Galaxy Zoo Talk

GZ 3D Blue Blob

  • Rick_Nowell by Rick_Nowell

    Reading a discussion in GZ 3D (link below), I looked for an alternative view for Subject 14715235 using the DECaLS browser and their SDSS selection. In the left of the two pictures below, there is a blue blob at the top at about 1 o'clock, which is shown in the right of the two pictures as a galaxy called SDSS J164657.81+390435.3. It is a GALEX UV source in the NED. Using the same DECaLS browser, the option 'MzLS+BASS DR6 images' has the object present.

    GZ 3D Subject 14715235

    DECaLS/SDSS J164657.81+390435.3

    SDSS J164657


  • JeanTate by JeanTate

    I think the SDSS name for the blue bar is SDSS J164657.81+390435.3. The DR13 zph is given as 0.089±0.0349, and its photometry indicates that it is indeed blue. Which is consistent with it being a GALEX source (both NUV and FUV).

    Without a spectrum, I think it's nigh on impossible to do more than reasonably speculate as to what this is. That said, here's my $0.02's worth:

    • it is likely near a distance consistent with its zph
    • the fact that it's not dramatically brighter (or fainter) in any of seven bands (FUV, NUV, u, g, r, i, and z; I guess similar for DECaLS) suggests that the colors are not primarily due to (narrow) emission lines
    • the fact that it appears blue throughout likely rules out an AGN as the primary source of the continuum radiation
    • so what's "extended blue" without dramatic emission lines?
    • widespread, fairly recent, star-formation

    So, a ~bulge-less, ~edge-on late-type spiral (Sd perhaps), with perhaps rather more ~recent star-formation than you might otherwise expect.

    What do you think?


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

    Good steer! Looking through Vroojes's Possible Bulgeless Collection some objects appear similar to the blue bar SDSS J164657.

    Certainly any of these three objects could be part of an answer:

    Image AGZ00054rp

    Image AGZ0006wwz

    Image AGZ0005f61


  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Should be helpful to get the DECaLS FITS files and try to get a cleaned-up image in Aladin Desktop.

    On a sidenote the greenish star in the GZ 3D target galaxy is an unreported #supernova.


  • vrooje by vrooje admin, scientist

    I agree it does look like a rotation-supported galaxy seen edge on... but it's also quite thick compared to make of the needle-like edge-on galaxies we usually see, and kind of squared off at the ends (truncated, perhaps). So it might be a dwarf galaxy that's still forming and hasn't yet made itself into a very thin disk. I agree spectra would probably be needed to get a better sense of it... and it would be interesting if we had very deep imaging to see if it had any interesting (but very faint, much too faint for SDSS) tidal features!