Download the evolutionary radial distributions of h2 density within spiral and

THE EVOLUTIONARY RADIAL DISTRIBUTIONS OF H2 DENSITY
WITHIN SPIRAL AND IRREGULAR GALAXIES AT HIGH REDSHIFT:
ESTIMATING CO DETECTION LIMITS
Mercedes Mollá (CIEMAT), Eduardo Hardy (NRAO), Ángeles I. Díaz (UAM)
Introduction.
Our models:
Chemical evolution models
The molecular interstellar medium plays a critical role
in the evolution of galaxies since it provides the
material from which stars form.
We have computed a wide grid of chemical evolution
models for a large set of theoretical spiral and irregular
galaxies of different total mass (Mollá & Díaz 2005).
There are observations of rotational transitions of
CO within 1.<z<6.5, mostly in QSO´s and EMG´s
(Solomon and Vanden Bout 2005), which demonstrate
that molecular clouds exist already at the epoch of
the galaxy formation.
The results provide the time evolution for specific radial
regions within each galaxy modeled, with the
corresponding mass in each gaseous or stellar phase, as
well as their elemental abundances.
However, the current sample is not very large for
z>3 where the study of the star and galaxy
formation is specially interesting. ALMA is the
instrument that may solve this problem.
The star formation model has 2
components:
components:
Star formation
in the halo:
SFR α Κ g 1.5
The gas falls on the equatorial plane forming out the disc
The gas infall rate depends on the total mass of the galaxy
τcol α Μ −1/2
In our models, the gas phase has two components:
diffuse (HI) and molecular (H2). This is why it is
possible to follow the evolution of the expected density
of H2 with redshift.
The total mass of the galaxy M
(R)
is calculated from the
universal rotation curve V(R) of
Steel et al. (1995)
We show here the predicted radial distributions of H2
gas density at high redshift, which might be detectable
with ALMA.
The collapse time scale as a
function of galaxy mass
Stars form
through cloud-cloud
collisions:
s α Ηc 2
Stars also form
from the interaction
between massive stars
and molecular clouds:
sαacs2
In the disc
molecular clouds
are formed from
the diffuse gas
c α µg 1.5
By using the multihase chemical evolution model from
Ferrini et al. (1992) we have simulated 440
theoretical
models,
with
44
different
radial
distributions of total mass and 10 star-forming
efficiencies for each radial distribution (see Mollá &
Díaz (2005) for details)
We have obtained the radial distributions of molecular
gas mass, which we computed separately from the
diffuse gas evolution.
Fig. 11- Radial
distributions of
molecular gas surface
density at z = 0.
0.
Labeled curves are for
different star
formation efficiencies
in the range 1 (highest)
to 10 (lowest).
The collapse time-scale
calibrated with the MWG
is
The colapse time scale is variable along
the galactocentric radius R
Fig.1 shows the radial distributions of present-time molecular gas density for different
SFRs. The lines in color correspond to the lowest star-forming efficiencies which maintain
a higher density than in cases where stars are formed more efficiently (black lines). The
distributions are close to exponential in the external regions but are flatter at the inner
regions, as is in fact observed in Nature.
HIGH REDSHIFT
Fig. 33-The expected radial distributions of molecular gas density
normalized to Rc (half optical radius) for three different redshifts
as labelled.
labelled.
For z=0 most galaxies show distributions with values higher than
the limiting value which may be detected with ALMA down to the
optical radius, while for high redshift only the inner disks have
have a
flux density sufficiently high to be detected.
z=0
z=2.9
Log(1+z)
Fig. 2.2.- The evolution of the total mass of molecular gas for
some of our models. Each panel represents the results for a
given galaxy total mass (or Vrot), using, for each one, 10
different SFRs.
SFRs. Lines mean the same as in Fig.1
z=4.25
The black lines correspond to the highest efficiencies.
The evolution of the total mass of molecular gas in
galaxies is represented in Fig.2 above.
The red points are the estimates from Solomon and
Vanden Bout (2005) for the Early Universe Molecular
Emission Line Galaxies (EMG´s) which might be ellipticals.
Our most massive spiral galaxies are in the bottom region
of the EMG´s data.
The molecular gas mass decreases for decresing z as in
Solomon & Vanden Bout (2005). Only the least massive
galaxies and those which are massive but with low starforming efficiencies are still producing molecular gas.
However, the most massive galaxies most efficient in
forming stars, consume very quickly the molecular gas.
The magenta crosses are those of Bertram et al. (2005)
for nearby QSO´s. The present time masses are similar
to those estimated for local objects.
The maximum value obtained for z=0 is in agreement with
the observational limit 1og(MH2)=10.3, shown by a green
line in the figure.
Observation of emission from CO rotational transitions
is the usual means of detecting interstellar molecular
clouds. CO is a very stable molecule and the most
abundant after H2. Following Combes (1991) we
consider levels J=4-5 to detect molecular gas at high
redshifts.
In order to verify if our densities of molecular gas
may be detected with ALMA, we need to transform
the limiting flux for this instrument to a limit in gas
density.
The flux limit depends on the observed frecuency. We
take the highest value from the report “Science with
ALMA”, and we use Eq. 1 from Solomon and Vanden
Bout (2005), to calculate the minimum LCO necessary
for detection. This luminosity is transformed into gas
mass by using a constant conversion factor a=0.8
(which assumes indpendence from z !).
The result is shown as a thick line in Fig.3
Fig.4Fig.4- Expected evolution of the flux density of CO lines as a
function of z , for an integrated galaxy, computed from the
molecular gas density σ(H2) using the method outlined,
outlined, for
spiral galaxies with four different rotation velocities. Line colors
colors
mean the same as in Fig.1
The most massive galaxies –panel d of Fig. 4– and those
with high
star formation efficiencies –black lines-maintain an almost constant H2 mass and flux density up to
z ~ 10.
Those with low-to-medium efficiencies or rotation
velocities Vrot < 50 km.s-1 ,on the other hand, show
increasing densities for decreasing z.
Conclusions
•At redshift z > 3 only the central regions of
galaxies might be observed in CO with ALMA
•EMG´s are not necessarily the most massive
galaxies.
References
Bertram, T., E, Eckart, A., Krips, M., et al. 2005,
Combes. F. 2001, Proc.of SF2A, EDPS Conf. Series in Astron. & Astroph.
Mollá, M. & Díaz, A.I. 2005, MNRAS
Solomon, P. M. & Vanden Bout, P. A. 2005, A.R.A.A.