Issue |
A&A
Volume 505, Number 2, October II 2009
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|
---|---|---|
Page(s) | 559 - 567 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200912605 | |
Published online | 03 August 2009 |
Is cold gas fuelling the radio galaxy
NGC 315?![[*]](/icons/foot_motif.png)
R. Morganti1,2 - A. B. Peck3,4 - T. A. Oosterloo1,2 - G. van Moorsel4 - A. Capetti5 - R. Fanti6,7 - P. Parma6 - H. R. de Ruiter6,8
1 - Netherlands Foundation for Research in Astronomy,
Postbus 2, 7990 AA, Dwingeloo, The Netherlands
2 - Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The
Netherlands
3 - Joint ALMA Office, Av El Golf 40, piso 18,
Santiago 7550108, Chile
4 - National Radio Astronomy Observatory,
Socorro, NM 87801, USA
5 - Osservatorio Astronomico di Torino,
Strada Osservatorio 25, 10025 Pino Torinese, Italy
6 - INAF,
Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
7 - Dipartimento di Fisica dell'Università di Bologna, Via
Irnerio 46, 40126 Bologna, Italy
8 - Osservatorio Astronomico di
Bologna, Via Ranzani, 1, 40127 Bologna, Italy
Received 31 May 2009 / Accepted 24 July 2009
Abstract
We present WSRT, VLA and VLBI observations of the H I absorption in the radio galaxy NGC 315. The main result is that
two H I absorbing systems are detected against the central
region. In addition to the known highly redshifted, very narrow
component, we detect relatively broad (FWZI 150 km s-1)
absorption. This broad component is redshifted by
80 km s-1 compared to the systemic velocity, while the narrow absorption is
redshifted
490 km s-1. Both H I absorption components are
spatially resolved at the pc-scale of the VLBI observations. The
broad component shows strong gradients in density (or excitation)
and velocity along the jet. We conclude that this gas is
physically close to the AGN, although the nature of the gas
resulting in the broad absorption is not completely clear. The
possibility that it is entrained by the radio jet (and partly
responsible of the deceleration of the jet) appears unlikely. Gas
located in a thick circumnuclear toroidal structure, with
orientation similar to the dusty, circumnuclear disk observed with
HST, cannot be completely ruled out although it appears difficult
to reconcile with the observed morphology and kinematics of the
H I. A perhaps more likely scenario is that the gas producing the
broad absorption could be (directly or indirectly) connected with
the fueling of the AGN, i.e. gas that is falling into the nucleus.
If this is the case, the accretion rate derived is similar
(considering all uncertainties) to that found for other X-ray
luminous elliptical galaxies, although lower than that derived
from the radio core luminosity for NGC 315. The data also show
that, in contrast to the broad component, the density distribution
of the narrow component is featureless. Moreover, in the WSRT
observations we do detect a small amount of H I in emission a few
kpc SW of the AGN, coincident with faint optical absorption
features and at velocities very similar to the narrow absorption.
This suggests that the gas causing the narrow absorption is not
close to the AGN and is more likely caused by clouds falling into
NGC 315. The environment of NGC 315 turns out to be indeed quite
gas rich since we detect five gas-rich companion galaxies in the
immediate vicinity of NGC 315.
Key words: Galaxies: active - Galaxies: individual: NGC 315 - radio lines: Galaxies
1 Introduction
1.1 HI 21-cm absorption in low-luminosity radio galaxies
The channeling of the gas to the very inner regions of a galaxy is considered to be the mechanism that can transform a ``starving'' black hole into an active nucleus. Mergers and interactions can play an important role in supplying the fuel and providing the conditions for the gas to reach the center (Wilson 1996). However, the relationship with mergers is not one-to-one and recent studies of radio galaxies have shown that the activity in some of these galaxies may be associated instead with the slow accretion of (hot) gas (Best et al. 2005,2007; Croton et al. 2006). In the case of radio-loud AGN, the way the accretion of gas proceeds can have important implications in determining the characteristics of the radio source that is associated with the AGN. In particular, the difference between powerful, edge-brightened and low-luminosity, edge-darkened radio galaxies could reflect a change in the mode of accretion: advective low efficiency/rate flow in the latter (Allen et al. 2006; Balmaverde et al. 2008) and standard optically thick accretion disks in the former. While the presence of thick tori predicted by unified schemes of AGN (see e.g. Antonucci 1993) is relatively well established for powerful radio galaxies through detections of obscuring atomic or molecular gas, X-ray absorption, or free-free absorption, the structure of the very inner regions of low-luminosity radio galaxies is still less defined. The study of obscuration toward the central regions of these galaxies (see e.g. Chiaberge et al. 1999; Chiaberge et al. 2002; Worrall et al. 2003; Balmaverde et al. 2006) has shown that the nuclear disks in these galaxies are geometrically and optically thin. This suggests that the standard pc-scale geometrically thick torus is not present in these low-luminosity radio galaxies.
On the other hand, the presence of gas in the nuclear regions can also affect the evolution of the radio sources (and in particular the radio jet) and the way the radio sources grow. Interaction between powerful radio jets and their environment is recognized to produce fast outflows that can be relevant in the evolution of the host galaxy (see e.g. Morganti et al. 2005; Holt et al. 2008). Entrainment of gas by the radio jet could also be crucial in slowing down the jet from relativistic to sub-relativistic velocities (see Laing et al. 2006).
Thus, it is clear that determining the gas distribution and kinematics in the nuclear regions is of paramount importance for understanding radio galaxies in general. Among the different gas components, the neutral hydrogen is particularly suited for this task. Atomic neutral hydrogen has been observed in absorption against the radio continuum in the central regions of a number of radio galaxies (see e.g. Heckman et al. 1983; Shostak et al. 1983; van Gorkom et al. 1990; Morganti et al. 2001; Vermeulen et al. 2003; Gupta et al. 2006). In some cases, though clearly not all, this H I absorption has been interpreted as due to gas distributed in circumnuclear disks/tori although there are surprisingly few examples where the spatially resolved signature of a rotating disk is actually observed. However, H I absorption is not always found at the systemic velocity of the galaxy and, as mentioned above, more sensitive observations and detailed studies show that in many cases the kinematics of the gas can be disturbed and affected by the presence of the radio jets (Morganti et al. 2005 and references therein). Understanding the effect of the radio jets on this gas requires high spatial resolution observations that are at present quite scarce.
Here we present the results from our detailed study of H I in the radio galaxy NGC 315. This includes Westerbork Synthesis Radio Telescope (WSRT), Very Large Array (VLA), and Global Very Long Baseline Interferometer (VLBI) observations. We have looked at this galaxy as part of a larger project to study, at both low and high resolution, the presence and the characteristics of the neutral hydrogen of a representative sample of low-luminosity radio galaxies (see Morganti 2002 for some details). The results of this statistical study will be presented in a forthcoming paper.
1.2 Why NGC 315?
NGC 315 is a bright elliptical galaxy. An accurate systemic
velocity of 4942 6 km s-1 (
z = 0.01648
) has been derived from stellar absorption
lines by Trager et al. (2000). In the optical band,
NGC 315 shows a highly inclined, very regular, circumnuclear disk,
seen in absorption in the HST image (see Fig. 1). The
dusty disk has a position angle of about 40
and it extends
to
pc with a mass of
(Forbes 1991) calculated from the
flux following Knapp et al. (1990). At its center
there is an unresolved optical compact core (Capetti
et al. 2000,2002). A number of discrete
optical absorption clouds, west of the nucleus at distances
ranging from 3 to 7 arcsec, have been detected by Butcher
et al. (1980). A mass limit of
has
been derived for these clouds. The HST image shows additional
absorption clouds to the SW. The dust disk is associated with a
disk of ionized gas which appears to be in ordered rotation
(Noel-Storr et al. 2003). CO emission was
detected by Leon et al. (2003). The inferred mass of
molecular hydrogen is
.
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Figure 1: Top: Image of NGC 315 obtained from HST (Capetti et al. 2000). The optical core is clearly visible as well as the dust disk and dust patches. The approximate direction of the radio jet is also indicated. Bottom: same image but highpass filtered and slightly smoothed afterward to highlight the dust patches around the nucleus. |
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NGC 315 hosts a giant (1 Mpc) radio source that is also
known as B2 0055+30 (Bridle et al. 1979; Mack
et al. 1998; Laing et al. 2006 and
references therein). The source has a radio power of
W Hz-1 and edge-darkened lobes (i.e. with
Fanaroff-Riley type I structure, see Fig. 2). On VLBI
scales, the source shows a core, a bright jet, and a faint
counterjet (Cotton et al. 1999). The main jet is
quite smooth. A multi-epoch study shows evidence for moving
features, indicating an accelerating, mildly relativistic jet
(Cotton et al. 1999). The jets show deceleration at
larger distances and detailed modeling indicates that the angle of
the jet to the line of sight is
38
(Canvin
et al. 2005).
Table 1: Instrumental parameters of the H I observations.
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Figure 2: Radio continuum image of NGC 315 at different angular resolutions (see Table 1): WSRT ( left), VLA ( middle) and VLBI ( right). The images were obtained from the line-free channels, see text for details. Contour levels of the WSRT image -1, 1, 2, 4, 8, 12, 16, 32, 64 and 128 mJy beam-1. |
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The nuclear X-ray emission of NGC 315 is seen through a moderate
intrinsic column density of
cm-2 (Worrall et al. 2003). Chandra
typically finds column densities below 1022 cm-2 in FRI
radio galaxy nuclei (e.g. Worrall et al. 2001),
suggesting that the X-ray emission is associated with the
sub-kpc-scale radio jets.
NGC 315 has been observed in H I by different authors (Dressel
et al. 1983; Heckman et al. 1983;
Shostak et al. 1983; Chamaraux
et al. 1987). In these studies, a narrow and
highly redshifted (490 km s-1 with respect to the systemic
velocity) H I absorption component has been detected. This H I absorption splits in two components when observed with high
velocity resolution. These two components are very narrow
(velocity width
2.5 km s-1) and very deep (
= 0.87 and
0.21) and separated by only
3 km s-1. A possible detection of
an other broader H I absorption closer to the systemic velocity
was reported by Heckman et al. (1983).
2 WSRT and VLA observations and results
The H I observations of NGC 315 reported in this paper have been carried out with various angular resolutions. The different observations have different purposes. In particular, we wanted to study in detail not only the H I absorption, but also investigate the possible presence of H I emission in and/or around this galaxy (Emonts et al., in prep.).
2.1 Observations
The low-resolution WSRT observations were done in order to investigate the presence of neutral hydrogen, both in emission and absorption. The relatively low spatial resolution of the WSRT is ideal when looking for low surface brightness and extended structures. The parameters of the observations are summarized in Table 1. 3C 147 was used as flux and bandpass calibrator. The data were calibrated and reduced using the MIRIAD package (Sault et al. 1995). The continuum subtraction was done using a linear fit through the line-free channels of each visibility record and subtracting this fit from all the frequency channels (``UVLIN''). The spectral-line cube was obtained using robust weighting set to 0.5 (Briggs 1995), i.e. intermediate between natural and uniform weighting. The data were Hanning smoothed to suppress the Gibbs ripples produced by the strong narrow absorption present in this galaxy. The resolution is


VLA observations of NGC 315 were obtained as part of a larger project aimed at studying the possible relation between the presence of H I absorption, the characteristics of the nuclear dust disks (seen by HST, Capetti et al. 2000) and the presence of optical cores (see Morganti et al. 2002 for preliminary results). The VLA observations have been carried out using the A-array to profit from the highest possible resolution with this telescope. The parameters are summarized in Table 1. The data reduction was done in a similar way as for the WSRT observations, again using the MIRIAD package. The continuum (from the line-free channels) and the line cube were produced with uniform weighting.
2.2 The two H I absorption systems
The first result of these observations is that two H I absorbing systems are detected in NGC 315. Apart from the very narrow and redshifted component (see Fig. 3), we also clearly detect the broad component that Heckman et al. (1983) reported as a probable detection. It is worth mentioning that there are only two other cases known of radio galaxies where two clearly separated H I absorbing systems have been observed: NGC 1275 (3C 84, see van Gorkom et al. 1989 and references therein) and 4C 31.04 (Mirabel 1990; Conway 1996).
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Figure 3: H I profile (in optical depth) obtained from the WSRT (solid line) and the VLA (dashed) data. The two H I absorption systems are clearly visible. The systemic velocity from Trager et al. (2000) is also indicated. The differences in the profile, in particular for the narrow component, is likely due to the different velocity resolution of the two observations. The bump in the WSRT profile at velocities just below those of the narrow absorption, corresponds to the H I emission detected in NGC 315. |
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Figure 4:
Left:
Contours (red) of the integrated H I emission derived from the
WSRT observations, on top of an optical image obtained from the
DSS. Contour levels are 4, 8, 16 and
|
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The two components in NGC 315 are detected in all observations,
also at the VLBI scale (see Sect. 6). Figure 3 shows the
profiles of the two components. As already known (Dressel
et al. 1983; Shostak et al. 1983), we
find that the narrow H I absorption is highly redshifted (by
500 km s-1) compared to the systemic velocity. The narrow
absorption is known to have a FWHM of only 2.5 km s-1 (Dressel
et al. 1983). Note that, the observed width of the
narrow, redshifted H I absorption is limited by the low velocity
resolution (and Hanning smoothing) of our data, therefore
appearing much larger than then
2.5 km s-1 detected with high
velocity resolution by Arecibo (Dressel
et al. 1983). Therefore, our low velocity resolution
observations do not provide any new insight on this component. The
integrated absorbed flux that we derive (977 mJy km s-1) is also
consistent with that obtained from Arecibo (998 mJy km s-1).
The broad H I absorption, detected in both the WSRT and the VLA
observations, has a full-width zero intensity (FWZI) in velocity
of 150 km s-1 (FWHM
80 km s-1) and it is centered on
5020 km s-1. Although the broader component is located closer
to the systemic velocity than the narrow component, it is
nevertheless redshifted by about 80 km s-1. For this galaxy an
accurate systemic velocity derived from stellar absorption lines
is available (
km s-1; Trager
et al. 2000). This estimate of the systemic velocity
is not affected by the disturbed kinematics that the ionized gas
may have, thus the offset in velocity is significant.
The characteristics of both H I absorption components, when
accounting for the different velocity resolution, are not
different between the WSRT and the VLA data (see
Fig. 3), indicating that the structures that produce the
absorption have a size smaller than 1 arcsec (0.5 kpc) and
therefore the study of their morphology requires VLBI observations
(see Sect. 6).
The peak of the continuum emission is 531 mJy in the WSRT data,
while the peak absorption of the broad component is
4.8 mJy and the integrated absorbed flux that we derive
is
353 mJy km s-1. The resulting optical depth is
0.009
and the corresponding column density of
cm-2 where f is the covering
factor. Similar values for the optical depth are derived from the
VLA data.
2.3 H I emission and the environment
One of the aims of the low-resolution WSRT observations was to look for H I in emission in and around NGC 315. We do indeed find such emission in NGC 315 (Fig. 4 right), at the same location as the HST images show faint optical absorption features. The emission is detected at velocities from



Apart from the emission in NGC 315, a number of neighboring galaxies are detected in H I. NGC 315 is part of the Zwicky cluster 0107.5+3212 (Zwicky et al. 1961) which is located in the Perseus-Pisces filament. Garcia (1993) lists 18 galaxies as part of this group. We detect H I emission in 5 galaxies within a few hundred km s-1 from the redshift of NGC 315. The objects and their characteristics are listed in Table 2. All galaxies have an optical counterpart (two of them are 2MASS sources) and are not part of the list of Garcia (1993).
Table 2: H I emission from other galaxies in the field of NGC 315.
In Fig. 4, we give the total H I intensity image,
showing the five companions. The environment of NGC 315 turns out
to be quite gas rich, the total amount of H I within 400 kpc of
NGC 315 is
.
It is clear that NGC 315 is
the dominant galaxy of the group. In such an environment it is not
unlikely that a small, gas-rich companion has fallen into NGC 315.
3 VLBI observations
Earlier VLBI H I observations of NGC 315 have been presented by Peck (1999). However, in these observations the observing band was centered on the velocity of the narrow H I absorption, as the presence of a broad absorption component was not yet established at the time. The new VLBI observations presented here use a broader bandwidth and a central frequency that allows us to detect both H I components.
NGC 315 was observed with the Global VLBI Network in November
2001. We used a bandwidth of 8 MHz in 512 channels. The velocity
resolution, after Hanning smoothing, is 6 km s-1. The rms
noise is
0.26 mJy beam-1 in the continuum image and
0.9 mJy beam-1 in the line channels.
The continuum image obtained from the line-free channels (beam
mas p.a. =
using natural weighting) is
shown in Fig. 2. A strong core and a jet are
detected, consistent with previous observations (see e.g. the
detailed observations presented in Cotton et al.
1999). The position angle of the jet is about
43
,
consistent with that of the large-scale jet and
perpendicular to dust disk observed by HST (see
Fig. 1). The peak of the continuum is about
190 mJy beam-1. In the following sections we present the
results obtained from the VLBI line data.
4 The H I absorption at pc-scale and the origin of the atomic neutral hydrogen
Both the narrow and the broad H I absorption systems are detected and spatially resolved by the VLBI observations. As we discuss below, both the kinematics and the column density distribution are quite different for the two components, suggesting that the two absorption systems have very distinct origins.
4.1 The broad H I absorption
An interesting but puzzling result from the VLBI observations is
the morphology and kinematics of the broad H I absorption
(Figs. 5 left and 6). Some absorption is
detected against the core, but there is a strong gradient in
column density, the column density being much higher further down
the jet. Together with the density gradient, there is also a
velocity gradient along the jet, with the gas further away from
the nucleus having a larger redshift. The derived column density
has a peak of
cm-2 K-1 (where the filling factor f has been taken as
1 given that the absorption is resolved), while the column density
in front of the nucleus is about a factor 10 lower. Further down
the jet, no absorption is detected, but this is most likely due to
the limited sensitivity of the VLBI observations. Indeed, about
30% of the flux detected for the broad absorption in the WSRT and
VLA observations is missing in the VLBI observations.
![]() |
Figure 5: Continuum (contours) and column density (grayscale) of the broad ( left) and narrow ( right) H I absorption as derived from the VLBI observations. The intensity scales given on the right side of each panel are in units of 1020 cm-2. The contours of the continuum are 2, 4, 8, 16, ... mJy beam-1. |
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Figure 6:
Position-velocity plot taken along the radio jet (p.a.
|
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One can attempt to make a rough estimate of the mass of the H I producing the broader absorption. Taking an average column density
of
cm-2 (for a canonical
K) and an area of
parsec (corresponding to
mas), the mass enclosed would be about 100
.
This number is most likely a lower limit because of a number of
uncertain parameters. Most uncertain is perhaps the size of the
gas cloud: part of the flux is missing and, therefore, the
absorption is likely to be more extended than can be recovered
from VLBI observations. In addition, of course, the H I detected
is only that part of the gas cloud that is in front of the jet.
Moreover, the assumed value for the
is uncertain.
The gas producing the H I absorption may be located very close to
the nucleus (see next section) and, therefore, the assumption of
K could be unrealistic. The
might well be at least a factor 10 higher (see also results on
PKS 1549-79 by Holt et al. 2006). X-ray observations
suggest that the AGN is seen through a medium with a column
density of
cm-2 (Worrall
et al. 2003), indeed suggesting that the spin
temperature is substantially higher than 100 K. Therefore,
1000
is a more likely lower limit to the H I mass. In
this context, it is quite conceivable that the observed gradient
in column density more reflects a gradient in
and
not a gradient in the actual density.
4.2 Nature of the cloud producing the broad H I absorption
The strong gradients in the H I column density and velocity of the broad absorption suggest that the gas is being influenced by the presence of the AGN, indicating that this gas is physically close to the core of NGC315. Moreover, as has been noted about broad H I absorption in other objects (see for example the case of Cygnus A described in Conway & Blanco 1995), a major problem of broad absorption being due to a chance superposition of a foreground cloud (e.g. similar to the dust patches SW of the core seen in the optical) is the large velocity width and gradient of the broad H I absorption. For clouds in the ISM of galaxies, even if one considers clouds that are ``wreckage'' of some accretion event, one would expect to observe velocity gradients much smaller than the 100 km s-1 over only 7 pc observed for the broad absorption.
However, although it is likely that the broad absorption occurs
close to the AGN, the nature of the gas resulting in the broad
absorption is not clear. One interpretation which may fit the data
is that of a circumnuclear torus. A configuration in which we are
looking through a section of a thick rotating torus comprised
mainly of atomic gas would result in a broad line width. In cases
where the radio source axis is in the plane of the sky and our
line of sight passes through the midplane of the torus, even
broader lines have been found, and in almost all cases detected to
date, absorption features attributable to a circumnuclear torus
are redshifted with respect to the systemic velocity of the host
galaxy (e.g. PKS 2322-123, FWHM 700 km s-1 redshifted
by
220 km s-1; Taylor et al. 1999). This
is probably due to streaming motions within the structure toward
the accretion disk and central supermassive black hole.
In a source which has a jet axis close to the plane of the sky, we would expect to see absorption from a torus toward the inner jets on both sides of the core (e.g. Peck et al. 1999), with a smaller optical depth possible toward the core due to much higher spin temperatures near the AGN, as described above. However in a radio source which is oriented more toward our line of sight, such as NGC 315, the counterjet is Doppler-dimmed, and the plane of the disk is not aligned with our vantage point. Thus we do not necessarily expect to have the sensitivity to detect absorption toward the inner counterjet, as the continuum source is very faint, and we are looking along a line of sight from the top through the middle of the torus, where much of the gas may lie in the region of higher spin temperature and thus yield a lower optical depth.
Where the circumnuclear torus model struggles in this source, however, is in explaining the compact region of higher column density shown in Fig. 5. Some circumnuclear tori have been shown to be clumpy on similar scales (Peck & Taylor 2001), but not with this range of density. Additionally, it would be hard to explain the velocity gradient along the jet unless this might be due to the gas above the midplane of the torus streaming down and inward toward the central mass. Such a scenario would result in an inward component of the velocity vector which would point more directly away from our vantage point as one went higher in scale height, yielding a larger redshift. The gas further down the jet would also have a component of inward streaming motion in addition to rotation, but given the orientation of the torus, these motions would not be directly away from us, and thus would not give rise to as large a redshift. Alternatively, one might explain both the compact region of high column density and the cospatial region of higher redshift with a discrete cloud falling toward a circumnuclear structure, but we do not have the spatial or velocity resolution to justify this level of detail in the model. Thus at present, we cannot rule out the possibility of a circumnuclear toroidal structure, but more corroborating data would be required before we could adopt this model.
Another possibility is that the broad absorption is due to gas entrained in the jet. NGC 315 does contain a number of small gas/dust clouds (see below), so it is, in principle, possible that one of these is now interacting with the jet. A structure of (molecular) gas aligned with radio jet has been found in M51 (Matsushida, Muller, & Lin 2007). Also in that object, a velocity gradient with blueshifted velocities closer to the nucleus has been observed. The authors argue that the observed structure and velocity gradient are due to molecular gas entrained by the radio jet. Naively, one would however expect that if gas is entrained in a jet, the velocities would appear more blueshifted (i.e. coming toward the observer dragged by the jet) as one goes along the jet, opposite to these observations. We note in passing that the broad absorption is redshifted with respect to the systemic velocity, but that it is blueshifted relative to the narrow absorption. If the velocity of the narrow absorption is representative for small clouds falling into NGC 315, the jet would then be pushing the gas out with (projected) blueshifts of about 400 km s-1. One complication is that it would be have to be a coincidence that the velocity of the outflowing gas ends up to be close to the systemic velocity of the galaxy. It would also still not explain the velocity gradient along the jet.
Finally, let us consider the possibility that the broad absorption
is somehow related to the process of providing (directly or
indirectly) material for fueling the activity in the nucleus,
i.e. gas (not necessarily part of a circumnuclear structure) that
is falling into the nucleus. One can estimate, albeit with
considerable uncertainty, the mass accretion rate that would be
associated to this process, simply from the H I mass and the
timescale for falling in. For these calculations it is, however,
important to keep in mind (as described in details above) that the
H I detected in absorption may be just a small fraction of the
H I present around the nucleus. This is because we are limited to
the gas in front of the radio continuum. This means that the
numbers derived here (in particular the accretion rate) can be
quite conservative lower limits. Taking 50 km s-1 as the infalling
velocity of the gas, the timescale would be
yr. This would give accretion rates of at least
10-4
yr-1 and more likely
10-3
yr-1, if we adopt the mass of the gas
estimated above with the assumption for
K.
An accretion rate of 10-3
yr-1 is very similar
to those derived for other X-ray luminous elliptical galaxies,
based on Chandra observations (Allen et al. 2006). On
the other hand, using the luminosity of the radio core, one can
estimate the power of the jet and from that the accretion rate
(see Balmaverde et al. 2008). Starting from a
radio core flux of 210 mJy, we find
erg s-1 Hz-1. Using the relationship between
core luminosity and jet power (Heinz et al. 2007) this
corresponds to
erg s-1 Hz-1. This can be converted in an
accretion power of
erg s-1 Hz-1 (Balmaverde
et al. 2008) implying a mass accretion rate
yr-1. This accretion rate is
quite high when compared to e.g. the sample in Allen
et al. (2006). This high feeding rate is consistent
with the high core power in NGC 315 relative to the galaxies in
that sample. This would suggest that in the case of NGC 315, the
hot gas dominates the accretion, unless the physical size of the
cloud responsible for the broad absorption is much larger (a
factor 100) than what we have assumed.
It is worth mentioning that in the case of the powerful radio galaxy Cygnus A, a cloud of molecular gas has been detected along the radio jet. This cloud (that has a velocity redshifted compared to the systemic velocity of the host galaxy) has been interpreted as falling into the nucleus and perhaps connected to fueling the AGN (Bellamy & Tadhunter 2004). These authors argue that the connection may also be indirect, with the process of capturing the cloud and the subsequent settling into the circumnuclear disk leading to radial motions in the disk and increased fueling rate. The tentative detection of an H I counterpart to this cloud in Cygnus A (Morganti et al. in preparation) makes the similarity to NGC 315 more evident.
4.3 The narrow H I absorption
Like the broad absorption, the narrow H I absorption is also
spatially resolved in the VLBI observations. This absorption
component covers about 20 mas of the source, 9 pc, from the
core to the first part of the jet (see Fig. 5 right).
The narrow absorption is different from the broad one in all
respects. The column density distribution is much more uniform,
the velocity is much narrower and no velocity gradient is
observed. The fact that, contrary to what is seen for the broad
absorption, the properties of the narrow absorption do not show
any relation with the location of the AGN, or gradient along the
jet, suggests that the cloud producing the narrow absorption is
not physically close to the AGN. The very narrow velocity width
also supports this (see also Dressel 1983).
Moreover, the narrow absorption is close in velocity (and perhaps
also in space, i.e. at large radii) to the H I emission detected
in the WSRT observation. This suggests that the narrow absorption
is due to a cloud similar to those seen in optical images as
absorbing clouds out to several kpc SW of the nucleus
(Fig. 1). Using HST data, de Ruiter et al.
(2002) derive a typical gas column density of
cm-2 for the gas clouds seen in NGC 315, a
value very similar to that found in our radio observations for the
H I in emission. We obtain the same column density for the narrow
absorption if we assume
K. This could further
support the idea that the gas is at larger radius since such value
for
is characteristic for the ISM not affected by
the active nucleus. If these clouds are the result of a small
accretion event, one would indeed expect a redshifted velocity
(Dressel 1983). Our low-resolution observations have
shown that the environment of NGC 315 contains several small,
gas-rich galaxies, so accretion of a small companion by NGC 315 is
likely to have happened.
There are only two other cases known of radio galaxies where two such distinct H I absorbing systems have been observed. NGC 1275 (3C 84, van Gorkom et al. 1989 and references therein) and 4C 31.04 (Mirabel 1990). In both cases, a redshifted narrow component has been observed in addition to a broad closer to the systemic. For both objects, the redshifted absorption has been explained as being due to material falling into the main galaxy and it appears that the same explanation also applies to NGC 315.
5 Summary
We have studied the properties of the H I in the radio galaxy
NGC 315. Two H I absorption components are present, a broad one
(FWZI 150 km s-1) redshifted (
80 km s-1) with
respect to the systemic velocity, and a very narrow component
(FWZI
8 km s-1). Interestingly, the two absorption
components have very different properties. We also detect H I emission in NGC 315, a few kpc SW of the nucleus. This emission is
likely associated with absorption patches that are observed in
optical images.
The broad absorption shows a strong gradient in column density (or spin temperature) along the jet, with the highest densities (or lowest spin temperatures) furthest away from the AGN. It also shows a strong velocity gradient (more than 100 km s-1 over 10 pc) with the more redshifted velocities away from the AGN. The properties of this broad component strongly suggest that the gas producing the absorption is physically close to the AGN. The possibility that it is entrained by the radio jet (and partly responsible of the deceleration of the jet) appears unlikely because of the redshifted velocities of the gas. Gas located in a thick circumnuclear toroidal structure, with orientation similar to the dusty, circumnuclear disk observed with HST, cannot be completely ruled out although it appears difficult to reconcile with the observed morphology of the absorption and it would require inward streaming motion in addition to rotation. The scenario we favor is that the gas producing the broad absorption could be (directly or indirectly) connected with the fueling of the AGN, i.e. gas that is falling into the nucleus. If this is the case, the accretion rate derived is similar (considering all uncertainties) to that found for other X-ray luminous elliptical galaxies, although lower than that derived from the radio core luminosity for NGC 315.
On the other hand, the properties of the narrow absorption are very uniform. Moreover, it tightly connects, in space and in velocity, with the H I emission in NGC 315. Most likely, the cloud responsible for the narrow absorption is quite far from the AGN and is likely due to material falling into NGC 315. Five nearby, small gas-rich companions are also detected in H I. This implies that the environment of NGC 315 is quite gas rich and that accretion of gas from the environment is quite likely.
Acknowledgements
We would like to acknowledge Natascha Boric. Part of this work was done during her ASTRON/JIVE Summer Student project 2002. We would like to thanks Jesús González for further inspecting the optical spectral of NGC 315 and provide us the most accurate value of the systemic velocity. The Westerbork Synthesis Radio telescope is operated by the ASTRON (Netherlands Institute for Radio Astronomy) with support of the Netherlands Foundation for Scientific Research (NWO). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research has made use of the NASA Extragalactic Database (NED), whose contributions to this paper are gratefully acknowledged. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166. The European VLBI Network is a joint facility of European, Chinese, South Africa, and other radio institutes funded by their national research councils.
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Footnotes
- ...
NGC 315?
- Based on observations with the Westerbork Synthesis Radio Telescope (WSRT), the Very Large Array (VLA) and Very Long Baseline Interferometer (VLBI).
- ... 0.01648
- Throughout
this paper we use a Hubble constant
km s-1 Mpc-1 and
and
. At the distance of NGC 315 this results in 1 arcsec = 0.335 kpc.
All Tables
Table 1: Instrumental parameters of the H I observations.
Table 2: H I emission from other galaxies in the field of NGC 315.
All Figures
![]() |
Figure 1: Top: Image of NGC 315 obtained from HST (Capetti et al. 2000). The optical core is clearly visible as well as the dust disk and dust patches. The approximate direction of the radio jet is also indicated. Bottom: same image but highpass filtered and slightly smoothed afterward to highlight the dust patches around the nucleus. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Radio continuum image of NGC 315 at different angular resolutions (see Table 1): WSRT ( left), VLA ( middle) and VLBI ( right). The images were obtained from the line-free channels, see text for details. Contour levels of the WSRT image -1, 1, 2, 4, 8, 12, 16, 32, 64 and 128 mJy beam-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: H I profile (in optical depth) obtained from the WSRT (solid line) and the VLA (dashed) data. The two H I absorption systems are clearly visible. The systemic velocity from Trager et al. (2000) is also indicated. The differences in the profile, in particular for the narrow component, is likely due to the different velocity resolution of the two observations. The bump in the WSRT profile at velocities just below those of the narrow absorption, corresponds to the H I emission detected in NGC 315. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Left:
Contours (red) of the integrated H I emission derived from the
WSRT observations, on top of an optical image obtained from the
DSS. Contour levels are 4, 8, 16 and
|
Open with DEXTER | |
In the text |
![]() |
Figure 5: Continuum (contours) and column density (grayscale) of the broad ( left) and narrow ( right) H I absorption as derived from the VLBI observations. The intensity scales given on the right side of each panel are in units of 1020 cm-2. The contours of the continuum are 2, 4, 8, 16, ... mJy beam-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Position-velocity plot taken along the radio jet (p.a.
|
Open with DEXTER | |
In the text |
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