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Modelling the Red Halos of Blue Compact
Galaxies Zackrisson, E1., Bergvall, N1.,
Marquart, T1., Mattsson, L1. Östlin, G.2 1
Uppsala Astronomical Observatory, Sweden
2 Stockholm Observatory, Sweden
Introduction to the Red Halo Problem Optical/near-
IR broadband photometry of the faint halos
surrounding Blue Compact Galaxies (BCGs) have
revealed a very red spectral energy distribution
(Bergvall Östlin 2002, Bergvall et al. 2003),
which cannot easily be reconciled with a
metal-poor stellar population like that in the
halo of the Milky Way. A similar problem has also
noted for the halos detected around edge-on disk
galaxies in stacked optical data from the Sloan
Digital Sky Survey (Zibetti et al. 2004),
suggesting that the red halo phenomenon may be
common to several types of galaxies. What is
the nature of these strange halos? Here, models
of spectral evolution are used to test three
different possible explanations for the red
excess High-metallicity stars, nebular emission
and a stellar population with a very bottom-heavy
initial mass function (IMF).
A High Metallicity? Well In Bergvall Östlin
(2002), high-metallicity stellar populations were
suggested as an explanation for the red excess of
BCG halos. In Fig. 1, we show that although this
solution does reasonably well when confronted
with the extended BCG halo data set of Bergvall
et al. (2003), the metallicity of many of the
halos has to be very high (solar or higher),
which is a strange result given the low
metallicity (10 solar) of the gas in the
central starburst of these objects. Also, this
explanation fails to explain the halos of edge-on
spirals (see dashed lines in Fig.
3b).
Fig 1. Colours of BCG halos (crosses) compared
to the stellar population evolution (solid lines)
predicted by PEGASE.2 (Fioc Rocca-Volmerange
1999) for a Salpeter IMF at different
metallicities. A star formation history typical
of an early-type system (SFR ? exp(-t/?) with ?1
Gyr) has been assumed. Only the highest
metallicities (red/black lines) provide a
reasonable fit.
In principle, the colours of BCG halos could
be affected by nebular emission originating in an
extended envelope of gas ionized by hot stars in
the central starburst. Since photoionization
models predict the spectrum of a complete
Strömgren sphere to be very blue, this
explanation for the red excess was dismissed by
Bergvall Östlin (2002). A problem with this
argument is however that current halo
observations do not probe the complete Strömgren
sphere, but only a small part of the ionized
region. For this reason, we have here used the
photoionization model Cloudy (Ferland 1996) to
predict colours for non-central lines-of-sight
through spherical nebulae (Fig 2). Despite a
substantial scatter, we find that even in this
case, nebular emission produces colours much too
blue to explain the red excess of BCG halos.
Nebular Emission? No!
A Bottom-Heavy Initial Mass Function? Yes?! In
Fig. 3a, we show that stellar populations with a
very bottom-heavy IMF (dN/dM?M-? with ?4.50,
M0.08120 Msolar) can explain the BCG halo
colours with low to intermediate stellar
metallicities. Interestingly, a stellar
population with the same IMF, age and metallicity
also succeeds in explaining the halos of edge-on
disks from the Sloan Digital Sky Survey (Fig 3b),
suggesting that such strange halos may be common
to galaxies of different types.
Fig 2. Colours of BCG halos (crosses) compared to
the colours predicted for various lines-of-sight
through photoionized starburst envelopes (blue
dots).
References Bergvall, N Östlin, G. 2004, AA
347, 556 Bergvall, N., Marquart, T. Persson, C.,
Zackrisson, E.. Östlin, G. 2003, In
Multiwavelength Mapping of Galaxy Formation and
Evolution, in press Ferland, G. J. 1996, HAZY, a
brief introduction to Cloudy, University of
Kentucky, Department of Physics and Astronomy
Internal Report Fioc, M. Rocca-Volmerange, B.
1999, astro-ph/9912179 Zibetti, S., White, S. D.
M. Brinkmann, J. 2004, MNRAS 347, 556
Fig 3b. Colours of the typical halo (cross)
detected around edge-on disks in the SDSS,
compared to the PEGASE.2 predictions for stellar
populations with a bottom-heavy IMF (solid), as
well as a Salpeter IMF (dashed).
Fig 3a. Colours of BCG halos (crosses) compared
to PEGASE.2 predictions (solid lines) for the
evolution of stellar populations with a
bottom-heavy IMF.