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Register a new userA Multi-Wavelength Study of the Young Star Clusters and Interstellar Medium in the Antennae Galaxies (NGC 4038/9)
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Qing Zhang
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We report on a multi-wavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X-rays to radio wavelengths) have become available in the past several years. Quantitative comparisons are made between the positions of the star clusters (broken into three age groups) and the properties of the interstellar medium by calculating the two-point correlation functions. We find that young star clusters are distributed in a clustered fashion. The youngest star clusters are associated with molecular cloud complexes with characteristic radii of about 1 kpc. In addition, there is a weak tendency for them to be found in regions with higher HI velocity dispersions. No dominant triggering mechanism is identified for the majority of the clusters in the Antennae. Feedback from young bright cluster complexes show large H_alpha bubbles and H_alpha velocity gradients in shells around the complexes. We estimate the current star formation rate to be 20 solar mass/yr, and the gas consumption timescale to be 700 Myr. The latter is comparable to the merging time scale and indicates that star formation has been enchanced by the merger event. Finally, we find that the Schmidt law, with index N=-1.4, is also a good description of the cluster formation triggered by merging in the Antennae. There is some evidence that feedback effects may modify the Schmidt law at scales below 1 kpc.
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The formation mechanism of super star clusters (SSCs), a present-day analog of the ancient globulars, still remains elusive. The major merger, the Antennae galaxies is forming SSCs and is one of the primary targets to test the cluster formation mechanism. We reanalyzed the archival ALMA CO data of the Antennae and found three typical observational signatures of a cloud-cloud collision toward SSC B1 and other SSCs in the overlap region; i. two velocity components with $sim$100 km s$^{-1}$ velocity separation, ii. the bridge features connecting the two components, and iii. the complementary spatial distribution between them, lending support for collisions of the two components as a cluster formation mechanism. We present a scenario that the two clouds with 100 km s$^{-1}$ velocity separation collided, and SSCs having $sim$10$^6$-10$^7$ $M_{rm odot}$ were formed rapidly during the time scale. {We compared the present results with the recent studies of star forming regions in the Milky Way and the LMC, where the SSCs having $sim$10$^4$-10$^5$ $M_{rm odot}$ are located. As a result, we found that there is a positive correlation between the compressed gas pressure generated by collisions and the total stellar mass of SSC, suggesting that the pressure may be a key parameter in the SSC formation.
To study the properties of the interstellar medium in the prototypical merging system of the Antennae galaxies (NGC 4038 and NGC 4039), we have obtained CO(1-0), (2-1) and (3-2) line maps, as well as a map of the 870 micron continuum emission. Our results are analysed in conjunction with data from X-ray to radio wavelengths. In order to distinguish between exact coincidence and merely close correspondence of emission features, we compare the morphological structure of the different emission components at the highest available angular resolution. To constrain the physical state of the molecular gas, we apply models of photon dominated regions (PDRs) that allow us to fit CO and [CII] data, as well as other indicators of widespread PDRs in the Antennae system, particularly within the super giant molecular cloud (SGMC) complexes of the interaction region (IAR) between the two galaxies. The modeled clouds have cores with moderately high gas densities up to 4 10^4 / cm^3 and rather low kinetic temperatures <=25K). At present, all these clouds, including those near the galactic nuclei, show no signs of intense starburst activity. Thermal radio or mid-infrared emission are all observed to peak slightly offset from the molecular peaks. The total molecular gas mass of the Antennae system adds up to ~10^10 M_sun. In the vicinity of each galactic nucleus, the moleculargas mass, 1-2 10^9 M_sun, exceeds that of the Galactic centre region by a factorof almost 100. Furthermore, the gas does not seem to deviate much from the N_{H_2}/I_CO ratio typical of the disk of our Galaxy rather than our Galactic centre.
We have used previously published observations of the CO emission from the Antennae (NGC 4038/39) to study the detailed properties of the super giant molecular complexes with the goal of understanding the formation of young massive star clusters. Over a mass range from 5E6 to 9E8 solar masses, the molecular complexes follow a power-law mass function with a slope of -1.4 +/- 0.1, which is very similar to the slope seen at lower masses in molecular clouds and cloud cores in the Galaxy. Compared to the spiral galaxy M51, which has a similar surface density and total mass of molecular gas, the Antennae contain clouds that are an order of magnitude more massive. Many of the youngest star clusters lie in the gas-rich overlap region, where extinctions as high as Av~100 imply that the clusters must lie in front of the gas. Combining data on the young clusters, thermal and nonthermal radio sources, and the molecular gas suggests that young massive clusters could have formed at a constant rate in the Antennae over the last 160 Myr and that sufficient gas exists to sustain this cluster formation rate well into the future. However, this conclusion requires that a very high fraction of the massive clusters that form initially in the Antennae do not survive as long as 100 Myr. Finally, we compare our data with two models for massive star cluster formation and conclude that the model where young massive star clusters form from dense cores within the observed super giant molecular complexes is most consistent with our current understanding of this merging system. (abbreviated)
(abridged) We report here a factor of 5.7 higher total CO flux in Arp~244 (the ``Antennae galaxies) than that previously accepted in the literature (thus a total molecular gas mass of 1.5x10$^{10}$ Msun), based on our fully sampled CO(1-0) observations at the NRAO 12m telescope. Our observations show that the molecular gas peaks predominately in the disk-disk overlap region between the nuclei, similar to the far-infrared (FIR) and mid-infrared (MIR) emission. The bulk of the molecular gas is forming into stars with a normal star formation efficiency (SFE) L_{IR}/M(H_2) approx 4.2 Lsun/Msun, same as that of giant molecular clouds in the Galactic disk. Additional supportive evidence is the extremely low fraction of the dense molecular gas in Arp~244, revealed by our detections of the HCN(1-0) emission. We estimate the local SFE indicated by the ratio map of the radio continuum to CO(1-0) emission. Remarkably, the local SFE stays roughly same over the bulk of the molecular gas distribution. Only some localized regions show the highest radio-to-CO ratios that we have identified as the sites of the most intense starbursts with SFE >~ 20 Lsun/Msun. These starburst regions are confined exclusively in the dusty patches seen in the HST images near the CO and FIR peaks where presumably the violent starbursts are heavily obscured. Nevertheless, recent large-scale star formation is going on throughout the system, yet the measured level is more suggestive of a moderate starburst (SFE >~ 10 Lsun/Msun) or a weak to normal star formation (SFE ~ 4 Lsun/Msun). The overall starburst from the bulk of the molecular gas is yet to be initiated as most of the gas further condenses into kpc scale in the final coalescence.
The Antennae Galaxies is one of the starbursts in major mergers. Tsuge et al. (2020) showed that the five giant molecular complexes in the Antennae Galaxies have signatures of cloud-cloud collisions based on the ALMA archival data at 60 pc resolution. In the present work we analyzed the new CO data toward the super star cluster (SSC) B1 at 14 pc resolution obtained with ALMA, and confirmed that two clouds show complementary distribution with a displacement of $sim$70 pc as well as the connecting bridge features between them. The complementary distribution shows a good correspondence with the theoretical collision model (Takahira et al. 2014), and indicates that SSC B1 having $sim$10$^{6}$ $M$$_{odot}$ was formed by the trigger of a cloud-cloud collision with a time scale of $sim$1Myr, which is consistent with the cluster age. It is likely that SSC B1 was formed from molecular gas of $sim$10$^{7}$ $M$$_{odot}$ with a star formation efficiency of $sim$10 % in 1 Myr. We identified a few places where additional clusters are forming. Detailed gas motion indicates stellar feedback in accelerating gas is not effective, while ionization plays a role in evacuating the gas around the clusters at a $sim$30-pc radius. The results have revealed the details of the parent gas where a cluster having mass similar to a globular is being formed.
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