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Register a new userMolecular and Ionized Hydrogen in 30 Doradus. I. Imaging Observations
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Sherry C. C. Yeh
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We present the first fully calibrated H$_2$, 1-0 S(1) image of the entire 30 Doradus nebula. The observations were conducted using the NOAO Extremely Wide-Field Infrared Imager on the CTIO 4-meter Blanco Telescope. Together with a NEWFIRM Br$gamma$ image of 30 Doradus, our data reveal the morphologies of the warm molecular gas and ionized gas in 30 Doradus. The brightest H$_2$-emitting area, which extends from the northeast to the southwest of R136, is a photodissociation region viewed face-on, while many clumps and pillar features located at the outer shells of 30 Doradus are photodissociation regions viewed edge-on. Based on the morphologies of H$_2$, Br$gamma$, $^{12}$CO, and 8$mu$m emission, the H$_2$ to Br$gamma$ line ratio and Cloudy models, we find that the H$_2$ emission is formed inside the photodissociation regions of 30 Doradus, 2 - 3 pc to the ionization front of the HII region, in a relatively low-density environment $<$ 10$^4$ cm$^{-3}$. Comparisons with Br$gamma$, 8$mu$m, and CO emission indicate that H$_2$ emission is due to fluorescence, and provide no evidence for shock excited emission of this line.
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Determining the efficiency with which gas is converted into stars in galaxies requires an accurate determination of the total reservoir of molecular gas mass. However, despite being the most abundant molecule in the Universe, H$_2$ is challenging to detect through direct observations and indirect methods have to be used to estimate the total molecular gas reservoir. These are often based on scaling relations from tracers such as CO or dust, and are generally calibrated in the Milky Way. Yet, evidence that these scaling relations are environmentally dependent is growing. In particular, the commonly used CO-to-H$_2$ conversion factor (X$_{rm CO}$) is expected to be higher in metal-poor and/or strongly UV-irradiated environments. We use new SOFIA/FIFI-LS observations of far-infrared fine structure lines from the ionised and neutral gas and the Meudon photodissociation region model to constrain the physical properties and the structure of the gas in the massive star-forming region of 30 Doradus in the Large Magellanic Cloud, and determine the spatially resolved distribution of the total reservoir of molecular gas in the proximity of the young massive cluster R136. We compare this value with the molecular gas mass inferred from ground-based CO observations and dust-based estimates to quantify the impact of this extreme environment on commonly used tracers of the molecular gas. We find that the strong radiation field combined with the half-solar metallicity of the surrounding gas are responsible for a large reservoir of CO-dark molecular gas, leaving a large fraction of the total H$_2$ gas (> 75%) undetected when adopting a standard X$_{rm CO}$ factor in this massive star-forming region.
We present $mathrm{^{12}CO}$ and $mathrm{^{13}CO}$ molecular gas data observed by ALMA, massive early stage young stellar objects identified by applying color-magnitude cuts to textit{Spitzer} and textit{Herschel} photometry, and low-mass late stage young stellar objects identified via H$mathrm{alpha}$ excess. Using dendrograms, we derive properties for the molecular cloud structures. This is the first time a dendrogram analysis has been applied to extragalactic clouds. The majority of clumps have a virial parameter equal to unity or less. The size-linewidth relations of $mathrm{^{12}CO}$ and $mathrm{^{13}CO}$ show the clumps in this study have a larger linewidth for a given size (by factor of 3.8 and 2.5, respectively) in comparison to several, but not all, previous studies. The larger linewidths in 30 Doradus compared to typical Milky Way quiescent clumps are probably due to the highly energetic environmental conditions of 30 Doradus. The slope of the size-linewidth relations of $mathrm{^{12}CO}$, 0.65 $pm$ 0.04, and $mathrm{^{13}CO}$, 0.97 $pm$ 0.12, are on the higher end but consistent within 3$mathrm{sigma}$ of previous studies. Massive star formation occurs in clumps with high masses ($> 1.83 times 10^{2};mathrm{M_{odot}}$), high linewidths (v $> 1.18;mathrm{km/s}$), and high mass densities ($> 6.67 times 10^{2};mathrm{M_{odot};pc^{-2}}$). The majority of embedded, massive young stellar objects are associated with a clump. However the majority of more evolved, low-mass young stellar objects are not associated with a clump.
New sensitive CO(2-1) observations of the 30 Doradus region in the Large Magellanic Cloud are presented. We identify a chain of three newly discovered molecular clouds we name KN1, KN2 and KN3 lying within 2--14 pc in projection from the young massive cluster R136 in 30 Doradus. Excited H$_2$ 2.12$mu$m emission is spatially coincident with the molecular clouds, but ionized Br$gamma$ emission is not. We interpret these observations as the tails of pillar-like structures whose ionized heads are pointing towards R136. Based on infrared photometry, we identify a new generation of stars forming within this structure.
Using observations obtained with the Wide Field Camera 3 (WFC3) on board the Hubble Space Telescope (HST), we have studied the properties of the stellar populations in the central regions of 30 Dor, in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterise and quantify this effect using young massive main sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main sequence (PMS) stars by looking for objects with a strong (> 4 sigma) Halpha excess emission and find about 1150 of them over the entire field. Comparison of their location in the Hertzsprung-Russell diagram with theoretical PMS evolutionary tracks for the appropriate metallicity reveals that about one third of these objects are younger than ~4Myr, compatible with the age of the massive stars in the central ionising cluster R136, whereas the rest have ages up to ~30Myr, with a median age of ~12Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very centre of the cluster. We attribute this latter effect to photoevaporation of the older circumstellar discs caused by the massive ionising members of R136.
Located in the Large Magellanic cloud and mostly irradiated by a massive-star cluster R$,$136, 30 Doradus is an ideal target to test the leading theory of the grain alignment and rotational disruption by RAdiative Torques (RATs). Here, we use publicly available polarized thermal dust emission observations of 30 Doradus at 89, 154, and 214$,mu$m using SOFIA/HAWC+. We analyse the variation of the dust polarization degree ($p$) with the total emission intensity ($I$), the dust temperature ($T_{rm d}$), and the gas column density ($N_{rm H}$) constructed from ${it Herschel}$ data. The 30 Doradus complex is divided into two main regions relative to R$,$136, namely North and South. In the North, we find that the polarization degree first decreases and then increases before decreasing again when the dust temperature increases toward the irradiating cluster R$,$136. The first depolarization likely arises from the decrease of grain alignment efficiency toward the dense medium due to the attenuation of the interstellar radiation field and the increase of the gas density. The second trend (the increase of $p$ with $T_{rm d}$) is consistent with the RAT alignment theory. The final trend (the decrease of $p$ with $T_{rm d}$) is consistent with the RAT alignment theory only when the grain rotational disruption by RATs is taken into account. In the South, we find that the polarization degree is nearly independent of the dust temperature, while the grain alignment efficiency is higher around the peak of the gas column density and decreases toward the radiation source. The latter feature is also consistent with the prediction of the rotational disruption by RATs.
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