Chemical Signatures of the Smallest Galaxies

filercaliforniaMechanics

Nov 14, 2013 (3 years and 9 months ago)

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Chemical
Signatures

of the
Smallest

Galaxies


Torgny Karlsson

SIfA
, School of
Physics
, The University of Sydney


Collaborators:

Joss

Bland
-
Hawthorn

and Ralph S. Sutherland

Outline

o
Ultra
-
faint

dwarfs



the
smallest

known

galaxies

o
A
hypothetical

object



ultra
low
-
mass

galaxy
:




o
Can
these

galaxies

explain

ultra
-
faint

dwarfs
?

o
Kinematics

o
Chemical
signatures


o
Conclusions



Smallest

DM halo that
can

retain

baryons

(
cold

gas) in the event of a


single

SN explosion”


talk by JBH

The
ultra
-
faint

dwarfs

(
UFDs
)



what

are
they
?

o
Very

dim

systems (10
3

<
L/L
sol

< 10
5

)

o
Very

dark
matter

dominated

(M
300


10
7

M
sol
)

o
Velocity

dispersions
σ
v



3


8 km/s

o
Very

few

number

of stars (N




1000


100 000)

o
Very

old

systems (stars
among

the
oldest

known
)

o
Very

metal
-
poor

(
-
2.5 <

[Fe/H]


<
-
2)

o
Relatively

close

to the
Milky

Way (
most

< 200
kpc
)

Belokurov

et al. (2006)/SDSS
collaboration

Boötes

I

ULMGs



ultra
low
-
mass

galaxies





Working

definition:

The
smallest

type

of
object

that
can

retain

cold

gas in the


event of a
single

supernova explosion



Gas
can

be
retained

in DM
haloes

of M < 10
7

M
sol

after SN explosions,
if

the


medium is
clumpy

(JBH, this morning)



Low
-
mass

stars
should

be
able



to form in the
enriched

gas



Should

be
found

further

out
,


far
away

from the
Milky

Way


t = 1 Myr t = 20 Myr (
M
vir

= 3
×
10
6

M
sol
)

Bland
-
Hawthorn
, Sutherland & Karlsson (2010)

Kinematics



a
poor

discriminant
?



Ultra
low
-
mass

galaxy
:



M
vir

= 3
×
10
6

M
sol



M
300

= 9
×
10
5

M
sol

(Einasto DM
profile
)



Motions in the gas
generated

by the SN


explosion
produce

a stellar
velocity



dispersion of
σ
v



4 km/s!





From a
kinematical

point

of
view
, the


ULMGs

appear

similar

to the
ultra
-
faint



dwarfs


Strigari

et al. (2008)

Karlsson,
Bland
-
Hawhtorn

& Sutherland (2010)

σ
v

= 3


8 km/s

Segue

1


what

is it?



Velocity

[km/s]

No. of stars

Radius

[
arcmin
]

Velocity

[km/s]

σ
v

= 4.3 km/s (
Geha

et al. 2009)

Chemical
enrichment

in

ultra
-
faint

dwarfs





Real
spread

in
metallicity

(
Kirby

et al. 2008)



Evidence

of a
spread

in stellar age (multiple
star

bursts
) for
few

UFDs

(
e.g
., de Jong et al. 2008)



Inhomogeneous

chemical

evolution
modelling
:

30
SNe

to
reach

[Fe/H]


-
2.5


-
2

Metal
-
rich

tail
!

Data bars:
Observed

metallicity

dis
-



tribution

function

in
UFDs


(
Kirby

et al. 2008)

Curve
:


Model

(cf. Karlsson 2005)

Star formation in

ultra
low
-
mass

galaxies





The
smallest

galaxies

have

been

enriched

by

1
-
10
SNe

In
ULMGs
,
low
-
number

statistics

drives the
chemical

evoultion
!


Chemical
signatures

in the
ULMGs



1
st

SN

2
nd

SN

3
rd

SN

4
th

SN

5
th

SN

t = 20 Myr

t = 1 Myr

[Fe/H]

[Fe/H]

Feltzing

et al. (2009)

Conclusions

o
ULMGs
:


Smallest

DM
haloes

that
can

retain

cold

gas in the event of a
single

SN”



This is
possible

in DM
haloes

< 10
7

M
sol

assuming

a
clumpy

medium

o
The
ULMGs

may

have

kinematics

and
luminosities

similar

to the
ultra
-
faint

dwarfs

(cf.
Segue

1)

o
However
:
Chemical
signatures



various

abundance

ratios
,
e.g
., [Mg/Ca], and
probably

a less
pronounced

[
Fe/H]
-
rich

tail



will

reveal

the
true

nature

of
these

galaxies

o
With the advent of the
next

gen.
telescopes

and
surveys
,
e.g
., the
ELTs
, LSST, and
Gaia
,
We

should

start
searching

for the
ULMGs

in the
outskirts

of the
Milky

Way!