Observation of a large sample of HII galaxies shows that the emission line ratios of the youngest objects change systematically with gaseous oxygen abundance, which was interpreted as resulting from changes in the initial mass function (IMF) of the ionizing cluster. Comparison with cluster/nebula models shows that both the slope and the upper mass limit of the cluster IMF vary with abundance. In HII galaxies with oxygen abundance about 1/10 that of Orion, the IMF for massive stars must have a slope which is about a factor of 2 smaller than in the Solar Neighborhood

Campbell, Alison W.

English

NASA Technical Reports Server

N87- 24332
THE INITIAL MASS FUNCTION IN HII GALAXIES
Alison W. Campbell
Center for Astrophysics
60 Garden Street
Cambridge, Mass
Abstract
Observation of a large sample of HII galaxies shows that the emission line
ratios of the youngest objects change systematically with gaseous oxygen abun-
dance, which we interpret as resulting from changes in the initial mass function
(IMF) of the ionising cluster. Comparison with cluster/nebula models shows that
both the slope and the upper mass limit of the cluster IMP vary with abundance.
In HII galaxies with oxygen abundance about 1/10 that of Orion, the IMP for mas-
sive stars must have a slope which is about a factor of 2 smaller than in the So-
lar Neighbourhood.
Introduction
"HII galaxies", dwarf galaxies with the spectra of giant HII regions, are
ionised by massive 0 stars formed during a recent intense burst of star forma-
tion. Although the very strong, narrow emission lines which characterise these
objects have been used in several studies to determine physical conditions in
the ionised region (eg Searle and Sargent 1972, French 1980, Kinman and David-
son 1981, Kunth and Sargent 1983, Campbell, Terlevich and Melnick 1986a; CTMa),
little is knownabout their ionising clusters.
The nebular excitation of low-abundance HII galaxies is higher than can be
explained by a solar neighbourhood IMP (Lequeuxet al 1981, Bergvall 1985). Gi-
ant HII regions in spiral galaxies show trends in emission line ratios which in-
dicate that the temperature of the ionising cluster, Tio, , increases smoothly as
the abundance decreases (eg Shields and Tinsley 1976, Stasinska 1980). The ob-
served change in Tion can arise if the IMF of the ionising cluster in giant HII
regions and HII galaxies is abundance-sensitive: Shields and Tinsley (1976)
proposed a systematic change in the upper mass limit of the IMP, while Terlevich
(1982) suggested that the slope of the IMF decreases with decreasing abundance.
Studies of the ionising cluster in HII galaxies have so far been based almost
entirely on UV/optical/infrared colours, mass-to-light ratios, and recombina-
tion line equivalent widths; the easily-observed emission line ratios, which
carry important information about the mass distribution and age of the ionis-
ing cluster, have been almost totally neglected. We report here the results of a
comparison between the line ratios of a sample of _ 50 HII galaxies and those of
model nebulae ionised by clusters with a variable IMF.
Modelling strategy
The ionising clusters of HII galaxies evolve very rapidly, on timescales
of order the lifetime of a massive star, a few Myr. As the cluster evolves, Tio.
falls and the "hardness" of the ionising spectrum decreases, leading to a rapid
change in the relative strength of lines from different ionisation stages of a
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 479
FEECEDING PAGE BLANK NOT FILMED
https://ntrs.nasa.gov/search.jsp?R=19870014899 2020-03-20T11:08:06+00:00Z
A. W. CAMPBELL
given element. A least-squares fit to the distribution of HII galaxies in a Tion
-sensitive emission line ratio will therefore always underestimate the T/on of
the cluster IMF (as distinct from the cluster's current Tion ) and consequently
cause us to either overestimate the IMF slope or underestimate its upper mass
limit. If we wish to study the abundance behaviour of the INF, it is therefore
crucial to remove the effect of cluster evolution.
In principle, there exists a set of HII galaxies, covering a range in abun-
dance, in which the ionising cluster is at close to zero age: a "zero age se-
quence" (ZAS) which will appear as an upper envelope in TioN -sensitive emission
line diagrams. Identification and modelling of this ZAS using zero-age main se-
quence cluster models will allow us to determine the abundance behaviour of the
HII galaxy IMF for stars more massive than _ 20 M® .
Model HII galaxies
We have used the cluster code of Melnick, Terlevich and Eggleton (1985) to
produce zero-age main sequence model clusters of mass 106 M® . The cluster mass
distribution is specified by a power-law IMF with slope G between O. 1 M® and a
maximum of 200 M® . We next pass the ionising part of the cluster's spectrum to
a model nebula code (Ferland and Truran 1981) and construct low-density, clumpy,
radiation-bounded HII regions with oxygen abundances spanning the range covered
by the data. The models are discussed in greater detail in Campbell, Terlevich
and Melnick (1986b; CTMb).
Comparison of models and data; the ([OIII]/[OII] , O/H) diagram
The Tion -sensitive line ratio [OIII]/[OII] is insensitive to O/H and thus
allows us to examine directly the behaviour of Tion with abundance. The ZAS. from
which objects evolve downwards, is easily visible in figure 1. The curves in
figure la are the loci of model HII galaxies ionised by clusters with a constant
IMF specified by the M_ and (x shown. All these models are poor fits to the data.
Those with a = 3.0 produce values of [OIII]/[OII] which fall nearly an order
of magnitude below the ZAS for O/H _ 4 x 10 -5 , while those with G = 1.0 grossly
overestimate the position of the ZAS for 6 x 10 -s . It is immediately evident that
for any Mu . a "flat" IMF (G _ 1.0) is required at low abundance. The steep turn-
down of the [OIII]/[OII] envelope with increasing abundance requires a system-
atic decrease in Mu or increase in G .
The strongest dependence of a and Mu on abundance which can be accommodated
without conflicting with their observed values at the Solar Neighbourhood oxygen
abundance are
M_ = 5.9 (0/H) -°'3 (1)
,_ = 1.2 log C0/H) + 6.9 C2)
The loci of models in which Mu and a vary according to equations (1) and (2) are
shown in figures lb and lc respectively. The fit of the variable-Mu models is
little better than that of constant IMF models, owing to the small change in the
cluster mass distribution caused by changes in Mu • Variable G models do much
better, but still overestimate the position of the envelope for O/H > 1.5 x 10 -4 .
It seems therefore that a systematic change with abundance of both G and Mu
is required to fit the [OIII]/[OII] envelope. We have constructed two such mod-
els, one in which a increases with abundance in the variable-Mu model (equation
(1)), and another in which Mu decreases with abundance in the variable-_ model
(equation (2)). Figure ld demonstrates that these closely similar models both
produce a very good fit to the ZAS.
480
ORIGINAL PAGE IS
OF POOR QUALITY
INITIAL MASS FUNCTION IN HII GALAXIES
l:
C_2s
(a)
Lo
•
_ _ o •
_' _- • I Zw 18 • e '
...... "x
1
-5 -45 -4 -35
LOG 0/H
(c)
"
B • o
f , %; o
o
° 0
o
-5 -45 -4 -35
cO
u_
o
_o
5
o
•o
x
o mA
o
° o
I I
-5 -45 -4 -35
LOG O/H
r
9Lm
• \\
• • \\
III ° _ • • o\
• • Oo
• oil _o
=am, • o
• BolD@
o
o
•o
(d)
- 5 -4 -3.5
LOG O/H LOG 0/II
Figure 1
The distribution of the sample in the ([OIII]l[OII], O/H) plane. Circles: data
from CTMa; squares: data from Kunth and Sargent (1983). IZwl8 is from Kinman and
Davidson (1981). The short dash-dot line represents the adopted zero-age enve-
lope for O/H > 1.5 x 10-4 , as explained in CTMb. The model loci are labelled as
follows: C398: constant IMF, a = 3.0, M= = g8 M® ; C3200: constant IMF, _ = 3.0,
M= = 200 M® ; C198: constant IMF, _ = 1.0, Ms = g8 M® ; C1200: constant IMF, a =
1.0, M= = 200 Me ; M3: IMF with (x= 3.0 and variable M= (eq.1); MI: IMF with G =
1.0 and variable M= ; _ 98: IMF with M= = 98 M® and variable G (eq.2) ; _ 200: IMF
withM= = 200M® and variable _x ; M G : the variable M= model (eq.1) with 1.0 <
< 2.7; aM: the variable amodel (eq.2) with 200 > M= > 73M® .
Discussion
An excellent fit to our adopted zero-age sequence in the ([OIII]/[OII].
O/H) diagram has been obtained by allowing the mass distribution of the ionis-
ing clusters of HII galaxies to vary systematically with abundance. Our models
indicate a very pronounced change in the IMF in the oxygen abundance range 2.5 x
481
A.W.CAMPBELL
10 -B to 2.5 x 10-4; G is required to vary from _ 1.5 to 2.7 and Mu from _ 200 to
75 M® . The dependence of the IMF slope on abundance is closely similar to that
derived (using different methods) for giant HII regions and HII galaxies by Ter-
1erich (1982).
Conclusions
Assuming trends in emission line ratios are due to changes in the stellar
mass distribution of the ionising clusters of HII galaxies, the main results of
the emission line modelling can be summarised as follows:
1) No constant IMF can reproduce the values of the Tio, -sensitive line ratio
[OIII]/[OII] observed in the youngest ("zero age") objects over the oxygen abun-
dance range 2.5 x 10 -5 < O/H < 2.5 x 10 -4 .
2) A systematic change with abundance of Mu alone also fails to reproduce the
zero age range in [OIII]/[OII], owing to the small effect which even large
changes in Mu have on the mass distribution of the ionising cluster.
3) Models in which only _ varies cover much more of the zero-age range in
[OIII]/[OII] and if M, _ 200 M e , are correct for oxygen abundances _ 1 x 10 -4 .
Above this they slightly overestimate the position of the [OIII]/[OII] enve-
lope.
4) Models in which both G and Mu vary with abundance reproduce the data envelope
very well over the full range in oxygen abundance of 2.5 x 10 -B to 2.5 x 10 -4 . The
detailed abundance behaviour of G and Mu is given in CTMb.
5) For the lowest-abundance HII galaxies, eg IZw18 (O/H _ 1.5 x IO-B), the pre-
dicted IMF slope is _ 1.2, irrespective of Mu . IZw18 is an evolved object in
which Tio, is currently considerably lower than its zero-age value.
Acknowledgements
I thank Jorge Melnick, Roberto Terlevich and Gary Ferland for access to their
model software, and Jack Baldwin and Belinda Wilkes for their helpful comments.
References
French H.B., 1980, Ap.J.
Kinman T. & Davidson K.,
Kunth D. & Sargent W.L.W
LequeuxJ., Joubert M.M.
Bergvall N., Ig85, A.&A. 146,269
Campbell A.W., TerlevichR.J. &Melnick J., 1986a, accepted byM.N.R.A.S. (CTMa)
Campbell A.W., Terlevich R.J. &Melnick J., 1986b, submitted to M.N.R.A.S. (CTMb)
Ferland G. &Truran J.W., 1981, Ap.J. 244,1022
, 240,41
1981, Ap.J. 243,127
., 1983, Ap.J. 273,81
, Deharveng J.M., Kunth D., 1981, A.a A. 103.305
Melnick J., Terlevich R.J. &Eggleton P.P., 1985, M.N.R.A.S. 216,255
Searle L. &Sargent W.L.W., 1972, Ap.J. 173,25
Shields G.A. & Tinsley B., 1976, Ap.J. 203,66
Stasinska G., 1980, A.a A. 84.320
Terlevich R.J.. 1982, Ph.D. Thesis, University of Cambridge
482


The initial mass function in HII galaxies

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server