Chemical signatures of the smallest galaxies
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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. Ultra-faint dwarfs – the smallest known galaxies A hypothetical object – ultra low-mass galaxy :

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Chemical Signatures of the Smallest Galaxies

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Chemical signatures of the smallest galaxies

Chemical Signatures of the Smallest Galaxies

Torgny Karlsson

SIfA, School of Physics, The University of Sydney

Collaborators:

JossBland-Hawthorn and Ralph S. Sutherland


Outline

Outline

  • Ultra-faintdwarfs – the smallestknowngalaxies

  • A hypotheticalobject – ultra low-massgalaxy:

  • Can thesegalaxiesexplainultra-faintdwarfs?

    • Kinematics

    • Chemical signatures

  • Conclusions

“Smallest DM halo that canretainbaryons (cold gas) in the event of a

single SN explosion” ➙ talk by JBH


The ultra faint dwarfs ufds what are they

The ultra-faintdwarfs (UFDs) – what are they?

Boötes I

  • Verydim systems (103 < L/Lsol < 105 )

  • Very dark matterdominated (M300 ∼107Msol)

  • Velocity dispersions σv ∼ 3 – 8 km/s

  • Veryfewnumber of stars (N✶ ∼ 1000 – 100 000)

  • Veryold systems (stars among the oldestknown)

  • Verymetal-poor (-2.5 < 〈[Fe/H]〉 < -2)

  • Relativelyclose to the Milky Way (most < 200 kpc)

Belokurov et al. (2006)/SDSS collaboration


Ulmgs ultra low mass galaxies

ULMGs – ultra low-massgalaxies

  • Working definition: The smallesttype of object that canretaincold gas in the

  • event of a single supernova explosion

  • Gas can be retained in DM haloes of M < 107Msolafter SN explosions, if the

  • medium is clumpy (JBH, this morning)

  • Low-mass stars should be able

  • to form in the enriched gas

  • Should be foundfurtherout,

  • far away from the Milky Way

t = 1 Myr t = 20 Myr (Mvir = 3×106Msol)

Bland-Hawthorn, Sutherland & Karlsson (2010)


Kinematics a poor discriminant

Kinematics – a poordiscriminant?

Karlsson, Bland-Hawhtorn & Sutherland (2010)

  • Ultra low-massgalaxy:

  • Mvir = 3×106Msol

  • M300 = 9×105Msol (Einasto DM profile)

  • Motions in the gas generated by the SN

  • explosion produce a stellar velocity

  • dispersion of σv ∼ 4 km/s!

  • ➙ From a kinematicalpoint of view, the

  • ULMGsappearsimilar to the ultra-faint

  • dwarfs

σv = 3 – 8 km/s

Strigari et al. (2008)


Segue 1 what is it

Segue 1 – what is it?

Velocity [km/s]

No. of stars

Radius [arcmin]

Velocity [km/s]

σv = 4.3 km/s (Geha et al. 2009)


Chemical enrichment in ultra faint dwarfs

Chemical enrichment in ultra-faintdwarfs

Data bars: Observedmetallicity dis-

tributionfunction in UFDs

(Kirby et al. 2008)

Curve:Model (cf. Karlsson 2005)

Metal-richtail!

  • Real spread in metallicity (Kirby et al. 2008)

  • Evidence of a spread in stellar age (multiple starbursts) for fewUFDs (e.g., de Jong et al. 2008)

  • Inhomogeneouschemical evolution modelling: ∼30 SNe to reach [Fe/H] ∼ -2.5 – -2


Star formation in ultra low mass galaxies

Star formation inultra low-massgalaxies

The smallestgalaxieshavebeenenriched by ∼1-10 SNe

In ULMGs, low-numberstatistics drives the chemicalevoultion!


Chemical signatures in the ulmgs

Chemical signatures in the ULMGs

t = 1 Myr

1st SN

[Fe/H]

t = 20 Myr

4th SN

2nd SN

5th SN

3rd SN

[Fe/H]

Feltzing et al. (2009)


Conclusions

Conclusions

  • ULMGs:

    “Smallest DM haloes that canretaincold gas in the event of a single SN”

    ➙ This is possible in DM haloes < 107Msolassuming a clumpy medium

  • The ULMGsmayhavekinematics and luminositiessimilar to the ultra-faintdwarfs (cf. Segue 1)

  • However: Chemical signatures – variousabundanceratios, e.g., [Mg/Ca], and probably a less pronounced [Fe/H]-richtail – willreveal the truenature of thesegalaxies

  • With the advent of the next gen. telescopes and surveys, e.g., the ELTs, LSST, and Gaia, Weshould start searching for the ULMGs in the outskirts of the Milky Way!


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