Radio Galaxy Zoo Talk

X-shaped source with unusual host galaxy

  • WizardHowl by WizardHowl

    As observed by zutopian, this appears to be an X-shaped source with host SDSS J081841.57+150833.5 at Z_sp=0.330. The host galaxy has an spectrum with an almost flat continuum and strong, broad emission lines which is classified as QSO starburst broadline, however the really unusual thing that struck me is that the H alpha lines shown in the interactive spectrum are split into two. I know a strong magnetic field can split lines but I don't remember seeing it on this scale and don't know why it would only show for H alpha.

    The host galaxy is extended in shape but it is not clear whether or not it is actually a disk. It has an obvious core but the object is fainter than I would expect for a QSO at this redshift and the spectrum does not show the continuum shape I would expect for a QSO, either. It is more like the spectrum of a starforming disk galaxy tilted a little so there is some absorption by dust at the blue end.

    Does anyone have any thoughts on what type of galaxy this might be and why the H alpha lines - and only the H alpha lines - are split?


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

    I agree that this is certainly a broadline spectrum! 😮 There is also a distinct narrow-line spectrum, esp the [OII] doublet at 3726. So we're looking down at the accretion disk, and have a clear view to both the broadline (BLR) and narrow-line (NLR) regions of the AGN. I don't see a split H-alpha; rather, a broadline [NII] 6583 (its fainter companion, [NII] 6548, is merely a slight bulge in the wings of the broad H-alpha). To me what's interesting is that several of the emission lines seem to be offset from where the estimated redshift says they should be; always to the red (the redshift estimate is, I understand, derived from the best cross-correlation fit to a template; specific lines are not used explicitly). Yet what few obvious absorption lines there are - Na, Mg, K (of the Ca HK doublet) - are not offset. Is this a hint of relative motion? The stars in the galaxy have a line-of-sight velocity component different from that of the NLR?

    If we're looking down, then the x-shape may be due to projection; it's two jets, but seen at high inclination.

    I agree that the galaxy looks disturbed, and that there are hints of a disk. Unfortunately, I'm not using my usual high-contrast monitor, so I can't really say more.

    At z=0.33, this isn't anywhere near bright enough to be called a QSO (these days "QSO" simply means an AGN brighter - in absolute magnitude terms - than a particular threshold).

    Very interesting find! 😄


  • ivywong by ivywong scientist, admin

    X-shaped sources are very unique and interesting class of objects but there aren't too many catalogued at the moment

    Halpha emission are typically observed near very strong AGN because typically, these AGN ionised the surrounding medium (consisting of Hydrogen) to ~ 10,000K so when we see Halpha emission in these cases, we are seeing the ionisation of the medium around the AGN. Halpha emission also arise from star-forming regions so the formation of the very large&hot O-type stars typically ionise their surroundings as well. Alternatively compact galaxy groups that are ripping each other apart can also shock ionise Hydroge 😃

    Hope this helps. 😃

    Thanks heaps again for all you dedicated RGZ-ites 😃