Dense CO in Mrk 71-A: Superwind Suppressed in a Young Super Star Cluster
M. S. Oey, C. N. Herrera, Sergiy Silich, Megan Reiter, Bethan L. James, A. E. Jaskot, Genoveva Micheva
DDraft version September 19, 2018
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DENSE CO IN MRK 71-A: SUPERWIND SUPPRESSED IN A YOUNG SUPER STAR CLUSTER
M. S. Oey, C. N. Herrera, Sergiy Silich, Megan Reiter, Bethan L. James, A. E. Jaskot, ∗ andGenoveva Micheva Department of AstronomyUniversity of Michigan1085 South University Ave.Ann Arbor, MI 48109-1107, USA Institut de Radioastronomie Millim´etrique300 Rue de la PiscineDomaine Universitaire38406, Saint-Martin-d’H`eres, France Insituto Nacional de Astrof´ısica, Optica y Electr´onicaAP 5172000 Puebla, Mexico Space Telescope Science Institute3700 San Martin DriveBaltimore, MD 21218, USA Department of AstronomyUniversity of MassachusettsAmherst, MA 01003, USA (Received 2017 September 12; Accepted 2017 October 8)
ABSTRACTWe report the detection of CO( J = 2 −
1) coincident with the super star cluster (SSC) Mrk 71-A in the nearbyGreen Pea analog galaxy, NGC 2366. Our NOEMA observations reveal a compact, ∼ M (cid:12) ) is similar to that of the SSC, consistent with a high star-formation efficiency, on the order of 0.5.There are two, spatially distinct components separated by 11 km s − . If expanding, these could be due to momentum-driven, stellar wind feedback. Alternatively, we may be seeing the remnant infalling, colliding clouds responsible fortriggering the SSC formation. The kinematics are also consistent with a virialized system. These extreme, high-density, star-forming conditions inhibit energy-driven feedback; the co-spatial existence of a massive, molecular cloudwith the SSC supports this scenario, and we quantitatively confirm that any wind-driven feedback in Mrk 71-A ismomentum-driven, rather than energy-driven. Since Mrk 71-A is a candidate Lyman continuum emitter, this impliesthat energy-driven superwinds may not be a necessary condition for the escape of ionizing radiation. In addition, thedetection of the nebular continuum emission yields an accurate astrometric position for the Mrk 71-A. We also detectfour other massive, molecular clouds in this giant star-forming complex. Keywords: stars: formation — stars: massive — ISM: bubbles — ISM: molecules — galaxies: starburst— galaxies: star clusters: general
Corresponding author: M. S. [email protected] ∗ Hubble Fellow a r X i v : . [ a s t r o - ph . GA ] O c t Oey et al. INTRODUCTIONHow and when do newborn super star clusters (SSCs), like the progenitors of globular clusters, emerge from densegas and clear their surroundings? The classical model for starburst mechanical feedback holds that stellar winds andsupernovae inside SSCs collide and merge to form strong superwinds (Chevalier & Clegg 1985). This hot ( (cid:38) K),shock-heated gas carries off most of the newly synthesized heavy elements produced by the massive stars and theirsupernovae. In dwarf galaxies, these metals could be ejected into the intergalactic medium, retarding chemical evolutionof the host galaxy. However, more recent developments in our understanding of both massive star feedback and chemicalabundance patterns suggest that this picture is more complex. Theoretical work suggests that catastrophic coolingand large ambient gas pressure may inhibit the development of superwinds and wind-driven superbubbles (Silich et al.2007; Silich & Tenorio-Tagle 2017), and radiation feedback is suggested to be important in the youngest and densestclusters (e.g., Freyer et al. 2003; Krumholz & Matzner 2009). On the observational side, multiple stellar populationswith differing abundances are now ubiquitous in individual globular clusters (e.g., Gratton et al. 2012), suggestingthat star formation may have recurred in these objects. So then what factors dominate the interaction between abrand-new SSC and its gaseous environment? Observations of newly born SSCs that are still clearing their gas arevital to revealing these processes. Here, we present remarkable CO observations revealing that a young, 10 M (cid:12) SSC,Mrk 71-A, located in a relatively clear environment, nevertheless coexists with an extremely compact, molecular cloudof comparable mass.
Figure 1.
Composite image of Mrk 71 using
HST /WFC3 data from James et al. (2016). Red, green, blue correspond to[
O ii ] λ O iii ] λ − , selecting pixels with emission > σ . Contour levels are 0.03,0.06, 0.09, 0.12, and 0.15 Jy/beam km/s. Black contours show the corresponding continuum emission integrated over 3.6 GHz,excluding 10 MHz of line emission, with contour levels at 2, 3, and 4 × − Jy/beam. North is up, east to the left.
Mrk 71 is a well-studied starburst complex in the nearby barred Im galaxy, NGC 2366, at a distance of 3.4 Mpc(Tolstoy et al. 1995). It hosts two SSCs, Mrk 71-A and B (Figure 1). Mrk 71-B is an exposed cluster with mass1 . × M (cid:12) (Micheva et al. 2017) and age of 3 − O in Mrk 71-A: Superwind Suppressed M (cid:63) = 1 . × M (cid:12) (Micheva et al. 2017; Drissen et al. 2000). To date, Mrk 71-A hasonly been detected as a compact, extreme excitation H ii region. No stellar features are spectroscopically detected,but strong nebular continuum is observed, including inverse Balmer and Paschen jumps (e.g., Gonz´alez-Delgado etal. 1994; Guseva et al. 2006). It is marginally spatially resolved with the
Hubble Space Telescope (HST), implyinga radius of (cid:46)
H ii region (Drissen et al. 2000), but hosting a 10 M (cid:12) newly-formed, embedded cluster.The Mrk 71/NGC 2366 system is especially interesting owing to its status as a local analog to extreme “GreenPea” galaxies. These are objects at z ∼ . O iii ] λ O ii ] λ (cid:38) β equivalent widths >
100 ˚A, and they are in many ways good analogs ofhigh-redshift starbursts. We showed that among these objects, those with the most extreme ionization parameters arestrong candidate Lyman continuum emitters (LCEs; Jaskot & Oey 2013). Our prediction was dramatically confirmedwhen Izotov et al. (2016a, b) detected Lyman continuum emission from all five of their targeted Green Peas, therebyinstantly doubling the number of confirmed local LCEs. The properties of Mrk 71 are quantitatively consistent withthose of Green Peas and, like those objects, Mrk 71 shows a variety of features that are consistent with optically thinLyman continuum (Micheva et al. 2017). Thus, Mrk 71/NGC 2366 may also offer a unique opportunity to study whatproperties facilitate the escape of ionizing radiation. CO OBSERVATIONSWe obtained observations of Mrk 71 on 2016 December 14 and 15, with the Northern Extended Millimeter Array(NOEMA) at Plateau de Bure, in the CO ( J = 2 −
1) line at 230.538 GHz, with a local standard of rest velocityof 90 km s − . The observations were carried out with 8 antennae in Configuration A, with baselines between 45 mand 760 m. We used the Widex correlator, with total bandwidth of 3.6 GHz and native spectral resolution 1.95 MHz(2.6 km s − ). The quasars 0716+714 and 0836+710 were observed as phase and amplitude calibrators, 3C 84 as band-pass calibrator, and the stars MWC 349 and LkHA 101 (1.91 Jy and 0.56 Jy at 230.5 GHz) as absolute flux calibrators.System temperatures were between 90 and 240 K, and the precipitable water vapor ∼ , using standard procedures.Images were reconstructed using natural weighting, resulting in a synthesized beam of 0 . (cid:48)(cid:48) × . (cid:48)(cid:48)
35 (PA=55.8 ◦ ) andrms noise in the CO map of 2.2 mJy beam − in a 2.6 km s − channel. The continuum map has rms noise of 75 µ Jybeam − .The 22 (cid:48)(cid:48) field of view encompasses the entire Mrk 71 complex. Figure 1 shows a 3-color image using HST /WFC3 datain [
O ii ] λ O iii ] λ HST absolute astrometryhas uncertainty of ∼ (cid:48)(cid:48) , while the NOEMA astrometry is good to 0.05 (cid:48)(cid:48) . However, since Mrk 71-A is a strong nebularcontinuum source, our detected mm source must correspond to this target. Flux densities for Mrk 71-A from the VLAreported at 3.6, 6, and 20 cm by Chomiuk & Wilcots (2009) with angular resolution of 3.7 (cid:48)(cid:48) yield a spectral index of–0.13, indicating a thermal H ii region continuum. Our NOEMA continuum observation of 0.71 mJy is less than thepredicted value of 3.35 mJy extrapolated from the VLA data. However, we only detect the high surface-brightness coreof Mrk 71-A in the NOEMA continuum (Figure 1). Since the VLA beam diameter is about 10 × larger than for ourobservations, the extended nebular emission dominates the flux in the large aperture. The non-detection of any otherNOEMA continuum source within the VLA beam and the agreement of the flux densities with a thermal power-lawindex therefore indicate that the observed emission must be due to the same source, Mrk 71-A. In Figure 1, we havetherefore aligned the data so that the mm continuum peak necessarily coincides with the optical H ii region centroid.The position for Mrk 71-A is thus: 07 h m s . , +69 ◦ (cid:48) (cid:48)(cid:48) .
07 (J2000) with uncertainty of 0.05 (cid:48)(cid:48) .Figure 1 shows a detection of CO(2–1) coincident with the thermal continuum source. The first two lines of Table 1show the source parameters based on calculating the flux above 2 σ and 3 σ . Mrk 71 has 12 + log(O / H) = 7 .
89 (Izotovet al. 1997). For this low, SMC-like value, the transition ratio R CO ≡ CO(2–1)/CO(1–0) is (cid:38)
1, observed in SMC
H ii regions (Rubio et al. 1993), and similar ratios are seen in 30 Doradus ( R CO = 0 . ± .
06; Sorai et al. 2001). Thus, Oey et al.
Table 1.
CO(2–1) Observations of Mrk 71Cloud a Size v LSR ∆ v b Peak Flux I CO Flux mass c Virial mass d pc [km/s] [km/s] [mJy] [mJy km/s] [K km/s] [10 M (cid:12) ] [10 M (cid:12) ]1 9.0 77.7 ± ± ± ±
25 13 ± ± ±
51 ( > σ ) 5.5 78.1 ± ± ± ±
18 15 ± ± ± ± ± ± ±
24 5.9 ± . ± ± ± ± ± ±
31 6.6 ± ± ± ± ± ± ±
36 12 ± ± ±
53 9.7 75.0 ± ± ± ±
28 14 ± ± ± ± ± ± ±
25 9 ± ± ± ± ± ± ±
19 7 ± ± ± a Values measured from pixels with fluxes > σ , unless otherwise specified. “Blue” and “red” denote thecomponents of Cloud 1. b FWHM of CO line. c Mass based on CO intensity estimated using X CO = 50 × cm − (K km / s) − . The errors showncorrespond to measurement uncertainty only, while X CO is uncertain to a factor of a few. d Virial mass calculated as 190 r (pc) × [∆ v (km / s)] (MacLaren et al. 1988), where the cloud radius r is halfthe value in Column 2. Errors shown are estimated only from the error in the fit of ∆ v . we take R CO = 1. The CO-to-H conversion factor from CO(1–0), X CO , is quite uncertain at this low metallicity;based on studies of other blue compact dwarfs (Amor´ın et al. 2016; Bolatto et al. 2013; Leroy et al. 2011), we adopt X CO = 50 × cm − / (K km s − ), yielding a total molecular gas mass of M g = 1 × M (cid:12) . This cloud mass issimilar to our estimated mass of M (cid:63) = 1 . × M (cid:12) for the enshrouded SSC, based on the H α luminosity (Michevaet al. 2017). Taken together, the masses agree well with the virial mass (Table 1).However, we caution that X CO is uncertain to a factor of a few. If the system is not virial, the cloud mass may be asubstantial upper limit. On the other hand, the Mrk 71-A system strongly resembles the molecular cloud NGC 5253-D1, which is associated with a radio supernebula and SSC (Turner et al. 2015, 2017). Turner et al. (2017) suggestthat the SSC may be overluminous for its mass; this may also apply to Mrk 71-A, where we indeed suggested possibleevidence for very massive stars (VMS) of >
100 M (cid:12) (Micheva et al. 2017). If these dominate the SSC luminosity, thenits stellar mass would be overestimated. In any case, the relative masses of the cloud and SSC are consistent with anextremely high star-formation efficiency (SFE), on the order of 0.5. This is again similar to NGC 5253-D1 (Turner etal. 2015) and SGMC 4/5-B1 in the Antennae (Herrera & Boulanger 2017).We detect four other sources at > σ . All are in the region south of Knot A, within 50 pc of the SSC (Figures 1,2), with no continuum detections. These are all molecular clouds similar in mass to Knot A, with parameters given inTable 1. Source 2 has a large line width, similar to Source 1 (Figure 2). Source 3 has the highest peak intensity, butshows no evidence of star formation. FEEDBACK IN MRK 71-AYoung SSCs are thought to clear their environment through massive star feedback, but the details of how and whenthis takes place are poorly understood. Alternatively, extremely high SFE may itself account for some gas clearing(Kruijssen 2012). Typically, feedback is characterized by a model in which the mechanical energy of stellar winds andsupernova ejecta is thermalized, leading to a high central overpressure. The hot, shocked gas flows away from the clusteras a superwind, colliding with the ambient ISM and forming a bubble that grows in an energy-conserving mode (Weaveret al. 1977; Mac Low & McCray 1988). Through photoionization and radiation pressure, Lyman continuum (LyC)photons from the hot, massive stars also generate feedback effects that can be of comparable magnitude to mechanicalfeedback, especially for the youngest and most compact SSCs (e.g., Freyer et al. 2003; Krumholz & Matzner 2009;
O in Mrk 71-A: Superwind Suppressed Figure 2.
Intensity map and CO(2–1) line profiles for all newly detected CO clouds shown in Figure 1. Source 1 is Mrk 71-A.
Dale 2015, but see Silich & Tenorio-Tagle 2013; Mart´ınez-Gonz´alez et al. 2014). It is essential to understand howthese processes interact to determine the duration of the clearing process, and how the gas properties and conditionsevolve. Mrk 71-A offers a unique opportunity to test feedback theories. Its mass, compactness, and extreme youthplace it in the critical regime where the dominant processes are unclear, and its proximity, with an unobscured line ofsight, allows high quality estimates of the physical parameters for quantitative modeling.The observed CO(2–1) emission line is double-peaked, with a velocity separation of 11.1 km s − (Figure 3a). Fig-ure 3b shows the spatial distribution of the two components, showing that they are not fully coincident. At face value,the peaks are offset from the continuum peak by about 1.7 pc (0.10 (cid:48)(cid:48) ) and 6.0 pc (0 . (cid:48)(cid:48) ) in projection, for the blue andred components, respectively (Figure 3b), although the beam size should be noted. These may be separate, distinctclouds, with 7 pc (0.42 (cid:48)(cid:48) ) separation, in projection. Both components appear to be extended, although the red com-ponent is marginally resolved at the 3 σ level. The blue component is resolved, and encompasses the continuum peakin projection, allowing for the possibility that much molecular gas is cospatial with the SSC stars. The kinematics,line fluxes, and implied molecular masses of the two components are given in Table 1, based on gaussian fitting of thespectral lines.To explain the presence of 10 M (cid:12) of molecular gas at such close quarters to the young SSC, a first impression is thatthe gas could simply be the cluster’s natal material that is still in the process of being destroyed through conventionalmechanical and radiation feedback. However, in high-density star-forming conditions, these feedback processes arecounteracted by radiative cooling and larger intra-cloud gas pressure. Our new observations support this dynamic,also seen in M82-A1 (Smith et al. 2006).If the two CO components represent an outflow, they could correspond to mechanical feedback with an expansionradius and velocity of R ∼ . v ∼ − , respectively. If the observed molecular gas corresponds to materialswept up within a 3.5-pc radius, the total molecular mass of M g ∼ M (cid:12) corresponds to an original uniform densityof n ∼ × cm − , a reasonable average value for the dense core of a massive, giant molecular cloud. At these highdensities, cooling dictates that either the wind cannot form (Silich & Tenorio-Tagle 2017), or the system transitionsrapidly from the energy-dominated to momentum-dominated regime (e.g., Mac Low & McCray 1988). The age, t (Myr), of a momentum-driven shell can be estimated as 0 . R/v = 0 . R is given Oey et al.
Figure 3.
Panel a (left) shows the CO(2–1) line profile for Mrk 71-A, extracted from an area of 0 . (cid:48)(cid:48)
68 size, defined as theemission > σ in the map integrated over 72 – 88 km s − . The two fitted gaussian components are shown with dashed blueand red lines, and the single fitted gaussian with the solid green line. Panel b (right) shows the spatial distribution of the twocomponents in blue and red contours, corresponding to the respective kinematic components in the left panel. The continuumflux is indicated by the color scale bar. The contours for the continuum correspond to 2, 3, 4, and 5 σ with rms 0.075 mJy/beam.The contours for the blue and red components are integrated over 72.4 – 77.6 km s − and 82.8 – 88.0 km s − , respectively, bothshowing contours starting at 2 σ in integer σ intervals. For the blue and red components, 1 σ = 12 . by (e.g., McCray & Snow 1979), R = 16 (cid:18) L n v (cid:19) / t / pc , (1)where L and v are the mechanical luminosity and wind velocity in units of 10 erg s − and 1000 km s − ,respectively, providing the impetus. For v = 1, appropriate to massive star winds, this yields L ∼ M (cid:63) ∼ M (cid:12) cluster at SMC metallicity. We further confirm thatthe observed dynamics are inconsistent with a conventional, energy-driven bubble. In this case, the expansion velocity v = 0 . R/t , implying an age of 0.4 Myr. For an adiabatic shell growth (e.g., McCray & Snow 1979), R = 27 (cid:18) L n (cid:19) / t / pc , (2)the same parameters imply L ∼
10, over an order of magnitude too small. Thus, energy-driven feedback doesnot dominate, supporting a catastrophic cooling scenario (Mac Low & McCray 1988; Silich et al. 2007; Krumholz &Matzner 2009). The importance of cooling is also confirmed by 3-D hydrodynamic simulations (Krause & Diehl 2014;Yadav et al. 2017). It is therefore likely that radiation dominates the SSC feedback (e.g., Freyer et al. 2003), especiallyif its age is ≤ α luminosity is 8 . × erg s − (Micheva et al. 2017), which isabout 10 × larger than the total wind power inferred above. Although the bipolar morphology implies only partialshells, this does not affect the basic mechanical feedback calculations; certainly adiabatic, pressure-driven feedback isimpossible with partial shells. We will quantitatively examine the possible feedback mechanisms in detail in a futurework.Alternatively, the two molecular components could be infalling, and perhaps be the remnants of two molecular cloudswhose collision triggered the formation of the SSC. This scenario has been suggested for other SSCs with similar COmorphology by, e.g., Fukui et al. (2016). Figure 4 shows a position-velocity (PV) diagram for Mrk 71-A, showing theprojected kinematic structure. Haworth et al. (2015) find from their simulated observations of colliding clouds thatthe existence of “broad bridge” material between the blue and red components is consistent with colliding clouds. Theexistence of emission between the blue and red components in Figure 4 is morphologically similar to their models,although our spatial resolution is relatively low. The components could also represent random accretion infall ofvestigial material from the natal molecular environment. O in Mrk 71-A: Superwind Suppressed Figure 4.
Position-velocity diagram for CO(2–1) in the region of Mrk 71-A designated in Figure 3b.
Taken as physically distinct clouds, their kinematics are also fully consistent with a virialized system, together withthe SSC, as noted above. The molecular gas is likely clumpy, perhaps dominated by the two unresolved, compact,massive clouds. Since both peaks are offset from the continuum peak (Figure 3b), this enhances the likelihood thatthe molecular clouds do not obstruct the escape of ionizing radiation in our line of sight, consistent with the low LyCoptical depth suggested by Micheva et al. (2017). If the clouds are infalling, then the blue component is likely to bebehind the SSC, which further enhances this scenario.Clarifying the timescale for gas retention and spatial relation to the SSC is critical for understanding the conditionsfor LyC escape and Green Pea-like systems. In particular, adiabatic superwinds have been suggested to be importantin clearing passages for ionizing radiation (e.g., Zastrow et al. 2013; Heckman et al. 2011). The action of mechanicalfeedback takes time, while after 3 Myr, the ionizing stellar population declines (Dove et al. 2000; Fujita et al. 2003).Thus, in this model, there is significant LyC escape only in a short period dominated by classical Wolf-Rayet starsaround age 3 – 5 Myr (Zastrow et al. 2013). However, Mrk 71-A is most likely younger than 3 Myr old, perhaps evenby a factor of 10, and it is a strong candidate LCE, quantitatively matching Green Pea properties in all respects,including extreme excitation and low optical depth (Micheva et al. 2017). Since we show that any mechanical feedbackin this system is not energy-driven, it suggests that superwinds are not a necessary condition for LyC escape. CONCLUSIONIn summary, our NOEMA CO(2–1) observations detect a compact, ∼ M (cid:12) molecular cloud coincident withthe SSC Mrk 71-A, which is of similar mass. At face value, the implied SFE is high, on the order of 0.5, as seen insimilar objects. In the extremely young, high-density star-forming conditions for Mrk 71-A, energy-driven feedbackwill be suppressed by strong, radiative cooling (e.g., Silich et al. 2007; Krause & Diehl 2014; Yadav et al. 2017). Thepresence of a massive, compact, molecular cloud cospatial with the SSC is fully consistent with this expectation, andwe quantitatively demonstrate that any mechanical feedback from the SSC must be momentum-driven. Under thesecircumstances, radiation feedback from the young SSC is likely the dominant feedback mode (e.g., Freyer et al. 2003;Krumholz & Matzner 2009). Given that Mrk 71-A is an extreme Green Pea analog and strong LCE candidate, ourresults suggest that superwinds are not necessary to clear gas for LyC escape.The CO(2–1) data appear to show two, spatially distinct, kinematic components separated by 11 km s − . If expand-ing, these could be due to momentum-driven, stellar wind feedback. Conversely, the components could be collidingclouds responsible for triggering the formation of the SSC, or simply random vestigial accretion. Finally, the kinematicsare also consistent with a virialized system.We also detect the nebular continuum in Mrk 71-A, allowing accurate measurement of its absolute coordinates, andfour additional molecular clouds of similar masses within 50 pc of the SSC. One is unresolved and extremely compact,but no continuum is detected. Oey et al.
We thank Jim Dale, Mark Krumholz, Eric Pellegrini, Linda Smith, and the anonymous referee for useful discussions.This work is based on observations carried out under project number W16BM with the IRAM NOEMA Interferometer.IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
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