1Can we use weak lensing
to directly probe
the baryonic content of galaxies?
Masato I.N. Kobayashi (Nagoya U)

Laboratory of galaxy formation and evolution, M2

Collaborators; Alexie Leauthaud, Surhud More (Kavli IPMU)

第一回銀河進化研究会, 2014.Jun.04 at NAOJ

0.Overview

1.Motivation & Introduction 2.Analysis & Data 3.Results 4.Summary 5.Future prospects2

Measurement of stellar mass
Stellar mass is a key to galaxy formation and evolution.
But how to measure?
1. SEDwith assumptions of galaxy age, metallicity, dust extinction,
initial mass function (IMF) …

2. Stellar kinematics

velocity dispersion anisotropy

3. Strong lensing

for massive galaxies}

Direct measurement of mass

4. Weak lensing ???
HSC is promising!
Consistency between 1 and 4 can put a constraint on IMF.

0.Overview

1.Motivation & Introduction 2.Analysis & Data 3.Results 4.Summary 5.Future prospects3

Goal of my project
Weak lensing allows us to study much wider samples ofgalaxies compared to strong lensing so that we caninclude spiral galaxies for example.
For this, we have to push weak lensing to small radialscales.

How well can we expect to measure stellar massusing the weak lensing technique?

0.Overview

1.Motivation & Introduction 2.Analysis & Data 3.Results 4.Summary 5.Future prospects4

Weak lensing on small radial scales
Gavazzi et al., 2007

Projected
2D surface
massdensity
Req
How well could we do these measurements with upcoming
weak lensing surveys? (e.g. HSC, Euclid)

0.Overview

1.Motivation & Introduction 2.Analysis & Data 3.Results 4.Summary 5.Future prospects5

Weak lensing S/N at Req
Number of lens source pairs
Number of lens * Effective area * Source density
Analytical factors
Volume effectReq (Angular diameter
distance, stellar to halo
mass relation etc.)
Critical surface density
for lensing…

Analytically difficult
Completeness limitOverlap of galaxies thatprevents shape measurement…

0.Overview

1.Motivation & Introduction 2.Analysis & Data 3.Results 4.Summary 5.Future prospects6

Overlap is non trivial

 Weak lensing uses background galaxies
 Galaxies are clustered As we go to small radial scales, there are more blends
between foreground and background galaxies. The numberdensity of usable source galaxies is difficult to estimate.

0.Overview 1.Motivation & Introduction

2.Analysis & Data 3.Results 4.Summary 5.Future prospects

Our solution!
Use real data (HST ACS observing COSMOS field) to“make predictions” on the number of source galaxies asa function of radius.
 To what small scale we can/have to push weak lensing?
How large area required to observe to achieve S/N = 2?
Uncertainty in mass coming from different IMF is about afactor 2. (e.g between Salpeter and Chabrier), so we aim
for at least S/N = 2 in weak lensing signal.

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0.Overview 1.Motivation & Introduction 2.Analysis & Data

3.Results 4.Summary 5.Future prospects

8

Results1: Examples from HST
Shapemeasurement
Okay! :)
Shape
measurement
Not okay… :(Green: Kron ellipse
Red: Collided Kron
Blue: Req

0.Overview 1.Motivation & Introduction 2.Analysis & Data

3.Results 4.Summary 5.Future prospects

N(<r)

Results2: Source number counts
around lenses

Physical Radius [kpc]0.62 < z < 0.72

Physical Radius [kpc]9

0.Overview 1.Motivation & Introduction 2.Analysis & Data

3.Results 4.Summary 5.Future prospects

Results3: Effective area
Area in between theKron ellipse and Req ofthe lens galaxy.

Req

Kron ellipse

100.Overview 1.Motivation & Introduction 2.Analysis & Data

3.Results 4.Summary 5.Future prospects

Results4: Required area to observe
HSTSubaru HSC
Preliminary

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0.Overview 1.Motivation & Introduction 2.Analysis & Data 3.Results

4.Summary 5.Future prospects

Summary• Stellar mass is crucial for the discussion of galaxy
properties, but still uncertain.
• We can exploit the weak lensing signal even on a small
scale about 10 kpc order as “first hand” information ofstellar mass in galaxies.
• Future surveys have an enough possibility to detect
weak lensing signal coming from stellar mass with atleast factor 2 precision. HSC is promising!

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0.Overview 1.Motivation & Introduction 2.Analysis & Data 3.Results 4.Summary

5.Future prospects 13

Future Prospects• Analyze ground based data to make better estimations
how well future ground based surveys can perform.
• Error from our assumptionsSo far only S/N, how about shear biased or the effect of the baryon profile
to the dark matter profile? …

• With those large statistics, towards galaxies withsmaller mass (gas contribution on the total massprofile).