Abstract
We present Hubble Space Telescope imaging and
spectroscopy, along with supporting Galaxy Evolution
Explorer and ground-based data, for the extended
high-ionization cloud known as Hanny's Voorwerp, near
the spiral galaxy IC 2497. Wide Field Camera 3 images
show complex dust absorption near the nucleus of
IC 2497. The galaxy core in these data is, within the
errors, coincident with the very long baseline
interferometry core component marking the active
nucleus. Space Telescope Imaging Spectrograph (STIS)
optical spectra show the active galactic nucleus (AGN)
to be a type 2 Seyfert galaxy of rather low luminosity.
The derived ionization parameter log U = –3.5 is in
accordance with the weak X-ray emission from the AGN.
We find no high-ionization gas near the nucleus, adding
to the evidence that the AGN is currently at a low
radiative output (perhaps with the central black hole
having switched to a mode dominated by kinetic energy).
The nucleus is accompanied by an expanding ring of
ionized gas ##IMG##
http://ej.iop.org/icons/Entities/ap.gif ≈ 500 pc
in projected diameter on the side opposite Hanny's
Voorwerp. Where sampled by the STIS slit, this ring has
Doppler offset ##IMG##
http://ej.iop.org/icons/Entities/ap.gif ≈
300 km s –1 from the nucleus, implying a kinematic
age <7 × 10 5 years. Narrowband O III and Hα+N II
Advanced Camera for Surveys images show fine
structure in Hanny's Voorwerp, including
limb-brightened sections suggesting modest interaction
with a galactic outflow and small areas where Hα is
strong. We identify these latter regions as regions
ionized by recent star formation, in contrast to the
AGN ionization of the entire cloud. These candidate
"normal" H II regions contain blue continuum
objects, whose colors are consistent with young stellar
populations; they appear only in a 2 kpc region toward
IC 2497 in projection, perhaps meaning that the star
formation was triggered by compression from a narrow
outflow. The ionization-sensitive ratio O III /Hα
shows broad bands across the object at a skew angle to
the galaxy nucleus, and no discernible pattern near the
prominent "hole" in the ionized gas. The
independence of ionization and surface brightness (SB)
suggests that there is substantial spatial structure
which remains unresolved, to such an extent that the SB
samples the number of denser filaments rather than the
characteristic density in emission regions; this might
be a typical feature of gas in tidal tails, currently
measurable only when such gas is highly ionized. These
results fit with our picture of an ionization echo from
an AGN whose ionizing luminosity has dropped by a
factor >100 (and possibly much more) within the last
(1-2) × 10 5 years; we suggest a tentative sequence of
events in IC 2497 and discuss implications of such
rapid fluctuations in luminosity for our understanding
of AGN demographics.
Links and resources
Tags