Very cool image, got me reading the Watkins et al. discovery paper which claims only a "double passage" for the galaxy interaction can explain the gas's distance and size. Apparently the ionization is a bit of a mystery, it's possibly like a Hanny's Voorwerp.
From Watkins et al. 2017 "The Cloud’s far northern location and high relative velocity provide the tightest constraints on the models - only the multi-passage model of Salo & Laurikainen (2000) reproduces any such gas. In this model, this gas is first elevated above NGC 5194’s disk plane in response to the companion’s initial passage, then is pulled downward along the line of sight by the second encounter.
While the Cloud may be tidally stripped, appealing to shock heating as its ionization source may also suffer from a timescale problem ... Alternatively, the high line ratios observed in the Cloud could be due to hard photoionization from either a central AGN or starburst driven winds. Similar to “Hanny’s Voorwerp” ... we find that the current nuclear X-ray luminosity of either galaxy is too faint by four orders of magnitude to account for the Cloud’s total LHα, implying that if the Cloud were ionized by AGN emission, the AGN has since faded significantly. This does not rule out past AGN activity, however, as the strength of such activity, and the jet angle, can evolve on <1 Myr timescales..."
Even the Deep Sky Collective astrophotography group's write-up mentions the high line-ratio: "Note that using wider bandpass Ha filters (5-7nm) will in fact be better than using a super narrow (3nm) one as they allow for Nii to pass through, which is roughly twice as bright as the Ha in these cliffs."
From Watkins et al. 2017 "The Cloud’s far northern location and high relative velocity provide the tightest constraints on the models - only the multi-passage model of Salo & Laurikainen (2000) reproduces any such gas. In this model, this gas is first elevated above NGC 5194’s disk plane in response to the companion’s initial passage, then is pulled downward along the line of sight by the second encounter.
While the Cloud may be tidally stripped, appealing to shock heating as its ionization source may also suffer from a timescale problem ... Alternatively, the high line ratios observed in the Cloud could be due to hard photoionization from either a central AGN or starburst driven winds. Similar to “Hanny’s Voorwerp” ... we find that the current nuclear X-ray luminosity of either galaxy is too faint by four orders of magnitude to account for the Cloud’s total LHα, implying that if the Cloud were ionized by AGN emission, the AGN has since faded significantly. This does not rule out past AGN activity, however, as the strength of such activity, and the jet angle, can evolve on <1 Myr timescales..."
Even the Deep Sky Collective astrophotography group's write-up mentions the high line-ratio: "Note that using wider bandpass Ha filters (5-7nm) will in fact be better than using a super narrow (3nm) one as they allow for Nii to pass through, which is roughly twice as bright as the Ha in these cliffs."
Statistics: Posted by florid_snow — Fri Dec 13, 2024 3:31 pm — Replies 2 — Views 215