, either is considered acceptable.

    • Hon. Salacious B. Crumb says

      Just like the English Teacher says; “It’s a question of individual style. Traditionally, the apostrophe has been used, but many modern writers and publishers prefer not to use the apostrophe.”

      The New York Times may have dumped it but the rest of the world may not have. It has become a problem on because of the internet and writing apostrophes in HTML. Smart A’s like you always think they should bring everyone into conformity, when the truth is it makes little difference. If Jon or anyone else wants use an apostrophe or not, it is no big deal and it is neither right or wrong.

      Jon made a little mistake on a number he wrote and individuals like you want to come in a lay in the boot. The old experiments of Pavlov’s Dog remind me of such behaviour. Simply pathetic.

  6. manjula says

    if there is an intelligence life there, how will they see the stars and the space? Only few number of stars and galaxies? Definitely that will be not an interesting place for me to live.

    • Member
      IVAN3MAN_AT_LARGE says

      It won’t make any difference to them if they have got the same sort of light pollution in their cities like we have on Earth!

  7. HeadAroundU says

    Let’s say it’s 10 times smaller than Milky Way. Then, it has maybe 30 giga stars. Local group’s radius is 4 Mly, around 30 galaxies. So, with galaxies is a bigger problem…

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