Arp102B is a nearby radio galaxy which displays the presence of double peaked
Balmer emission lines. Sub-arcsec Keck mid-infrared imaging and Spitzer
spectroscopy reveal a spatially compact mid-infrared source which displays
tentative evidence for variability. The F$_{\nu}\propto\nu^{-1.2}$ spectral
energy distribution is suggestive of an advection dominated accretion flow. The
absence of dust features over the 5-40 micron range make it unlikely that
thermal dust emission dominates the mid-infrared luminosity. We also detect the
presence of molecular hydrogen in emission which is asymmetrically redshifted
by ~500-1000 km/s from the systemic velocity of the galaxy. Since the
forbidden, low ionization lines in this galaxy are at the systemic velocity, we
suggest that the molecular hydrogen emission arises from a rotating molecular
gas structure surrounding the nuclear black hole at a distance of ~1 pc.Comment: 4 pages, 3 figures, Conference proceedings to appear in "The Central
  Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San
  Francisco: ASP

Chary, Ranga-Ram

English

arXiv

Arp 102B is a nearby radio galaxy which displays the presence of double-peaked Balmer emission lines. Sub-arcsec Keck mid-infrared imaging and Spitzer spectroscopy reveal a spatially compact mid-infrared source which displays tentative evidence for variability. The F_v ∝ ν^(−1.2) spectral energy distribution is suggestive of an advection-dominated accretion flow. The absence of dust features over the 5−40 μm range make it unlikely that thermal dust emission dominates the mid-infrared luminosity. We also detect the presence of molecular hydrogen in emission which is asymmetrically redshifted by ~500-1000 km/s from the systemic velocity of the galaxy. Since the forbidden, low-ionization lines in this galaxy are at the systemic velocity, we suggest that the molecular hydrogen emission arises from a rotating molecular gas structure surrounding the nuclear black hole at a distance of ~1 pc

Caltech Authors - Main

The Central Engine of Active Galactic Nuclei
ASP Conference Series, Vol. 373, 2007
L. C. Ho and J.-M. Wang
Arp 102B: An ADAF and a Torus?
Ranga-Ram Chary
Spitzer Science Center, MS220-6, Caltech, Pasadena, CA 91125
Abstract.
Arp 102B is a nearby radio galaxy which displays the presence of double-
peaked Balmer emission lines. Sub-arcsec Keck mid-infrared imaging and Spitzer
spectroscopy reveal a spatially compact mid-infrared source which displays ten-
tative evidence for variability. The Fν ∝ ν−1.2 spectral energy distribution
is suggestive of an advection-dominated accretion flow. The absence of dust
features over the 5−40 µm range make it unlikely that thermal dust emission
dominates the mid-infrared luminosity. We also detect the presence of molecu-
lar hydrogen in emission which is asymmetrically redshifted by ∼500–1000 km/s
from the systemic velocity of the galaxy. Since the forbidden, low-ionization
lines in this galaxy are at the systemic velocity, we suggest that the molecular
hydrogen emission arises from a rotating molecular gas structure surrounding
the nuclear black hole at a distance of ∼1 pc.
1. Introduction
Arp 102B is an E0, radio-loud galaxy at a distance of 104.9 Mpc (Eracleous &
Halpern 2004). Stellar dynamical measurements indicate a velocity dispersion
of 188±8 km s−1 (Barth et al. 2002), which implies a black hole mass of ∼ 108
M⊙ (see also Newman et al. 1997). The AGN in Arp 102B is of the “double-
peaked” emitter class. The simplest possible explanation for this spectral profile
identifies an accretion disk as the source of these lines. The AGN also shows
strong low-ionization lines and very weak/absent high-ionization lines (Stauffer
et al. 1983; Halpern et al. 1996). The weakness of the high-ionization lines and
absence of double-peaked structure in them supports the accretion disk origin
under the assumption that the outer parts of the thin disk are invisible to the
photoionizing source. The absence of a strong UV bump, presence of a hard
X-ray source and low Eddington luminosity of this object are indicative of an
advection-dominated flow (ADAF; Ho et al. 2000).
In this paper, we present the results from high-spatial resolution mid-
infrared imaging and moderate resolution infrared spectroscopy of the nucleus
of Arp 102B. Mid-infrared fine structure emission lines (e.g., [Ne II], [Ne III],
[O IV]) are less affected by extinction and help determine if shocks or photoion-
ization are responsible for the line excitation mechanism. The mid-infrared is
also the regime of molecular H2 rotational lines which provide an independent
measure of temperature and mass of the surrounding gas.
443
444 Chary
2. Broad-band Properties
In the Keck/LWS observations undertaken in 2000, the AGN in Arp 102B was
found to have a flux density of 90±20 mJy at 12.5 µm. The source was un-
detected at 17.9 µm with a 3 σ flux density limit of 50 mJy. In the Spitzer
observations in 2005, the source has decreased in brightness by more than a
factor of 2 at 12 µm since the Keck observations and has shown a change in the
infrared spectral energy distribution (Figure 1). The Spitzer measurements are
also consistent with the Puschell et al. (1986) value of 23±7 mJy derived from
10.6 µm observations made in 1981. As a result, it is unclear if the Keck data
suffer from photometric calibration errors or if there actually is a factor of ∼2
variability in the brightness of the source.
The broad-band flux density in the Spitzer data can be best represented by
the form Fν(mJy)=1.7×λ1.23±0.15 with evidence for a turnover at λ > 20 µm.
Such a spectrum is remarkably similar to that observed in NGC 4258 (Chary
et al. 2000) where the emission was thought to arise from self-Comptonized syn-
chrotron radiation. The 5−40 µm luminosity of the source is 6.7×109 L⊙, which
is comparable to the 0.2–10 keV luminosity measured by ROSAT and ASCA
in 1991 and 1998, respectively (Eracleous et al. 2003). The bolometric lumi-
nosity derived by integrating over the full SED including the radio/millimeter
measurements by Puschell et al. (1986) is 8×1043 ergs s−1. This implies that
the Eddington ratio of the 108 M⊙ black hole is ∼6×10−3, about 5 times larger
than previous estimates and marginally within the ADAF limit.
3. Emission-line Properties
3.1. Low-ionization Lines
Figure 2 shows the emission lines that are detected in the spectrum of Arp
102B. As noted from optical/UV spectroscopy, the spectrum of Arp 102B is
dominated by low-ionization metallic fine structure lines with a notable absence
of high-ionization lines. [O IV], which has an ionization potential of 55 eV, is
the most prominent of these features but is known to exist in starburst galaxies.
Adopting the simplified model of Sturm et al. (2002), the [O IV]/[Ne II] ratio of
0.21±0.05 and absence of high-ionization lines would indicate that that 90% of
the bolometric luminosity in Arp 102B is powered by a starburst. The measured
[Fe II]/[O IV] ratio of 0.86±0.25 is also remarkably similar to that of NGC 6240,
which is thought to be a starburst-dominated source (Lutz et al. 2003).
However, Arp 102B shows an absence of strong polycyclic aromatic hydro-
carbon dust emission or silicate features. PAHs are commonly thought to be
present in starbursts. It is unclear if the PAHs are absent because they are de-
stroyed in the hard radiation field around an AGN/super starburst or because
there is no dust in the vicinity of the AGN. The latter is a less likely option
given the presence of molecular gas emission in the infrared spectrum.
With a [Ne II]/[S III] ratio of 2.3±0.5 and [Ne III]/[Ar II]> 3.1, Arp 102B is
out of the starburst-powered regime and intermediate between the shock-excited
and photoionization-excited regimes. The pure photoionization models in Spin-
golio & Malkan (1992) reproduce the observed [Ne III]/[O III] λ5007 ratios very
well but underestimate the [Ne II]/[Ne III] ratio. Ionizing shocks from starbursts
Arp 102B: An ADAF and a Torus? 445
and supernova remnants, can account for the observed [Fe II]/[O IV] ratio as
well as the high [Ne II] line flux. Thus, we conclude that the low-ionization line
spectrum of Arp 102B is a composite, dominated by photoionization but with
∼20–30% contribution from slow shocks.
3.2. Molecular Hydrogen Emission
One of the striking features of the spectrum are the S1 and S3 rotational lines of
H2, which appear to be offset in velocity from the systemic velocity of the galaxy.
The ratio of the S(3)/S(1) line fluxes is 1.6, indicative of warm molecular gas at
a temperature of ∼400 K. None of the other H2 lines are detected. Although the
forbidden lines are observed at the systemic velocity of the galaxy, the H2 lines
are offset by > 2σ at both the orders. This is unlikely to be due to wavelength
calibration uncertainties. We suggest that the molecular hydrogen might be in a
rotating molecular gas structure with the redshifted emission stronger than the
blueshifted emission due to a warp. We do not detect the blueshifted emission of
the S(3) line because of inadequate wavelength coverage in the spectrum. The
size of the structure is about ∼1 pc, ranging from 0.5−2.7 pc depending on the
adopted velocity range.
Although the mid-infrared lines are offset, the near-infrared H2 lines in
ground-based spectra appear to be at the systemic velocity of the galaxy
(Rodŕıguez-Ardila et al. 2005). This is surprising. One possibility is that the
near-infrared continuum and line emission are being dominated by the starlight
while the mid-infrared is being dominated by the region around the AGN. Fur-
thermore, the strength of the near-infrared lines is far too strong for the gas
temperature that we derive from the ratio of the mid-IR H2 lines. Thus, unless
there are unknown wavelength calibration uncertainties in the mid-IR spectrum,
there must be a hotter gas component which dominates the near-IR molecular
gas emission.
Acknowledgments. I would like to acknowledge my collaborators, Luis
Ho, Harlan Devore, Eric Becklin and Nick Scoville for their assistance with dif-
ferent aspects of the observations. I would also like to acknowledge the organizers
for a very enjoyable and productive meeting. These observations were partly a
component of the Spitzer Teacher’s Education program.
References
Barth, A. J., et al. 2002, AJ, 124, 2607
Chary, R., et al. 2000, ApJ, 531, 756
Eracleous, M. & Halpern, J. P. 2004, ApJS, 150, 181
Eracleous, M., Halpern, J. P., & Charlton, J. C. 2003, ApJ, 633, 644
Halpern, J. P., Eracleous, M., Filippenko, A. V., & Chen, K. 1996, ApJ, 464, 704
Ho, L. C., et al. 2000, ApJ, 541, 120
Lutz, D., et al. 2003, A&A, 409, 867
Newman, J. A., Eracleous, M., Filippenko, A. V., & Halpern, J. P. 1997, ApJ, 485, 570
Puschell, J. J., et al. 1986, AJ, 91, 751
Rodŕıguez-Ardila, A., Riffel, R., & Pastoriza, M. G. 2005, MNRAS, 364, 1041
Spingolio, L., & Malkan, M. A. 1992, ApJ, 399, 504
Stauffer, J., Schild, R., & Keel, W. 1983, ApJ, 270, 465
Sturm, E., et al. 2002, A&A, 393, 821
446 Chary
Figure 1. Infrared spectral energy distribution of Arp 102B from Spitzer
2005 data and Keck 2000 data (solid circles). The luminosity of the nuclear
source has decreased by a factor of 2.5 at 12 µm between the Keck and Spitzer
observations while its spectral shape has inverted, suggestive of a transition
from a thin disk to an ADAF. However, calibration uncertainties are a possible
source of error for the Keck data.
Figure 2. Spectral lines seen in the Spitzer/IRS spectra with a best-fit Gaus-
sian and constant baseline. Also shown is the ±1σ uncertainty in the wave-
length calibration for the SH and LH modules. Although the forbidden lines
are consistent with no velocity offset to within the calibration uncertainty, the
H2 lines are offset by 500−1000 km s−1 from the systemic velocity. Given the
mass of the black hole, it implies the molecular hydrogen is ∼1 pc from the
nucleus.


Arp 102B: An ADAF and a Torus?

The Central Engine of Active Galactic Nuclei
ASP Conference Series, Vol. 373, 2007
L. C. Ho and J.-M. Wang
Arp 102B: An ADAF and a Torus?
Ranga-Ram Chary
Spitzer Science Center, MS220-6, Caltech, Pasadena, CA 91125
Abstract.
Arp 102B is a nearby radio galaxy which displays the presence of double-
peaked Balmer emission lines. Sub-arcsec Keck mid-infrared imaging and Spitzer
spectroscopy reveal a spatially compact mid-infrared source which displays ten-
tative evidence for variability. The Fν ∝ ν−1.2 spectral energy distribution
is suggestive of an advection-dominated accretion ﬂow. The absence of dust
features over the 5−40 µm range make it unlikely that thermal dust emission
dominates the mid-infrared luminosity. We also detect the presence of molecu-
lar hydrogen in emission which is asymmetrically redshifted by ∼500–1000 km/s
from the systemic velocity of the galaxy. Since the forbidden, low-ionization
lines in this galaxy are at the systemic velocity, we suggest that the molecular
hydrogen emission arises from a rotating molecular gas structure surrounding
the nuclear black hole at a distance of ∼1 pc.
1. Introduction
Arp 102B is an E0, radio-loud galaxy at a distance of 104.9 Mpc (Eracleous &
Halpern 2004). Stellar dynamical measurements indicate a velocity dispersion
of 188±8 km s−1 (Barth et al. 2002), which implies a black hole mass of ∼ 108
M⊙ (see also Newman et al. 1997). The AGN in Arp 102B is of the “double-
peaked” emitter class. The simplest possible explanation for this spectral proﬁle
identiﬁes an accretion disk as the source of these lines. The AGN also shows
strong low-ionization lines and very weak/absent high-ionization lines (Stauﬀer
et al. 1983; Halpern et al. 1996). The weakness of the high-ionization lines and
absence of double-peaked structure in them supports the accretion disk origin
under the assumption that the outer parts of the thin disk are invisible to the
photoionizing source. The absence of a strong UV bump, presence of a hard
X-ray source and low Eddington luminosity of this object are indicative of an
advection-dominated ﬂow (ADAF; Ho et al. 2000).
In this paper, we present the results from high-spatial resolution mid-
infrared imaging and moderate resolution infrared spectroscopy of the nucleus
of Arp 102B. Mid-infrared ﬁne structure emission lines (e.g., [Ne II], [Ne III],
[O IV]) are less aﬀected by extinction and help determine if shocks or photoion-
ization are responsible for the line excitation mechanism. The mid-infrared is
also the regime of molecular H2 rotational lines which provide an independent
measure of temperature and mass of the surrounding gas.
443
444 Chary
2. Broad-band Properties
In the Keck/LWS observations undertaken in 2000, the AGN in Arp 102B was
found to have a ﬂux density of 90±20 mJy at 12.5 µm. The source was un-
detected at 17.9 µm with a 3 σ ﬂux density limit of 50 mJy. In the Spitzer
observations in 2005, the source has decreased in brightness by more than a
factor of 2 at 12 µm since the Keck observations and has shown a change in the
infrared spectral energy distribution (Figure 1). The Spitzer measurements are
also consistent with the Puschell et al. (1986) value of 23±7 mJy derived from
10.6 µm observations made in 1981. As a result, it is unclear if the Keck data
suﬀer from photometric calibration errors or if there actually is a factor of ∼2
variability in the brightness of the source.
The broad-band ﬂux density in the Spitzer data can be best represented by
the form Fν(mJy)=1.7×λ1.23±0.15 with evidence for a turnover at λ > 20 µm.
Such a spectrum is remarkably similar to that observed in NGC 4258 (Chary
et al. 2000) where the emission was thought to arise from self-Comptonized syn-
chrotron radiation. The 5−40 µm luminosity of the source is 6.7×109 L⊙, which
is comparable to the 0.2–10 keV luminosity measured by ROSAT and ASCA
in 1991 and 1998, respectively (Eracleous et al. 2003). The bolometric lumi-
nosity derived by integrating over the full SED including the radio/millimeter
measurements by Puschell et al. (1986) is 8×1043 ergs s−1. This implies that
the Eddington ratio of the 108 M⊙ black hole is ∼6×10−3, about 5 times larger
than previous estimates and marginally within the ADAF limit.
3. Emission-line Properties
3.1. Low-ionization Lines
Figure 2 shows the emission lines that are detected in the spectrum of Arp
102B. As noted from optical/UV spectroscopy, the spectrum of Arp 102B is
dominated by low-ionization metallic ﬁne structure lines with a notable absence
of high-ionization lines. [O IV], which has an ionization potential of 55 eV, is
the most prominent of these features but is known to exist in starburst galaxies.
Adopting the simpliﬁed model of Sturm et al. (2002), the [O IV]/[Ne II] ratio of
0.21±0.05 and absence of high-ionization lines would indicate that that 90% of
the bolometric luminosity in Arp 102B is powered by a starburst. The measured
[Fe II]/[O IV] ratio of 0.86±0.25 is also remarkably similar to that of NGC 6240,
which is thought to be a starburst-dominated source (Lutz et al. 2003).
However, Arp 102B shows an absence of strong polycyclic aromatic hydro-
carbon dust emission or silicate features. PAHs are commonly thought to be
present in starbursts. It is unclear if the PAHs are absent because they are de-
stroyed in the hard radiation ﬁeld around an AGN/super starburst or because
there is no dust in the vicinity of the AGN. The latter is a less likely option
given the presence of molecular gas emission in the infrared spectrum.
With a [Ne II]/[S III] ratio of 2.3±0.5 and [Ne III]/[Ar II]> 3.1, Arp 102B is
out of the starburst-powered regime and intermediate between the shock-excited
and photoionization-excited regimes. The pure photoionization models in Spin-
golio & Malkan (1992) reproduce the observed [Ne III]/[O III] λ5007 ratios very
well but underestimate the [Ne II]/[Ne III] ratio. Ionizing shocks from starbursts
Arp 102B: An ADAF and a Torus? 445
and supernova remnants, can account for the observed [Fe II]/[O IV] ratio as
well as the high [Ne II] line ﬂux. Thus, we conclude that the low-ionization line
spectrum of Arp 102B is a composite, dominated by photoionization but with
∼20–30% contribution from slow shocks.
3.2. Molecular Hydrogen Emission
One of the striking features of the spectrum are the S1 and S3 rotational lines of
H2, which appear to be oﬀset in velocity from the systemic velocity of the galaxy.
The ratio of the S(3)/S(1) line ﬂuxes is 1.6, indicative of warm molecular gas at
a temperature of ∼400 K. None of the other H2 lines are detected. Although the
forbidden lines are observed at the systemic velocity of the galaxy, the H2 lines
are oﬀset by > 2σ at both the orders. This is unlikely to be due to wavelength
calibration uncertainties. We suggest that the molecular hydrogen might be in a
rotating molecular gas structure with the redshifted emission stronger than the
blueshifted emission due to a warp. We do not detect the blueshifted emission of
the S(3) line because of inadequate wavelength coverage in the spectrum. The
size of the structure is about ∼1 pc, ranging from 0.5−2.7 pc depending on the
adopted velocity range.
Although the mid-infrared lines are oﬀset, the near-infrared H2 lines in
ground-based spectra appear to be at the systemic velocity of the galaxy
(Rodr´ıguez-Ardila et al. 2005). This is surprising. One possibility is that the
near-infrared continuum and line emission are being dominated by the starlight
while the mid-infrared is being dominated by the region around the AGN. Fur-
thermore, the strength of the near-infrared lines is far too strong for the gas
temperature that we derive from the ratio of the mid-IR H2 lines. Thus, unless
there are unknown wavelength calibration uncertainties in the mid-IR spectrum,
there must be a hotter gas component which dominates the near-IR molecular
gas emission.
Acknowledgments. I would like to acknowledge my collaborators, Luis
Ho, Harlan Devore, Eric Becklin and Nick Scoville for their assistance with dif-
ferent aspects of the observations. I would also like to acknowledge the organizers
for a very enjoyable and productive meeting. These observations were partly a
component of the Spitzer Teacher’s Education program.
References
Barth, A. J., et al. 2002, AJ, 124, 2607
Chary, R., et al. 2000, ApJ, 531, 756
Eracleous, M. & Halpern, J. P. 2004, ApJS, 150, 181
Eracleous, M., Halpern, J. P., & Charlton, J. C. 2003, ApJ, 633, 644
Halpern, J. P., Eracleous, M., Filippenko, A. V., & Chen, K. 1996, ApJ, 464, 704
Ho, L. C., et al. 2000, ApJ, 541, 120
Lutz, D., et al. 2003, A&A, 409, 867
Newman, J. A., Eracleous, M., Filippenko, A. V., & Halpern, J. P. 1997, ApJ, 485, 570
Puschell, J. J., et al. 1986, AJ, 91, 751
Rodr´ıguez-Ardila, A., Riﬀel, R., & Pastoriza, M. G. 2005, MNRAS, 364, 1041
Spingolio, L., & Malkan, M. A. 1992, ApJ, 399, 504
Stauﬀer, J., Schild, R., & Keel, W. 1983, ApJ, 270, 465
Sturm, E., et al. 2002, A&A, 393, 821
446 Chary
Figure 1. Infrared spectral energy distribution of Arp 102B from Spitzer
2005 data and Keck 2000 data (solid circles). The luminosity of the nuclear
source has decreased by a factor of 2.5 at 12 µm between the Keck and Spitzer
observations while its spectral shape has inverted, suggestive of a transition
from a thin disk to an ADAF. However, calibration uncertainties are a possible
source of error for the Keck data.
Figure 2. Spectral lines seen in the Spitzer/IRS spectra with a best-ﬁt Gaus-
sian and constant baseline. Also shown is the ±1σ uncertainty in the wave-
length calibration for the SH and LH modules. Although the forbidden lines
are consistent with no velocity oﬀset to within the calibration uncertainty, the
H2 lines are oﬀset by 500−1000 km s−1 from the systemic velocity. Given the
mass of the black hole, it implies the molecular hydrogen is ∼1 pc from the
nucleus.


http://authors.library.caltech.edu/19554/1/Chary2007p8683Central_Engine_Of_Active_Galactic_Nuclei.pdf

Arp102B: An ADAF and a Torus ?

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