We had our final group meeting and presentations of the semester yesterday. This fantastic group has done so much incredible research this semester, all while being a supportive, welcoming, and friendly group – even having fun together at times. Thank you all for being a part of the Milky Way Laboratory! ☺️🤩🥳

 

Postdoc Daniel Walker in our group has published a paper reporting some of the very first detections of protostellar outflows¹ in our Galaxy’s Center! These outflows are found toward “The Brick” cloud (G0.253+0.016), which previously showed tentative signs of active star formation. Using ALMA Band 6 data, Dr. Walker found 18 continuum sources, 9 of which are driving bipolar molecular outflows in SiO (5-4) emission. Somewhat amazingly though, there is NO evidence for high-mass protostars within this field, the observations instead being consistent with a cluster of low- to intermediate-mass protostars. These incredible new data demonstrate that we are likely not missing any high-mass star formation in our estimates (meaning the low star formation rate of the CMZ is probably real!) and also provide some of the first examples of protostellar outflows toward our Galactic Center, for detailed study of the physics of protostellar accretion in this environment. Congrats, Dan!

¹See also recent work by collaborator Dr. Xing Lu and members of our group on another sample of protostellar outflows in the Galactic Center.

An image highlighting the outflows as traced by SiO (5-4) emission. Blue is velocities from 29-42 km/s and red is 43-56 km/s. Continuum sources are shown by white ellipses.

 

From the UConn Astronomy meeting room whiteboard (left) to ApJ letters (right) in just over a year! Read all about the distribution of star formation and gas in Galaxy Centers from cosmological zoom-in simulations. We find that there are two classes of centers: bursty and smooth. The bursty centers are the only ones able to simultaneously match the Milky Way’s CMZ star formation rate and gas density, supporting the idea that star formation in the CMZ may be bursty. This might help to explain the unexpectedly low star formation rate of our Galaxy’s Center at the current time.

The CMZoom Catalog Paper is published! 🤩🥳 Read the full paper on ADS here: https://ui.adsabs.harvard.edu/abs/2020ApJS..251…14H/abstract. The related data and catalogs can be downloaded on our dataverse page: https://dataverse.harvard.edu/dataverse/cmzoom

Well done H Perry Hatchfield! Read a summary of the paper, by H Perry below:

The Milky Way’s central region (the central molecular zone or CMZ) contains a modern astrophysical enigma. Despite its impressive reservoir of fuel for star formation, hosting 80% of all the high density gas in the Milky Way, the CMZ appears to be forming far fewer stars than we would expect. The CMZoom survey was designed to take a complete look at the high density clouds of the CMZ and to generate a catalog of as many sites of possible high mass star formation as possible to help us understand how stars form in this extreme environment. 

The CMZoom survey uses the Submillimeter Array to map all material in the CMZ above a column density of 10²³ cm^-2, an unprecedented broad look at this anomalous environment. In this paper, we use CMZoom’s 1.3mm dust continuum emission to catalog the compact substructure within the surveyed region. We construct the dust continuum catalog using a pruned dendrogram algorithm, a hierarchical clustering method that produces a “tree” of related sources, with the highest level emission being the “leaves”. This tree of sources is “pruned” by removing leaves that are not sufficiently significant relative to the local noise level. Figure 1 shows the distributions of these dendrogram leaves from the robust version of the catalog, along with zoom-ins of two sample regions. Figure 2 shows a histogram of the Herschel column densities at which SMA sources are found, a distribution with a sharp uptick around 1-2×10²³ cm^-2. This sudden increase in compact structure may support the existence of an environmentally variable density threshold for gravitational collapse and star formation.

While the catalog does not investigate which of these objects host signatures of star formation, we plan to investigate their properties further in future work. By simulating interferometric observations, we find the catalog is complete to >99% of places where a massive star (M>8M_⊙) could form. Considering all these possible sites of star formation, this catalog reveals the CMZ to be extreme in its variety of dense gas morphology, with many extremely dense clouds exhibiting little to no compact substructure, while nearby regions are overflowing with sites of ongoing and future star formation.

Fig 1. A column density map from Herschel with overlaid blue contours representing the CMZoom SMA footprint, and identified sources overplotted in red. A kernel density estimation is shown for the galactic longitude and latitude, representing the normalized distribution of the robust catalog’s sources. The highest concentration of sources is in the Sgr B2 complex, the Dust Ridge, and the 50 km s^-1 cloud, while many regions are devoid of compact substructure despite their high column densities. The two zoom-in panels show the 1.1 Degree Cloud Complex (left), which displays very little substructure, and the Three Little Pigs (right), which are named for their seemingly sequential increase in substructure complexity. The catalog sources are shown as white contours on top of the SMA 1.3mm dust continuum.

 

Fig 2. The column density distribution of Herschel pixels that correspond to an SMA 1.3mm dust continuum source from the CMZoom robust catalog. The grey histogram represents the column density of all Herschel pixels within the CMZoom footprint, and the blue histogram represents the subset of these which house a robust catalog source. The black line is the fraction of Herschel pixels at each column density bin which have CMZoom sources. This fraction increases sharply past a column density threshold of 1-2×10²³ cm^-2, suggesting that compact substructure is more readily able to condense and persist at these high column densities.

2020… the year of zoom, sourdough starters, and finding your bubble has brought with it many challenges, but also many new joys. For us, a major new joy is three new members joining the Milky Way Laboratory! We have an awesome new postdoc, Dr. Daniel Walker, and two first year graduate students, Dani Lipman and Yiyan Kuang.

Dr. Walker joins us from the Atacama Large Millimeter Array (ALMA) in Chile where he worked as a postdoctoral fellow after graduating with his PhD from Liverpool John Moores University where he did his PhD with Dr. Steve Longmore. Dan will be leading the kinematics, spectral line analysis, and Bayesian distance estimator for “3-D CMZ: Unveiling the Structure of our Galaxy’s Central Molecular Zone.” 

Dani Lipman joins us as a first year graduate student. Last year Dani was on Fulbright Research Award in China to study faculty participation in science outreach, before, y’know, Covid-19 had anything to say about that. Dani completed her Bachelor’s in 2019 at the University of Iowa. She will be leading the dust extinction study as part of “3-D CMZ: Unveiling the Structure of our Galaxy’s Central Molecular Zone.”

Yiyan Kuang joins us as a first year graduate student. He earned his Bachelor’s degree in 2020 from Allegheny College. Yiyan is experience in numerical simulations and has studied the evolution of star clusters using N-body simulations. He will bring this expertise to studying the formation of the pre-stellar Core Mass Function (CMF), and comparing simulations and observations.

Welcome Yiyan, Dani, and Dan!

The CMZoom Survey Overview paper is now published: https://ui.adsabs.harvard.edu/abs/2020ApJS..249…35B/abstract!

This paper presents the overview of the CMZoom survey, presents its first data release (data publicly available here: https://dataverse.harvard.edu/dataverse/cmzoom), and describes first key results.

CMZoom is the first complete and unbiased survey of dense gas in the Central Molecular Zone (CMZ: inner 500 pc of our Galaxy) at wavelengths that are sensitive to young, forming stars. CMZoom is a large survey (550 hours) on the Submillimeter Array (SMA) at 1.3 mm in dust continuum and spectral lines. CMZoom observed all of the highest column density gas (N(H₂) > 10²³ cm^-2) in the CMZ and a few additional regions of interest. The spatial resolution of CMZoom is about 3″ or 0.1 pc at the distance of the Galactic Center. In this paper, we report on and release the dust continuum data (available here) but we also describe the observation and data reduction of the spectral line data, which will be released in a future publication (Callanan et al. in prep.)

This Figure shows an overview of the CMZoom dust continuum, thought to be tracing young, forming stars. The middle panel shows the full dust continuum emission over the entire survey, with zoom-ins toward select regions. The images are all on the same color scale.

While CMZoom detected many regions with rich and complex substructure, its key result is an overall deficit in compact substructures on 0.1-2 pc scales (the compact dense gas fraction: CDGF). In the figure below, the CDGF is shown on the y-axis and the column density on the x-axis. Colored in red are sources in the Galactic disk and in blue are CMZ sources. In comparison with clouds in the Galactic disk, the CDGF in the CMZ is substantially lower, despite having much higher average column densities. CMZ clouds with high CDGFs are well-known sites of active star formation. 

The inability of most gas in the CMZ to form compact substructures is likely responsible for the dearth of star formation in the CMZ, surprising considering its high density. The factors responsible for the low CDGF are not yet understood but are plausibly due to the extreme environment of the CMZ, having far-reaching ramifications for our understanding of the star formation process across the cosmos.