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Thread: Absolute magnitude of the Milky Way: what is it?

  1. #1
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    Absolute magnitude of the Milky Way: what is it?

    This University of Arizona webpage has a nice, detailed set of tables (and more) giving the absolute magnitude of the Sun.

    Which got me wondering, what is the absolute magnitude of our Milky Way galaxy (MW)? In various wave-bands (a.k.a. filters)? And how was such a thing estimated anyway, given that we're deep inside the MW? Also, wouldn't the apparent magnitude of the MW (from which the absolute magnitude is estimated) depend on the viewing angle (at least in bluer bands); you know, inclination, the disk's dust lanes, etc?

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    From Lord Google:

    Absolute magnitude is a measure of the luminosity of a celestial object, on a logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles.

    I am not sure the Milky Way (or any galaxy or star cluster) can have an absolute magnitude, as the distance required for measuring is far too short to make the measurement meaningful. If I am proven wrong in this, I will admit my error and go sulk.

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    Quote Originally Posted by Roger E. Moore View Post
    From Lord Google:

    Absolute magnitude is a measure of the luminosity of a celestial object, on a logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly 10 parsecs (32.6 light-years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles.

    I am not sure the Milky Way (or any galaxy or star cluster) can have an absolute magnitude, as the distance required for measuring is far too short to make the measurement meaningful. If I am proven wrong in this, I will admit my error and go sulk.
    We don't need to observe an object from 10 parsecs to determine its luminosity. We measure the amount of light from whatever distance and calculate how bright a compact object emitting the same total amount of light would appear at 10 parsecs. The latter is by standard definition the absolute magnitude. Of course we will not get that much light from a galaxy if we are 10 parsecs from any part of it, because most of it will be much farther away.

    From published distance and apparent magnitude information, my estimate for M31 is about -21. The Milky Way should be close to that.

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    According to the Annual Reviews of Astronomy and Astrophysics, 2016, the absolute visual magnitude of the Milky Way Galaxy is -21.37. See the table on page 534.

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    Quote Originally Posted by SagittariusAStar View Post
    According to the Annual Reviews of Astronomy and Astrophysics, 2016, the absolute visual magnitude of the Milky Way Galaxy is -21.37. See the table on page 534.
    This seems to be in the paper in an Annual Reviews document, by Joss Bland-Hawthorn and Ortwin Gerhard: "The Galaxy in Context: Structural, Kinematic, and Integrated Properties" (link). Unfortunately, for me, this is paywalled. Does anyone have a way to access the content, for free?

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    One thing I'm quite curious about: to what extent is viewing angle (or inclination) considered, when estimating a galaxy's absolute magnitude?

    Relevant for ellipticals, but particular so for spirals, Sb and Sc (and SBb and SBc), because they contain lots of dust. Which selectively absorbs shorter wavelengths ("bluer") and re-radiates it (predominantly) in the FIR.

    Do astronomers try to "correct" for inclination? If so, how?

    Clearly this effect is far greater on absolute magnitude estimates in a given band (say, u or i) than for absolute bolometric estimates (where there is an attempt to add up all the emitted light).

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    Quote Originally Posted by Jean Tate View Post
    One thing I'm quite curious about: to what extent is viewing angle (or inclination) considered, when estimating a galaxy's absolute magnitude?

    Relevant for ellipticals, but particular so for spirals, Sb and Sc (and SBb and SBc), because they contain lots of dust. Which selectively absorbs shorter wavelengths ("bluer") and re-radiates it (predominantly) in the FIR.

    Do astronomers try to "correct" for inclination? If so, how?
    (1) Yes.

    (2) That depends... The effect of dust on galaxies' emerging radiation has been (off and on) a major topic (overlapping galaxy system...). Working out how to correct for these effects requires understanding the sample selection in considerable detail (as Dave Burstein once said in exactly this context, a sample of a million galaxies gives no statistical power if the selection is biased). Broadly, the limits are (a) galaxies with no internal dust have the same apparent luminosity viewed from all inclination angles, and (b) opaque galaxies have the same surface brightness wen viewed form ay direction, as we see only an outer layer at each point (and the depth of that layer is wavelength-dependent). In practice, we see something in between. One can find various prescriptions, sometimes explicitly including bulge properties as less affected by dust concentrated to the disk. For example, there is a fairly elaborate prescription in the Third Reference Catalog of Bright Galaxies from de Vaucouleurs and colleagues used to derive inclination-corrected total magnitudes.

    <Hobbyhorse alert>
    A question now being addressed is how much the dust effects can be estimated from other galaxy properties, and how much scatter there is for, say, fixed Hubble type and luminosity. For example, resonance rings draw the ISM including dust from a broader annulus into a narrow ring, producing a nearly transparent zone on one or both sides. As an off-the-wall example, this ringed spiral in Abell 2218 is backlit by a gravitationally-lensed arc, showing how much light comes all the way through the disk immediately inside and outside the ring's spiral segments of dust.
    Attached Images Attached Images

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    Quote Originally Posted by Jean Tate View Post
    This seems to be in the paper in an Annual Reviews document, by Joss Bland-Hawthorn and Ortwin Gerhard: "The Galaxy in Context: Structural, Kinematic, and Integrated Properties" (link). Unfortunately, for me, this is paywalled. Does anyone have a way to access the content, for free?
    Yes. Abstract and pdf (large).

    https://arxiv.org/abs/1602.07702
    https://arxiv.org/pdf/1602.07702.pdf

    Also, I admit to being wrong about the absolute magnitude of a galaxy. New one on me.

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    Thanks ngc3314 and Roger E. Moore.

    Fleshing out SagittariusAStar's post, the table is Table 2, "Global magnitudes, colour indices and mass-to-light ratios for the Galaxy." which gives Absolute magnitudes, Color indices, and Mass-to-Light ratios in ten bands (SDSS's u, g, r, i, and z; and U, B, V, R, and I). There are some very important words attached to the table; "a" is a footnote on Absolute magnitudes, and "b" on Color indices:

    Magnitudes and colours derived for the Galaxy from Milky Way analogues drawn from the sdss survey using the Kroupa initial mass function. All values assume R0 = 8.2 kpc and Rd = 2.6 kpc for the Galaxy.
    aThe sdss and Johnson photometry are calibrated (typical errors ∼ 0.1 mag) with respect to the AB and Vega magnitude systems, respectively;
    bDifferent calibration schemes are needed for the sdss total magnitudes and the unbiassed galaxy colours which leads to inconsistencies between magnitude differences and colour indices (courtesy of Licquia, Newman & Brinchmann 2015).
    As I haven't yet read the Bland-Hawthors&Gerhard paper, I don't yet know how they deal with inclination effects on estimates of absolute magnitude.

    Another factor: an AGN. Our galaxy's is (currently) pretty quiet, but in other galaxies the AGN can be the dominant source of light. And viewing angle can really make a difference, at least in optical bands: the same galaxy can look like a BL Lac object or merely a type 2 QSO.

  10. #10
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    So, we are in Milky Way disc - how much would Milky Way be brighter for an observer near axis?

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