Imaging the Molecular Outflows of the Prototypical ULIRG NGC 6240 with ALMA
T. Saito, D. Iono, J. Ueda, D. Espada, K. Sliwa, K. Nakanishi, N. Lu, C. K. Xu, T. Michiyama, H. Kaneko, T. Yamashita, M. Ando, M. S. Yun, K. Motohara, R. Kawabe
aa r X i v : . [ a s t r o - ph . GA ] D ec MNRAS , 1–6 (2017) Preprint 22 December 2017 Compiled using MNRAS L A TEX style file v3.0
Imaging the Molecular Outflows of the PrototypicalULIRG NGC 6240 with ALMA
T. Saito, , ⋆ D. Iono, , J. Ueda, D. Espada, , K. Sliwa, K. Nakanishi, , N. Lu, , C. K. Xu, , T. Michiyama, , H. Kaneko, , T. Yamashita, M. Ando, , M. S. Yun, K. Motohara, and R. Kawabe , National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, 69117 Heidelberg, Germany The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA National Astronomical Observatories of China, Chinese Academy of Sciences, Beijing 100012, China China-Chile Joint Center for Astronomy, Chinese Academy of Sciences, Camino El Observatorio, 1515 Las Condes, Santiago, Chile Nobeyama Radio Observatory, National Astronomical Observatory of Japan, Minamimaki, Minamisaku, Nagano 384-1305, Japan Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We present 0 . ′′ × . ′′
53 (470 pc ×
250 pc) resolution CO ( J = 2–1) observations to-ward the nearby luminous merging galaxy NGC 6240 with the Atacama Large Millime-ter/submillimeter Array. We confirmed a strong CO concentration within the central700 pc, which peaks between the double nuclei, surrounded by extended CO featuresalong the optical dust lanes ( ∼
11 kpc). We found that the CO emission around thecentral a few kpc has extremely broad velocity wings with full width at zero inten-sity ∼ − , suggesting a possible signature of molecular outflow(s). In order toextract and visualize the high-velocity components in NGC 6240, we performed a mul-tiple Gaussian fit to the CO datacube. The distribution of the broad CO componentsshow four extremely large linewidth regions ( ∼ − ) located 1-2 kpc away fromboth nuclei. Spatial coincidence of the large linewidth regions with H α , near-IR H ,and X-ray suggests that the broad CO (2–1) components are associated with nuclearoutflows launched from the double nuclei. Key words: galaxies: individual (NGC 6240) — galaxies: interactions — galaxies:evolution — galaxies: active
Gas-rich galaxy mergers play a major role in the forma-tion and evolution of galaxies by triggering intense starformation and changing their morphology as suggestedby numerical simulations (e.g., Barnes & Hernquist 1991).During the process of a galaxy merger, radial stream-ing can feed gas to the central supermassive black hole(SMBH) (e.g., Hopkins et al. 2008), possibly reaching at thequasar-phase after dispersing the obscuring gas and dust(Urrutia et al. 2008). Powerful winds or outflows, driven bythe active galactic nucleus (AGN) or the surrounding star-bursts ((ultra-)luminous infrared galaxy, (U)LIRG), are pre- ⋆ E-mail: [email protected] dicted to suppress gas feeding to the central SMBH andits host galaxy’s spheroidal component in order to explainthe empirical correlation between the stellar velocity disper-sion of a galaxy bulge and the mass of the central SMBH(Costa et al. 2014).Observational studies of such galactic winds/outflowsin (U)LIRGs have been frequently done by spectralanalyses of hot molecular gas, atomic gas, and ionizedgas emission/absorption lines (e.g., Bellocchi et al. 2013;Veilleux et al. 2013). Those tracers are bright enough toprobe generally faint, broad outflowing gas profiles, althoughthey are not likely to be major constituents of galactic out-flows. Cold molecular gas is likely to dominate the outflowin terms of mass, and thus it is the key to understandingfundamental feedback effects of galactic outflows on the sur- © Toshiki Saito et al. rounding interstellar medium (ISM) (Feruglio et al. 2010;Cicone et al. 2014; Fiore et al. 2017).NGC 6240 is a well-studied close galaxy pair in the lo-cal Universe (z = 0.024480, 1 . ′′ L ⊙ ( L IR = 10 . L ⊙ ; Armus et al. 2009). This system has a dou-ble nucleus detected in a variety of wavelengths from X-rayto radio (e.g., Komossa et al. 2003; Hagiwara et al. 2011;Stierwalt et al. 2014; Ilha et al. 2016), indicating the pres-ence of two separated AGNs. The cold molecular ISM inNGC 6240 has been observed through many rotational tran-sitions of CO, HCN, and HCO + , showing a strong gas con-centration between the nuclei (e.g., Nakanishi et al. 2005;Iono et al. 2007; Papadopoulos et al. 2014; Scoville et al.2015; Tunnard et al. 2015; Sliwa & Downes 2017). Althoughthe properties shown above are indeed suitable for repre-senting nearby ULIRGs, past observations have revealed adifferent side of NGC 6240, which makes it rather unique.Lu et al. (2015) reported that NGC 6240 shows an orderof magnitude higher L CO ( − ) / L IR ratio than other local(U)LIRGs. Such an extreme CO line-to-continuum ratio canbe explained by a simple C-shock model (Meijerink et al.2013). Furthermore, deep and wide Subaru observations re-vealed a vastly extended, filamentary H α nebulae ( ∼
90 kpc;Yoshida et al. 2016) coinciding with soft X-ray emission.Contrary to those extended features, the H α and X-ray ob-servations have also detected a compact “butterfly” nebula( ∼ ≤ − ) toward the center(Tacconi et al. 1999; Ohyama et al. 2003; Iono et al. 2007;Feruglio et al. 2013a,b). However, the spatial and velocitystructures of the central kpc of NGC 6240 are still unclearmainly because of the limited sensitivity, angular resolution,and/or bandwidth of previous observations. Also, the com-plex velocity structures due to the ongoing violent mergingevent prevent us from modeling and revealing the underlyinggaseous structures.In this letter, we present Atacama Large Millime-ter/submillimeter Array (ALMA) Band 6 observations ofNGC 6240 in the CO (2–1) line emission and its underly-ing continuum with sub-arcsecond resolution. The goal ofthis letter is to reveal the complete spatial and velocity dis-tribution of the molecular outflow, which has been previ-ously only partly mapped, in NGC 6240. We assumed H = 70 km s − Mpc − , Ω M = 0.3, Ω Λ = 0.7 throughout thisletter. NGC 6240 was observed on 2016 June 26 for the ALMACycle 3 program ID = 2015.1.00003.S using forty-two 12 mantennas. The Band 6 receiver was tuned to cover CO (2–1) ( ν obs = 225.02928 GHz). The field of view, on-sourcetime, and baseline lengths are ∼ ′′ , ∼ ∼ − )with a bandwidth of 1.875 GHz. The single sideband sys-tem temperature after flagging is 50–100 K with two peaks of ∼
190 K at atmospheric absorptions. Precipitable wa-ter vapour during the observations toward NGC 6240 andall calibrators are 0.68–0.82 mm. Titan, J1550+0527, andJ1651+0129 were observed as the amplitude, bandpass, andphase calibrators, respectively. We used the delivered cali-brated u v data. The flux of J1550+0527 at each spw is inagreement with the ALMA Calibrator Source Catalogue (a flux measurement obtained on the same date). Thus, weregard the accuracy of the absolute flux calibration as 5%throughout this paper.The data reduction was carried out using CASA version4.5.3 (McMullin et al. 2007). All maps are reconstructedwith natural weighting. We used the
CASA task clean inmulti-scale mode to make use of the multiscale CLEAN de-convolution algorithm (MS-CLEAN; Cornwell 2008). MS-CLEAN can reduce extended low surface brightness struc-tures in the residual image, and thus recover extended emis-sion significantly larger than the synthesized beam size. Forthe deconvolution process using clean , we performed an it-erative auto-masking procedure as described in the CASAguide . In order to improve the image fidelity, we carried outa two-rounded phase self-calibration after the normal ALMAcalibration process. We chose the bright compact source cor-responding to the peak of the CO spectrum ( ∼ − )for the model used in the self-calibration. Continuum emis-sion was subtracted in the u v -plane by fitting the line-freechannels in both lower and upper sidebands with a first or-der polynomial function, and then we made a CO (2–1) datacube with 5 km s − resolution. The synthesized beam sizeand sensitivity are 0 . ′′ × . ′′
53 (PA = 64 . ◦
5) and 2.5 mJybeam − (at 5 km s − bin), respectively. The CO (2–1) integrated intensity contours overlaid on avelocity field map are shown in Figure 1a. It shows a brightnuclear concentration ( ∼
700 pc, a rough size of the contourwhich has 50% of the peak value by eye) and extended fil-amentary structures (length ∼
11 kpc), which coincide withthe dust lanes in the optical image (Yoshida et al. 2016).Both features match the CO (1–0) emission as reported byFeruglio et al. (2013a). The extended CO emission is com-pact compared to the H α distribution ( ∼
90 kpc) and theremnant of the main stellar disk ( ∼
20 kpc) (Yoshida et al.2016). There are extremely broad, asymmetric velocitywings, ranging from ∼ ∼ − in the radioLSRK velocity frame (full width at zero intensity (FWZI) ∼ − ), within the central 4 ′′ aperture (Figure 1aand 1b). This linewidth is broader than that previously re-ported for CO lines (e.g., Feruglio et al. 2013a), because ofthe higher sensitivity and wider bandwidth of our CO data.The observed total integrated intensity inside the 3 σ con-tour is 1283 ±
64 Jy km s − , which is 86 ±
15 % of thesingle-dish value (Papadopoulos et al. 2014). We also de-tected CN (2 / –1 / ) and CS (3–2) lines, which will be pre-sented in a future paper. https://almascience.nrao.edu/sc/ https://casaguides.nrao.edu/index.php/M100 Band3 Combine4.3000
15 % of thesingle-dish value (Papadopoulos et al. 2014). We also de-tected CN (2 / –1 / ) and CS (3–2) lines, which will be pre-sented in a future paper. https://almascience.nrao.edu/sc/ https://casaguides.nrao.edu/index.php/M100 Band3 Combine4.3000 , 1–6 (2017) olecular Outflows of NGC 6240 with ALMA a) V e l o c i t y D i s pe r s i on ( k m s - ) b) Figure 1. (a) CO (2–1) velocity field (moment 1) image of NGC 6240. The velocity field in color scale ranges from 6600 to 7600 km s − .Contours show the CO (2–1) integrated intensity (moment 0) map. The contours are 208 × (0.125, 0.25, 0.5, 1.0, 2.0, 4.0, 8.0, 16.0, 32.064.0, and 96.0) Jy beam − km s − . The crosses indicate the peak positions of the Band 6 continuum emission. (b) CO (2–1) spectrumtoward the central 4 ′′ aperture, which is shown in Figure 1a, with a zoomed panel to emphasize the wings. eastsouth westnorthdataresidual Figure 2.
CO (2–1) spectrum (blue), residual (green), and two-component Gaussian model (red = narrow, purple = broad, andblack = narrow + broad) within 2 . ′′ We implemented one and two Gaussian fits to disen-tangle outflow signatures from the chaotic velocity field ofNGC 6240 (Figure 1a). A hexagonal Nyquist sampling with2 . ′′ . ′′
0) was performed on the CO (2–1)cube to decrease the number of spectra (from 500 ×
500 pix-els to 40 ×
40 apertures) for the fit. We fit the data as follows:(1) perform a one-component fit for spectra with a peak S/N > >
5, perform a two-component fit to the data (notthe residual). Thus, the resultant best-fit model could haveone or two Gaussian component(s) at each aperture. For theone-component fit, the three initial parameters (i.e., peak,velocity, and FWHM) were chosen from the peak flux, peakvelocity, and the number of channels, which satisfy peak S/N >
5. For the two-component fit, initial parameters of the firstGaussian are obtained from the one-component fit, althoughthose of the second Gaussian were chosen from the residualof the one-component fit following the procedure used for estimating the initial guess of the one-component fit. Ex-amples of multiple Gaussian fits that require two Gaussiancomponents are shown in Figure 2. The peaks seen in theresidual of the two-component fit are statistically insignifi-cant (peak S/N < ≤
240 km s − ) are mainly found along the optical dust lanes,which is similar to that of giant molecular associations foundin the nearby merging galaxy the Antennae ( ≤
210 km s − ;Espada et al. 2012; Ueda et al. 2012). On the other hand,the large linewidth components reach unusually large valuesof ∼ − . We performed a Gaussian fit to this plot.The best-fit Gaussian (green line) shows a peak at 49 km s − and the dispersion of 160 km s − (i.e., + σ ≃
210 km s − ).There are 12% outliers larger than the + σ limit (at FWHM ∼
530 km s − ) of the Gaussian population, and thus we sim-ply define data whose linewidths are larger than 500 km s − as “broad components”. The broad components account for .
10% of the total integrated intensity. The linewidth im-age of the broad components is shown in Figure 3b and 3c.The image area of Figure 3b roughly corresponds to that ofFigure 1a.
The origin of the CO velocity wings in the central region ofNGC 6240 has been discussed for years, and recent studiesbased on interferometric observations suggested the presenceof massive molecular outflow(s) (see Section 1). However, thespatial and velocity distributions of the molecular outflowsare unclear yet mainly because of the complicated CO ve-locity structures and the instrumental limits.The broad component is located around the central3.4 kpc, although large linewidth regions are not locatedat the nuclear positions. The linewidth of the broad com-ponents peak at four positions around the nuclei, whose
MNRAS , 1–6 (2017)
Toshiki Saito et al. a) histogram c) FWHM (broad component)b) FWHM (broad component) east northsouth west F W H M ( k m s - ) Figure 3. (a) Distribution of the CO (2–1) linewidth in FWHM (km s − ) from every Gaussian component fit. The green line shows thebest fit Gaussian to the histogram, showing that there are outliers higher than 500 km s − (i.e., broad component). The black dashed linemarks where FWHM = 500 km s − . (b) CO (2–1) linewidth map of the broad component in FHWM (km s − ). The green contour showsthe outline of the H α (“butterfly nebula”). Black dots show the center position of each aperture. (c) Zoomed-in CO (2–1) linewidth mapof the broad components with the color bar showing the linewidth range. a) velocity field (broad component) east northsouth west S ys t e m i c V e l o c i t y ( k m s - ) V sys = 7122 km s -1 V sys = 7164 km s -1 Figure 4.
Peak velocity map of the broad component. The ve-locity field in color scale ranges from 6850 to 7350 km s − . Datapoints around the four peaks in Figure 3c are shown. spectra are shown in Figure 2. The broad components showa clearly different spatial distribution with respect to thedense gas tracer, HCN (4–3), which likely traces nuclear ro-tating disks (Scoville et al. 2015). The four peak positionsof the broad components roughly coincide with peaks ofH α (Gerssen et al. 2004), near-IR H (Max et al. 2005), and0.5-1.5 keV X-ray (Komossa et al. 2003), indicating thatthe CO broad components arise from a multi-phase ISM.This is also supported by the maximum velocity of theCO (2–1) emission (FWZI/2 ∼ − ; Figure 1b),which is consistent with the maximum velocity of a warmmolecular outflow traced by blueshifted OH absorption( ∼ − ; Veilleux et al. 2013) and H α emission ( ≥ − ; Heckman et al. 1990). As seen in other LIRGs(e.g., NGC 1068; Garc´ıa-Burillo et al. 2014, NGC 3256;Sakamoto et al. 2014, and NGC 1614; Saito et al. 2017),molecular outflows are often spatially extended (a few kpc)and concomitant with (or entrained by) ionized gas outflow.Considering all evidences described above and some similar-ities with other LIRGs, we suggest that the four peaks ofthe broad CO (2–1) components trace molecular outflows inNGC 6240. We derive physical parameters of the molecular outflowsfound in NGC 6240. The spatially-resolved map (Figure 3c)allows us to estimate the mass and projected distance of thelinewidth peaks from the launching points. We assume thelaunching points are the double nuclei, that is, the north-ern and eastern (southern and western) components of themolecular outflow come from the northern (southern) nu-cleus (see gray arrows in Figure 4). This spatial configura-tion is supported by radio continuum observations with verylong baseline interferometers (Gallimore & Beswick 2004;Hagiwara et al. 2011), showing that the northern nucleushas an east-west bipolar structure. The direction of thosebipolar structures are similar to our assumed morphology ofthe molecular outflows. We also assume that the inclination( α ), CO (2–1)/CO (1–0) line intensity ratio, and CO (1–0)luminosity to H mass conversion factor of the molecularoutflows in NGC 6240 are 45 ◦ , unity, and 0.8 M ⊙ (K km s − pc ) − , respectively, for the sake of simplicity.To estimate the mass outflow rate ( Û M H , out ), we employthe expression, Û M H , out = v out , proj M H , out R out , proj tan α, (1)where v out , proj is the projected velocity of the outflow, M H , out is the gas mass of the outflow, R out , proj is the distance fromthe launching point to the outflowing gas, and α is the in-clination of the outflow. This equation assumes a uniformlyfilled cone geometry (Maiolino et al. 2012). All derived pa-rameters related to Û M H , out are listed in Table 1. v out , proj ofthe northern and eastern (southern and western) outflowsare derived by using the projected velocity of the outflow-ing gas (Figure 4) and the systemic velocity of the northern(southern) nucleus of 7122 (7164) km s − in the radio LSRKvelocity frame (Hagiwara 2010).The derived mass of each broad component is ∼ . − . M ⊙ ( ∼ . M ⊙ in total, which is consistent withthe value derived from unresolved CO data; Cicone et al.2014). Since the ionized gas mass of the butterfly nebula is < × M ⊙ (Yoshida et al. 2016), >
70 % of the total gasmass in the outflow consists of molecules. The total Û M H , out of ∼ M ⊙ yr − is 3.5 times lower than that estimated byCicone et al. (2014). This is due to the difference of the as- MNRAS000
70 % of the total gasmass in the outflow consists of molecules. The total Û M H , out of ∼ M ⊙ yr − is 3.5 times lower than that estimated byCicone et al. (2014). This is due to the difference of the as- MNRAS000 , 1–6 (2017) olecular Outflows of NGC 6240 with ALMA Table 1.
NGC 6240 molecular outflow properties.Property east south west north total R out , proj (pc) 1960 1240 1370 830 ... v out , proj (km s − ) −
70 +150 −
270 +180 ...age (Myr) 27 8 5 5 ... log M H , out ( M ⊙ ) 7.9 8.1 8.2 8.2 8.7 Û M H , out ( M ⊙ yr − ) 8 43 86 94 231 sumed geometry. Cicone et al. (2014) assumed a sphericalgeometry because their data did not spatially resolve thebroad CO component, and we assumed a conical geometrywith a certain inclination. Using the nuclear SFR of ∼ M ⊙ yr − derived from radio-to-FIR SED fitting (Yun & Carilli2002), the mass loading factor ( Û M H , out divided by SFR) is ∼
4, indicating that either the AGN, or the starburst, or bothare the main drivers of the outflows. We also estimated theage (= R out , proj / v out , proj assuming the inclination of 45 ◦ ) ofeach broad component, and found that it ranges from 5 to27 Myr. This is consistent with the age of the central H α structures from the butterfly nebula (8.4 Myr) to the “hour-glass” (24 Myr) (Yoshida et al. 2016), indicating that, again,the H α and CO (2–1) outflows are colocated.Note that, although the spatial characteristics of thebroad CO components in NGC 6240 could also be explainedby inflow motions (e.g., Gaspari et al. 2017) rather than out-flow, we favor the latter because the absorption profile of theOH doublet toward the central 9 ′′ shows that a high veloc-ity component faster than 1000 km s − is only detected onthe blueshifted side (Veilleux et al. 2013). We present the ALMA high-resolution observations ofCO (2–1) line toward the prototypical ULIRG NGC 6240,which is known to have broad CO profiles around the center,although no one has succeeded in extracting their spatial dis-tributions. Our high sensitivity and wide-band ALMA datarevealed the presence of extremely broad CO wings (FWZI ∼ − ), which are as fast as the OH and H α outflows.We performed multiple Gaussian fitting for the CO datacubeto visualize the morphology of the wings. We found that forthe first time the broad component presents four peaks 1-2kpc away from the double nuclei. We also found a spatialconnection between nuclear bipolar structures in radio, CO,and the H α and X-ray emission, and thus we suggest thatthe CO wings are associated with twin east-west bipolarmolecular outflows launched from the nuclei. ACKNOWLEDGEMENTS
The authors thanks an anonymous referee for commentsthat improved the contents of this paper. T.S. thanks T.H. Saitoh and R. Maiolino for useful discussion. T.S. andthe other authors thank the ALMA staff for their kind sup-port. This paper makes use of the following ALMA data:ADS/JAO.ALMA
REFERENCES
Armus, L., Mazzarella, J. M., Evans, A. S., et al. 2009, PASP,121, 559Barnes, J. E., & Hernquist, L. E. 1991, ApJ, 370, L65Bellocchi, E., Arribas, S., Colina, L., & Miralles-Caballero, D.2013, A&A, 557, A59Cicone, C., Maiolino, R., Sturm, E., et al. 2014, A&A, 562, A21Cornwell, T. J. 2008, IEEE Journal of Selected Topics in SignalProcessing, 2, 793Costa, T., Sijacki, D., & Haehnelt, M. G. 2014, MNRAS, 444,2355Espada, D., Komugi, S., Muller, E., et al. 2012, ApJ, 760, L25Feruglio, C., Maiolino, R., Piconcelli, E., et al. 2010, A&A, 518,L155Feruglio, C., Fiore, F., Maiolino, R., et al. 2013, A&A, 549, A51Feruglio, C., Fiore, F., Piconcelli, E., et al. 2013, A&A, 558, A87Fiore, F., Feruglio, C., Shankar, F., et al. 2017, A&A, 601, A143Gallimore, J. F., & Beswick, R. 2004, AJ, 127, 239Garc´ıa-Burillo, S., Combes, F., Usero, A., et al. 2014, A&A, 567,A125Gaspari, M., Temi, P., & Brighenti, F. 2017, MNRAS, 466, 677Gerssen, J., van der Marel, R. P., Axon, D., et al. 2004, AJ, 127,75Hagiwara, Y. 2010, AJ, 140, 1905Hagiwara, Y., Baan, W. A., & Kl¨ockner, H.-R. 2011, AJ, 142, 17Heckman, T. M., Armus, L., & Miley, G. K. 1990, ApJS, 74, 833Hopkins, P. F., Hernquist, L., Cox, T. J., & Kereˇs, D. 2008, ApJS,175, 356-389Ilha, G. d. S., Bianchin, M., & Riffel, R. A. 2016, Ap&SS, 361,178Iono, D., Wilson, C. D., Takakuwa, S., et al. 2007, ApJ, 659, 283Komossa, S., Burwitz, V., Hasinger, G., et al. 2003, ApJ, 582,L15Lu, N., Zhao, Y., Xu, C. K., et al. 2015, ApJ, 802, L11Maiolino, R., Gallerani, S., Neri, R., et al. 2012, MNRAS, 425,L66Max, C. E., Canalizo, G., Macintosh, B. A., et al. 2005, ApJ, 621,738McMullin, J. P., Waters, B., Schiebel, D., Young, W., & Golap,K. 2007, Astronomical Data Analysis Software and SystemsXVI, 376, 127Meijerink, R., Kristensen, L. E., Weiß, A., et al. 2013, ApJ, 762,L16Nakanishi, K., Okumura, S. K., Kohno, K., Kawabe, R., & Nak-agawa, T. 2005, PASJ, 57, 575Ohyama, Y., Yoshida, M., & Takata, T. 2003, AJ, 126, 2291Papadopoulos, P. P., Zhang, Z.-Y., Xilouris, E. M., et al. 2014,ApJ, 788, 153Saito, T., Iono, D., Xu, C. K., et al. 2017, ApJ, 835, 174Sakamoto, K., Aalto, S., Combes, F., Evans, A., & Peck, A. 2014,ApJ, 797, 90Scoville, N., Sheth, K., Walter, F., et al. 2015, ApJ, 800, 70Sliwa, K., & Downes, D. 2017, arXiv:1704.03766Stierwalt, S., Armus, L., Charmandaris, V., et al. 2014, ApJ, 790,124Tacconi, L. J., Genzel, R., Tecza, M., et al. 1999, ApJ, 524, 732Tunnard, R., Greve, T. R., Garcia-Burillo, S., et al. 2015, ApJ,815, 114Ueda, J., Iono, D., Petitpas, G., et al. 2012, ApJ, 745, 65Urrutia, T., Lacy, M., & Becker, R. H. 2008, ApJ, 674, 80-96Veilleux, S., Mel´endez, M., Sturm, E., et al. 2013, ApJ, 776, 27Yoshida, M., Yagi, M., Ohyama, Y., et al. 2016, ApJ, 820, 48MNRAS , 1–6 (2017)
Toshiki Saito et al.
Yun, M. S., & Carilli, C. L. 2002, ApJ, 568, 88This paper has been typeset from a TEX/L A TEX file prepared bythe author. MNRAS000