Properties of galaxy disks in hierarchical hydrodynamical simulations: Comparison with observational data
A. S\'aiz
R. Dom\'{\i}nguez-Tenreiro
We analyze the structural and dynamical properties of disk-like objects formed in fully consistent cosmological simulations with an inefficient star formation algorithm. Comparison with data of similar observable properties of spiral galaxies gives satisfactory agreement.
saiza.tex
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\markboth{S\'aiz et al.}{Properties of disks in simulations}
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\begin{document}
\title{Properties of galaxy disks in hierarchical hydrodynamical simulations:
  Comparison with observational data}
\author{A. S\'aiz, R. Dom\'{\i}nguez-Tenreiro}
\affil{Depto.\ de F\'{\i}sica Te\'orica, C-XI.\@ Universidad Aut\'onoma de
  Madrid, Madrid, E-28049, Spain}
\author{P. B. Tissera}
\affil{I.A.F.E., Casilla de Correos 67, Suc.\ 28, Buenos Aires, 1428,
  Argentina}
\author{S. Courteau}
\affil{University of British Columbia, Dept.\ of Physics and Astronomy,
  Vancouver, BC, Canada V6T 1Z1}
\begin{abstract}
  We analyze the structural and dynamical properties of disk-like objects
  formed in fully consistent cosmological simulations with an
  inefficient star
  formation algorithm. Comparison with data of similar observable properties
  of spiral galaxies gives satisfactory agreement.
\end{abstract}
We present results of a detailed comparison
between the parameters characterizing the structural and dynamical
properties of a sample of 29 simulated disk-like objects (DLOs)
and those measured
in observed spiral galaxies. The DLOs have been identified in AP3M-SPH
fully consistent hierarchical hydrodynamical simulations,
where an {\em inefficient\/} Schmidt law-like algorithm to model the
stellar formation processes has been implemented.

A surface density bulge-disk decomposition was performed
on the DLOs, using a double-exponential profile. The
%simulated 
resulting
bulge and disk scale lengths,
%A surface density bulge-disk decomposition has been performed on the DLOs,
%and a double exponential profile assumed.
%The bulge and disk scale lengths we have obtained, 
$R_{\rm b}$ and $R_{\rm d}$,
and their ratio $R_{\rm b}/R_{\rm d}$, are consistent with available data
(Courteau, de Jong, \& Broeils 1996; de Jong 1996;
Moriondo, Giovanelli, \& Haynes 1999). See Fig.~1(a).
\begin{figure}
  \plottwo{saiza1.eps}{saiza2.eps}
  \caption{The scale lengths $R_{\rm b}$ and $R_{\rm d}$ (a),
    and the $V_{2.2}$ velocities versus $R_{\rm d}$ (b).
    Filled symbols correspond to
    DLOs formed in our simulations. Dots in (a) are from 1D decompositions
    of surface brightness profiles given by Courteau, de Jong, \& Broeils
    (1996) and Courteau (1997). Dots in (b) are data from the Courteau--Faber
    (H$\alpha$ spectra) sample. Open circles are data from spirals with
    \hbox{H\,{\sc i}} rotation curves from Casertano \& van Gorkom (1991).}
%    and Rhee (1996).}
  \vspace{-0.3cm}
\end{figure}

Our rotation curve %(RC)
analysis and shape modeling uses a
parameterization based on the spatial scale
%The rotation curves $V_{\rm cir} (R)$ for the DLOs
%have been analyzed and their shapes
%parametrized through
$R_{2.2} = 2.2 \, R_{\rm d}$ %as
%spatial scale, and 
and the velocity (mass) estimate 
$V_{2.2} = V_{\rm cir} (R_{2.2})$. % as mass size.
These parameters were compared with those measured for the
Courteau--Faber sample of bright Sb--Sc field spirals with
long-slit H$\alpha$ rotation curves, %(RCs), 
%RCs,
and the Broeils
(1992) compilation of late-type spirals with extended 
\hbox{H\,{\sc i}} %HI
%RCs.
rotation curves.  
Deep surface photometry is available for both samples.
%These parameters have been compared with those measured at
%Sb--Sc field spirals drawn from the Courteau--Faber sample, with long-slit
%H$\alpha$ spectrum and $r$-band photometry, and
%Sb--Sc spirals from Broeils (1992) compilation of extended
%\hbox{H\,{\sc i}} 
%RCs with surface photometry.
In contrast
to findings
in other fully consistent
hydrodynamical simulations
(Navarro \& Steinmetz 2000 and references therein),
we find DLO $V_{2.2}$
values that are consistent with the observational data.
%DLO $V_{2.2}$ velocities have been found to be consistent with
%observational data. 
This is a consequence of disk formation
with conservation of the specific angular momentum $j$.
%with specific angular momentum, $j$, conservation. 
See Fig.~1(b).

In conclusion, the comparison between DLOs produced in our simulations and
observational data allows us to affirm that %they 
DLOs
have counterparts
in today spiral galaxies.
This agreement suggests that the process
operating in Fall \& Efstathiou (1980) standard
model for disk formation (i.e., gas cooling and collapse with
$j$ conservation) is also at work in the
{\em quiescent\/} phases of DLO formation in these simulations. 
However,
{\em violent\/} episodes (i.e., interactions and merger events) also occur
and play an important role in DLO assembly (Dom\'{\i}nguez-Tenreiro,
Tissera, \& S\'aiz 1998). %; S\'aiz, Tissera, \& Dom\'{\i}nguez-Tenreiro 1999).
To provide the right conditions for disk regeneration
after the last violent episode of DLO assembly, a compact central stellar
bulge is needed; this will ensure the axisymmetric character
of the gravitational potential well at scales of some kpcs at all
times, avoiding excessive $j$ losses in violent events.
A second condition is necessary: the availability of gas at low~$z$,
to form the disk.
The good match of our DLO parameters with obervational
data suggests that our {\em inefficient \/} star formation
algorithm meets both requirements.
%The good behaviour of our DLO sample as compared with
%observations suggests that the {\em inefficient\/} SF algorithm used
%in the simulations
%has met both requirements.
This global agreement with observations also represents
an important step towards making numerical approaches more widely used
for the study of
%to learn about 
galaxy formation and evolution
in a cosmological framework, i.e., from primordial fluctuations.
\begin{references}
  \reference Broeils, A. H. 1992, Ph.D. thesis, Univ.\ Groningen
  \reference Casertano, S., \& van Gorkom, J. H. 1991, \aj, 101, 1231
  \reference Courteau, S. 1997, \aj, 114, 2402
  \reference Courteau, S., de Jong, R. S., \& Broeils, A.H. 1996,
  \apj, 457, L73
  \reference de Jong, R. S. 1996, \aap, 313, 45
  \reference Dom\'{\i}nguez-Tenreiro, R., Tissera P. B., \& S\'aiz, A. 1998,
  \apj, 508, L123
  \reference Fall, S. M., \& Efstathiou, G. 1980, \mnras, 193, 189 
  \reference Moriondo, G., Giovanelli, R., \& Haynes, M. P. 1999,
  \aap, 346, 415
  \reference Navarro, J. F., \& Steinmetz, M. 2000, %astro-ph/0001003 preprint
  \apj, 538, 477
%  \reference Rhee, M. H. 1996, Ph.D. thesis, Univ.\ Groningen
%  \reference S\'aiz, A., Tissera, P. B., \& Dom\'{\i}nguez-Tenreiro, R. 1998,
%  \apss, 263, 43
\end{references}
\end{document}
