The
Ratio
of
Molecular
tu
Atomic Gas
in
Spiral Galaxies
as a function of Morphological Type
Patricia M. Knezek and Judith
S.
Young
University
ctf
Mass.,
Amherst, and Five College Radio Astronomy Observatory
I.
Introduction
-
1
.-
In
order to gain an understanding of the global processes which influence cloud and star
formation
in
disk galaxies,
it
is
necessary to determine the relative amounts of atomic, molecular,
and ionized gas both
as
a
function
of
position
in
galaxies and Prom galaxy to galaxy. While atomic
gas studies of galaxies have been underway for
30
years, the relative youth of the field of
extragalactic molecular studies has meant that knowledge of the relative amounts of molecular and
atomic gas
in
galaxies
is
limited by the relatively small number of galaxies observed
in
CO.
Ever since the pioneering work
in
the
1950's
and
1960's,
it
has been recognized that there is a
morphological type dependence to the atomic gas content of galaxies.
In
particular, Roberts
(1969)
showed
that
the
HI
mass
to
blue luminosity ratio, M(HI)/ increases by a factor
of
5
among spiral
he
CO
distributions
in
over
200
galaxies
now completed as part of the
FCRAO
Extragalactic
CO
S
),
we are finally
in
a position to determine the type dependence of the molecular content
of
spir
the ratio of molecular to atomio;_ga<as
a
function of type.
Do
late type spi
galaxies from types Sa through Scd.
With
observations
>*I
r.i
n early types when the molecular gas content is Included?
1
',
11.
Sample
'@rid
Rnalysis
\
The galaxqs
i?l
observed as part of the
FCRAO
Extragalactic
CO
Survey are either
(1)
brighter
than BT*
=
12.5
observations at
W'
(cf. Kenney and You
were taken from
M
111.
Results
an
the blue, or
(2)
brighter than
20
Jy at
100
Mm. From major axis
CO
.
on and spacing
in
over
200
galaxies, we have derived global
CO
fluxes
8);
Hz masses were derived using the conversion factor
1.
(19831,
and mprphological types are from RC2.
N(Hz)/I
~0=2.8%f
(K
km
s-l)
(Bloemen et al.
1986).
HI masses for the sample galaxies
Within
our galaxy sample, whconfirm the previously found
trend
of M(HI)/LB
to
increase
with increasing type from Sab-Sdm. We find the mean ratio of
M(Hz)/Ls
to be roughly constant
1
53
-
for types Sa-Sc, with a decrease
of
a factor
of
>3
for types Scd-Sdm. The combination
of
these
two effects
is
that
the mea? va/ue uf the rafio of mu/ecu/af
tu
afumic gas decredses
smmfh&
by
d
facior
of
-20
ds
a
function
of
mopho/ugica/
type
for
types
Sa-Sd
as shown
in
Figure
1.
We have verified that the observed H2/HI ratio variation with type is not due to the inclusion
of
Virgo cluster spiral galaxies, many of which are HI-deficient early type galaxies with high H2/HI
ratios (van Gorkom and Kotanyi
1985;
Kenney and Young
1986, 1988;
Stark et al.
1986).
We have
also verified that the observed decrease
of
H2/HI is not the result
of
a Malmquist bias, since the
observed trend
is
also found
in
the subset
of
galaxies more nearby than the Virgo cluster. We
argue that metallicity variations with type among the galaxies
in
our sample will be small, since the
mean mass for types Sa-Scd changes by only a factor of
4
and the [O/H] ratio shows almost no
variation with mass for spirals between
lolo-
and
1OI2
Mo (Pagel and Edmunds
1981).
molecular cloud properties, and thereby
in
the CO-rHz conversion as a function of morphological
type, we have determined the mean dust temperature for the galaxies
in
our sample as a function
of
type.
We
find Tdust to be
1.2
times higher
in
the Sa-Sab galaxies
in
the sample than
in
the Sb-Sdm
galaxies.
If
we assume that the gas and dust temperatures are equal, then we will have
overestimated M(H2)
in
the early type spirals by a factor
of
1.2,
since M(H;I)
=
LC-JI;;/Tgas
(Dickman, Snell, and Schloerb
1986;
Maloney and Black
1988;
Elmegreen
1989).
Correcting the
observed M(Hz)/M(HI) ratio
in
each galaxy by the observed dust temperature leaves
a
factor
of
17
variation
in
the ratio M(H2)/M(HI) with morphological type.
In
order to ascertain whether the observed H2/HI variation is due to systematic changes
in
IV.
Conclusions
We conclude that there
is
than an order of magnitude decrease
in
the ratio of molecular
to atomic gas mass as a func
more molecular than atomic g erage Sc has less. Therefore,
the
tufa/infersf@//dfgds
fype
from
Sd-Sd.
The dominant effect we find
is
that
the
pndse
of
the
gas
in
the cool
ISM
varies
along
the
Hubble sequence.
molecular-dominated
ISM
through the collection
of
more gas
in
the
potentlal. That gas may then
form
molecular clouds
when
a
critical density
is
exceeded. The picture which these observations
support
is
one
in
which
the
conversion
of
atomic gas to molecular gas
is
a global process which
depends
on
large scale dynamics (cf. Wyse
1986).
Rmong interacting and merging systems,
we
find
considerable scatter
in
the
M(Hz)/M(HI)
ratio,
with
the mean ratio similar
to
that
in
the early tup<- These high global ratio
of
molecular to atomic gas could result from the removal of HI gas, the enhanced conversion
of
HI into
Ha,
or
both.
hological type from Sa-Sd; an average Sa galaxy has
s*"
mdss
fo
b/ue
/um/husitfy
fdfiu, /hCfedSSS
by
/eSS tbdff
d
fdCfUf
Of
fW0
dS
d
fU?7Cfk7ff
Uf
Me suggest
that
the more massive and centrally concentrated galaxies are able
to
achieve a
I
,
I
54
References
Bloemen, J.B.G., et al.
1986,
Rstr.RD.,
159,
25.
Dickman,
R.L.,
Snell, R.L., and Schloerb,
F.P.
1986,
Flu.,
309,
326.
Elmegreen, B.G.
1989,
R7.,
338,
178.
Huchtmeier, W.K., Fiichter,
0.-G.,
Bohnenstengel, H-D., and Hauschildt, M.
1983,
€SO
preprint.
Kenney, J.D.P., and Young, J.S.
1988,
RD.J.SUDDI.,
66,
261.
Maloney,
P.,
and Black, J.H.
1988,
AU.,
325,
389.
Pagel, B.E.J., and Edmunds,
M.G.
1981,
Ann.Rev.Rstr.RD.,
19,
77.
Roberts,
M.S.
1969,
Ad.,
7Y,
859.
van Gorkom,
J.,
and Kotanyi, K.
1985,
in
ESO
Workshoo on the
Vim0
Cluster
of
Galaxies, p.
61.
Wyse, R.
1986,
AD.J.fLetters),
311,
LYl.
Young, J.S., Claussen,
M.,
Devereux,
N.,
Huang, Y., Kenney,
J.,
Knezek,
P.,
Tacconi,
L.,
Tacconi-
3.
Garman,
L.,
Schloerb,
P.,
Viscuso,
P.,
and Xie,
S.
1989,
in
preparation.
Sb
-
Sbc
SC
-
Scd
I
Sd
Interacting
Pairs
Merger
Remnants
Dwarfs
and
Io's
3
P
fm,
0
5
10
Galaxy
Type
bh
Fiaure
1.
Ratio
of
molecular to atomic gas mass as a function
of
morphological type among
200
galaxies. We have combined RC2 types
0
and
1,
types
7
and
8,
and types
9
and
10.
also
included
in
category
9-10
are dwarf galaxies and
I0
galaxies (i.e.
M82).
Type
12
represents interacting
galaxy pairs and type
13
represents merger remnants. The error bars shown represent the error
in
the mean M(H2)/M(HI)
ratio
for
each type.
55