Powerful Outflows In Seyfert Galaxies: Wind- vs. Jet-Driven Models
Drake University Department of Physics & Astronomy
Kory Kreimeyer, Deanna Berget, Julie Leifeld, Jordan Mirocha, Charles Nelson
Abstract
Analysis
Seyfert galaxies are active galaxies characterized by luminous outflows from a
central super massive black hole. The outflow mechanism has not been fully
determined, but the two primary competing systems are a wind-driven conical
outflow and a jet-driven cylindrical outflow. We are developing models with a
variety of parameters, including inclination, cone opening angle, etc. By varying
these parameters, we can add detail to the model to more completely match the
observed velocities. We compare the models to the Hubble Space Telescope
long-slit spectra of the Seyfert galaxy NGC 1068 in an effort to more fully
understand the actual outflow mechanism of these galaxies.
Background
Kinematical Modeling
To analyze the HST data, we used the software package IRAF (Image Reduction and Analysis Facility),
which is widely used for data analysis in astronomical research. The spectra consist of multiple image
files, each providing spectral and positional data for one of the slits (Figure 3). Although the Hβ and 4959
lines are weaker, they share the same velocity structure, seen as the varying horizontal displacement of the
features at different positions along the slit. Rightward indicates a redward shift, or recessional velocity,
which can be observed below the center of the image for all three emission lines. Above the center, all
emission lines have changed to exhibit large blue shifts. The narrow vertical emission above the main
structure is not associated with the NLR but arises from gas in the disk of the host galaxy. Using the
“splot” task in IRAF, we produced plots of wavelength vs. flux (not shown) for each position along the slit.
Gaussian fitting and statistical analysis of the [OIII] lines then allowed for the determination of gas cloud
velocities at each point along the slit.
While the black hole accretion disk model applies for all AGN, the unique
appearance of Seyfert galaxies is due to the presence of two additional areas
within the nucleus. The innermost region surrounding the black hole is known as
the broad line region (BLR), from which we observe broad emission lines due to
the large velocity dispersion near the continuum source. This area is surrounded
by a dense torus of gas and dust, which obscures our view of the BLR if seen
edge on. Even if this is the case, the narrow line region (NLR) will still be
visible. This region consists of gas clouds lying in the path of the nuclear
outflows, perpendicular to the plane of the accretion disk and the BLR. In type I
Seyfert’s both of these regions are visible, while in type II’s only the NLR is
observable. Type I and type II Seyfert galaxies are thought to be the same
phenomena, with the only difference being orientation with respect to earth’s line
of sight (Figure 1).
NGC 1068
NGC 1068 is a spiral
galaxy located in the
constellation Cetus, and is
the prototypical type II
Seyfert galaxy (narrow
emission lines only). It
lies at a distance of about
45 million light years,
which allows for high
resolution study of the
central regions. At this
distance, one arcsecond
corresponds to about 220
light years.
Fig. 1 – Seyfert Galaxy Model
Observations
Long slit spectroscopy is the primary data collection technique for the study of
active galactic nuclei. Our spectra were taken with the Space Telescope Imaging
Spectrograph (STIS) which is one of the instruments onboard Hubble Space
Telescope (HST). For NGC 1068, five slit positions were placed over the central
region of the galaxy, positioned strategically in order to capture as many features
of the narrow line region as possible (Figure 2). Each slit preserves the vertical
spatial dimension, while providing spectral data for each point along its length
(Figure 3). Three prominent AGN emission lines lie within this spectral range:
one hydrogen Balmer line, Hβ (4861Å), and two lines from doubly-ionized
oxygen, [OIII] (4959 Å) and [OIII] (5007 Å). In the vertical dimension, each
pixel corresponds to 0.05 arcseconds and so provides good spatial resolution. In
the end there are five data files for NGC 1068, each with spectral and positional
data of clouds in the galaxy’s narrow line region.
Fig. 2 – Narrow Line Region of NGC 1068 shown
in [OIII].
We have developed a computer model to produce theoretical images in an identical form to
the observed spectrum (Figure 3), but based on an idealized velocity field. As mentioned
previously, the two competing outflow theories are a wind-driven conical system and a jetdriven cylindrical system, and these two mechanisms are the primary study of the model.
By defining a perfectly conical or perfectly cylindrical outflowing velocity field, it is
possible to observe the associated features of each system. In addition, the model allows
for the variation of a number of parameters, including inclination toward observer, cone
opening angle, maximum outflow velocity, and velocity-distance relation. To create the
pictures below, we used input parameters obtained from previous studies. The opening
angle used was 65º, from Evans et al. (1991). Also, our inclination angle is 15º toward
Earth, as given by Cecil et al. (1990).
Two wind-driven conical outflow models are presented below. Figure 4 shows a
constant conical outflow, while Figure 5 depicts a velocity fall-off with increasing distance
from the center. Similarly, the lower two pictures show jet-driven cylindrical outflow
models. Figure 6 is a cylindrical constant velocity field, and Figure 7 involves a decreasing
velocity with increasing distance from the jet axis.
Note that the model produces an image of only the brightest emission line ([OIII]
5007 Å). The other lines exhibit identical velocity structure and could be creating by
simply shifting and scaling the 5007 Å line.
Fig. 3 – STIS spectrum: from left to right, Hβ (4861Å),
[OIII] (4959 Å), [OIII] (5007 Å)
Understanding The Model
Fig. 4 – Wind model with constant
velocities
Fig. 5 – Wind model with velocities
decreasing as r-1/2
The model begins by laying down a 3-dimensional grid of volume elements called “voxels.” At each of these points in
space, it determines the magnitude and direction of the velocity vector caused by the particular choice of velocity field,
either conical or cylindrical. It also assigns an intensity and a velocity spread to this point. The velocity vector then is
inclined to the line of sight, and the final radial velocity is used as the center of a velocity-intensity Gaussian distribution.
Each voxel has a corresponding Gaussian distribution, but to simulate the actual observations, all depth positions along the
same line have to be added together. This allows for multiple components of velocity within one line, something seen
frequently in the actual data.
It is important to note that the model does not allow for a partially filled NLR. Any point that lies within the desired
spatial region is given an intensity; however, we know the NLR is not completely filled with gas and does contain regions
of empty (and therefore dark) space. Obscuration by dust clouds within the emitting region produces a similar effect. This
was an acceptable omission at this time, since our main interest is in determining what observations could be explained by a
conical or cylindrical velocity field.
Fig. 6 – Jet model with constant velocities
Discussion
The differences in the two models can be characterized in the following way. The jet models show line-splitting into blue
and red shifted components at all points along the slit, since we are seeing emission from both approaching and receding
gas at any given location. The wind models show blue shifts towards the top of the slit and red shifts toward the lower part
of the slit, as expected for radial outflow in a cone slightly inclined to the line of sight. Although correspondence between
the observed spectrum and our simple models is poor, the overall velocity gradient in the data is best matched by the wind
models with constant velocity. However, there does seem to be some line-splitting in the data, albeit from fainter emitting
regions. This indicates that though the wind may dominate, both wind and jet forces may be influencing the gas dynamics
of the NLR. More detailed modeling, involving varying the intensity, the influence of dust and filling factor, will reveal
more information about the ionized gas flow.
Fig. 7 – Jet model with velocities decreasing
as r-1/2
Summary
Our analysis of the gas cloud kinematics in NGC 1068 has provided evidence of powerful outflows within the
galaxy’s nuclear regions. In addition to revealing the magnitude of nuclear outflows, the STIS spectra
provide detailed information about the narrow line region’s velocity distribution. Our models attempt to
recreate these high velocity outflows by defining a perfectly conical or perfectly cylindrical velocity field,
representing a powerful galactic wind or jet-gas interaction, respectively. Although wind models seem to be a
better match to the data, there is evidence that both processes are at least partially responsible for the
observed kinematics in the narrow line region. To test if one mechanism truly dominates, a detailed mapping
of the gas cloud velocity and brightness distribution in the region will be required. In collaboration with Dr.
Tim Urness, we plan to use computer data visualization techniques to facilitate this analysis.