Data from ESA's Gaia mission is re-writing the history of our galaxy

A dwarf galaxy is a collection of between thousand and several billion stars. For decades it has been widely believed that the dwarf galaxies that surround the Milky Way are satellites, meaning that they are caught in orbit around our galaxy, and have been our constant companions for many billions of years. Now the motions of these dwarf galaxies have been super computed with unprecedented precision thanks to data from Gaia’s early third data release and the results are surprising.

François Hammer, Observatoire de Paris - Université Paris Sciences et Lettres, France, and colleagues from across Europe and China, used the Gaia data to calculate the movements of 40 dwarf galaxies around the Milky Way. They did this by supercomputing a set of quantities known as the three-dimensional velocities for each galaxy, and then using those to calculate the galaxy’s orbital energy and the angular (rotational) momentum. Dwarf galaxies around the Milky Way

They found that these galaxies are moving much faster than the giant stars and star clusters that are known to be orbiting the Milky Way. So fast, that they couldn’t be in orbit yet around the Milky Way, where interactions with our galaxy and its contents would have sapped their orbital energy and angular momentum.

Our galaxy has cannibalized many dwarf galaxies in its past. For example, 8-10 billion years ago, a dwarf galaxy called Gaia-Enceladus was absorbed by the Milky Way. Its stars can be identified in Gaia data because of the eccentric orbits and range of energies they possess.

More recently, 4-5 billion years ago, the Sagittarius dwarf galaxy was captured by the Milky Way and is currently in the process of being pulled to pieces and assimilated. The energy of its stars is higher than those of Gaia-Enceladus, indicating the shorter time that they have been subject to the Milky Way’s influence.

In the case of the dwarf galaxies in the new study, which represents the majority of the dwarf galaxies around the Milky Way, their energies are higher still. This strongly suggests that they have only arrived in our vicinity in the last few billion years.

The discovery mirrors one made about the Large Magellanic Cloud (LMC), a larger dwarf galaxy so close to the Milky Way that it is visible as a smudge of light in the night sky from the southern hemisphere. The LMC was also thought to be a satellite galaxy of the Milky Way until the 2000s when astronomers measured its velocity and found that it was traveling too fast to be gravitationally bound. Instead of a companion, LMC is visiting for the first time. Now we know that the same is true for most dwarf galaxies too.

So will these newcomers settle into orbit or simply pass us by? “Some of them will be captured by the Milky Way and will become satellites,” says François.

But saying exactly which ones is difficult because it depends on the exact mass of the Milky Way, and that is a quantity that is difficult for astronomers to calculate with any real accuracy. Estimates vary by a factor of two.

The discovery of the dwarf galaxy energies is significant because it forces us to re-evaluate the nature of the dwarf galaxies themselves.

As a dwarf galaxy orbits, the Milky Way’s gravitational pull will try to wrench it apart. In physics, this is known as a tidal force. “The Milky Way is a big galaxy, so its tidal force is simply gigantic and it's very easy to destroy a dwarf galaxy after maybe one or two passages,” says François.

In other words, becoming a companion to the Milky Way is a death sentence for dwarf galaxies. The only thing that could resist our galaxy’s destructive grip is if the dwarf had a significant quantity of dark matter. Dark matter is the mysterious substance that astronomers think exists in the universe to provide the extra gravity to hold individual galaxies together.

And so, in the traditional view that the Milky Way’s dwarfs were satellite galaxies that had been in orbit for many billions of years, it was assumed that they must be dominated by dark matter to balance the Milky Way’s tidal force and keep them intact. The fact that Gaia has revealed that most of the dwarf galaxies are circling the Milky Way for the first time means that they do not necessarily need to include any dark matter at all, and we must re-assess whether these systems are in balance or rather in the process of destruction.

“Thanks in large part to Gaia, it is now obvious that the history of the Milky Way is far more storied than astronomers had previously understood. By investigating these tantalizing clues, we hope to further tease out the fascinating chapters in our galaxy’s past,” says Timo Prusti, Gaia Project Scientist, ESA.

Mirabilis Design System-Level AI Modeler reduces latency, power by 2-6X

Mirabilis Design has released VisualSim AI Processor Designer. VisualSim AI accelerates time-to-market of new AI technology, configures high-performance computing systems, eliminates under and over-design, and provides an interactive reference design for end-users to create new applications. 

VisualSim can be used for the architecture evaluation of AI processor hardware, partition AI algorithms on a System-on-Chip (SoC), test the AI/ML implementation, and measure power and performance of an AI processor in automotive, medical, and data center applications. The Intellectual Property available in the VisualSim AI brings together processor cores, neural networks, accelerators, GPU, and DSP. At the system level, VisualSim AI can be integrated with a network model and FPGA boards for full system verification.

“The best processor configuration depends on the application, price point, and the expected performance. Trying to predict the feasibility before building the first prototype requires modeling IP, which is never readily available. The intense competition in the marketplace makes the delay in detecting performance limitation, a major detriment to a successful new product introduction”, says Deepak Shankar, Vice President – Technology, Mirabilis Design Inc. “The complex model requires configurable IPs and an integrated simulation environment.”

The AI Designer enables an architect to rapidly construct a graphical model using parameterized IP and integrating around an interconnect such as a Network-on-Chip, Quantum Nodes, or in-Memory elements. The user can accurately simulate AI workloads and real-life interface traffic. The model can vary task allocation between cores, neural networks, and accelerators; size the system parameters; create an equilibrium between response time and power consumption, and select the scheduler and buffer strategy. The combination of the large model capacity, fast model construction, an extremely fast simulator, and a programmable analytics engine, enables users to rapidly arrive at the most suitable architecture. 

Users can run software on the VisualSim AI architecture to measure response times, power, network throughput, cache hit-ratio, and memory bandwidth. VisualSim AI enables companies to optimize and validate the SoC system specification, and system companies to select the right SoC for the target application. A number of beta customers have utilized this platform to design AI SoC for the data center and automotive applications. Other applications that can use this platform are autonomous driving, radars’ processing, defense systems, flight avionics, medical instruments, high-performance computing, and infotainment systems.

Availability

VisualSim AI designers have tested the accuracy of the IP blocks for task latency and power consumption across multiple projects. The platform works on VisualSim version 2140b. OS supported includes Windows, Linux, and Mac OS. To learn more, register for a private session in Booth 2441 at Design Automation Conference 2021 in San Francisco by registering at https://calendly.com/mirabilisdesign/dac or contact Mirabilis Design at info@mirabilisdesign.com

Using MD simulations, MIT chemists show how molecular clusters in the nucleus interact with chromosomes

A new study finds the clusters form small, stable droplets and may give the genome a gel-like structure

A cell stores all of its genetic material in its nucleus, in the form of chromosomes, but that’s not all that’s tucked away in there. The nucleus is also home to small bodies called nucleoli — clusters of proteins and RNA that help build ribosomes.

Using supercomputer simulations, MIT chemists have now discovered how these bodies interact with chromosomes in the nucleus, and how those interactions help the nucleoli exist as stable droplets within the nucleus.

Their findings also suggest that chromatin-nuclear body interactions lead the genome to take on a gel-like structure, which helps to promote stable interactions between the genome and transcription types of machinery. These interactions help control gene expression.

“This model has inspired us to think that the genome may have gel-like features that could help the system encode important contacts and help further translate those contacts into functional outputs,” says Bin Zhang, the Pfizer-Laubach Career Development Associate Professor of Chemistry at MIT, an associate member of the Broad Institute of Harvard and MIT, and the senior leader of the study. MIT graduate student Yifeng Qi is the head of the study.

Modeling droplets

Much of Zhang’s research focuses on modeling the three-dimensional structure of the genome and analyzing how that structure influences gene regulation.

In the new study, he wanted to extend his modeling to include nucleoli. These small bodies, which break down at the beginning of cell division and then re-form later in the process, consisting of more than a thousand different molecules of RNA and proteins. One of the key functions of the nucleoli is to produce ribosomal RNA, a component of ribosomes.

Recent studies have suggested that nucleoli exist as multiple liquid droplets. This was puzzling because, under normal conditions, multiple droplets should eventually fuse into one large droplet, to minimize the surface tension of the system, Zhang says.

“That’s where the problem gets interesting, because in the nucleus, somehow those multiple droplets can remain stable across an entire cell cycle, over about 24 hours,” he says.

To explore this phenomenon, Zhang and Qi used a technique called molecular dynamics simulation, which can model how a molecular system changes over time. At the beginning of the simulation, the proteins and RNA that make up the nucleoli are randomly distributed throughout the nucleus, and the simulation tracks how they gradually form small droplets.

In their supercomputer simulation, the researchers also included chromatin, the substance that makes up chromosomes and includes proteins as well as DNA. Using data from previous experiments that analyzed the structure of chromosomes, the MIT team calculated the interaction energy of individual chromosomes, which allowed them to provide realistic representations of 3D genome structures.

Using this model, the researchers were able to observe how nucleoli droplets form. They found that if they modeled the nucleolar components on their own, with no chromatin, they would eventually fuse into one large droplet, as expected. However, once chromatin was introduced into the model, the researchers found that the nucleoli formed multiple droplets, just as they do in living cells.

The researchers also discovered why that happens: The nucleoli droplets become tethered to certain regions of the chromatin, and once that happens, the chromatin acts as a drag that prevents the nucleoli from fusing.

“Those forces essentially arrest the system into those small droplets and hinder them from fusing together,” Zhang says. “Our study is the first to highlight the importance of this chromatin network that could significantly slow down the fusion and arrest the system in its droplet state.”

Gene control

The nucleoli are not the only small structures found in the nucleus — others include nuclear speckles and the nuclear lamina, an envelope that surrounds the genome and can bind to chromatin. Zhang’s group is now working on modeling the contributions of these nuclear structures, and their initial findings suggest that they help to give the genome more gel-like properties, Zhang says.

“This coupling that we have observed between chromatin and nuclear bodies is not specific to the nucleoli. It’s general to other nuclear bodies as well,” he says. “This nuclear body concentration will fundamentally change the dynamics of the genome organization and will very likely turn the genome from a liquid to a gel.”

This gel-like state would make it easier for different regions of the chromatin to interact with each other than if the structure existed in a liquid state, he says. Maintaining stable interactions between distant regions of the genome is important because genes are often controlled by stretches of chromatin that are physically distant from them.