arXiv:1612.06488v3 [astro-ph.GA] 20 Mar 2018 ilytefraino asv tr n trclusters, and massive of espe- formation and the regions to central cially galactic order in In SF star- the pressure. understand in and surface gas dispersion, of velocity molecular values density, clouds extreme the exhibits the environments disks, life- to burst ’s galactic Compared host typical the 2013). in of al., gas fraction et molecular (Tacconi tiny all time a turn is to stars) (needed into time depletion the hr F a es ag (10 large so , be starburst can SF SFR in of where epoch found conditions dynamic conditions this the the in resembled that (ISM) medium evidence Dickinson, interstellar & is and (Madau There old years billion 2013). few a only was t aiu tredshift reached at density (SFR) maximum rate its that formation reveal star galaxies cosmic of distant the evolution of the Observations in role key galaxies. a plays (SF) formation Star Introduction 1. 1 oyih 06b h author(s). the Environ- by 2016 Different Copyright in 01. Formation Star ments the of Proceedings MIYA Yusuke and NAKAI, Naomasa Gaku 2-1 University, Gakuin TOMIYASU, Kwansei Physics, of Yuto Department SALAK, [email protected] Dragan ut-ieIaigo h trus aayNC10 ihALM with 1808 NGC Galaxy Starburst the of Imaging Multi-line inars omchsoy nti ae,w rsn our present we ( paper, forma- high-resolution this star recent In of history. process cosmic starburst the across in understand tion to gas key molecular is of galaxies properties the Revealing aatcnrcrdu of radius galactocentric a iesas,a ela nosadotoso turbulent pc. of 100 outflows central and the inflows in as gas mas- well from as star winds stars), nuclear sive and intense pres- explosions by the ( generated suggests formation likely disk shocks, circumnuclear of the ence in (2-1) SiO iisaohrpa of peak another hibits ewe . n nte50p ig 3 h ratio the (3) ring; pc 500 HCO the to is in (1-0) (1-0) HCN 1 CO of to and (3-2) 0.3 CO between traced of gas, ratio to intensity CO elevated of line excitation the X- the by by (2) revealed AGN observations; with low-luminosity ray coincident the CO, of and location mm the 0.87 at continuum dust of h rsneo oeua a ou ihardu of radius a with torus gas molecular indicating a r disk of circumnuclear CO the presence of in the distributions (2-1) the CS in discov- and revealed the (3-2) peak (1) double a findings: of main ery 1808, the NGC of some galaxy highlight starburst and barred nearby, the of tions iisarda rdet hl h ai erae from decreases ratio the while ∼ gradient: radial a hibits ∼ 1 . CS,QyNo,Venm 06 SD:volume PSFDE: 2016. Vietnam, Nhon, Quy ICISE, , 0p;isd h ou,w on opc source compact a found we torus, the inside pc; 30 ntecne to center the in 5 ∼ ntesa-omn ik h ai is ratio the disk; star-forming the in 1 Abstract + ∼ ∼ ∼ 10 ntecnrl1kcex- kpc 1 central the in (1-0) 1 0 1 r . z . ′′ narn-iesrcueat structure ring-like a in 2 nte50p ik tex- it disk, pc 500 the in 9 ∼ r5 c LAobserva- ALMA pc) 50 or ∼ 0 c h eeto of detection The pc. 300 ,we h Universe the when 2, ∼ 10 3 M ⊙ yr − 1 that ) us aaisa ihrslto ( star- resolution of high at studies galaxies observational burst detailed Ar- (ALMA), Millimeter/submillimeter ray Large Atacama Before galaxies. molecular starburst of in properties clouds the understand to essential is it s 0-csae 3 h itiuino es a tracers gas at dense lines HCO of and (1-0) distribution (1-0) CO HCN the and (3) (3-2) scale, CO 100-pc the of ratio intensity ris eetdmsl rmcmatsucs(SF sources compact from mostly dust Detected thermal in of originates distribution grains. that the emission continuum) shows in mm 1d GMCs (0.87 Figure SF by 1c). (denoted pattern Fig clouds multi-arm a molecular in giant the organized and in is gas al., disk molecular pc et that 400 (Salak shows central work image new previous The our of 2016). in As distribution reported 1b,c). the (1-0) resembles (Fig. CO ring structure pattern pc spiral the 500 a the expected, and including CND disk the pc SF between 500 a pc), and 100 central pseudoring, (CND; disk circumnuclear the oeua a ou nteglci etr(eta 100 a (central center of with galactic discovery outline traced the the pc) we in (1) torus presentation, gas on: this molecular focus In 1993; and observations (Phillips, the 2016). 1a) al., (Fig. starburst outflow et nucleus gas Salak barred and the Mpc), lanes from target dust (11 emerging polar The nearby peculiar with a 2). galaxy (cycle 1808, using operations NGC galaxy early is starburst its a in an of represents sit- ALMA work observations The this of and objects. example changed, nearby now few has to uation limited were sitivity 32 t358Gzb LAi hw nFg 1b. unprece- 0 Fig. at of in region resolution shown the angular is throughout dented ALMA detected by was CO GHz in CO observed 345.8 1808 at NGC of (3-2) kiloparsec 1 central The the in core and torus gas Molecular 2.1. Discussion and Results 2. prep). in al. in et presented (Salak be paper will forthcoming analysis De- a comprehensive 1808. more well NGC as a reduction, in as data (2-1) and observations and SiO the disk, about of pc tails detection 500 first central the the in (4) gradient radial a hibits estvt f8mybeam mJy 8 of sensitivity − 1 eeln h itiuino oeua lusin clouds molecular of distribution the revealing , n ad,6913 yg,Japan Hyogo, 669-1337 Sanda, en, eta 0 pc 100 central 12 O( CO + J 10,woeitniyrtoex- ratio intensity whose (1-0), 3 = − − 1 )adC 21,()the (2) (2-1), CS and 2) . nacanlo km 5 of channel a in 6 ′′ < ( ∼ 0 c n sen- and pc) 100 0p) iha with pc), 30 MOTO A Multi-line Imaging of the Starburst Galaxy NGC 1808 with ALMA

7 GMCs), the distribution correlates very well with CO; order of ∼ 1 × 10 M⊙. The ISM mass of the same or- these dusty GMCs are the nurseries of newly-formed der of magnitude is obtained from the 0.87 mm contin- massive star clusters revealed in infrared observations uum, with an assumed dust temperature of Td = 25 K, (e.g., Busch et al. 2016). The major difference be- and applying the calculation method in Scoville et al. tween the continuum and CO is observed in the CND (2014). An order of magnitude higher Td would yield (central 100 pc; Fig. 1e). In this region, we reveal approximately an order of magnitude lower ISM mass, for the first time that molecular gas exhibits a dou- implying an even lower value of XCO. In either case, ble peak, whereas the dust continuum is strongest be- the conversion factor in the starburst nucleus of NGC tween the CO peaks, and coincident with a nuclear X- 1808 is at least a factor of two lower than the standard ray source within uncertainty (Jim´enez-Bail´on et al., Galactic value (Bolatto et al., 2013), and the GMC 6 7 2005). This structure can be explained if the con- masses are in the range of 10 -10 M⊙. This is com- tinuum source, denoted by the “core” in Fig. 1f, is parable to the masses of super star clusters in galaxy surrounded by an inclined molecular gas torus (diam- centers. eter ∼ 61 pc if the galaxy is at distance 10.8 Mpc). The core is not at the center of the torus, but offset 2.3. Dense gas tracers HCN (1-0) and HCO+ by ∼ 10 pc; this structure is similar to the nucleus (1-0) of the NGC 1068 (Garc´ıa-Burillo et al., 2016). The scenario of a rotating torus is supported We also observed the dense gas tracers HCN (1- + ′′ ∼ by the velocity information inferred from the CO data 0) and HCO (1-0) at a resolution of 1 ( 50 cube. The molecular gas in the core exhibits a sepa- pc). Both lines were detected throughout the cen- rate velocity component, possibly due to presence of tral 1 kpc, exhibiting an overall structure similar to CO (3-2). In Fig. 2c, we show the distribution of a massive central object (supermassive black hole, nu- + clear cluster), warped disk, or gas inflow/outflow. the line intensity ratio of HCN (1-0) to HCO (1- 0), Rdg ≡ IHCN/IHCO+ . This ratio has been used as a diagnostic tool for the physical conditions of 2.2. High-resolution CO excitation image of molecular gas in active galactic nuclei (AGN), where the starburst region a high Rdg has been explained in terms of den- Using our new CO (3-2) data and the CO (1-0) data sity and temperature effects, X-ray dominated chem- from cycle 1, we derived an intensity ratio of CO (3-2) istry, shocks, and infrared pumping (Kohno et al., to CO (1-0), defined as R31 ≡ I32/I10, where I is the 2001; Imanishi et al., 2006; Meijerink et al., 2006; integrated intensity in brightness units (Fig. 2a). Both Harada et al., 2010; Aalto et al., 2012; Izumi et al., CO images were produced by combining the data ac- 2013; Tafalla et al., 2010). The ratio was imaged quired with the 12-m array, Atacama Compact Array, in NGC 1808 before, although at low resolution and Total Power array, thereby recovering the total (Green et al., 2016). Averaged over image pixels, the flux. The CO (3-2) image was smoothed to the angu- ratio is Rdg = 1.45 ± 0.08 in the CND region and lar and velocity resolution of CO (1-0). Fig. 2a shows Rdg = 0.90 ± 0.03 in the 500 pc ring. The variation that R31 is high (∼ 1) in the central 200 pc, reaching of the ratio is shown in Fig. 2e as an azimuthally a peak of R31 ≃ 1.0 in the galactic center (Fig. 2b); averaged radial profile derived at a resolution of 1.5 this region coincides with the SF disk revealed with ra- arcseconds. Throughout the 500 pc ring and outer re- dio continuum observations (Salak et al., 2016). The gions the ratio is close to unity. However, Fig. 2c 500 pc ring on average exhibits a much lower ratio of shows that Rdg is not constant in the ring, where R31 ∼ 0.6, decreasing to R31 < 0.4 in the outermost values between 0.5 and 1.3 can be found. The ratio parts. Higher CO gas excitation in SF regions can is not constant in the CND either; Fig. 2c,d shows be explained as a consequence of higher gas density that although the value in the galactic center is rel- and kinetic temperature, typical for starburst environ- atively high (1.5), similar to what has been observed ments. For instance, a similar trend of decreasing R31 recently in the Seyfert galaxies NGC 1068 and NGC with radius has also been found in the starburst galaxy 1097 (Garc´ıa-Burillo et al., 2014; Mart´ınet al., 2015), M82 (Salak et al., 2013). the ratio exhibits a secondary peak in a ring-like struc- ture with a radius of r ∼ 300 pc. There are important From the CO excitation image (Fig. 2a), we find implications from this result: (1) We found no correla- R ≃ 0.8 in the GMCs in the inner 400 pc (SF 31 tion between R and X-ray luminosity. While X-ray GMCs in Fig. 1c). Using a conversion factor of dg − regulated chemistry has been proposed to explain the X = 0.8 × 1020 cm−2 (K km s 1)−1, derived by CO elevated R in Seyfert nuclei, the absence of correla- applying radiative transfer modeling on the central 1 dg tion in NGC 1808, which harbors a hard X-ray source kpc (Salak et al., 2014), we obtain GMC masses of the Multi-line Imaging of the Starburst Galaxy NGC 1808 with ALMA

(AGN candidate), implies that Rdg is dominated by galaxy, demonstrating the revolutionary capabilities of another process. (2) Rdg can be enhanced in regions ALMA. While the achieved high resolution yielded a of higher density and temperature, including shocks. fascinating view on the nuclear spiral pattern, distri- The gradient of Rdg in Fig. 2d across the major axis bution of individual GMCs, and the resolved intensity ◦ of the CND (position angle 155 ) indicates that Rdg is ratio of different molecular lines, the high sensitivity elevated in the outer regions of the CND, where molec- allowed detections of weak lines such as SiO (2-1), es- ular gas is exposed to the feedback from the circumnu- sential to understand the physical conditions of molec- clear SF, although optical depth effects may be impor- ular gas. A more comprehensive presentation of the tant in this region. Applying the diagnostic method data will be given in a forthcoming paper (Salak et al. of Kohno et al. (2001) and Imanishi et al. (2006), the in prep.), while future ALMA observations at higher nucleus of NGC 1808 lies in the boundary region be- resolution will be able to resolve the core and reveal tween “pure AGN” and “starburst” classes, possibly a its structure and kinematics. composite. Acknowledgments 2.4. Dense gas tracers CS (2-1) and SiO (2-1) This paper makes use of the following ALMA data: In Fig. 2f, we show the distributions of CS (2-1) and ADS/JAO.ALMA#2013.1.00911.S. ALMA is a part- SiO (2-1), observed at 98.0 and 86.8 GHz, respec- nership of ESO (representing its member states), tively. While CS (2-1) is a reliable dense gas tracer, NSF (USA), and NINS (Japan), together with NRC SiO (2-1) is known as a tracer of fast shocks, prob- (Canada) and NSC and ASIAA (Taiwan), in cooper- ∼ 5 −3 ing the regions of dense (nH 10 cm ) molec- ation with the Republic of Chile. The Joint ALMA ular gas where the sputtering of dust grains occurs Observatory is operated by ESO, AUI/NRAO, and (Mart´ın-Pintado et al., 1992; Schilke et al., 1997). NAOJ. The National Radio Astronomy Observatory is Note that CS (2-1) in the CND exhibits the same dou- a facility of the National Science Foundation operated ble peak structure as CO (3-2), apparently coexisting under cooperative agreement by Associated Universi- in the molecular torus. On the other hand, SiO (2-1), ties, Inc. D.S. was supported by the ALMA Japan detected for the first time in NGC 1808, is more cen- Research Grant of NAOJ Chile Observatory, NAOJ- trally distributed, suggesting the presence of shocked ALMA-59 and NAOJ-ALMA-98. molecular gas in the inner regions of the CND in the vicinity of the continuum core. The shocks may have References been generated by intense SF activity in the nucleus, Aalto, S., et al. 2012, A&A, 537, A44 resulting in supernova explosions, as well as inflows Bolatto, A., et al. 2013, ARA&A, 51, 207 and outflows of gas (Salak et al., 2016; Busch et al., Busch, G., et al. 2016, arXiv:1611.07868 2016). Although SiO (2-1) emission could also be a Garc´ıa-Burillo, S., et al. 2010, A&A, 519, A2 product of an X-ray dominated region (XDR), as dis- Garc´ıa-Burillo, S., et al. 2014, A&A, 567, A125 cussed for the case of NGC 1068 (Garc´ıa-Burillo et al., Garc´ıa-Burillo, S., et al. 2016, ApJ, 823, L12 2010), we prefer the shock scenario because the galaxy Green, C.-E., et al. 2016, MNRAS, 457, 2470 center in NGC 1808 seems to be dominated by SF ac- Harada, N., et al. 2010, ApJ, 721, 1570 tivity with evidence of noncircular motions and large Imanishi, M., et al. 2006, AJ, 131, 2888 velocity dispersion. Izumi, T., et al. 2013, PASJ, 65, 100 Further investigations that include more dense gas Jim´enez-Bail´on, E., et al. 2005 A&A, 442, 861 tracers and their isotopologues, as well as evaluation Kohno, K., et al. 2001, ASPC, 249, 672 of optical depths and column densities will clarify the Madau, P. and Dickinson, M. 2014, ARA&A, 52, 415 physical conditions and abundance variations of dif- Mart´ın, S., et al. 2015, A&A, 573, A116 ferent molecular species in GMCs. NGC 1808 will re- Mart´ın-Pintado, J., et al. 1992, A&A, 254, 315 main an interesting target galaxy for further studies Meijerink, R., et al. 2006, ApJ, 650, L103 with ALMA. Phillips, A. C. 1993, AJ, 105, 486 Salak, D., et al. 2013, PASJ, 65, 66 Salak, D., et al. 2014, PASJ, 66, 96 3. Concluding Remarks Salak, D., et al. 2016, ApJ, 823, 68 These first high-resolution, multi-line observations Schilke, P., et al. 1997, A&A, 321, 293 of NGC 1808 have given us an insight into the Scoville, N., et al. 2014, ApJ, 783, 84 SF process and its feedback in a nearby starburst Tacconi, L., et al. 2013, ApJ, 768, 74 Tafalla, M., et al. 2010 A&A, 522, A91 Multi-line Imaging of the Starburst Galaxy NGC 1808 with ALMA Multi-line Imaging of the Starburst Galaxy NGC 1808 with ALMA 퐼

(a) (b) " (c) J beam [Jy SF GMCs

, + CND

ms km 500 pc spiral pseudoring , + ]

500 pc

(d) (e) (f) Molecular torus radius 푟= 32 ± 5 pc 푆 " J beam [Jy

CND , + ]

Core 50 pc (radio continuum, molecular gas, X-ray emission)

Figure 1. (a) An optical image (B band) of NGC 1808 (NED). (b) CO (3-2) contours (0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 0.95) times the maximum within the rectangle region in panel (a). (c) Illustration of the main structures of the CO (3-2) distribution in panel (b). (d) 0.87-mm continuum (color) with CO (3-2) contours. (e) Enlargement of panel (d). (f) Illustration of the molecular gas distribution in the circumnuclear disk (central 100 pc). The “core” exhibits radio, X-ray, and CO emission, marking the location of the AGN, whereas the double peak seen in panel (e) indicates the presence of a gaseous torus. The 0.87 mm continuum peak coincides with a hard X-ray source within the uncertainty.

(a) CO (3-2)/(1-0) intensity ratio (b) CO (3-2)/(1-0) intensity ratio (c) HCN/HCO+ (1-0) intensity ratio

500 pc 500 pc 50 pc

(d)HCN/HCO+ (J=1-0) intensity ratio (e) (f) SiO (2-1) intensity, CS (2-1) contours

50 pc 50 pc

Figure 2. (a) The intensity ratio of the CO (3-2) and CO (1-0) lines (each line intensity in brightness units; clipped at ′′ 5 σ). The ratio image was derived after adjusting the angular resolution (2 or ∼ 100 pc); CO (3-2) contours at (0.025, − − 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 0.95) times 133.4 Jy beam 1 km s 1. (b) Enlargement of figure (a). (c) The intensity ratio of the HCN (1-0) and HCO+ (1-0) lines at 1′′ resolution with CO (3-2) contours. (d) Enlargement of (c). (e) The azimuthally averaged radial profile of the HCN (1-0) to HCO+ (1-0) ratio at a resolution of 1.5 arcsecs. (f) SiO (2-1) − − integrated intensity and CS (2-1) contours (0.1, 0.2, 0.4, 0.6, 0.8 times 0.744 Jy beam 1 km s 1). The black cross marks the location of the radio continuum core.