A Nascent Tidal Dwarf Galaxy Forming within the Northern H i Streamer of M82

We identify a ∼600 pc wide region of active star formation located within a tidal streamer of M82 via Hα emission (F Hα ∼ 6.5 × 10−14 erg s−1 cm−2), using a pathfinder instrument based on the Dragonfly Telephoto Array. The object is kinematically decoupled from the disk of M82 as confirmed via Keck/LRIS spectroscopy and is spatially and kinematically coincident with an overdensity of H i and molecular hydrogen within the “northern H i streamer” induced by the passage of M81 several hundred Myr ago. From H i data, we estimate that ∼5 × 107 M ⊙ of gas is present in the specific overdensity coincident with the Hα source. The object’s derived metallicity (12+ log ( O / H ) ≃ 8.6 ), position within a gas-rich tidal feature, and morphology (600 pc diameter with multiple star-forming clumps), indicate that it is likely a tidal dwarf galaxy in the earliest stages of formation.

Star formation in ‘the Brick’: ALMA reveals an active protocluster in the Galactic centre cloud G0.253+0.016

ABSTRACT G0.253+0.016, aka ‘the Brick’, is one of the most massive (>105 M⊙) and dense (>104 cm−3) molecular clouds in the Milky Way’s Central Molecular Zone. Previous observations have detected tentative signs of active star formation, most notably a water maser that is associated with a dust continuum source. We present ALMA Band 6 observations with an angular resolution of 0.13 arcsec (1000 AU) towards this ‘maser core’ and report unambiguous evidence of active star formation within G0.253+0.016. We detect a population of eighteen continuum sources (median mass ∼2 M⊙), nine of which are driving bi-polar molecular outflows as seen via SiO (5–4) emission. At the location of the water maser, we find evidence for a protostellar binary/multiple with multidirectional outflow emission. Despite the high density of G0.253+0.016, we find no evidence for high-mass protostars in our ALMA field. The observed sources are instead consistent with a cluster of low-to-intermediate-mass protostars. However, the measured outflow properties are consistent with those expected for intermediate-to-high-mass star formation. We conclude that the sources are young and rapidly accreting, and may potentially form intermediate- and high-mass stars in the future. The masses and projected spatial distribution of the cores are generally consistent with thermal fragmentation, suggesting that the large-scale turbulence and strong magnetic field in the cloud do not dominate on these scales, and that star formation on the scale of individual protostars is similar to that in Galactic disc environments.

RCW98: a dust enshrouded HII region

RCW 98 is an HII region with an active star formation in its immediate neighbourhood. At least three early-type stars ionize it and it contains the bright rimmed cloud. Our observations of this region in the optical regime shows that RCW 98 is a complex and dynamical region. We have created a simple model of the dust distribution in this object, but it is not fully consistent with the optical observations. We also study the effects of ionizing stars on the dust.

Diffuse star-forming galaxies: thermal emission in radio continuum at 1.4 GHz

We determine the fraction of thermal radio emission in the total flux at the frequency of 1.4 GHz in 30 diffuse star-forming galaxies using the approach described in the paper by Parnovsky and Izotova (Astronomische Nachrichten, 2015 Vol.336, Issue 3, p.276). We use the sample of diffuse galaxies with active star formation selected from the Data Release 12 (DR12) of the Sloan Digital Sky Survey (SDSS). The SDSS spectroscopic data for diffuse galaxies were supplemented by radio data from the FIRST and NVSS catalogues. From the fluxes in the Hβ emission line corrected for extinction and aperture we estimate the fluxes of thermal component of the radio emission at the 1.4 GHz using the result of the paper by J. Caplan and L. Deharveng (Astron. Astrophys. 1986, V. 155, P.297) and compare them with total fluxes from the FIRST and NVSS. In the sample of diffuse galaxies with active star formation, the distribution of the fraction of thermal radio emission A is similar to the lognormal one with the median value of 6 %. This is less than the median value which was derived earlier by Parnovsky and Izotova for a sample of compact star-forming galaxies, but both distributions of A are similar. We study the dependence of A on the W equivalent width of the Hβ emission line and on the I colour index g-r. The fraction of thermal component increases with increasing of the equivalent width of the Hβ emission line if a colour index is constant or an increasing of the colour index at a fixed equivalent width W.

The properties of molecular clouds across the Magellanic System

Most stars form in Giant Molecular Clouds (GMCs) and regulate the evolution of galaxies in various respects. The formed stars affect the surrounding materials strongly via their UV photons, stellar winds, and supernova explosions, which lead to trigger the formation of next-generations of stars in the GMCs. It is therefore crucial to reveal the distribution and properties of GMCs in a galaxy. The Magellanic System is a unique target to make such detailed comprehensive study of GMCs. This is because it is nearby and the LMC is nearly face-on, making it feasible to unambiguously identify associated young objects within GMCs. Recent millimeter and sub-millimeter observations in the Magellanic System have started to reveal the distribution and properties of the individual GMCs in detail and their relation to star formation activities. From the NANTEN CO surveys, three types of GMCs can be classified in terms of star formation activities; Type I is starless, Type II is with H ii regions only, and Type III is associated with active star formation indicated by huge H ii regions and young star clusters. The further observations to obtain detailed structure of the GMCs by Mopra and SEST and to search for the dense cores by ASTE and NANTEN2 in higher tansition lines of CO have been carried out with an angular resolution of about 5 to 10 pc. These observations revealed that the differences of the physical properties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation. Type I and II GMCs are at the early phase of star formation where density does not yet become high enough to show active star formation, and Type III GMCs represent the later phase where the average density is increased and the GMCs are forming massive stars.