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Register a new userGASTON: Galactic Star Formation with NIKA2. Evidence for the mass growth of star-forming clumps
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Determining the mechanism by which high-mass stars are formed is essential for our understanding of the energy budget and chemical evolution of galaxies. By using the New IRAM KIDs Array 2 (NIKA2) camera on the Institut de Radio Astronomie Millimetrique (IRAM) 30-m telescope, we have conducted high-sensitivity and large-scale mapping of a fraction of the Galactic plane in order to search for signatures of the transition between the high- and low-mass star-forming modes. Here, we present the first results from the Galactic Star Formation with NIKA2 (GASTON) project, a Large Programme at the IRAM 30-m telescope which is mapping $approx$2 deg$^2$ of the inner Galactic plane (GP), centred on $ell$=23.9$^circ$, $b$=0.05$^circ$, as well as targets in Taurus and Ophiuchus in 1.15 and 2.00 mm continuum wavebands. In this paper we present the first of the GASTON GP data taken, and present initial science results. We conduct an extraction of structures from the 1.15 mm maps using a dendrogram analysis and, by comparison to the compact source catalogues from Herschel survey data, we identify a population of 321 previously-undetected clumps. Approximately 80 per cent of these new clumps are 70 $mu$m-quiet, and may be considered as starless candidates. We find that this new population of clumps are less massive and cooler, on average, than clumps that have already been identified. Further, by classifying the full sample of clumps based upon their infrared-bright fraction - an indicator of evolutionary stage - we find evidence for clump mass growth, supporting models of clump-fed high-mass star formation.
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Understanding where and when the mass of stars is determined is one of the fundamental, mostly unsolved, questions in astronomy. Here, we present the first results of GASTON, the Galactic Star Formation with NIKA2 large programme on the IRAM 30m telescope, that aims to identify new populations of low-brightness sources to tackle the question of stellar mass determination across all masses. In this paper, we focus on the high-mass star formation part of the project, for which we map a $sim2$ deg$^2$ region of the Galactic plane around $l=24^circ$ in both 1.2 mm and 2.0 mm continuum. Half-way through the project, we reach a sensitivity of 3.7 mJy/beam at 1.2mm. Even though larger than our target sensitivity of 2 mJy, the current sensitivity already allows the identification of a new population of cold, compact sources that remained undetected in any (sub-)mm Galactic plane survey so far. In fact, about 25% of the $sim 1600$ compact sources identified in the 1.2 mm GASTON image are new detections. We present a preliminary analysis of the physical properties of the GASTON sources as a function of their evolutionary stage, arguing for a potential evolution of the mass distribution of these sources with time.
We study the star formation (SF) law in 12 Galactic molecular clouds with ongoing high-mass star formation (HMSF) activity, as traced by the presence of a bright IRAS source and other HMSF tracers. We define the molecular cloud (MC) associated to each IRAS source using 13CO line emission, and count the young stellar objects (YSOs) within these clouds using GLIMPSE and MIPSGAL 24 micron Spitzer databases.The masses for high luminosity YSOs (Lbol>10~Lsun) are determined individually using Pre Main Sequence evolutionary tracks and the evolutionary stages of the sources, whereas a mean mass of 0.5 Msun was adopted to determine the masses in the low luminosity YSO population. The star formation rate surface density (sigsfr) corresponding to a gas surface density (siggas) in each MC is obtained by counting the number of the YSOs within successive contours of 13CO line emission. We find a break in the relation between sigsfr and siggas, with the relation being power-law (sigsfr ~ siggas^N) with the index N varying between 1.4 and 3.6 above the break. The siggas at the break is between 150-360 Msun/pc^2 for the sample clouds, which compares well with the threshold gas density found in recent studies of Galactic star-forming regions. Our clouds treated as a whole lie between the Kennicutt (1998) relation and the linear relation for Galactic and extra-galactic dense star-forming regions. We find a tendency for the high-mass YSOs to be found preferentially in dense regions at densities higher than 1200 Msun/pc^2 (~0.25 g/cm^2).
Star-forming clumps dominate the rest-frame ultraviolet morphology of galaxies at the peak of cosmic star formation. If turbulence driven fragmentation is the mechanism responsible for their formation, we expect their stellar mass function to follow a power-law of slope close to $-2$. We test this hypothesis performing the first analysis of the stellar mass function of clumps hosted in galaxies at $zsim 1-3.5$. The clump sample is gathered from the literature with similar detection thresholds and stellar masses determined in a homogeneous way. To overcome the small number statistics per galaxy (each galaxy hosts up to a few tens of clumps only), we combine all high-redshift clumps. The resulting clump mass function follows a power-law of slope $sim -1.7$ and flattens at masses below $2times 10^7$ M$_{odot}$. By means of randomly sampled clump populations, drawn out of a power-law mass function of slope $-2$, we test the effect of combining small clump populations, detection limits of the surveys, and blending on the mass function. Our numerical exercise reproduces all the features observed in the real clump mass function confirming that it is consistent with a power-law of slope $simeq -2$. This result supports the high-redshift clump formation through fragmentation in a similar fashion as in local galaxies, but under different gas conditions.
With the spatial resolution of the Atacama Large Millimetre Array (ALMA), dusty galaxies in the distant Universe typically appear as single, compact blobs of dust emission, with a median half-light radius, $approx$ 1 kpc. Occasionally, strong gravitational lensing by foreground galaxies or galaxy clusters has probed spatial scales 1-2 orders of magnitude smaller, often revealing late-stage mergers, sometimes with tantalising hints of sub-structure. One lensed galaxy in particular, the Cosmic Eyelash at $z=$ 2.3, has been cited extensively as an example of where the interstellar medium exhibits obvious, pronounced clumps, on a spatial scale of $approx$ 100 pc. Seven orders of magnitude more luminous than giant molecular clouds in the local Universe, these features are presented as circumstantial evidence that the blue clumps observed in many $zsim$ 2-3 galaxies are important sites of ongoing star formation, with significant masses of gas and stars. Here, we present data from ALMA which reveal that the dust continuum of the Cosmic Eyelash is in fact smooth and can be reproduced using two Sersic profiles with effective radii, 1.2 and 4.4 kpc, with no evidence of significant star-forming clumps down to a spatial scale of $approx$ 80 pc and a star-formation rate of $<$ 3 M$_odot$ yr$^{-1}$.
In this work, we aim to characterise high-mass clumps with infall motions. We selected 327 clumps from the Millimetre Astronomy Legacy Team 90-GHz (MALT90) survey, and identified 100 infall candidates. Combined with the results of He et al. (2015), we obtained a sample of 732 high-mass clumps, including 231 massive infall candidates and 501 clumps where infall is not detected. Objects in our sample were classified as pre-stellar, proto-stellar, HII or photo-dissociation region (PDR). The detection rates of the infall candidates in the pre-stellar, proto-stellar, HII and PDR stages are 41.2%, 36.6%, 30.6% and 12.7%, respectively. The infall candidates have a higher H$_{2}$ column density and volume density compared with the clumps where infall is not detected at every stage. For the infall candidates, the median values of the infall rates at the pre-stellar, proto-stellar, HII and PDR stages are 2.6$times$10$^{-3}$, 7.0$times$10$^{-3}$, 6.5$times$10$^{-3}$ and 5.5$times$10$^{-3}$ M$_odot$ yr$^{-1}$, respectively. These values indicate that infall candidates at later evolutionary stages are still accumulating material efficiently. It is interesting to find that both infall candidates and clumps where infall is not detected show a clear trend of increasing mass from the pre-stellar to proto-stellar, and to the HII stages. The power indices of the clump mass function (ClMF) are 2.04$pm$0.16 and 2.17$pm$0.31 for the infall candidates and clumps where infall is not detected, respectively, which agree well with the power index of the stellar initial mass function (2.35) and the cold Planck cores (2.0).
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