The Ghost of Mirach

By Phil Plait | December 30, 2008 12:55 pm

Most objects in the sky are fairly well scattered, separated from each other. Through a telescope, which magnifies images, this becomes even more apparent. Bright, big, nearby galaxies usually float alone in the image, unless accompanied by satellite galaxies (or they live in a cluster).

But there are exceptions. Here’s a good one:

On the left is a visible light image of Mirach, a medium-bright naked-eye star 200 light years away in the constellation of Andromeda. It’s a red giant, a star more massive than the Sun that is nearing the end of its life. It’s hugely over-exposed in the image. Look just above it and to the right: see that smudge? It’s The Ghost of Mirach (technically NGC 404*), a galaxy 11 million light years away overwhelmed by the light of the much closer star. If that galaxy were by itself someplace, it would be a real showpiece, easily seen in small telescopes. As it is, it’s like a firefly sitting on the edge of a spotlight.

But wait! That’s visible light, the kind our eye is sensitive to. What if we look at other wavelengths?

In infrared things would be worse; Mirach pours out long wavelength light, being a red giant at all. But what about bluer light? Like ultraviolet, say? Well, that’s the image on the right. Taken by the Galaxy Evolution Explorer, or Galex, it’s a UV image of the same region of sky. Mirach is gone! For all its sound and fury in the red, a star like Mirach doesn’t put out much UV, so it essentially vanishes, and we can see past it to the Ghost beyond.

What is seen is a bit surprising. The Ghost is a lenticular, a lens-shaped disk galaxy. These are weird galaxies; they are flattened disks like our Milky Way, but generally are devoid of gas, so they don’t have any ongoing star formation in them (like elliptical galaxies). Usually, UV light from a galaxy is a product of gas actively forming stars; massive stars get formed which blast out UV radiation that in turn makes the gas clouds glow. Lenticulars, being gas-poor, shouldn’t be bright in the UV.

But look at the Ghost of Mirach! It’s clearly seen putting out UV light, and in fact you can trace a ring pattern too. Rings usually indicate a galaxy got disturbed somehow, usually by the gravity of a nearby galaxy passing it or colliding with it. And, as it happens, earlier observations of The Ghost using radio telescopes indicated a ring of hydrogen just where the UV ring is, and the authors of that study think that this galaxy did suffer a collision, nearly a billion years ago. Perhaps back then there was more gas in the galaxy, and the collision triggered a massive wave of star formation that used up almost — but not quite all — of the gas. All that remains now is this semi-stable ring, still making stars, but at much lower rates.

However it happened, NGC 404 is an object worthy of study, even if sitting in a natural source of light pollution. Astronomers prefer the dark, but sometimes, like anyone else searching for something, we’re forced to look where the light is bright.

* You gotta love the punning and appropriate catalog number for this galaxy. NGC 404, for a galaxy that is hard to see and therefore usually not found?

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (15)

  1. Thor

    Good call on the 404.

  2. I keep getting a song stuck in my head reading this. I’ll resist the urge to post the entire lyrics to Manfred Mann’s Earth Band “Blinded by the light” though.

  3. Astronomynut

    NGC 404 is easily seen in a small telescope. It lies just far enough away from Mirach to be able to slide the bright star out of the field of view, and there it shines brightly. In my 17.5″, from a dark sky site, it’s really bright. Not a lot of detail, but worth taking a look. I think it’s on the Herschel 400 list.

  4. KC

    I’ve heard NGC 404 referred to as the Phantom Galaxy (a name which these days is applied more to M74 than NGC 404). I think its an appropriate nickname as you really have to use averted vision to see it.

  5. [pedant]

    According to Wikipedia, at least two techniques have been used to measure distances to NGC 404. The infrared surface brightness fluctuations distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. Using this technique in 2003, the distance measured is 9.9 ± 0.5 Mly (3.03 ± 0.15 Mpc).

    However, NGC 404 is close enough that red super-giants can be imaged as individual stars. The light from these stars, in comparison to similar stars within the Milky Way galaxy, enables direct measurement of the distance to the galaxy. This method is referred to as the tip of the red giant branch (TRGB) method. The estimated distance to NGC 404 using this technique is 10.0 ± 1.2 Mly (3.1 ± 0.4 Mpc). Averaged together, these distance measurements give a distance estimate of 10.0 ± 0.7 Mly (3.07 ± 0.21 Mpc) or approximately 10,300,000 light-years.

    Click on my name for the link to the Wikipedia article.


  6. Tom English

    I’ve heard this object referred to as “Komorowski’s Comet.” Supposedly an amateur astronomer happened upon it one night, and after consulting his sky atlas and not finding an object there, initiated contacts with professionals to confirm his “discovery.” I’ve forgotten the details of the story, but the school in NC where I used to teach owned one of Komorowski’s telescopes, and I was told the tale by members of an area astronomy club. What was most memorable about the story was the last line: “…later his wife shot him.” Presumably not about the comet misidentification.

  7. KC Says:
    I’ve heard NGC 404 referred to as the Phantom Galaxy

    So would that be the galaxy in George Lucas’ STAR WARS series? It is far, far away (at least *I* consider 11 million LY far, far away).


  8. the universe is violent and pretty. I like the name “Ghost of Mirach”, it does poetic justice (no, not THAT kind of poetic justice) to the beauty and mystery of space.

  9. I wonder what the sky would look like from a planet in a typical gas poor lenticular galaxy – something like the “Milky Way” but without the dark clouds obscuring most of the stars, perhaps? Could be quite impressive…

  10. Nice writeup Phil. I happened upon this galaxy while recalibrating my pointing system in late November, and as usual have to rush to finish processing.

    Here’s my take on Mirach’s Ghost

  11. Ted

    If only I had read this before LAST night. Was out in the back yard with binoculars hunting for M31 and Mirach happened to be at the exact zenith while I was observing. It made a nice starting point for hopping over to see the “cool” stuff.

    Looks like NGC 404 is 11th magnitude though, so no casual observing for me. Still pretty cool to know it’s there!

  12. Gary Ansorge

    I have a slight confusion. How do we tell the difference between a red shift due to velocity, as in for distant quasars and a red shift due to a strong Gravity field? From what I can recall of relativity theory, there should be no difference,,,

    GAry 7

  13. @ Gary Ansorge,

    This is an extract from Wikipedia:

    Gravitational Redshift vs. Gravitational Time Dilation

    When using special relativity’s relativistic Doppler relationships to calculate the change in energy and frequency (assuming no complicating route-dependent effects such as those caused by the frame-dragging of rotating black holes), then the Gravitational redshift and blue-shift frequency ratios are the inverse of each other, suggesting that the ‘seen’ frequency-change corresponds to the actual difference in underlying clock-rate. Route-dependence due to frame-dragging may come into play, which would invalidate this idea and complicate the process of determining globally-agreed differences in underlying clock rate.

    While gravitational redshift refers to what is seen, gravitational time dilation refers to what is deduced to be ‘really’ happening once observational effects are taken into account.

    (Click on my name for the link to the full article.)

  14. marko

    I have one of those old-fashioned bitmap editors where you can combine clippins with “xor”, “not or” operations. I’ve done this with these two images, and most stars in it really correspond on a per-pixel basis. Amazing, how the visible and UV version differ and at the same time resemble each other.

  15. So here’s the ghost of Mirach;
    An echo near a star.
    A fuzzy light that’s a galaxy
    Backgrounded, its light boneheaded from afar. (Yet relatively near.)

    The star, it is so ancient,
    A dying, glowing red
    The galaxy it hides is older even dead
    Lenticular its hidden shape
    Is crammed with just such stars,
    As that that’s blots its foreground place
    And binds it in our heads.


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