Radio Galaxy Zoo Talk

Any idea what the four+ unmarked, narrow emission lines are?

  • JeanTate by JeanTate

    At ~907.6, 833.9, 669.1, and 429.7 nm. None correspond to any skylines (though several other, fainter, unmarked emission lines do).

    Posted

  • ivywong by ivywong scientist, admin

    Was talking to Zookeeper Kevin about this and he thinks that these could be unrelated emission lines from a confused background emission line galaxy perhaps? Maybe there were some lensing involved? Unfortunately we are speculating at this point because we will require higher resolution observations before we can constrain what is really going on.

    Posted

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

    Thanks ivywong.

    I don't think there's a second redshift system there; here's why:

    Take two emission lines, say [OIII] 5008.24 Å (vacuum wavelength) and Hα 6564.61 Å (ditto). Call the ratio of their rest (lab) wavelengths r, 1.31076 in this case.

    Now redshift them both, by z. The observed wavelengths will be the rest wavelengths multiplied by (1+z). The ratio of these two observed wavelengths will the same as for the rest/lab wavelengths, because the (1+z) factor is common, and so cancels out.

    So if two of the unmarked emission lines are [OIII] 5007 and Hα the ratio of their observed wavelengths will be ~1.3108 (there will be some uncertainty in estimating the observed wavelengths in the SDSS spectrum).

    What are the strongest emission lines, seen in SDSS spectra of galaxies and quasars? Assuming a redshift of < ~3, there are not many (rest, vacuum, wavelengths in brackets, in Å; source): Lyα (1215.24), C IV (1549.48), C III] (1908.734), Mg II (2799.117), [OII] (3727.092)*, Hβ (4862.68), [OIII] (5008.24), and Hα (6564.61).

    The ratios of pairs of these, sorted by size (up to ~3**; 4 decimal places), are: 1.0299, 1.3047, 1.3108, 1.3315, 1.3437, 1.3500, 1.4665, 1.5818, 1.7372, 1.7892, 1.8065, 1.9527, 2.3033, 2.3452, 2.4054, 2.5476, 2.6239, 3.0670.

    None of the ratios of the four prominent, unmarked lines in the SDSS spectrum of SDSS J103928.21+053613.6 (ignoring skylines) are close to any of these ratios.

    So perhaps they're artifacts?

    Here's the object itself, a pretty normal-looking elliptical:

    enter image description here

    *this is actually a doublet, not always resolved in SDSS spectra

    **this includes the range of all SDSS spectra

    Posted

  • ivywong by ivywong scientist, admin

    Fair argument but if the spectra we obtain consists of emission mainly dominated by the foreground object then most of the effects from the background galaxy will be washed out sufficiently that you won't see the difference in ratios especially if the background source does not have many of the strong lines you mentioned and/or if our sensitivity is insufficient.

    In any case, it's fair to treat them as unimportant at this stage.

    Posted