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Issue
A&A
Volume 533, September 2011
Article Number L10
Number of page(s) 4
Section Letters
DOI https://doi.org/10.1051/0004-6361/201117730
Published online 30 August 2011

© ESO, 2011

1. Introduction

Large-scale feedback effects such as jets and outflows from active galactic nuclei (AGN) are thought to be capable of affecting the formation of new stars in their host galaxies. The triggering of star formation by compression of gas (e.g., van Breugel et al. 1985), as well as the suppression of star formation by heating of gas that prevents its further collapse (e.g., Nesvadba et al. 2010) have been observed in local AGN. Cosmological simulations have suggested that AGN feedback effects, which are often associated with mergers, could make galaxies appear red, or even explain the observed luminosity functions of galaxies (Croton et al. 2006; Hopkins et al. 2006). Combined with multi-wavelength observations indicating that the star-formation history and the black-hole-accretion history of the Universe peak at comparable redshifts, between 1  ≲  z  ≲  3 (Marconi et al. 2004; Merloni et al. 2004), this suggests that AGN feedback could have affected the shape of present-day galaxies considerably.

Extensive tests of the role of AGN feedback on the interstellar medium (ISM) of galaxies require a detailed kinematic study of outflowing gas in local sources. Signs of massive gas outflows have been detected for ionized atomic gas (e.g., Veilleux et al. 1995; Emonts et al. 2005; Holt et al. 2006; Müller-Sánchez et al. 2006), neutral atomic gas (e.g., Morganti et al. 2005; Rupke et al. 2005), and molecular CO and OH gas (e.g., Curran et al. 1999; Das et al. 2005; García-Burillo et al. 2009; Sakamoto et al. 2009; Feruglio et al. 2010; Fischer et al. 2010; Sturm et al. 2011). In this letter we present evidence for the first detection of highly turbulent motions of H2 gas at a temperature of a few hundred Kelvin as seen with Spitzer for several local AGN. We adopt H0 = 70 km s-1 Mpc-1, ΩM = 0.3, and ΩΛ = 0.7 throughout this work.

thumbnail Fig. 1

Normalized, continuum-subtracted molecular and ionized gas line profiles in local AGN with resolved H2 emission. The filled gray area represents a Gaussian with FHWM equal to the resolution of the IRS.

Table 1

Fluxes and widths of resolved H2 lines in local AGN.

2. The sample selection

We queried for turbulence in the warm H2 gas in local AGN using mid-IR spectra obtained with Spitzer in high-resolution mode. The full archival sample comprises 298 sources that are classified as AGN based on optical spectroscopy. It is presented in Dasyra et al. (2011) together with the data reduction techniques and the extracted spectra.

To look for turbulent H2 gas motions, we examined the profiles of the purely rotational (0−0)S0 28.22 μm, (0−0)S1 17.04 μm, (0−0)S2 12.28 μm, and (0−0)S3 9.66 μm lines. We searched for either resolved profiles with velocity dispersion σ  ≳  200 km s-1, or for profiles with asymmetric wings that are characteristic of outflows. To ensure the reliability of our results, we only examined sources with at least two lines of signal-to-noise (S/N) ratio  >5. We also requested that at least two lines suggest a similar kinematic pattern, i.e. a wing or a resolved profile. To consider a line resolved we requested that its full width at half maximum (FWHM) value minus the FWHM error exceeds the instrumental resolution R plus the resolution error at the observed-frame wavelength of the line (Dasyra et al. 2011). The average R value in the 12.0 − 18.0 μm range, which comprises the H2 S1 and S2 transitions, is 507  ±  66 km s-1.

3. Results: sources with highly turbulent or rapidly moving H2 gas in the warm phase

Of the 298 sources 62 had at least two H2 lines detected with S/N  >  5. The profiles of two or more lines were spectrally resolved in only five sources, namely 3C 236, 3C 293, IRAS 09039+0503, MCG-2-58-22, and Mrk 463E (Fig. 1). Their velocity dispersions are in the range 220  ≲  σ  ≲  280 km s-1 (Table 1). Because all these sources are radio galaxies and/or interacting systems, the high turbulence in their warm H2 motions can be driven by AGN feedback mechanisms, by gravitational instabilities, or by supernova winds. Still, no mechanism is efficient enough to kinematically distort a detectable mass of warm H2 gas to velocity dispersions exceeding 300 km s-1. We computed the excitation temperature T and mass of the turbulent gas (Table 1) using the detected S1, S2, and S3 line fluxes as in Higdon et al. (2006). At temperatures of 300 − 400 K, its mass is typically on the order of 1% of the cold H2 gas mass indicated by CO observations.

Further outflow or inflow signatures were sought for in the H2 line wings and in the difference of the H2 recession velocity from the systemic velocity, Vsys. The latter was determined from the [Ne ii] line, emitted by ions that are abundant in star-forming regions and in the AGN vicinity owing to their low ionization potential, 21.56 eV. The H2 recession velocity agreed within the errors with Vsys for all sources, including those with massive outflows of the gas that is photoionized by the AGN and that is traced by the [Ne v] or [O iv] lines (i.e., 3C 273, IRAS 13342+3932, IRAS 05189-2524, IRAS 15001+1433, IRAS 23060+0505, Mrk 609; Dasyra et al. 2011).

thumbnail Fig. 2

Upper panel: normalized, continuum-subtracted molecular and ionized gas line profiles for 4C 12.50. In addition to the profiles of the MIR lines, the 5007 Å [O iii] profile that is convolved to the resolution of IRS is presented for comparison. The blue wing is detected in all lines except for the unresolved S3. Lower panel: H2 excitation diagram constructed separately for the primary Gaussian component (open diamonds) and the secondary Gaussian component (stars) of each H2 line.

The only source with wing signatures in its H2 line profiles was 4C 12.50, also known as IRAS 13451+1232 or PKS 1341+12. The S1 and S2 line wings (Fig. 2; upper panel), detected with S/N  ≳  3, point at two warm molecular gas kinematic components in this source. The peak of the secondary Gaussian that is needed to fit both profiles is blueshifted by  ~640 km s-1 from the primary Gaussian at Vsys. The flux ascribed to the primary Gaussian is only 2.6 and 2.7 times higher than that ascribed to the secondary Gaussian of the S1 and S2 transitions, respectively. Using the actual flux in each Gaussian (Table 1), we separately calculated the H2 gas mass for each kinematic component. We find the excitation temperature of the bulk of the gas to be 374  ±  12 K. It is equal to the inverse of the slope of the line that best fits the excitation diagram points (Rigopoulou et al. 2002). The excitation diagram (Fig. 2; lower panel) shows the natural logarithm of the number of electrons ne that descended from the upper to the lower state, normalized by the statistical weight g of the transition, as a function of the temperature that corresponds to the energy of the upper state Eup divided by the Boltzmann constant k. The value of ne is computed as L/(αhν), where h is the Planck constant, α is the Einstein coefficient of the transition, and ν is the frequency of the emitted line. For a single temperature of 374 K we find that the mass of the H2 gas at systemic velocity is 1.4  ×  108 M⊙ (see also Higdon et al. 2006). Assuming (for simplicity) that T is the same for both H2 kinematic components (see Fig. 2), we find that the mass of the H2 gas moving at 640 km s-1 is 5.2  ×  107 M⊙. This is 0.3% of the cold H2 gas mass in the west nucleus of 4C 12.50, which was found from CO observations to be 1.5  ×  1010 M⊙ (Evans et al. 2002).

4. Discussion: an AGN-driven molecular gas outflow in 4C 12.50?

Even though 4C 12.50 is an IR-bright system of two interacting galaxies (Axon et al. 2000), the secondary Gaussian of Fig. 2 is unlikely to be tracing gas in the east nucleus, which is mostly located outside the IRS slit (Fig. 3). Any residual gas from the east nucleus inside the slit would be moving at a velocity comparable to the difference in the recession velocity of the two nuclei,  ~200 km s-1. This difference is computed from ionized gas kinematics (Holt et al. 2003), and it is confirmed by stellar kinematics from CO absorption features presented in Dasyra et al. (2006). Besides the two nuclei residing in a common bulge, 4C 12.50 also has off-nuclear gas concentrations and super star clusters in tidal tails. The blueshifted H2 emission could arise from gas in a tidal tail inside the IRS slit (Fig. 3), which is entering a shock front created during the collision (see Cluver et al. 2010). The tidal tail could be moving faster or be at a different inclination angle i than its corresponding nucleus, of  ~20° in either case. Because the deprojected tail velocity would be equal to 640/sin(i) km s-1, it could reach a value as high as  ~2000 km s-1. This scenario is also unlikely given that no off-nuclear, large-scale (<20 kpc) kinematic component has been observed to be moving faster than  ± 250 km s-1 from Vsys along the line of sight (Holt et al. 2003; Rodríguez Zaurín et al. 2007).

thumbnail Fig. 3

Composite Hα (cyan) and 5900 Å continuum (red) image of 4C 12.50, constructed from Hubble Space Telescope data (Batcheldor et al. 2007). To enhance the visibility of low surface brightness structures, we removed all symmetric galaxy components (i.e., a common bulge and two residual disks located at the position of the two nuclei) from the 5900 Å image with GALFIT (Peng et al. 2002). Both disks were found to have an inclination of  ~ 20°. Irregularly moving components are found within the IRS slit, marked with a yellow box for both nod positions. (A) marks the position of two super star clusters with velocities that are blueshifted by up to  ~250 km s-1 from systemic velocity (Rodríguez Zaurín et al. 2007).

A scenario that agrees better with existing observations is that the H2 gas is moving toward us driven by feedback mechanisms (e.g., Alatalo et al. 2011). Optical spectroscopy indicated the presence of an outflow from the west nucleus of 4C 12.50 by revealing the existence of three kinematic components for the nuclear [O i], [S ii], and [O iii] emission (Holt et al. 2003). Most of the [O iii] emission is blueshifted by 400 km s-1, while its broadest component (of FWHM  ~  1900 km s-1) is blueshifted by 2000 km s-1 from Vsys. MIR spectroscopy suggested an AGN-related nuclear outflow of ionized gas. Blue wings were observed in the profiles of the [O iv] 25.89 μm and the [Ne v] 14.32 μm lines (Spoon & Holt 2009; Dasyra et al. 2011), emitted by ions that are primarily found in hard radiation fields. Radio observations revealed an HI absorption line blueshifted by  ~1000 km s-1 from Vsys (Morganti et al. 2004). Because a background radio source is required for HI absorption to be seen, the hydrogen clouds are likely to be located between the observer and the AGN or its jet. Estimates of the outflow mass range from 8  ×  105 M⊙ for the ionized 104 K gas (Holt et al. 2011) to 5.6  ×  108 M⊙ for the neutral gas traced by Na iD (Rupke et al. 2005), bracketing our mass estimate, Mout, of 5.2  ×  107 M⊙ for the outflowing  ~400 K H2 gas.

If the outflow is caused by AGN radiation pressure or winds (Holt et al. 2011), it can be considered spherical. If the gas is also distributed in a sphere, and if its density is falling with the inverse square of the distance from the center, its mass outflow rate, Ṁ, will be given from the product MoutVoutR-1. For an outflow velocity Vout of 640 km s-1 and for a radius R of 270 pc, as estimated for the CO gas assuming that it is thermalized to the dust temperature (Evans et al. 2002) and as converted to the adopted cosmological distance, Ṁ will be 130 M⊙ yr-1. An outflow of these properties is unlikely to entirely suppress star formation in the west nucleus of 4C 12.50, whose star-formation rate (SFR) is estimated to be between 370 − 1380 M⊙ yr-1. The lower value is found from the CO mass using a gas consumption timescale of 4  ×  107 yrs, while the upper value is found by folding the CO-based H2 mass and radial extent in the Schmidt-Kennicutt law (Evans et al. 2002). If the outflow were symmetric, both a blue and a red H2 line profile wing should exist unless the gas moving away from the observer is obscured by dust. This is plausible for a source with E(B − V) of 1.44 mag (Holt et al. 2011) and 9.7 μm optical depth of 0.59 (Veilleux et al. 2009), which could translate into a 10 − 20% absorption of the total flux at 17 μm (Li & Draine 2001), and which could preferentially suppress the red line wing for a circumnuclear dust distribution.

Alternatively, a radio jet encountering clouds on its path could be driving the outflow (e.g., Dietrich & Wagner 1998). A jet is indeed known to emerge from the west nucleus of 4C 12.50. It extends out to 45 pc in projection in the north and 170 pc in the south (Stanghellini et al. 1997), and it propagates close to the speed of light at a small angle from the line of sight (Lister et al. 2003). A previous flare of the jet, undetected in the radio because of its weak signal, could be associated with the shocked gas whose extended X-ray emission peaks at 20 kpc south of the nucleus (Siemiginowska et al. 2008). The scenario of a jet-driven outflow can easily explain the observed line profiles. The detection of a blue or a red wing is random since it depends on the location of the clouds with respect to the jet propagation axis.

5. Summary and concluding remarks

We queried the archival catalog of 298 optically-selected AGN observed with Spitzer IRS in high-resolution mode (Dasyra et al. 2011), aiming to identify sources with turbulent motions of their warm molecular gas. We examined the profiles of the H2 S0, S1, S2, and S3 lines and found only five radio and/or interacting galaxies with σ  >  200 km s-1 but no source with σ  >  300 km s-1. In a sixth source, 4C 12.50, the S1 and S2 lines have a blue wing that points at warm gas moving toward us with 640 km s-1. Its mass, 5.2  ×  107 M⊙, corresponds to an impressively high fraction,  ~ 1/4, of the total  ~400 K H2 gas mass. While it could be tracing shock regions along tidal tails, it is more likely to be tracing an AGN jet or wind-driven outflow, known from ionized and neutral gas kinematic studies. Even if all of this gas is entrained by an outflow, it is unlikely to entirely suppress star formation in 4C 12.50. Additional tests of the role of AGN feedback mechanisms in increasing the turbulence of the molecular gas require observations of high-rotational-number transitions of CO molecules that can be mostly excited by the AGN (van der Werf et al. 2010). An essential role in revisiting this question will also be played by the Atacama Large Millimeter Array, which, via high-resolution studies of the cold gas, will enable a comparison never performed before: the

computation of the warm-to-cold molecular gas mass ratio in an outflow vs. the rest of the ISM.

Acknowledgments

K.D. acknowledges support by the European Community through a Marie Curie Fellowship (PIEF-GA-2009-235038) awarded under the 7th Framework Programme (2007−2013).

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All Tables

Table 1

Fluxes and widths of resolved H2 lines in local AGN.

In the text

All Figures

thumbnail Fig. 1

Normalized, continuum-subtracted molecular and ionized gas line profiles in local AGN with resolved H2 emission. The filled gray area represents a Gaussian with FHWM equal to the resolution of the IRS.
In the text
thumbnail Fig. 2

Upper panel: normalized, continuum-subtracted molecular and ionized gas line profiles for 4C 12.50. In addition to the profiles of the MIR lines, the 5007 Å [O iii] profile that is convolved to the resolution of IRS is presented for comparison. The blue wing is detected in all lines except for the unresolved S3. Lower panel: H2 excitation diagram constructed separately for the primary Gaussian component (open diamonds) and the secondary Gaussian component (stars) of each H2 line.
In the text
thumbnail Fig. 3

Composite Hα (cyan) and 5900 Å continuum (red) image of 4C 12.50, constructed from Hubble Space Telescope data (Batcheldor et al. 2007). To enhance the visibility of low surface brightness structures, we removed all symmetric galaxy components (i.e., a common bulge and two residual disks located at the position of the two nuclei) from the 5900 Å image with GALFIT (Peng et al. 2002). Both disks were found to have an inclination of  ~ 20°. Irregularly moving components are found within the IRS slit, marked with a yellow box for both nod positions. (A) marks the position of two super star clusters with velocities that are blueshifted by up to  ~250 km s-1 from systemic velocity (Rodríguez Zaurín et al. 2007).
In the text

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