EDE 2024

Le clustering des galaxies: observation 1/4

Ecole Rodolphe Cledassou 2024

Corentin Ravoux

LPCA, CNRS/IN2P3, Clermont-Ferrand

ACCEL² simulation

Cosmological context

How did the Universe evolve to its current state?
What are the fundamental constituents of our Universe?
How is matter distributed in the Universe?

The Story of Our Universe - NASA

You are here

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Global content of our Universe

Evolution of the Universe described by the ΛCDM model
Composition today:
- Only 5 % of its content is known
- Properties of components constrained by observations

What is the nature of dark energy
and dark matter?

Large scale structures

Initial density perturbations in the primordial Universe.
Perturbations grows to form the cosmic web: Nodes, filaments, walls and voids
Large-scale distribution of matter characterized by the linear power spectrum

Walther et al. 2021

Chabanier et al. 2019

How can we probe the cosmic web?
What can we learn from it?

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Outline

1. Properties of observed objects

2. Light path from source to detector

3. Galaxy survey properties

4. An overview of galaxy surveys

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

1. Properties of observed objects

2. Light path from source to detector

3. Galaxy survey properties

4. An overview of galaxy surveys

1. Properties of observed objects

Outline

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Astrophysical objects

In a galaxy survey, we generally observe:
- Stars (Milky Way, galaxy component)
- Galaxies
- Quasars
Very complex objects presenting a wide range of astrophysical phenomena

Arizona desert sky on a clear night

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Stars: spectra

Source: ESO

The stars are basically black bodies, i.e. follows Planck radiation law.
Surface temperature given by Wien's law

Luminosity given by Stefan-Boltzmann's law

For the same class of stars, the temperature, lifetime and luminosity mainly depend on the mass (degeneracy)

Spectral radiance: power emitted by the emitting surface per unit area, per unit solid angle and per spectral unit

\lambda_{\text {peak }} T=0.2898 \mathrm{~cm} \mathrm{~K}

L=4 \pi R_*^2 \sigma T_*^4

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Stars: stellar populations

Source: ESO

Different class of stars located in the Hertzprung Russel (HR) diagram
- Different diagram forms (color, temperatures)
- Theoretical and observational (e.g. Gaia)
- Large luminosity range
- Complex evolution on this diagram

Source: Gaia DR3

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Source: ESO

Deformed black body spectra
Classification based on the surface temperature (O, B, A...)
Contains emission and absorption lines (mainly absorption) caused by elements in the star atmosphere
Sawtooth profile of reddest stars (M, M giant) caused by molecules formed in the star atmosphere due to low surface temperature

Stars: real spectra

Source: SDSS

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxies

Source: ESO

A galaxy is a collection of:
- A central supermassive Black Hole
- Stars (> 10⁸)
- Gas (of various temperature and composition)
- Dust
- Dark matter
Largest fraction of the luminous matter in the form of stars.

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxies: diversity

Very historically-oriented classification of shapes: Hubble-de Vaucouleurs
Galaxy types:
- Elliptical
- Spirals and barred
- Irregulars
Very simplified in comparison to recent classifications

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Source: ESO

Galaxies: spectra

The galaxy spectra shape depends mainly on its stellar population, and thus on:

The initial distribution of stellar masses
The history of star formation in the galaxy
The age of the galaxy
The merging history of the galaxy

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Source: ESO

Galaxies: continuum

Combination of many Black-Body spectra spanning a range in temperatures
This produces a fairly flat overall spectrum
A characteristic feature is the 4000Å break caused by:
- Accumulation of absorption from metals in the stellar atmospheres
- The lack of hot blue stars
Hence, older galaxies present a strong 4000Å break

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Source: ESO

Galaxies: lines

Absorption

Emission

Need metals in stellar atmospheres or cold gas in the interstellar medium
Old stellar population = old
galaxy
Ellipticals galaxies or spiral bulge

Emission from high ionized regions around stars (recombination, collision)
Need very hot gas and O and B
type stars (bluer ones)
Newly formed stars =
star-forming/young galaxy
Spiral Disks or Irregular galaxies

Old galaxies

Young galaxies

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Source: ESO

Galaxies: population summary

Color:

Star age:

SFR:

HI gas:

Dust:

Red

Blue

Old

Young

Low

High

Low

High

Low

High

Old, no star production, red, 4000Å break, absorption lines

Young, star production, blue, no 4000Å break, emission lines

"Early-type"

"Late-type"

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxy spectra examples

Source: ESO

SDSS DR5 template spectrum

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxy spectra examples

Source: ESO

4000Å break, absorption lines,no emission lines

Elliptical "Early-type"

SDSS DR5 template spectrum

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxy spectra examples

Source: ESO

SDSS DR5 template spectrum

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxy spectra examples

Source: ESO

no 4000Å break, emission lines

Spiral "Late-type"

SDSS DR5 template spectrum

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Quasars

Source: ESO

Quasar = supermassive black hole + accretion disk
Brightest objects in the Universe (up to 1000 x Milky Way)
Observed up to redshift z~7

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Quasars: subtype of Active Galaxy Nuclei

Source: ESO

Quasar enters into the classification of Active Galaxy Nucleus (AGN)
High gravitational energy
Heating of accreted matter by complex non-linear processes

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Quasars: spectra

Wavelength [Angstrom]

Flux

DESI quasar at z = 1.11

Strong emission in the visible, UV, X-ray and gamma-ray
Can emit radio depending on its orientation

Very large emission lines due to thermal and collisional process in the accretion disk

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Blind test on real data

DESI EDR data

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Blind test on real data

DESI EDR data

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Blind test on real data

DESI EDR data

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Blind test on real data

DESI EDR data

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Blind test on real data

DESI EDR data

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

1. Properties of observed objects

2. Light path from source to detector

3. Galaxy survey properties

4. An overview of galaxy surveys

Outline

2. Light path from source to detector

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Life of a photon

Source

Cosmic web

Telescope

Atmosphere

Milky Way

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Life of a photon

Source

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Redshift

Source

Emitted spectra is redshifted due to Universe expansion and galaxy peculiar velocity

(1+z)=\left(1+z_{\exp }\right)\left(1+z_{\text {vpec }}\right)

Probe universe expansion

Probe cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Life of a photon

Cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Traveling the cosmic web

Galaxy/Quasar light travels over large distances through:
- Intergalactic medium (IGM)
- Other galaxies/clouds
All these elements leave an imprint on the spectrum that can be used to probe the cosmic web

Cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: the Lyman-α forest

Cosmic web

Ravoux et al. 2023

Lines in quasar spectra at caused by absorbers in the intergalactic medium at

\lambda_{\mathrm{obs}} = \left(1 + z_{\mathrm{abs}}\right) \lambda_{\mathrm{Ly\alpha}}

z_{\mathrm{abs}}

DESI quasar z = 3.42

Tracer of neutral hydrogen between the quasar and us

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

The Lyman-α forest

Cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: the Lyman-α forest

Lyman-α forest

Observer

Quasar

Cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: the Lyman-α forest

Quasar

Lyman-α forest

\delta_F = \frac{F}{\langle F \rangle} - 1

= Fraction of transmitted flux

Observer

Correlation

\xi_\alpha(\vec{r})=<\delta_F(\vec{x}) \delta_F(\vec{x}+\vec{r})>_x

3D auto correlation

Cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: the Lyman-α forest

Lyman-α forest

\delta_F = \frac{F}{\langle F \rangle} - 1

= Fraction of transmitted flux

Observer

Correlation

\xi_{\alpha q}(\vec{r})=<\delta_F(\vec{x}) \delta_{\mathrm{qso}}(\vec{x}+\vec{r})>_x

Cosmic web

Quasar

3D cross correlation with quasars

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: the Lyman-α forest

3D auto correlation

Separation (Mpc/h)

BAO scale

Measurement of the BAO scale with the Y1 data of DESI

DESI collaboration 2024

3D cross correlation with quasars

Cosmic web

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Life of a photon

Milky Way

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Absorption: dust

Milky Way

Dust and gas affect the
transfer of radiation through a galaxy (scattering & absorption)
Impact generally expressed in term of color difference in B (blue) and V (visual) bands, called redenning:

Light that cross dust appears redder than emitted light

E(B-V) = (B-V)_{\text{obs}} - (B-V)_{0}

Johnson/Cousins UBVRI system

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Absorption: dust extinction

Milky Way

Total extinction in the visual band (V)

Extinction for a specific wavelength:

a and b functions have complex profiles, depending on the galaxy
Affects light both in the MW and in the host galaxy

A_V = R_V \times E(B-V)

A(\lambda) = A_V \times \left[ a\left(\frac{1}{\lambda}\right) + \frac{b\left(\frac{1}{\lambda}\right)}{R_V} \right]

= ratio of total to selective extinction

R_V

~ 3.1 for Milky way but can vary depending on dust property

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Absorption: Milky Way dust

Milky Way

E(B-V) dust extinction map in the Milky Way
Major contaminant of CMB surveys
For galaxy surveys: High values of E(B-V) can bias the clustering of pre-targetted galaxies

Additional absorption lines of specific elements: CaII H-K lines, Na doublet line..

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Improved SFD map

Emission: background

Milky Way

Background for JWST

Yellow = out of FoV
blue = galactic + zodiacal
red = self emission

Galactic emission (ranked by intensity):
- Synchrotron: e⁻ in galactic magnetic field
- Thermal free-free: e⁻ scatter off ions
- Dust, molecular emissions
Zodiacal emission: Infrared radiation by heated particles in our Solar System

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: correcting angular galaxy clustering

Milky Way

Chaussidon et al. 2022

Quasar density fluctuations are contaminated by photometric properties, including Milky Way dust E(B-V)
In DESI: systematics mitigated using various Machine Learning methods

Initial density map

After correcting all photometric systematics

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Life of a photon

Atmosphere

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Atmosphere

For ground based surveys, need to account for atmosphere effects

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Some astronomy definitions

Transparency/Opacity: transmission of the atmosphere on the line of sight.
Sky level: Sky brightness from natural and artificial light sources (mag.arcsec⁻²)
Airmass: Amount of air in the line-of-sight (normalized to zenith).
Seeing: Blurring caused by atmosphere, quantified by FWHM of a star image (in arcsec)
Often normalized to a nominal atmosphere on the considered observation site

Atmosphere

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Atmosphere absorption

Atmosphere

Atmosphere absorption determines the wavelength range of a survey

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Atmosphere absorption

Atmosphere

Absorptions caused by different molecules in the atmosphere
Rayleigh scattering caused by molecules and aerosols (particulate matter)
Some absorptions varies during the night (water, aerosols)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Atmosphere emissions

Atmosphere

Airglow: natural upper atmosphere emission
Light pollution
Moon light especially in the UV
Molecular emissions

Sky emission spectrum taken by DESI

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Atmosphere continuum

Atmosphere

In the optical and NIR, emission dominated by airglow (OH in particular)
Thermal radiation from atmosphere and telescope dominates above 2.2 μm

Noll et al. 2012

Sky model in Cerro Paranal

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Atmosphere sky lines

Atmosphere

Sky emission highly dominated by OH hydroxyl radicals (continuum + lines)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Example: characterizing atmosphere

Atmosphere

Auxiliary telescope near LSST measuring star spectra
Atmospheric properties obtained by forward modeling the atmosphere taking into account telescope transmission and out-of-spectra atmosphere
Goal: Live correction of measured brightness

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Life of a photon

Telescope

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Instrument: optics

Telescope design (refracting, reflecting, both)
Lenses for focal plane focalisation and optical corrections
Telescope responses (sometimes including atmosphere):
- Throughput: Total transmission of the telescope as a function of wavelength
- PSF (Point Spread Function): image of a point-like source (with or without seeing)

Telescope

Jee et al. 2011

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Detection: CCD

Telescope

See "Concept instrumental des photomètres visibles" Y1 P. Astier

LSST focal plane

CCD = Charged-Coupled Device
Convert photons into e⁻ through photoelectric effect
Collect them with a p-n semiconductor junction (p=holes, n=excess)
Transfer them line by line with adequate clocking
Focal plane = several CCD mounted
Different readout and communication systems

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Detection: quantum efficiency

Telescope

Quantum efficiency = number of electrons / number of incoming photons (Sensor response)
Factors influencing QE:
- Reflection coefficient (anti-reflection coatings)
- Absorption coefficients
- Thickness
- Width of the depletion layer (layer for charge transfer)
- Structure of the electrodes
- Temperature...

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Detection: CMOS

Telescope

CMOS = Complementary Metal-Oxide-Semiconductor
Each pixel (photodiode) operate independently and convert into voltage
CMOS vs. CCD:
- Lower quantum efficiency
- Higher readout noise
- Higher readout speed
- No blooming (spilling of saturated pixel)
- Integration of processing functions
- Lower electrical consumption
- Lower price
Increasing interest in astronomy applications due to integration and readout

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Detection: infrared detectors

Telescope

Thermal detectors (bolometers): detection of resistance change induced by temperature
Photoelectric detectors: material absorbs infrared radiation and convert them to e⁻, generally connected to a CMOS Readout Integrated Circuit (ROIC)
- Material dependent on wavelength range (HgCdTe [1-14 µm] InSb [3–5 µm] ...)
- Example: HgCdTe infrared detectors in Euclid NISP

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

ACCEL² simulation

Galaxy surveys contains a wide variety of complex objects
From the source to the observer, a lot of effects (astrophysical, instrumental) can impact clustering measurement

Annexes

Annexes

Le clustering des galaxies: observation 2/4

Ecole Rodolphe Cledassou 2024

Corentin Ravoux

LPCA, CNRS/IN2P3, Clermont-Ferrand

ACCEL² simulation

Summary

Source

Cosmic web

Telescope

Atmosphere

Milky Way

Galaxies
Stars
Quasars

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

1. Properties of observed objects

2. Light path from source to detector

3. Galaxy survey properties

4. An overview of galaxy surveys

Outline

3. Galaxy survey properties

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Surveys

Generally, surveys tends to be either:
- Static or dynamic (transient)
- Wide & Faint or Small & deep
- Photometric or spectrometric
Very recent surveys achieve several functions at once
A survey covers a fixed wavelength range (from gamma to radio)
In galaxy surveys for this course: optical/NIR, wide, static surveys

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Properties: field of view

Field of view (Ω): area of the sky visible through the telescope camera
Depends on telescop optics (design, focal length, lenses, correctors) and camera size
Collecting area surface (A): larger mirror = higher photon flux
Etendue (E=AΩ): characterize the quantity of accessible information on the sky

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Properties: volume of the survey

Footprint: observed size on the sky at the end of the survey
Expressed in healpix format (hierarchical equal area isolatitude pixelation)
Density: Number of object observed per square degree
Redshift range dz
Volume ∝ Footprint × dz
Python package: healpy

N_{\mathrm{pix}} = 12 N_{\mathrm{side}}^2

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Properties: depth

Magnitude: logarithmic scale for object fluxes for one given filter
Apparent magnitude (m): magnitude on the detector (including extinction)
Absolute magnitude (M): magnitude at 10 parsec
Zero point calibration:
- Vega system: Vega star magnitude null for all wavelength
- AB system: Reference to a constant flux object (3631 janskys)

M = m + 5 - 5 \log _{10}(d)

m_1-m_2=-2.5 \log _{10}\left(\frac{F_1}{F_2}\right)

m_{\mathrm{Vega}}(\mathrm{Obj})=-2.5 \log \left[\frac{\int f_\lambda(\mathrm{Obj}) W_\lambda d \lambda}{\int f_\lambda(\mathrm { Vega }) W_\lambda d \lambda}\right]

m_{\mathrm{AB}}(\mathrm{Obj})=-2.5 \log \left[\frac{\int f_\lambda(\mathrm{Obj}) W_\lambda d \lambda}{\int W_\lambda d \lambda}\right] - 48.6

= filter response/window

W_\lambda

Survey depth: magnitude (redshift) of the faintest (farther) object observed
Signal-to-noise ratio (SNR): characterize the quality of one observation

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Calibration images

Zero/Bias

Dark

Flat

Arc

All surveys need calibration images
Zero/Bias: minimum response time image
- Measure the CCD intrinsic readout noise
Dark: Image with the same exposure time as science image without any light
- Use to characterize noise sources

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Calibration images

Flat: Focal plane illuminated with a known smooth spectrum lamp
- Use to correct relative CCD responses under illumination
Arc: Image produced by known monochromatic lamps
- Use for wavelength calibration (only for spectra)
Master: A stack/coadd of several calibration image

Zero/Bias

Dark

Flat

Arc

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric vs Spectroscopic surveys

Source: DESI communication figures

Measure the spectra of objects (selected or not)
Examples: SDSS, 6dFGS, DESI, 4MOST, Euclid NISP (spectro)

Measure the object flux through a filter
Examples: DES, ZTF, CFHT, LSST, Euclid VIS, Euclid NISP (photo)

Same detection method (CCD, IR)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric surveys: filters, filters and filters

Filter: Precise transmission shape and width vs wavelength
"Standard" filters: Johnson/Cousins (UBVRI), Bessel, Gunn, IR (JHK...)
Full/Synthetic filter: Filter x throughput x quantum efficiency x standard atmosphere
Images taken with different filters (fast changer for recent surveys)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric surveys: filters, filters and filters

A lot of different systems of filters are used...
Practical website for filter profiles:
- https://astroquery.readthedocs.io/en/latest/svo_fps/svo_fps.html
- https://speclite.readthedocs.io/en/latest/filters.html

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric surveys: photometric redshift

Only a partial part of the spectra is measured
Classification and redshift methods:
- Template fitting
- Machine learning
- Additional environment informations
Outliers: redshift & object type confusion
Recent precision:
Need to be calibrated with spectroscopic redshift

\sigma(z) \sim 10^{-2} (1+z)

\sigma(z) \sim 10^{-4} / 10^{-5}

Lee et al. 2024

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric surveys: image calibration

Photometry: providing a precise estimate of object magnitudes/brightness
After the CCD, e⁻ are converted into digital counts, or Analog-Digital Units (ADUs) by ADC (Analog To Digital Convertor)
Photon transfer curve: ADU variance of uniform exposures vs flux
Gain: N input ADU / N output ADU
Measuring stability of gain and saturation is essential !

Photons on CCD follows Poisson law

(full well capacity)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy

Diffraction grating: periodic structure
Interference condition: path difference between AB and A'B‘ is equal to an integer multiple of λ

m \lambda= a (n_1 \sin \alpha+n_2 \sin \beta)

Wavelength dependent dispersion
Zeroth order corresponds to the specular reflection off the grating

n_2 \beta = - n_1 \alpha

= distance between adjacent grooves

\beta(\lambda)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy

Pattern of the gratting have an impact on the diffraction scheme
Adjusting the slit profile can maximize efficiency in one order and minimize / cancel it in the other orders
Littrow configuration:
Achieve maximum grating efficiency in a given diffraction order m

\beta \simeq \alpha = \theta_{\mathrm{B}}

Blazed gratting

m \lambda= 2 a \sin \theta_{\mathrm{B}}

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: dispersion

Grism: Transmission grating attached to a prism
Simple to engineer
Quasi-Littrow configuration - no variable distortion
Inefficient for number grooves > 600/mm due to groove shadowing and other effects

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: dispersion

They work by a mechanism similar to Bragg diffraction (crystal diffraction)
- Periodic refractive index modulations in a volume of gelatin
- Interference pattern created by two coherent laser beams inside a photosensitive medium
Transmission gratings with very low fringe spacings can be made

Glass

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: spectral resolution

Ability to distinguish spectral features separated by Δλ
Resolving power:
- Low resolution
- High-resolution
Choice depends on the spectral features
- Broad features (e.g. because of velocity dispersion) or narrow (object properties)
Higher resolution need larger exposure time for the same SNR

R = \frac{\lambda}{\Delta \lambda}

R \sim 100

R \sim 50000

R \sim 5000

R \sim 50000

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: type of spectrograph

Slitless spectrograph (no mask, sky background)

Multi-slit spectrograph (focal plane mask)

Multi-fiber spectrograph (optical fibers)

Integral field spectrograph

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: multi-object spectrographs

Multi-split = insertion of slits (masks) and of a grating
Typically, for small fields of view (up to 10 arcmin)
Drilled in masks (hundreds of slits): Free positioning. Possibly multiple spectra in dispersion direction.
Movable slitlets (tens of slits): Slits on top of each other. Positioning along the dispersion direction only.
Multi-Shutter Assembly (Example: NIRSpec JWST) : Configurable shutters

VLT-FORS

ESO

JWST MSA

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: multi-object spectrographs

Multi-fiber = decoupling of telescope and spectrographs
Fields of view up to degree scales
Up to thousands of fibres
Various positioner geometries:
- One motor (or a couple) positions fibres everywhere in the focal plane
- One motor per fibre, with limited patrol field per fibre

DESI

WEAVE

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: slitless spectroscopy

See "La spectrométrie IR" Y3 Y. Copin

Slitless spectroscopy with a prism or grism in the beam produces spectra for all objects in the field of view
Pros: all objects get a spectrum
Cons:
- Cross contamination
- Mixing of spatial and spectral information
- High background, same as within an imaging filter of same bandpass as the grism
Recent example: Hubble, JWST, Euclid, Roman space telescop

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopy: target selection

For multi-object spectrographs: select measured objects to measure from photometric surveys
Magnitude cuts: select brighter objects
Color (magnitude difference) cuts:
- Select a given population of object (LRG, ELG, QSO, LBG)
- Homogeneous sample for clustering measurements
- Reject stars and unwanted objects

Chaussidon et al. 2022

Galaxies

Stars

Zhou et al. 2022

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

1. Properties of observed objects

2. Light path from source to detector

3. Galaxy survey properties

Outline

4. An overview of galaxy surveys

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Galaxy observations

964: Abd al-Rahman al-Soufi (persian astronomer) observes first galaxies (Andromeda, Large magellanic cloud)
1750/1755: Thomas Wright (english astronomer) & Emmanuel Kant postulate that nebulae are other galaxies like the Milky Way
- "Une nébuleuse n’est pas un unique et énorme soleil, mais un système de nombreux soleils, rassemblés en raison de leur distance dans un espace si étroit, que leur lumière, qui serait imperceptible pour chacun d’eux isolément, parvient, grâce à leur innombrable quantité, à produire une blancheur pâle et uniforme.” – Emmanuel Kant, Histoire générale de la nature et théorie du ciel, 1755

1910-20's:
- Leavitt & Hubble observations confirmed the existence of other galaxies
- Hubble & Lemaitre discovers expansion

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric surveys - "Stage I"

POSS I (1949 - 1958)
UKST (1974 - 1999)
POSS II (1985 - 2000)
First digital surveys:
- Digitized Sky Survey (1994) scanning of POSS I/II and UKST
- 2MASS (1997 - 2001): 3-band NIR photometric survey (also considered "stage II")

photographic plate of POSS I

copy-paste from the mighty Arnaud

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Photometric surveys - "Stage II/III"

Deep surveys:
- Hubble space telescop (1990-)
- VLT (1998-)
Wide surveys:
- DES (2013-2019)
- HSC (2014-)
- PanStarrs (2008-)
- CFHT (1979-)
- VISTA VHS (2009-2015)

COSMOS

VLT

UNIONS

DES Imager

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopic surveys - "Stage I"

First spectrographic surveys:
- CfA I (1977 - 1982), 1100 redshifts, structures formed by galaxies
- CfA II (1985 - 1995), 18 000 redshifts, Great Wall discovery

copy-paste from the mighty Arnaud

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopic surveys - "Stage II"

SDSS-I SDSS-II (1998-2008)
2dFGS 6dFGS (1997-2009)
WiggleZ (2006-2011)
Goal: BAO detection

6dFGS map

SDSS

WiggleZ BAO

Blake et al. 2011

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Spectroscopic surveys - "Stage III"

SDSS BOSS/eBOSS (2008-2020)
HETDEX (2017-2024)
PFS (2014 – 2020)
Goal: improving BAO

IFU

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Moore's law for spectroscopic galaxy survey

III

copy-paste from the mighty Arnaud

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Present experiments: "Stage IV"

Ground photo:
- LSST (2025-)
Ground spectro:
- DESI (2021-)
- 4MOST (2024-)
Space (spectro/photo):
- Euclid (2023-)
- JWST (2023-)
- Roman space telescop (2025-)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

The Dark Energy Spectroscopic Instrument (DESI)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

4 m telescope at Kitt Peak Observatory
2021 - 2026
3 years of data secured

DESI collaboration 2022

DESI instrument

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI survey forecast

BGS: Bright Galaxy Survey
LRG: Luminous Red Galaxy
ELG: Emission Line Galaxy
QSO: Quasar
- z > 2.1: with a Lyman-α forest
- z < 2.1: as tracers

40 million redshifts in 5 years

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI BAO

DESI collaboration 2024

See "Conférence DESI" A. de Mattia

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI focal plane

5 μm positioning in few sec. for 5000 targets
All fibers connected to the spectrographs

Silber et al. 2022

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI spectrographs

10 Spectrograph modules with cryostats containing 3 CCD

DESI collaboration 2022

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI science image

Raw image over one DESI CCD of one spectrograph

Identify some features

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI science image

Fibers side by side

Wavelength

Cosmics

Atmosphere emission lines

Extracted spectrum

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI calibration images

Zero/Bias

Dark

Flat

Arc

DESI calibration images

Minimal exposure time with dome closed

Science exposure time with dome closed

Telescope pointed to a uniformly illuminated screen in the dome

Screen in the dome illuminated with calibration emission lamps

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI spectral extraction

Raw image calibrated with bias/flat/dark/arc
Cosmics and bad pixels detected and flagged by software
Spectra extracted with the "spectroperfectionism" algorithm:

\vec{p}=\mathbf{A} \vec{f}+\vec{n}

Camera signal

Flux

Noise

Spectral noise evaluated with calibration images
Spectrum level normalized using sky fibers (fixed, or broken positioners) and standard stars

A = matrix measured with arc images, containing full 2D PSF modeling

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

DESI spectral extraction

Object type and

redshift obtained by:

Visual inspection
Template fitting
Machine learning

\left(\mathrm{RA},\mathrm{DEC},z\right)

\left(\mathrm{RA},\mathrm{DEC},z,f_{\lambda}\right)

for galaxies

for Lyman-alpha forest

Correlation function and power spectrum measurements

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Legacy Survey of Space and Time (LSST)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

LSST acronyms

Vera Rubin Observatory (VRO)

Legacy Survey of Space and Time (LSST)

Dark Energy Science Collaboration (DESC)

+ 7 (Transients, SL, AGN...)

Observatory/Telescop

Survey organism

Science collaborations

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Rubin observatory

30 m diameter dome

Telescope mount assembly

AuxTel: 1.2m atmospheric telescop

Cerro Pachon

Service and maintenance facility

Control room

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Rubin observatory: mirrors

Modified three-mirror Paul-Baker design: M3 and M1 polished from a single substrate
8.4 m primary mirror
9.6 square degrees field of view

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Rubin observatory: mechanical structure

Telescop Mount Assembly (~15 m)
Can move 3.5° in 4 seconds
Mirrors mounted on hexapods and correctors: counteract the changing distortions produced by changes in the gravitational orientation

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Rubin observatory: camera

Largest numerical camera in the world (3.2 Gpixels)
2s readout time
Integrated cryostat modules (~120°C)
3060 kg

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Rubin observatory: camera

189 4k x 4k science CCD
Corner sensor for guiding and wave-front analysis
CCD packed by raft (3x3) with dedicated electronic treatment system

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Rubin observatory

Integrated filter wheel and filter changer (french contribution)
6-band ugrizy survey

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

LSST survey

10-year photometric survey of half of the sky (~ 20 000 deg²)
In commissioning, expected start in september 2025
Complete coverage every 4 nights during 10 years (static and transient survey)
15 TB data per night, 100 PB in total, 10 million alerts per night in average
Deliverables:
- 20 billion galaxies with photometric redshifts up to z=3
- 17 billion resolved stars
- ~ 1 million type Ia supernovae (with ~ 300,000 cosmological SNe Ia)

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

LSST survey: observation strategy

Strategy elaborated for 10 years to satisfy 8 science collaboration and a lot of science cases
Full survey simulation night by night
Creation of precise metrics
Example: full SNIa simulation and impact of rolling cadence
5 Deep Drilling fields for photo-z & WL calibration, high redshift SNIa ...

XMM-LSS

COSMOS

EUCLID

CDFS

ELAIS-S1

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

LSST survey: science

Dark matter & Dark energy

- Strong & Weak lensing

- BAO (angular and photo-z)

- Clusters, Supernovae cosmology

Mapping the Milky Way

- Structure and evolution

- Stellar properties

Solar System science

- Comets & asteroids

- Small body census

Transient sky

- Supernovae, variable stars

- Rare events (kilonovae, TDE)

- New classes of transients

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Euclid telescope

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Euclid telescope

VIS

NISP

See "La mission Euclid, objectifs et organisation" F. Bernardeau

Two modes: photometric and slitless spectro (grisms)
16 2k × 2k NIR HgCdTe detectors

Visual imager (photometric)
36 4k x 4k CCD with one single very broad band filter

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Euclid: First light

VIS

NISP (Spectro)

NISP (Photo)

+ Early release observation

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Euclid: NISP slitless spectroscopy

HST WFC3 images

Euclid-like image: HST WFC3
Slitless spectroscopy over one axis:
- Source confusions
- Spectroscopic order superpositions
Decontaminate
spectral overlap: Grism wheel with different orientations

Ecole Rodolphe Cledassou 2024 - galaxy clustering: observation

Euclid: NISP slitless spectroscopy

3 "red" grisms with different rotations, 1 "blue" grism
1 red grism did not met requirement: correction of source confusion by tilting the two other grisms