arXiv:1111.3614v1 [astro-ph.CO] 15 Nov 2011
The Spectral Energy Distribution of Galaxies
Proceedings IAU Symposium No. 284, 2011
R.J. Tuffs & C.C.Popescu, eds.
c ? 2011 International Astronomical Union
Ionization of the diffuse gas in galaxies :
Hot low-mass evolved stars at work
N. Flores-Fajardo1†, C. Morisset2,3, G. Stasi´ nska4, L. Binette2
1Centro de Radioastronom´ ıa y Astrof´ ısica, Universidad Nacional Aut´ onoma de M´ exico;
Antigua Carretera a P´ atzcuaro no. 8701 Col. Ex Hacienda San Jos´ e de la Huerta, Morelia,
Michoac´ an C.P. 58089, M´ exico
2Instituto de Astronom´ ıa, Universidad Nacional Aut´ onoma de M´ exico; Apdo. postal 70–264;
Ciudad Universitaria; M´ exico D.F. 04510; M´ exico.
3Instituto de Astrof´ ısica de Canarias; C/ V´ ıa L´ actea, s/n, E38205, La Laguna (Tenerife),
4LUTH, Observatoire de Paris, CNRS, Universit´ e Paris Diderot; Place Jules Janssen 92190
Abstract. We revisit the question of the ionization of the diffuse medium in late type galaxies,
by studying NGC 891. The most important challenge for the models considered so far was
the observed increase of [O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα with increasing distance to the
galactic plane. We propose a scenario based on the expected population of massive OB stars
and hot low-mass evolved stars (HOLMES) in this galaxy to explain this observational fact.
In the framework of this scenario we construct a finely meshed grid of photoionization models.
For each value of the galactic latitude z we look for the models which simultaneously fit the
observed values of the [O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα ratios. For each value of z we find a
range of solutions which depends on the value of the oxygen abundance. The models which fit
the observations indicate a systematic decrease of the electron density with increasing z. They
become dominated by the HOLMES with increasing z only when restricting to solar oxygen
abundance models, which argues that the metallicity above the galactic plane should be close
to solar. They also indicate that N/O increases with increasing z.
Keywords. galaxies: individual (NGC 891) — galaxies: ISM — galaxies: abundances — stars:
AGB and post-AGB
The Diffuse Ionized Medium (DIG) was detected through its optical line emission out-
side the classical H ii regions (Reynolds (1971)) and turns out to be a major component
of the interstellar medium in galaxies (Reynolds (1991)). Most specialist agree that OB
stars in galaxies likely represent the main source of ionizing photons for the DIG (see
Haffner et al. (2009)). However, an additional ionizing source is suggested by reported
increase of such emission line ratios as [N ii]/Hα, [S ii]/Hα with galactic height. Several
sources of additional ionization/heating have been suggested without complete success.
2. Proposed Scenario
The extraplanar DIG is destributed in clouds (for simplicity represented by rectangles)
that are ionized by two star populations:1) OB stars located in the thin disk and whose
ionizing radiation escape from the disk through small “holes”, which ionize the “bottom”
part of the gas clouds. They constitute only a small fraction of the entire population of
† E-mails:NahieFlores@Gmail.com, Chris.Morisset@Gmail.com, firstname.lastname@example.org
2 Flores-Fajardo et al.
Figure 1. Schematic representation of the extraplanar gas and ionizing stars. Ionized gas is
represented in blue, neutral gas in red, with a lighter shade for less dense regions. For both
components, a darker shade indicates gas of higher densty. The big blue stars in the plot represent
OB stars. The small purple stars represent teh HOLMES, which can be either central satrs of
present-day planetary nebulae, or hot pre-white dwarfs.
Figure 2. Observed values of [O iii]/Hβ, [O ii]/Hβ and [N ii]/[O ii]in NGC 891, as a function
of the distance to the galactic plane. The data are from Otte et al. (2001).
the OB stars in the disk. 2) HOLMES, which are distributed in the galaxy thick disk and
halo. Their influence with respect to that of OB stars increases away from the galacti
plane. Figure 1 shows the datails of the proposed scenario.
3. Observational Data
We chose to focus on the edge on spiral galaxy NGC 891, a galaxy that has been
extensively observed, especially in optical emission lines (Figure 2), providing the best
diagnostics for our scenario.
The ionizing spectral energy distribution (SED) from OB stars is obtained using the
code Starburst99 (Leitherer et al. (1999)), considering a continuous star formation. The
SED of HOLMES is obtained using the code PEGASE (Fioc & Rocca-Volmerange(1997))
considering a instantaneous starburst at look back after 10 Gyr. The photoionization
models for the DIG were computed with Cloudy (Ferland et al. (1998)). Each one is
defined by the ratio of the surface fluxes: ΦHOLMES/ (ΦOB+ ΦHOLMES), the ionization
parameter U, O/H and N/O.
Figure 3 shows the location of the observaional points in the [O iii]/Hβ vs [O ii]/Hβ
diagram, with respect to our “reduced” grid of models. The dashes lines join models with
same values of log ΦOB(equal to 3.5, 4, 4.5, 5, 5.5, 6, 6.5), while the continuous line join
models with the same values of U (equal to -4, -3.5, -3). Form left to right, each panel
corresponds to a different value of log O/H (-3.9, -3.3, -2.9). The value of N/O is the
same in all the panels, namely -0.5 dex. The observational points that have the highest
values of [O iii]/Hβ and [O ii]/Hβ correspond to the largest values of ΦHOLMES/Φtotal.
Ionization of the diffuse gas in galaxies
Figure 3. “Reduced” grids (the full grid is 5 time more resolved) of models in the [O iii]/Hβ
vs. [O ii]/Hβ plane, for various values of O/H. See text for details on the varying parameters.
The observational points are superimposed on the grid.
Figure 4. ΦHOLMES/ΦOB, ne, U and N/O vs z for the models that fit the observational
[O iii]/Hβ, [O ii]/Hβ, and [N ii]/Hα simultaneously. The green bars indicate the range of the
values for wich the models in the grid fit the observations. The dark bars show the same, but
restricting to solar abundances models.
For each observational point in the DIG of NGC 891 we select the models of our
finely meshed grid that reproduce simultaneously the values of [O iii]/Hβ, [O ii]/Hβ
and [N ii]/Hα within the observational uncertainties. We find solutions for each value of
z. In Figure 4 we show the acceptable ranges of ΦHOLMES/Φtotal, U, neand N/O as a
function of z. The models clearly indicate a systematic decrease of the electron density
with increasing z. If we restrict to solar metallicity (dark bars in Fig. 4), we find that
the models which fit the observations become dominated by the HOLMES as z increases.
Turning the argument around, this might be an indication that the metallicity of the DIG
is roughly solar. Our models also indicate that N/O increases with increasing destance
to the galactic plane, at least until |z| ∼ 1.5 kpc.
Our scenario, which considers both the population of OB stars and that of HOLMES, is
able to explain the long standing problem of the ionization of the DIG (see Flores-Fajardo
et al. (2011)).
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