Image of the Week

The chemical trace of Galactic stellar populations as seen by Gaia

 

Gaia low-resolution spectra of nearly 1 million giant stars within 4 kpc (~13,000 lyr) of the Sun (position indicated by ⊙) reveal the global distributions of different metals which encode the formation history of the Milky Way. The colour coding shows the stars’ iron content ([Fe/H], left) and the alpha-to-iron ratio ([α/Fe], right), a compositional property of stars that allows astronomers to tell apart different stellar populations. The Galactic disk is mostly metal-rich and α-poor, while the regions above and below the Galactic midplane become progressively more metal-poor and α-rich. Image created by Alvin Gavel, Andreas Korn, Rene Andrae and Morgan Fouesneau.

The Milky Way evolves both dynamically and chemically, fusing nuclei in thousands of generations of stars. Decades of star counts and spectroscopic follow-up have established the existence of a few major Galactic building blocks called stellar populations: the thin and thick disk, the halo and the central bulge. Gaia’s spectrophotometry records how much starlight we receive from the blue to the red for most stars observed by Gaia.

The analysis presented here shows that astronomers can use Gaia’s low-resolution spectrophotometry to assign stars to stellar populations across most of the Galaxy! Unravelling the assembly history of the Milky Way requires not only knowing the positions and motions of its stars, but also their intrinsic properties, such as their surface temperatures, luminosities, chemical compositions and ages.

Determining these parameters is the task of DPAC Coordination Unit 8 (CU8) responsible for the "Astrophysical Parameters". CU8 uses all of the data available from the Gaia space telescope – astrometry, optical spectrophotometry, near-infrared spectroscopy – to characterize the observed stars (and extragalactic objects). The DPAC Coordination Unit 5 (CU5) deals with the processing of the spectrophotometry – the low-resolution (R~80) optical spectra of stars that encode key stellar parameters like surface temperature and chemical composition. CU5 processed and delivered all the spectra on which this work is based.

In particular, CU5 recently provided spectra for a set of 200,000 stars used to train a stellar-parameter and chemical-composition model based on machine-learning algorithms. This training set was compiled from the Australian-led GALAH (GALactic Archeology with HERMES) survey providing us with known stellar parameters and compositions from their high-resolution (R~30,000) ground-based spectroscopy.

The above figures are Galactic maps showing the application of such a model to 945,384 giant stars within 4 kpc (~13,000 lyr) from the Sun, a distance corresponding to half the distance to the Galactic center. The colour coding represents the stellar iron content (left) and one additional key stellar observable, the alpha-to-iron ratio, labelled [α/Fe] (right). The latter quantity (indirectly) tells astronomers about the time scale on which a given stellar population was formed. [α/Fe] varies across the different Galactic populations. A stellar population that came into existence over a short (t < 1 Gyr) timescale will end up having high values of [α/Fe]. Conversely, a population of stars that formed over several Gyr will have lower [α/Fe] values. The Sun, the product of some 4 Gyr of thin-disk evolution, is a typical low-[α/Fe] star.

Clearly, the vertical regions in which stars of the Galactic thin disk dominate (z < 1 kpc) are α-normal (in reference to our Sun) and metal rich while the regions above and below the disk are markedly α-rich and metal poor. This is what astronomers expect from the dominance of α-rich stellar populations contributing to the Galactic thick disk and to the halo at high Galactic latitudes (z > 1 kpc) which formed early and rapidly in the history of the Milky Way while the metal rich thin disk formed over a long time period. There are other global features that this work qualitatively reproduces, e.g. the fact that the Galactic disk flares at large Galactocentric distances.

In Gaia Data Release 3 CU5 will publish millions of low-resolution spectra of stars all across the sky and CU8 will extract from them as well as from the integrated photometry and the parallaxes various intrinsic properties such as effective temperature, [Fe/H] and surface gravity. This work is a demonstration that the quality of Gaia’s low-resolution spectra allows the community to extract other quantities from these data that are not provided as part of the release, for example the [α/Fe] abundance ratios presented here.

 

Technical details

The results presented above have been derived from an ExtraTrees model trained on ~200,000 stars taken from the GALAH (GALactic Archeology with HERMES) survey catalog. The model was finally applied to a sample of 3,380,241 stars (6 < G magnitude < 15) observed with Gaia within 4 kpc of the Sun.

From these, 945,384 giant stars were selected via a surface-gravity criterion (logarithmic surface gravity below 3.3). The precision of the surface-gravity inference is high enough to secure a homogeneous sample of luminous stars out to fairly large distances.

α is a placeholder for a group of elements that is created by adding α particles (helium nuclei) to carbon. Prominent members of this group of elements, mostly fused in core-collapse (type II) supernovae of massive stars, are oxygen, magnesium, silicon and calcium.

Iron, the other element discussed above, is instead mostly fused in type Ia supernovae, that is, white dwarfs accreting matter from a companion thereby surpassing a stability mass limit.

 

Credits: ESA/Gaia/DPAC, A. Gavel, A. Korn, R. Andrae, M. Fouesneau, all of Coordination Unit 8 (CU8) of Gaia DPAC. We wish to thank Coordination Unit 5 (CU5) and Gaia Data Processing Centre at the Institute of Astronomy in Cambridge (DPCI) for producing the high-quality spectrophotometry on which this work rests.

[Published: 16/03/2020]

 

Image of the Week Archive

2020
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.