Image of the Week

 

A Milky-Way relic of the formation of the Universe

 

Figure 1: The Milky Way globular clusters on top of our Galaxy as seen by the Gaia space telescope. Like the insert image, each cluster is a grouping of thousands to millions of stars. The colour of the symbols represents their “metallicity,” i.e., their fraction in heavy atomic elements compared to the Sun. The stars of the C-19 stream (star symbols) have a 2,500 times lower density of heavy elements when compared to the Sun. C-19 is the Milky-Way structure with the lowest metallicity known to date. Image credit: N. Martin; Observatoire astronomique de Strasbourg; Canada-France-Hawaii Telescope / Coelum; ESA/Gaia/DPAC

 

An international team of researchers discovered the remnants of a star cluster whose stars are uniquely pristine. The stars in this cluster have an extremely low metallicity and must have formed in the metal-poor early Universe. Stars gradually enrich the interstellar gas with heavy elements which allows future stars born from this enriched interstellar gas to contain more heavy elements. The found cluster constitutes of only very early generations of stars hence provides a remarkable relic from when the first stellar structures were assembling. Some theories hypothesized clusters of such pristine stars could not form at all, others that they would have all vanished by now. This finding makes this a fundamental discovery for our understanding of how stars form in the early Universe.

To study the formation of the first stellar structures in the universe, astronomers can look at the most distant galaxies. As light takes time to travel, the observations we make now originate from a long time ago, from a time these galaxies and their constituent stars were in their infancy. Alternatively, astronomers may study the oldest structures in our galaxy, the Milky Way, in great detail. This approach is called “Galactic Archeology.” Like the Sun, most stars surrounding us formed in our Galaxy. However, favourably found in its outskirts, a small fraction of the Milky Way’s stars come from smaller galaxies that passed nearby, and donated stars and stellar clusters like comets leave debris behind them. One of these encounters left behind the discovered cluster that now sheds a stream of stars travelling through the strong Galaxy’s tides.

The team led by Nicolas Martin (CNRS/Strasbourg University) explored the unprecedentedly detailed map of the location and movement of stars seen by Gaia with a novel algorithm to isolate the rare groupings of stars moving in concert. One of the structures discovered is a new candidate stellar stream called “C-19”. In parallel, at the Canada-France-Hawaii Telescope (CFHT) in Hawaii, the Pristine survey mapped the sky to systematically measure the fraction of heavy elements in millions of stars (see Figure 1). When combined, these two studies revealed the surprising fact that C-19 contains stars with an extremely low heavy element fraction.

"It is only through the combination of the exquisite Gaia data and observations from multiple ground based telescopes that the Pristine team unveiled this remarkable stellar stream," said Dr. Nicolas Martin, Observatoire astronomique de Strasbourg and Pristine team lead. "This exciting discovery illustrates the power of bringing together observers and theorists from across the world."

Follow-up spectroscopic observations with the Gemini North Telescope in Hawaii and Gran Telescopio Canarias in La Palma allowed the astronomers to measure the chemical content of these stars. They found that they share a chemical signature specific to star clusters. The spectra also confirmed that their heavy element fraction, otherwise called “metallicity,” is 2,500 times lower than the Sun’s, making C-19’s level well below any other known structure in the universe.

This remnant from ancient times opens a direct and unique window into the early epochs of star formation in the Universe and the build-up of stellar structures in these very early times. The continuously improving maps of the Milky Way from the Gaia mission, the Pristine survey, and future telescopes will help the discoveries of such exceptional groupings of stars as precious fossils from ancient times.

Gaia Data Release 3, expected in Q2 of 2022, will provide us with astrophysical parameters determined from the BP/RP and RVS spectra. These will open many more dimensions to explore to find rare chemo-dynamical structures allowing us to better understand the formation and evolution of our Galaxy.

 

The “metallicity” of stars

The Sun is composed of 98% of the two lightest elements in the universe: hydrogen and helium. The sum of all the other heavier atomic elements (carbon, oxygen, iron, etc.) is what astronomers call “metallicity” and represents only 1.5% of the total mass of our star. Massive stars (5 times the Sun and above) produce most of these heavy elements. As these stars reach their final stages of evolution, they release these elements into the interstellar gas through winds or when they explode as supernovae. Like the Sun 4.5 billion years ago, newborn stars from this interstellar gas contain the elements formed in the previous generations of stars; they are metal-enriched. Consequently, stars impoverished in metals must be of the first generations in the entire Universe. The stars that make up the C-19 stream show abundances in heavy elements 2,500 times lower than those found in our Sun.

 

Animation of the C-19 stellar stream

An animation was created to show the evolution of the C-19 stream. This animation was created by Gabriel Pérez Díaz (SMM, IAC).

 

Animation of the formation and evolution of the stellar stream C-19 / Animación de la formación y evolución del remanente C-19. Credits / Crédito: Gabriel Pérez Díaz (SMM, IAC). Origin of the animation: website of the Instituto de Astrofísica de Canarias

 

 

References and further reading:

 

Credits: Nicolas F. Martin, M. Fouesneau, et al., 2021

 

Published: 05/01/2022

 

Image of the Week Archive

2022

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe

2021

23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4

2020

22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy

2019
20/12: The lost stars of the Hyades
06/12: Do we see a dark-matter like effect in globular clusters?
12/11: Hypervelocity star ejected from a supermassive black hole
17/09: Instrument Development Award
08/08: 30th anniversary of Hipparcos
17/07: Whitehead Eclipse Avoidance Manoeuvre
28/06: Following up on Gaia Solar System Objects
19/06: News from the Gaia Archive
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
2018
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.