Application of STREAMFINDER onto ESA/Gaia DR2 w/ Rodrigo A. Ibata - - PowerPoint PPT Presentation

Stellar Stream map of the Milky Way Halo : Application of STREAMFINDER onto ESA/Gaia DR2

w/ Rodrigo A. Ibata and Nicolas F. Martin

Khyati Malhan PhD Student Supervisor: Dr. Rodrigo Ibata

Outline

• What are stellar streams important?
• The STREAMFINDER algorithm.
• Stellar Stream maps of the Milky Way halo

from ESA/Gaia DR2.

Stellar Streams

Pal 5 stream. Discovered by

• Odenkirchen et al. (2001). This map was

created by Bernard et al (2016)

Stellar Streams

• Malhan, Ibata & Martin (2018). ZEA projection of the Galactic sky

containing all of the known streams Milky Way streams (~40). The plot was created using galstreams package (Mateu et al 2017)

>40 Previously discovered Milky Way Streams

Stellar Streams

Pal 5 stream. Discovered by

• Odenkirchen et al. (2001). This map was

created by Bernard et al (2016)

Orbital Structure of Streams

Stellar Streams

Stellar streams are great archaeological tools :

• Constraining the φMW(x), MW dark-matter distribution,

lumpiness of the halo (Ibata et al 2002, Law & Majewski 2010, Koposov et al 2010, Bovy et al 2015, Carlberg 2013, Erkal & Belokurov 2015)

• Estimating fundamental parameters (Rsun, Vsun) of the MW

Galaxy (Majewski et al 2006, Kupper et al 2015, Malhan & Ibata 2017)

• Radial migration process in the disk (Martell et al 2016)
• Quantifying pre-merging history of our Galaxy.

…..So they are useful. And the more, the merrier.

STREAMFINDER

Maximize stream detection by:

• Use all the prior information about the stellar stream

(knowledge of phase-space-color-magnitude distribution)

• Use all the information delivered by Gaia DR2

Malhan & Ibata (2018)

STREAMFINDER

Maximize stream detection by:

• Use all the prior information about the stellar stream

(knowledge of phase-space-color-magnitude distribution)

• Use all the information delivered by Gaia DR2

Best way to detect stream : LOOK ALONG THE ORBITS

Malhan & Ibata (2018)

STREAMFINDER

Maximize stream detection by:

• Use all the prior information about the stellar stream

(knowledge of phase-space-color-magnitude distribution)

• Use all the information delivered by Gaia DR2

Best way to detect stream : LOOK ALONG THE ORBITS……Hmm?

Malhan & Ibata (2018)

STREAMFINDER

Maximize stream detection by:

• Use all the prior information about the stellar stream

(knowledge of phase-space-color-magnitude distribution)

• Use all the information delivered by Gaia DR2

Assumption : stream members contained in a 6D hypertube and its 6D volume ~ f(σw, σv, torbit)

Malhan & Ibata (2018)

STREAMFINDER

Malhan & Ibata (2018)

Testing algorithm with N-body simulated stream

• Simulated a globular cluster stream in a realistic galactic model

(Dehnen & Binney 1998)

• Retained only 50 objects in the stream, ΣG =33 mag arcsec-2 (faint stream)
• Retained only 4D phase-space information – l , b, μl ,μb (with errors)

vrad and ω information was deleted.

• Convolved Gaia like errors in proper motions.
• Also assigned a SSP model to the stream of ([Fe/H], Age)= (-1.5, 10 Gyr)

STREAMFINDER

Malhan & Ibata (2018)

GAIA-like ERRORS PERFECT STREAM

Testing algorithm with N-body simulated stream

STREAMFINDER

Malhan & Ibata (2018)

• Stream (50 stars) + GUMS (330,000 stars) = Data. 0.015% stream stars.
• vrad and ω information was deleted. Convolved Gaia like errors.

STREAMFINDER

Malhan & Ibata (2018)

Orbit sampling

• Blindness and uncertainty in stellar phase-space position.

a) Sample orbits in distance space (consistent with a SSP model) b) Sample orbits in vrad space (vtotal< vescape) c) Sample orbits in proper motion space

• ~30,000 orbits for every datum.

Sampled orbits Data orbit Perfect orbit

STREAMFINDER

Malhan & Ibata (2018)

Ldatum = Lkinematics + LLF + Lcontinuity

• Log-likelihood of a star being associated with a stellar stream.

Data-orbit comparison (given the observed errors) Luminosity Function criteria Stream continuity criteria

STREAMFINDER

Malhan & Ibata (2018)

Ldatum = Lkinematics + LLF + Lcontinuity

• Log-likelihood of a star being associated with a stellar stream.
• L acts as ``weight’’ for every star used to obtain stream density plot

Data-orbit comparison (given the uncertainties) Luminosity Function criteria Stream continuity criteria

STREAMFINDER

Malhan & Ibata (2018)

• Stream output – loglikelihood density plot

STREAMFINDER

Malhan & Ibata (2018)

Multiple stream case :

• Age = 9-10 Gyrs
• [Fe/H] = -1.5 to -2.5
• 50 stars per stream
• Surface brightness

~ 33 mag arcsec-2 (very faint)

STREAMFINDER

Malhan & Ibata (2018)

Multiple stream case :

• Age = 9-10 Gyrs
• [Fe/H] = -1.5 to -2.5
• 50 stars per stream
• Surface brightness

~ 33 mag arcsec-2 (very faint)

• STREAMFINDER

using different SSP models

MW Stellar Stream map from Gaia DR2

• Gaia DR2 released on 25 April 2018
• 1.7 billion sources down to G~20.7.
• Positions +Proper motions+ parallaxes (not so good) +G,BP,RP for

all sources.

MW Stellar Stream map from Gaia DR2

Malhan, Ibata & Martin (2018)

Gaia DR2 dataset

• |b|>30 ◦
• GCs and DGs

masked

STREAMFINDER

results…?

Inner halo. Distance =[5, 15] kpc. Malhan et al (2018)

Phase-space-luminosity coherence of the structures. Malhan et al (2018)

Observations Orbital Solutions

Testing the phase-space-luminosity coherence of the structures. Malhan et al (2018)

Intermediate halo. Distance =[15, 30] kpc. Malhan et al (2018)

Inner halo. Distance =[5, 15 kpc]. Malhan et al (2018)

Outer halo. Distance =[20, 100] kpc. Malhan et al (2018)

Sgr stream

STREAMFINDER

Malhan & Ibata (2018)

Main advantages of the algorithm :

• Completing the 6D DF(x,v) of detected stellar streams -useful in

the context of galaxy formation .

• Stream Orbital properties

GD-1 stream

• bservations (Koposov et al 2010)

STREAMFINDER solutions

• ``Chemical tagging’’ of stars based on SSP model.
• STREAMFINDER BLOBFINDER (find blob of stars – star clusters and

dwarf galaxies)