The Red Halo Puzzle – and how to solve it Erik

The Red Halo Puzzle
– and how to solve it –
Erik Zackrisson
Uppsala Astronomical Observatory
The Swedish Red Halo Team
Nils Bergvall (Uppsala)
Anna Blomqvist (Uppsala)
Brady Caldwell (Uppsala)
Maria Leksell (Uppsala)
Daniel Malmberg (Uppsala)
Thomas Marquart (Uppsala)
Genoveva Micheva (Stockholm)
Göran Östlin (Stockholm)
Erik Zackrisson (Uppsala)
Outline
Introduction
to the red halo phenomenon
A possible explanation for the red excess
Important issues for BCGs
Future prospects
Blue Compact Galaxies (BCGs)
ESO 338-04
(HST)
He 2-10
(NTT)
Red halos around Blue Compact Galaxies
Bergvall & Östlin 2002,
A&A 390, 891
Normal stellar populations
Zackrisson et al. 2004, astro-ph/0411537
Zackrisson et al. 2005, ApJ, submitted
Bottom-heavy initial mass functions
dN
∝ M − 4.5
dM
Zackrisson et al. 2004, astro-ph/0411537
Zackrisson et al. 2005, ApJ, submitted
Halos of edge-on disk galaxies in the SDSS I
Zibetti et al. 2004,
MNRAS 347, 556
Halos of edge-on disk galaxies in the SDSS II
Bottom-heavy
IMFs
Normal IMFs
Zackrisson et al. 2004, astro-ph/0411537
Zackrisson et al. 2005, ApJ, submitted
Implications
Stellar halos dominated by low-mass stars
common to galaxies of very different
Hubble types?
Mass-to-light ratio: M/L ≥ 30
→ Effectively baryonic dark matter
Mundane explanations
Nebular emission?
Photoionization models → No!
Nebular emission would make the halo bluer!
Dust reddening?
Balmer decrement → No!
Dust emission in the K-band?
ISO observations → No!
Some instrumental effect?
Very unlikely. Same red excess seen with
different telescopes and instruments. New
tests in progress…
Nebular emission
Nebular
components
Important issues I: Colour profiles
Continuous slope
Colour
Colour
Plateau
Radius
Radius
No plateau →
Radial gradient in halo population properties
or
Contamination from the central starburst non-negligible to the
outermost radius probed
Important issues II:
Surface brightness profiles
Are these hosts / starburst envelopes / LSB
components really halos?
Milky Way stellar halo: ρ ∝ R -2.7 to R -3.5
M31 stellar halo ρ ∝ R -3
Model predictions for stellar halo: ρ ∝ R -3 to R -4
(Abadi et al. astro-ph/0506659)
SDSS average halo ρ ∝ R -3
CDM halos ρ ∝ R -1 (centre) to R -3 (outskirts)
Also important:
1. Halo shape
2. Correct for extended nebular emission
Important issues III:
Scaling relations
What do Lhalo, µhalo,0 and hhalo scale with?
Ltot?
µtot,0?
Mtot?
Which objects have the brightest halos?
Important issues IV:
What are BCGs?
Suggestions:
BCG = Central starburst + Red halo?
BCG = Central starburst + Normal host + Red halo?
BCG = Central starburst + Normal host
(Red excess explained by mundane effects)
Tests of Bottom-Heavy IMFs I:
Counting giants
A
LA & assumed IMF →
Expected number
of giant stars
Just count and check!
Strategy: Map red halos
of nearby BCGs
already imaged by HST
NGC 5907 – The first red halo detection
(now considered a dubious case)
Tests of Bottom-Heavy IMFs II:
Deep I-band photometry
Salpeter VS bottom−heavy IMFs in the I−band
1.2
1
Age: 1 Gyr
Age: 5 Gyr
Age: 14 Gyr
c)
0.8
B−V
0.6
Solid lines: Salpeter IMF
populations with Z=0.001, 0.004
0.008, 0.020 & 0.040
(overlapping)
0.4
0.2
0
Dashed lines: Bottom−heavy IMF
populations with Z=0.0001, 0.001,
0.004 & 0.008 (left to right)
−0.2
−0.4
−0.5
V-I
0
0.5
1
V−I
1.5
2
Ongoing work
Observational searches for red halos around:
A
larger sample of BCGs (PI Östlin)
Elliptical galaxies (PI Bergvall)
Post-starburst galaxies (PI Zackrisson)
Development of more advanced image
processing techniques (Micheva)
Improved models for stellar populations with
bottom-heavy IMFs (Zackrisson)
Post-starbursts as Probes of Red
Halos
Starburst
Post-starburst
Telescope time granted on both the NTT (Oct 05)
and the NOT (Jan/Feb 06)
Undergraduate projects
Searches for post-starburst galaxies in the
SDSS (Daniel Malmberg, Anna Blomqvist)
Searches for halos around low surface
brightness galaxies in the SDSS (Brady
Caldwell)
Other potential projects
Stacking K-band images of edge-on disks
in 2MASS (Östlin)
Stacking BCGs in SDSS (Marquart)
Stacking all our deep BCG data average
BCG halo (Östlin)
Summary
Red halos detected around BCGs and
edge-on disks
Possible explanation: A stellar population
dominated by low-mass stars
Several observational projects underway
to search for red halos around other types
of galaxies