The missing link: tracing molecular gas in the outer filament of Centaurus A
Raffaella Morganti, Tom Oosterloo, J. B. Raymond Oonk, Francesco Santoro, Clive Tadhunter
aa r X i v : . [ a s t r o - ph . GA ] J u l Astronomy & Astrophysicsmanuscript no. 28950_Final c (cid:13)
ESO 2018July 30, 2018
The missing link: Tracing molecular gas inthe outer filament of Centaurus A Ra ff aella Morganti , , Tom Oosterloo , , J. B. Raymond Oonk , ,Francesco Santoro , and Clive Tadhunter ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands.e-mail: [email protected] Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden Department of Physics and Astronomy, University of She ffi eld, She ffi eld, S7 3RH, UKJuly 30, 2018 ABSTRACT
We report the detection, using observations of the CO(2-1) line performed with the Atacama Pathfinder EXperiment (APEX), ofmolecular gas in the region of the outer filament of Centaurus A, a complex region known to show various signatures of an interactionbetween the radio jet, an H i cloud, and ionised gas filaments. We detect CO(2-1) at all observed locations, which were selected torepresent regions with very di ff erent physical conditions. The H masses of the detections range between 0 . × and 1 . × M ⊙ ,for conservative choices of the CO to H conversion factor. Surprisingly, the stronger detections are not coincident with the H i cloud,but instead are in the region of the ionised filaments. We also find variations in the widths of the CO(2-1) lines throughout the region,with broader lines in the region of the ionised gas, i.e. where the jet–cloud interaction is strongest, and with narrow profiles in theH i cloud. This may indicate that the molecular gas in the region of the ionised gas has the momentum of the jet–cloud interactionencoded in it, in the same way as the ionised gas does. These molecular clouds may therefore be the result of very e ffi cient coolingof the down-stream gas photo- or shock-ionised by the interaction. On the other hand, the molecular clouds with narrower profiles,which are closer to or inside the H i cloud, could be pre-existing cold H cores which manage to survive the e ff ects of the passing jet. Key words. galaxies: active - galaxies: individual: Centaurus A - ISM: jets and outflow - radio lines: galaxies
1. Introduction
Radio-loud active galactic nuclei (AGN) are known to inject en-ergy into the surrounding interstellar medium (ISM) via plasmajets. The impact this has on the host galaxy is relevant onboth large and small scales (see e.g. McNamara & Nulsen 2012;Morganti et al. 2013). The induced compression – and subse-quent cooling – of gas disturbed by the transit of a radio jet caninduce the formation of new stars (e.g. van Breugel & Dey 1993;Dey et al. 1997; Oosterloo & Morganti 2005; Croft et al. 2006),but AGN-driven gas outflows can also occur which remove, orat least redistribute, the cold gas and / or create highly turbulentconditions which inhibit star formation (e.g. Alatalo et al. 2011;Combes et al. 2013; García-Burillo et al. 2014; Morganti et al.2013; Guillard et al. 2015; Morganti et al. 2015).One of the important results of recent work is that moleculargas often is the most massive component in jet-cloud interac-tions (e.g. Feruglio et al. 2010; Alatalo et al. 2011; Cicone et al.2014; Morganti et al. 2015). Thus, tracing this phase of the gasprovides key information on how these processes work and howthey depend on the properties of the gas and of the radio jet.When it comes to studying the interaction between a radiojet and the ISM of the host galaxy, the nearest radio-loud AGN,Centaurus A (Cen A, D =
2. Special region: The outer filament
The outer filament was discovered as a filament of highlyionised gas outside the optical body of Cen A, well alignedwith the radio jet of Cen A (Blanco et al. 1975; Graham & Price1981; Morganti et al. 1991, 1999; Morganti 2010). Later workidentified regions of ongoing star formation in the filament(Mould et al. 2000; Rejkuba et al. 2001), while bright UV emis-sion was detected with GALEX (Ne ff et al. 2015). About 2kpc west of the ionised gas, a large H i cloud was found(Schiminovich et al. 1994), likely the remnant of a major ac-cretion by Cen A. In this H i cloud, at locations closest to theradio jet, H i with velocities very di ff erent from those of theregular rotation of the H i cloud around Cen A was detected(Oosterloo & Morganti 2005), which was taken as evidence thatthe radio jet is a ff ecting the H i cloud at those locations. Thisis further confirmed by integral field spectroscopy with VI-MOS and MUSE (Santoro et al. 2015a,b), showing the pres-ence of even more disturbed kinematics in the ionised gas. TheMUSE data reveal three morphologically and kinematically dis- Article number, page 1 of 5 & Aproofs: manuscript no. 28950_Final tinct components in the ionised gas, which are thought to cor-respond to di ff erent stages in the jet-cloud interaction. The dataalso show that, overall, the ionisation of the gas is due to ionisingphotons from the AGN, but that locally star formation also playsa role (Santoro et al. 2016). Interestingly, the rate at which theavailable gas reservoir is turned into stars is low, possibly con-nected to increased turbulence powered by injection of kineticenergy by the jet (Salomé et al. 2016). The Balmer decrement(H α / H β ) across the MUSE fields confirms that the ionised gas isdusty. The presence of very cold dust ( T ∼
13 K) in and around theouter filament is also seen in Herschel data (Auld et al. 2012).The above shows that the region around the outer filament isvery rich in phenomena and that it shows the full complexity ofthe interaction of a jet with a large gas cloud. This interaction isclearly stirring up a gas cloud and is destroying it (partially), butat the same time star formation is happening with low e ffi ciency.To complete this picture, information on a key compo-nent is missing: the molecular gas. The only observationsavailable around this region are those done with SEST byCharmandaris et al. (2000) of locations in the H i cloud andALMA observations of a single location just north of the H i cloud (Salomé et al. 2016). They have detected molecular gas as-sociated with the northern region of this cloud and estimate thatthe molecular gas is the most massive component at the observedlocations. These observations, however, only sample regions rel-atively distant ( ∼ i velocities. Here we report observationswhich have a more complete coverage of the outer filament.
3. APEX observations
Observations with the Atacama Pathfinder EXperiment (APEX)12 m antenna were conducted between 7 and 10 April 2016, us-ing the APEX-1 instrument with the XFFTS backend tuned to230.5486 GHz, the frequency of CO(2-1) corresponding to thevelocity of the gas in the outer filament ( V hel ∼
400 km s − ). Fivepointing positions were selected to cover di ff erent regions acrossthe outer filament in order to sample very di ff erent conditions ofthe gas and related phenomena. Their locations are shown in Fig.1 and are listed in Table 1. Positions F3 and F4 were chosen to liein the regularly rotating part of the H i cloud in order to samplethe quiescent H i gas. To sample a location in the main ionisedfilament where the gas is clearly a ff ected by the jet–cloud in-teraction, we have chosen a position (F6) where the ionised gasconsists of at least two distinct kinematically and morphologi-cally distinct components (Santoro et al. 2015b). Finally, loca-tions F2 and F5 lie just outside the H i cloud at locations wheresome recent star formation has occurred.The observations were done in good weather conditions(for these frequencies). The precipitable water vapour (PWV)was between 2.4 and 3.3 mm. The observations were made us-ing 32768 channels covering a total band of 2.5 GHz ( ∼ Table 1.
Locations of the APEX pointing positions and the noise of thespectra.
Position R.A. (J2000) Dec (J2000) Noise(h m s) (d m s) (mK)F2 13 26 21.9 −
42 48 58.5 3.2F3 13 26 13.1 −
42 50 03.1 1.8F4 13 26 18.4 −
42 49 18.1 1.2F5 13 26 16.4 −
42 47 59.5 3.3F6 13 26 28.8 −
42 49 52.5 2.5
F6 F3F2 F4F5
Fig. 1.
Locations of the APEX pointings (see Table 1) on the GALEXFUV image of the outer filament (Ne ff et al. 2015). The black contoursgive the distribution of the H i cloud with contour levels 1, 4, 7, 10,13, and 16 × cm − (Oosterloo & Morganti 2005). The dashed linesapproximately delineate the path of the radio jet (Morganti et al. 1999);the arrow shows the flow direction of the jet.
200 300 400 500V
LSR (km s −1 )−10010203040 T ∗ A ( m K ) F3
200 300 400 500V
LSR (km s −1 ) F4
200 300 400 500V
LSR (km s −1 ) F5
200 300 400 500V
LSR (km s −1 )−10010203040 T ∗ A ( m K ) F2
200 300 400 500V
LSR (km s −1 ) F6 Fig. 2.
CO(2-1) spectra for the observed locations. Red lines show thesingle-component Gaussian fits, while the blue line for F6 shows thedouble-Gaussian fit. km s − ) with a velocity resolution of 0.076 km s − . However, thefinal spectra were smoothed to 10 km s − bins. The data were re-duced with the CLASS software from the Gildas package usingthe standard scripts provided by ESO. From the individual scansof each of the XFFTS units a linear baseline was subtracted be-fore adding all spectra. The noise levels of the spectra are listedin Table 1. Table 2 presents the parameters of the CO(2-1) pro-files for the five positions derived from Gaussian fits. At the fre-quency of our observations, the spatial resolution of APEX is ∼ ′′ ( ∼ − .
4. CO(2-1) in the region of the outer filament
The first important result (Fig. 2 and Table 2) is that we detectCO(2-1) at all five locations spread over the entire region : in the http: // / IRAMFR / GILDASArticle number, page 2 of 5organti et al.: The missing link: Tracing molecular gas in the outer filament of Centaurus A
Position Peak Velocity Dispersion Integrated flux H mass H / H i (mK) ( km s − ) ( km s − ) (mK km s − ) ( M ⊙ )F2 22.3 ± ± ± ± × > . ± ± ± ± × . ± ± ± ± × . ± ± ± ± × > . ± ± ± ± × > . ± ± ± ± × F6-2 12.3 ± ± ± ± × Table 2.
Parameters of the CO(2-1) profiles derived using Gaussian fits, derived molecular masses, and ratios of molecular to atomic mass for thefive positions. For position F6 the results of a two-Gaussian fit are also given and are labelled F6-1 and F6-2. The molecular masses have beenestimated using CO(2-1) / CO(1-0) = α CO = . M ⊙ (K km s − pc ) − . H i masses arederived from the data presented in Oosterloo & Morganti (2005) using the same aperture as the APEX beam. H i cloud (F3 and F4), in the main region of ionised gas (F6),but also at locations in between the H i cloud and the ionised gas(F2 and F5). The first direct consequence is that the moleculargas is the only gas tracer that is present inside the H i cloud,in the ionised filament, and in between the two, while H i andionised gas are detected in mutually excluding locations. Thisunderlines, as in other cases of jet–cloud interactions, that themolecular gas is a crucial component in such phenomena.Although limited by the small number of pointings, a trendcan be seen in the width of the CO(2-1) profile. The widths in-crease going from west to east, from inside the H i cloud to out-side it, and finally to the region of ionised gas. This mirrors whatis seen in the H i (which has narrow profiles only ∼
20 km s − wide; Oosterloo & Morganti 2005) and the ionised gas (withmuch more complex kinematics with line widths >
100 km s − ;Morganti et al. 1991; Santoro et al. 2015b). The velocities andwidths of the profiles F3, F4, and F5 are consistent with thoseof the regularly rotating part of the H i cloud and with what wasobtained by Charmandaris et al. (2000) for a region near F5. Incontrast, the detections towards and in the region of the ionisedfilament (F2 and F6) are much broader and show gas at veloci-ties corresponding to the high-velocity, disturbed components ofthe ionised gas (see Santoro et al. 2015a) and are deviating fromthe extrapolation of the regularly rotating H i . The profile at F6 isbroadest with a FWHM =
72 km s − , is asymmetric and showssome indications of having multiple components. The possiblepresence of multiple components at position F6 may be relatedto the presence of di ff erent kinematical components seen in theionised gas, in which case it would imply a close connection be-tween the two phases.It has been noted by several groups that the star forma-tion rate in the outer filament is low and that the rate atwhich the available cold gas is turned into stars is also low(Mould et al. 2000; Rejkuba et al. 2004; Oosterloo & Morganti2005; Salomé et al. 2016). Salomé et al. (2016) suggest that tur-bulence due to kinetic energy injection from the AGN jet leadsto molecular gas reservoirs not forming stars e ffi ciently and toquenching of star formation. The di ff erences in line widths wedetect do indeed suggest that turbulence is increased due to thejet–cloud interaction. However, star formation only occurs in theregion outside the H i cloud, albeit at low rates, while it seems tobe completely absent in the region inside the H i cloud. It there-fore seems that the star formation that is occurring is not beingquenched, but instead is stimulated by the passage of the radiojet. It is, on the other hand, rather puzzling that no star formationis occurring in the H i cloud where there are no indications thatconditions are particularly adverse to star formation. The rela-tive locations of the ionised gas and the H i are also suggestive that the jet plays a role in the star formation (see Fig. 1). The gen-eral possibility of jet-induced star formation has been consideredby theoretical models (Mellema et al. 2002; Fragile et al. 2004;Gaibler et al. 2012), but no study has been made for conditionssimilar to those of the outer filament.In order to derive molecular masses from our CO(2-1) de-tections, we have made assumptions about the CO(2-1) / CO(1-0) ratio and the conversion factor CO-to-H . For the former wehave assumed the factor obtained by Charmandaris et al. (2000)for their Shell S1 region, which is located close to F5: CO(2-1) / CO(1-0) = .
55. This may not be correct for all regions cov-ered by our observations and the di ff erences in total line fluxbetween the positions inside and outside the H i cloud couldpartly be due to di ff erent ratios at di ff erent locations with di ff er-ent conditions. However, until observations of more transitionsare available, it is di ffi cult to apply a varying ratio.The choice of the conversion factor for CO to H requiressome considerations. Charmandaris et al. (2000) have used astandard value, i.e. α CO = . M ⊙ (K km s − pc ) − . How-ever, more conservative assumptions have been used for de-tections in radio AGN. For example, Evans et al. (2005) andSmolˇci´c & Riechers (2011) have used α CO = . M ⊙ (K km s − pc ) − for CO detected in radio galaxies. In Table 2 we list theestimated molecular gas masses using the more conservative as-sumption. Nevertheless, the relatively low metallicity of the gasmay suggest that larger conversion factors would be more real-istic, resulting in higher values for the H masses. However, aneven more complex scenario is conceivable where di ff erences inconversion factors exist between di ff erent regions owing to theirvery di ff erent physical conditions (e.g. velocity structure of thegas, influence of the jet). Ignoring such complications (whichcan only be addressed by more detailed observations) and us-ing the more conservative assumption, the masses of the molec-ular gas range between 0 . × and 1 . × M ⊙ and, likethe kinematics, they show di ff erences between the regions coin-cident with the H i , having the lower H masses, and the onesoutside, showing higher H masses. Again we want to note thatthe apparent di ff erence in masses between inside and outside theH i cloud could be due in part to not taking di ff erent physicalconditions into account. A direct comparison of the masses de-rived above with those of the detections of Charmandaris et al.(2000) is not possible because of the larger area covered by theirmosaic observation. However, the peak fluxes and widths of theprofiles shown by Charmandaris et al. (2000) are very similar tothose of our spectra.In Table 2 we also present the ratio between molecular andatomic gas masses, M H / M H i , where we have derived the H i masses from the data of Oosterloo & Morganti (2005) using an Article number, page 3 of 5 & Aproofs: manuscript no. 28950_Final aperture of the same size as the APEX beam for each position.We find that for the two regions coincident with the H i cloudthis ratio is ∼ > M H / M H i = . i cloud. Consider-ing the di ff erence in the CO-to-H2 conversion factor used, ourresults appear to be consistent with theirs. Despite the uncertain-ties in the conversion factors used, the di ff erence in M H / M H i wefind is too large to be due to these uncertainties alone. In fact, thedistinctive factor for the di ff erence in mass ratios is the lack ofH i at some locations, more so than the di ff erences in molecularmass. We therefore conclude that the large contrast in mass ra-tios suggests that the atomic hydrogen is much more a ff ected bythe jet–cloud interaction than the molecular gas.
5. Origin and fate of the molecular gas
One of the main puzzles of our results is the overall presenceof molecular gas, despite the large range of conditions char-acterising this region near Cen A. One scenario to explain theobservations is that molecular clouds originally residing insidethe H i cloud are not much a ff ected by the jet–cloud interaction,while the more tenuous atomic hydrogen of the H i cloud is. Themolecular gas in the H i cloud is likely to be in denser clumpswhich would, in this scenario, manage to survive the e ff ects ofthe passing jet. On the other hand, the H i itself, being less dense, is a ff ected by the jet and is blown away more easily. The H i isionised while flowing downstream, resulting in high values of M H / M H i . The overall kinematics of the molecular clouds wouldbe very similar to that of the H i cloud (or the extrapolation of itto the regions devoid of H i ). The increased profile widths ofthe molecular gas down the flow would mean that, although thisgas survives, increased turbulence is somehow induced by theinteraction. It has been suggested that the increased turbulencecauses the low star formation e ffi ciency in the outer filament(Salomé et al. 2016). This scenario would explain the relativedistribution of the di ff erent gas phases, the very di ff erent ratios M H / M H i , and some of the kinematics. This scenario may occurif, as suggested by the overall kinematics of the ionised gas, onlya mild interaction occurs between a slowly moving radio jet andthe ISM.An alternative scenario is similar to what is proposed forother cases of jet–cloud interactions (e.g. Tadhunter et al. 2014).Here it is assumed that all the gas (atomic and molecular) getsphoto- or shock-ionised when entering the region of the radio jet.However, denser clumps rapidly reform in the wake of the inter-action and cool very e ffi ciently, forming new clouds of moleculargas downstream which ultimately start forming stars. In this sce-nario H i is only a very short transient phase in the cooling of thegas from ionised to molecular and its column densities are be-low the detection limit. As a result, the molecular gas is the mostmassive component in this region because it is the end product ofa fast cooling process and it accumulates over time, giving highvalues for M H / M H i . Since the molecular clumps form from theionised gas, the kinematics of the molecular gas would mirrorthat of the ionised gas and less that of the atomic gas.With the available data, it is not easy to distinguish betweenthe two scenarios. The large profile widths measured outside theH i cloud, as well as the possible presence of multiple compo-nents in the same velocity range as those seen in the ionised gas,could suggest that the kinematics of the H at these locationsis similar to that of the ionised gas, which covers a larger ve-locity range than that of the H i and which consists of separatecomponents. This may indicate that the molecular gas has the momentum of the interaction encoded in it, in a similar way tothe ionised gas, which would argue for the second scenario. Onthe other hand, the narrow width of profile F5 just outside theH i cloud matches that of F3 and F4 inside it and would better fitthe first scenario of exposed, pre-existing cold H cores. It mightwell be that one scenario is more important in one region and theother scenario at other locations, depending on the local strengthand geometry of the jet–cloud interaction.More extensive observations of the molecular gas, includinghigher resolution imaging, will be required to study this further.Imaging the region at high spatial resolution with ALMA wouldallow us to investigate the turbulence and velocity structure ofthe gas on the scales of individual clouds and as a function of po-sition, at very similar resolution to the optical data. This wouldmake it possible to obtain a detailed view of the connection be-tween the di ff erent gas phases and of the processes which occurin the jet–cloud interaction in the outer filament of Cen A.Note added in proof. During the evaluation of thismanuscript, we became aware of a manuscript by Salome et al.(arXiv:1605.05986) that also discusses CO(2-1) APEX observa-tions of this region of Centaurus A. It mostly addresses distinctissues but is consistent / compatible with our conclusions. Acknowledgements.
We would like to thank Carlos de Breuck and Theresa Nils-son for their support for the observations. This publication is based on data ac-quired with the Atacama Pathfinder EXperiment (APEX) which is a collabora-tion between the Max-Planck-Institut für Radioastronomie, ESO, and the OnsalaSpace Observatory. We acknowledge the use of the GILDAS software for thedata reduction. The research leading to these results has received funding fromthe European Research Council under the European Union’s Seventh FrameworkProgramme (FP / / ERC Advanced Grant RADIOLIFE-320745.
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