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Accueil > Anciennes pages d’équipe > Dynamique des galaxies > DAGAL > DAGAL@LAM > Gas velocity fields in barred galaxies > Gas velocity fields in barred galaxies

 Gas velocity fields in barred galaxies

In this section we will present a few results from the literature on how fluid-dynamical models of the gas flows can help constrain the dynamical properties of barred galaxies.


The results for this object are based on Lin et al. (2013). NGC1097 (see Figure 1) is a nearby barred galaxy with an active nucleus, classified as a Seyfert 1.

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Figure 5
Composite image of NGC1097 observed with the VIMOS instrument on UT3 at the ESO’s VLT facility.

Hydrodynamical simulation including self-gravity of gas have been performed to explain the presence of a circumnuclear disk, nuclear ring, dust lanes, spiral arms and characteristic velocity features in the context of a coherent dynamical model. The simulations managed to reproduce the most relevant features and thereby constrained the galaxy properties, in particular : bar strength, angular speed and position of potential minimum. Figure 2 and 3 show the comparison between the simulated and observed results for the density distribution and velocity fields, respectively.

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Figure 2
Left panel : projected surface density distribution of the simulation result in logarithmic scale in units of solar masses over squared pc. Middle panel : replica of the galaxy image. Right panel : superposition of the left two figures.
The bright nuclear starburst ring, the northwest off-centered dust lane, two lateral spiral arms along the bar region, as well as the southwest main spiral arm are reproduced by the simulation in both their shapes and location.
© Lin et al. (2013)
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Figure 3
Comparisons of simulated and observed velocity fields. Left panel : isovelocity curves from the simulated velocity field. Middle panel : isovelocity curves from the observations, overplotted on the galaxy image. Right panel : superposition of the left two.
© Lin et al. (2013)


The results for this object are based on Lin et al (2011). NGC4945 (see Figure 4) is a nearby spiral galaxy hosting a Seyfert 2 nucleus with a strong and variable hard X-ray emission.

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Figure 4
Composite image of NGC4945 from observations taken with the MPG/ESO 2.2-m telescope at La Silla.

The velocity field observed via CO emission is marked by a clear S-shaped asymmetry, pointing to the presence of spirals in the nuclear regions. Two-dimensional hydrodynamical simulations were carried out to understand the structure and kinematics of this region in detail. The simulation reasonably reproduce the results and the S-shaped feature provided a small, weak and rapidly rotating bar is present at the centre. Figure 5 shows the observed and simulated results.

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Figure 5
Top panel : intensity-weighted mean CO velocity maps at the centre of the galaxy. The map shows a velocity gradient from redshift to blueshift, as indicated by the color bar. Bottom panels show the projected simulation results of the gaseous disk
with self-gravity. Bottom left : isovelocity contours of the simulated velocity field (km/s). Bottom right : the simulated density distribution. The color map denotes the surface density distribution in logarithmic scale in units of solar masses over squared pc.
© Lin et al. (2011)


NGC1365 (see Figure 6) is a nearby object known as the Great Barred Spiral Galaxy. Its I-band (old stars) light distribution is highly symmetric, yet the observed dust/gas distribution and gas flow have strong asymmetries that extend throughout the galaxy. A review on the properties of this object is given in Lindblad (1999).

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Figure 6
Composite image of NGC1365 observed with the FORS1 instrument on UT1 at the ESO’s VLT facility.

Lindblad, Lindblad & Athanassoula (1996) performed a 2D hydrodynamical modelling of NGC1365. They managed to reproduce the main features of the general morphology and velocity field of the object with a model featuring a disk, a bar and a spiral structure. The spatial position and extent of the dust lanes (gas lanes in the simulation), the strong velocity gradients across the dust lanes and the general motion within the bar region are all reproduced at reasonable levels in the model. Figure 7 and 8 show the results.

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Figure 7
Density contour map from the disk+bar+spiral model overlaid on the total HI column density map (left) and on the optical map (right) of NGC1365.
© Lindblad, Lindblad & Athanassoula (1996)
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Figure 8
The optical radial velocity field in the inner region of NGC1365 (left) and the corresponding velocity field from the model with disk+bar (right). The thick contour corresponds to the zero velocity and the interval between the contours is 20 km/sec. The straight lines mark the bar’s major axis and the galaxy’s minor axis.
© Lindblad, Lindblad & Athanassoula (1996)

Zanmar Sanchez et al. (2008) presented new photometric images, Fabry-Perot spectroscopy, as well as a detailed reanalysis of the neutral hydrogen observations of NCG1365. They performed hydrodynamic simulations of the gas flow pattern in the bar region in order to constrain the mass-to-light ratio. For each type of halo adopted, they run a grid of simulations covering a range of both mass-to-light ratios and pattern speeds for the bar. Their best model is obtained for bar parameters in excellent agreement with those found by Lindblad, Lindblad & Athanassoula (1996). As for the mass-to-light ratio, the results come with large uncertainties. They suggest a massive, although not fully maximal disk and an unusually concentrated NFW halo.


Weiner, Sellwood & Williams (2001) use fluid-dynamical models of gas flows in the barred galaxy NGC 4123 (Figure 9) to constrain its dynamical properties : disk mass-to-light ratio, bar pattern speed and the central density and scale radius of the dark halo.

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Figure 9
NGC4123. Image from

The results from four hydrodynamical models are shown in Figure 10 (gas density) and 11 (gas velocity field). The comparison of the model results to optical/radio emission-line observations of gas flows favours a massive disk with a fast-rotating bar. The resulting disk is predicted to be 80%-100% of the maximal disk. Lighter disks can be excluded because they do not produce shocks as strong as those observed. At the same time, slower bars produce shocks in a location that does not match the observations.

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Figure 10
Gas density fields in four representative simulations. The bar runs approximately from left to right and the offset regions of high gas density along the bar are the loci of shocks. Panels correspond to : (a) heavy disk, M/L=2.25, fast bar ; (b) heavy disk, M/L=2.25, slow bar ; (c) light disk, M/L=1.0, fast bar ; (d) light disk, M=1.0, slow bar.
© Weiner, Sellwood & Williams (2001)
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Figure 11
Four simulated gas velocity fields in the bar region. The color scale shows the line-of-sight velocity field. The bar runs approximately from left to right, and the shocks are the large velocity gradients perpendicular to the bar. The area between the ellipses is that used for comparison to the data. Panels are as in Figure 10 : (a) heavy disk, M/L=2.25, fast bar ; (b) heavy disk, M/L=2.25, slow bar ; (c) light disk,M/L=1.0, fast bar ; (d) light disk, M=1.0, slow bar.
© Weiner, Sellwood & Williams (2001)


NGC5383 is a classical example of a barred spiral galaxy (Figure 12).

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Figure 12
© Roger Lynds/NOAO/AURA/NSF

Duval & Athanassoula (1983) obtained photometric information for this galaxy and concluded that the following components are present : a classical bulge, an exponential disk, a bar and a lens. They then model the gas distribution and velocity field in order to determine the mass-to-light ratio for each of these components. To this purpose, they compare the simulated velocities to observational results obtained via multiple spectra. They find the following sequence in mass-to-light ratios : bulge (small M/L), bar and lens (intermediate M/L), outer disk (high M/L). The low value found for the bulge is consistent with strong star formation. The high value for the outer disk can be explained by the presence of a dark-halo component. The similar value found for the bar and the lens suggests a common origin for the two structures. Figure 13 and 14 show, respectively, the model results (gas velocity field and density distribution) and the original isophotal maps for NGC5383.

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Figure 13
Results from the hydrodynamical modelling.
© Duval & Athanassoula (1983)
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Figure 13
Isophotal map of NGC5383.
© Duval & Athanassoula (1983)

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