Image of the Week

QSO emission lines in low-resolution BP/RP spectra

 

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Gaia low-resolution BP and RP spectra (blue and red, respectively) of ten known quasars selected with apparent G magnitudes between 17 and 18. The QSO spectra are plotted in their rest-frame – the wavelengths corrected for the expansion of the Universe. The estimated redshift values from the DPAC Apsis/QSOC pipeline were used, as indicated on the right side of each source. In this wavelength system, some major emission lines of the QSOs align and correspond to well-known elements that are indicated by the dashed lines. From the redshift estimate, also indicated is the lookback time, which tells us how long ago the light from a quasar was emitted in our direction. Image created by Ludovic Delchambre, René Andrae, Morgan Fouesneau, Orlagh Creevey, Rosanna Sordo.

 

The Gaia mission’s primary objective is to measure the parallaxes and proper motions of stars in the Milky Way. Yet, Gaia also relies on very distant objects to define its reference frame. In fact, Gaia also sees several millions of distant galaxies and in particular quasi-stellar objects, or quasars (QSOs). Along its journey through the cosmos, the light from distant quasars encounters a wavelength elongation that is due to the expansion of the universe. As a consequence, the emitted light appears redder and covers a more expanded wavelength range than when it was initially emitted, billions of years ago.

QSOs are believed to be powered by the accretion of matter onto massive black holes at the centers of galaxies, a process that emits more energy than thermonuclear reactions. Quasars are amongst the most luminous objects in the universe and the most important tracers to study the accretion history of supermassive black holes, the early structure formation, and the history of cosmic reionization.

However, finding and characterising QSOs is extremely challenging because of their low spatial density and high contamination rate from Galactic objects. The Gaia Data Processing and Analysis (Coordination Unit 8) Astrophysical Parameters Inference System (Apsis, 2013A&26A...559A..74B) analyses the observations from the blue and red spectro-photometers of most of the objects seen by Gaia, the so-called low-resolution BP/RP spectra. In particular, Apsis identifies sources with high probabilites of being quasars and estimates their cosmological redshift with its QSOC module (2018MNRAS.473.1785D).

The figure shows ten known QSOs from the SDSS DR12Q (2017A&26A...597A..79P) and 2QZ (2004MNRAS.349.1397C) catalogues selected with apparent G magnitude between 17 and 18 magnitude that were successfully analyzed by QSOC (∼80% of the total number of QSOs in this magnitude range). This figure illustrates the quality of the Gaia low-resolution BP/RP spectrophotometry, which in this case enables us to estimate the spectroscopic redshifts of QSOs. All selected QSOs exhibit several strong emission features at the rest-frame wavelengths where they are expected, regardless of redshift and despite the low resolution and broad line-spread function of the BP/RP spectrophotometry.

As Gaia observes higher redshift QSOs, they can also appear dimmer because they are more distant. As a result, their BP/RP spectra become noisier and almost all spectral features are washed-out except for the very strongest emission lines (H-alpha and Ly-alpha), which still suffice for QSOC to remain capable of estimating their redshifts. Through the redshift effect, scientists can estimate the lookback time – how long it took the photons emitted from the QSO to travel through the intergalactic medium towards Gaia – providing us with “pictures” of the Universe as it was billions of years ago. QSOs pushed back the limits of the observable universe significantly in both distance and lookback-time.

In Gaia Data Release 3 – planned for the first half of 2022 – Gaia's Coordination Unit 5 will publish millions of low-resolution spectra all across the sky and Gaia's Coordination Unit 8 will provide us with their associated object classification and astrophysical parameters from Apsis, which include these QSO redshifts (see Gaia target release scenario).

 

Some technical details

The results presented above have been derived from the DPAC/CU8 Apsis analysis of the BP/RP spectra processed by DPAC/CU5. Determining the QSO classification and redshift parameters is the task of DPAC Coordination Unit 8 (CU8) responsible for the “Astrophysical Parameters”. CU8 uses all data available from the Gaia space telescope – astrometry, optical spectrophotometry, and near-infrared spectroscopy – to characterize the observed extragalactic objects (and stars). The DPAC Coordination Unit 5 (CU5) deals with the processing of the spectrophotometry – the low-resolution (R∼80) optical spectra of sources that encode key features such as the Lyα or Hβ emission lines.

The redshift estimates provided by QSOC were converted to lookback time for this figure under the assumption of a Λ-CDM cosmological model with parameters H0= 67.37 km s-1 Mpc−1, Ωm= 0.3147, ΩΛ= 0.6853, Ωk=0.0007 (Planck,2020A&A...641A...6P).

 

Credits: ESA/Gaia/DPAC, L. Delchambre, R. Andrae, M. Fouesneau, O. Creevey, R. Sordo, and all of Coordination Unit 5 and 8 (CU5/CU8) of Gaia DPAC. We wish to thank the Gaia Data Processing Centre at the Institute of Astronomy in Cambridge (DPCI) for producing the high-quality spectrophotometry and Centre National d’Etudes Spatiales (CNES; DPCC) upon which this work rests.

[Published: 22/12/2020]

Image of the Week Archive

2021

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.