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