Extreme star formation in the Milky Way: luminosity distributions of young stellar objects in W49A and W51

Abstract

We have compared the star-formation properties of the W49A and W51 regions by using far-infrared data from the Herschel infrared Galactic Plane Survey (Hi-GAL) and 850-μm observations from the James Clerk Maxwell Telescope (JCMT) to obtain luminosities and masses, respectively, of associated compact sources. The former are infrared luminosities from the catalogue of Elia et al., while the latter are from the JCMT Plane survey source catalogue as well as measurements from new data. The clump-mass distributions of the two regions are found to be consistent with each other, as are the clump-formation efficiency and star-formation efficiency analogues. However, the frequency distributions of the luminosities of the young stellar objects are significantly different. While the luminosity distribution in W51 is consistent with Galaxy-wide samples, that of W49A is top heavy. The differences are not dramatic and are concentrated in the central regions of W49A. However, they suggest that physical conditions there, which are comparable in part to those in extragalactic starbursts, are significantly affecting the star-formation properties or evolution of the dense clumps in the region.

Characteristic scale of star formation – I. Clump formation efficiency on local scales

“…This is consistent with the results of Eden et al (2015), who found an increased ratio of L/M, a proxy of SFE (e.g. Moore et al 2012;Eden et al 2015Eden et al , 2018, in the Perseus arm compared to the Scutum-Centaurus and Sagittarius spiral arms. However, this was found to be due to distinctly different time gradients for star formation across the spiral arms, with the Perseus star formation at a more advanced stage.…”

LEGO II: A 3 mm molecular line study covering 100 pc of one of the most actively star-forming portions within the Milky Way Disc

“…As the W49 region is known to contain significantly higher densities on smaller spatial scales that are not resolved by the observations presented in this work (e.g. Vastel et al 2001;Eden et al 2018). The line emission could then emit from these compact regions that are heavily beam diluted in the observations presented in this work.…”

“…Higher resolution observations have shown that much sub-structure exists within W49A on scales much smaller than this beam size, which would then not be resolved with these observations (Galván-Madrid et al 2013). For example, using James Clerk Maxwell Telescope and higher-resolution Herschel observations, Eden et al (2018) identified ∼ 50 compact sources within the central ∼ 60 pc of W49A that have effective radii ranging from 10−20 (∼ 0.2 -0.3 pc). These cores have masses of > 10 3 M , which when assuming a spherical geometry approximately have column densities of > 10 23 cm −2 .…”

“…We propose that the number density using the spatial resolution of the Herschel observations can also be obtained by limiting the three-dimensional structure to a single spatial element along the line-of-sight: or in other words, by assuming that the measured column density at each position originates from a 20 × 20 × 20 cube (or ∼ 1 pc 3 at ∼ 11 kpc). We determine an uncertainty on the model density by varying the cube size by 1) the linear size based on the uncertainty from the distance estimate, and 2) varying the depth of the box between 0.5 − 3 pc, which represent the average size of the sub-structures identified by Eden et al (2018), and the size of the angular resolution of the molecular line observations (∼60 ), respectively.…”

“…Column density values of 10 23 cm −2 are only observed towards the centre of W49A within the ∼ 60 smoothed Herschel maps (see Figure 1). The dense region identified by Eden et al (2018) would then be beam diluted within our smoothed maps, along with any molecular line emission originating from these regions. It is plausible that this would cause molecular line emission predominately originating from isolated compact, high (column) dense regions to appear to originate from low column densities when diluted.…”