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Title: Highly efficient star formation in NGC 5253 possibly from stream-fed accretion
Author: J. L. Turner, S. C. Beck, D. J. Benford, S. M. Consiglio, P. T. P. Ho, A. Kovács, D. S. Meier, J.-H. Zhao

A local dwarf galaxy, NGC 5253, has a young super star cluster that may provide an example of highly efficient star formation. Here we report the detection and imaging, with the Submillimeter Array, of the J= 3-2 rotational transition of CO at the location of the massive cluster associated with the supernebula. The gas cloud is hot, dense, quiescent, and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by Wolf-Rayet stars within the embedded star cluster. Its star formation efficiency exceeds 50%, ten times higher than clouds in the Milky Way: this cloud is a factory of stars and soot. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.

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NGC 5253 is an irregular galaxy in the constellation Centaurus. It was discovered by John Frederick William Herschel on 15 March 1787
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Title: HeI in the central Giant HII Region of NGC 5253. A 2D observational approach to collisional and radiative transfer effects
Authors: Ana Monreal-Ibero (1), Jeremy R. Walsh (2), Mark S. Westmoquette (2), Jose M. Vilchez (1), ((1) Instituto de Astrofisica de Andalucia (CSIC), (2) European Southern Observatory)

NGC5253 is an ideal laboratory for detailed studies of starburst galaxies. We present for the first in a starburst galaxy a 2D study of the spatial behaviour of collisional and radiative transfer effects in He^+. The HeI lines are analysed based on data obtained with FLAMES and GMOS. Collisional effects are negligible (i.e. 0.1-0.6%) for transitions in the singlet cascade while relatively important for those in the triplet cascade. In particular, they can contribute up to 20% of the flux in the HeIl7065 line. Radiative transfer effects are important over an extended and circular area of 30pc in diameter centred at the Super Star Clusters. HeI abundance, y^+, has been mapped using extinction corrected fluxes of six HeI lines, realistic assumptions for T_e, n_e, and the stellar absorption equivalent width as well as the most recent emissivities. We found a mean of 10^3 y^+ ~81.6 over the mapped area. The relation between the excitation and the total helium abundance, y_tot, is consistent with no abundance gradient. Uncertainties in the derivation of He abundances are dominated by the adopted assumptions. We illustrated the difficulty of detecting a putative He enrichment due to the presence of Wolf-Rayet stars in the main GHIIR. Data are marginally consistent with an excess in the N/He ratio in the N enriched area of the order of both, the atmospheric N/He ratios in WR stars and the uncertainties estimated for the N/He ratios.

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Title: The NGC 5253 star cluster system. I. Standard modelling and infrared-excess sources
Authors: Richard de Grijs (1,2), Peter Anders (1), Erik Zackrisson (3), Göran Ostlin (3) ((1) KIAA, Peking University, China, (2) Kyung Hee University, Rep. of Korea, (3) Stockholm University, Sweden)

Using high-resolution Hubble Space Telescope data, we reexamine the fundamental properties (ages, masses and extinction values) of the rich star cluster population in the dwarf starburst galaxy NGC 5253. The gain in resolution compared to previous studies is of order a factor of two in both spatial dimensions, while our accessible wavelength range transcends previous studies by incorporation of both near-ultraviolet and near-infrared (IR) passbands. We apply spectral synthesis treatments based on two different simple stellar population model suites to our set of medium-, broad-band and H-alpha images to gain an improved physical understanding of the IR-excess flux found for a subset of young clusters (30 of 149). With the caveat that our models are based on fully sampled stellar mass functions, the NGC 5253 cluster population is dominated by a significant number of relatively low-mass (M_cl <= a few 10^4 solar masses) objects with ages ranging from a few x 10^6 to a few x 10^7 yr, which is in excellent agreement with the starburst age of the host galaxy. The IR-excess clusters are almost all found in this young age range and have masses of up to a few x 10^4 solar masses. The IR excess in the relatively low-mass NGC 5253 clusters is most likely caused by a combination of stochastic sampling effects and colour variations due to the presence of either luminous red or pre-main-sequence stars. We also find a small number of intermediate-age (~ 1 Gyr-old), ~10^5 solar mass clusters, as well as up to a dozen massive, ~10 Gyr-old globular clusters. Their presence supports the notion that NGC 5253 is a very active galaxy that has undergone multiple episodes of star cluster formation.

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Title: Piecing together the puzzle of NGC 5253: abundances, kinematics and WR stars
Authors: M. S. Westmoquette (ESO), B. James (STScI/IoA), A. Monreal-Ibero (IAA), J. R. Walsh (ESO)

We present Gemini-S/GMOS-IFU optical spectroscopy of four regions near the centre of the nearby (3.8 Mpc) dwarf starburst galaxy NGC 5253. This galaxy is famous for hosting a radio supernebula containing two deeply embedded massive super star clusters, surrounded by a region of enhanced nitrogen abundance that has been linked to the presence of WR stars. We detected 11 distinct sources of red WR bump (CIV) emission over a 20" (~350 pc) area, each consistent with the presence of ~1 WCE-type star. WC stars are not found coincident with the supernebula, although WN stars have previously been detected here. We performed a multi-component decomposition of the H\alpha\ line across all four fields and mapped the kinematics of the narrow and broad (FWHM = 100-250 km/s) components. These maps paint a picture of localised gas flows, as part of multiple overlapping bubbles and filaments driven by the star clusters throughout the starburst. We confirm the presence of a strong H\alpha\ velocity gradient over ~4.5" (~80 pc) coincident with the region of N/O enhancement, and high gas density known from previous study, and interpret this as an accelerating ionised gas outflow from the supernebula clusters. We measure the ionised gas abundances in a number of regions in the outer IFU positions and combine these with measurements from the literature to assess the radial abundance distribution. We find that the O/H and N/H profiles are consistent with being flat. Only the central 50 pc exhibits the well-known N/O enhancement, and we propose that the unusually high densities/pressures in the supernebula region have acted to impede the escape of metal-enriched hot winds from the star clusters and allow them to mix with the cooler phases, thus allowing these freshly processed chemicals to be seen in the optical.

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Hubble Spots a Peculiar Compact Blue Dwarf Galaxy

710074main1_ngc5253-673.JPG

The NASA/ESA Hubble Space Telescope captured an impressive image of the irregular galaxy NGC 5253.
NGC 5253 is one of the nearest of the known Blue Compact Dwarf (BCD) galaxies, and is located at a distance of about 12 million light-years from Earth in the southern constellation of Centaurus. The most characteristic signature of these galaxies is that they harbour very active star-formation regions. This is in spite of their low dust content and comparative lack of elements heavier than hydrogen and helium, which are usually the basic ingredients for star formation.

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Title: The ionised gas in the central region of NGC 5253: 2D mapping of the physical and chemical properties
Authors: Ana Monreal-Ibero (1), Jeremy R. Walsh (2), Jose M. Vilchez (1), ((1) Instituto de Astrofisica de Andalucia (CSIC), (2) European Southern Observatory)

 NGC5253 was previously studied by our group with the aim to elucidate in detail the starburst interaction processes. Some open issues regarding the 2D structure of the main properties of the ionised gas remain to be addressed. Using IFS data obtained with FLAMES, we derived 2D maps for different tracers of electron density (n_e), electron temperature (T_e) and ionisation degree. The maps for n_e as traced by several line ratios are compatible with a 3D stratified view of the nebula with the highest n_e in the innermost layers and a decrease of n_e outwards. To our knowledge, this is the first time that a T_e map based on [SII] lines for an extragalactic object is presented. The joint interpretation of our two T_e maps is consistent with a T_e structure in 3D with higher temperatures close to the main ionising source surrounded by a colder and more diffuse component. The highest ionisation degree is found at the peak of emission for the gas with relatively high ionisation in the main GHIIR and lower ionisation degree delineating the more extended diffuse component. Abundances for O, Ne and Ar are constant over the mapped area within <0.1 dex. The mean 12+log(O/H) is 8.26 while the relative abundances of log(N/O), log(Ne/O) and log(Ar/O) were ~ -1.32, -0.65 and -2.33, respectively. There are two locations with enhanced N/O. The first (log(N/O) ~ -0.95) is associated to two super star clusters. The second (log(N/O) ~ -1.17), reported here for the first time, is associated to two moderately massive (2-4x10^4 Solar masses) and relatively old (~ 0 Myr) clusters. A comparison of the N/O map with those produced by strong line methods supports the use of N2O2 over N2S2 in the search for chemical inhomogeneities within a galaxy. The results on the localised nitrogen enhancement were used to compile and discuss the factors that affect the complex relationship between Wolf-Rayet stars and N/O excess.

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Title: Intermediate Old Star Clusters in a Young Starburst: The case of NGC 5253
Authors: D. Harbeck, J. Gallagher III, D. Crnojevic

We investigate the star cluster population in the outer parts of the starburst galaxy NGC 5253 using archive images taken with the Hubble Space Telescope's Advanced Camera for Surveys. Based on the F415W, F555W, and F814W photometry ages and masses are estimated for bona-fide star cluster candidates. We find three potentially massive (\ge 10 x 10^5 solar mass) star clusters at ages of order of 1-2 Gyr, implying, if confirmed, a high global star formation rate in NGC 5253 during that epoch. This result underlines earlier findings that the current star burst is just one episode in an very active dwarf galaxy.

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