Context: Tidal Dwarf Galaxies

During galaxy interactions tidal forces expel large amount of gas from the galaxies. This can be seen in Fig.1 where the superimposed blue color shows the 21 cm emission from atomic hydrogen, which is a good tracer of tidal features.

In some systems, this gas may re-collapse and form new stellar systems, which can be massive enough to be considered as dwarf galaxies and are usually called Tidal Dwarf Galaxies (TDGs). In Fig.1, star forming regions are shown in pink and the yellow labels indicate tidal dwarf galaxies. They are however different from the other known types of dwarf galaxies for several reasons:

- they do not have an old stellar component and are forming their first stellar population,
- they have a much higher metallicity, around half-solar, as they are formed from recycled material,
- they are devoid of dark matter.

Their particular physical conditions make them very intersting to study. In particular, they have a high-gas fraction, metallicity and turbulence which are similar to that of high-redshift galaxies, and have a great advantage of being observed in the local Universe where new instruments can resolve them in details.

Fig.2: The Multi-Unit Spectroscopic Explorer (MUSE) instrument.

Our Work:

We studied one TDG in particular: NGC5291N, located on top of the left panel of Fig.1, which is one the most massive and star forming known TDG. To investigate the ioinization processes at play in this systems we used the brand-new MUSE instrument, mounted on the Very Large Telescope in Chile. The instrument is shown in Fig.2, while it was being developed in Lyon.

This instrument is an integral-field spectrograph: it provides a spectrum for each spatial pixel. With MUSE one can thus compute the spatial variations of lines, ratio of lines or width of lines, which makes it a wonderful tool to investigate the internal processes of a resolved galaxy. A map of the galaxy using various emission lines in shown in Fig.3: in this case we see that the distribution is quite uneven with some regions dominated by [OIII] emission and others by [NII].

We quantitatively studied the variation of these emission line ratios with the help of BPT diagrams (which stands for Baldwin, Phillips, Terlevich; see e.g Baldwin et al. 1981 ). These diagrams were first used on spectra of galaxies and succeeded in discriminating whether the gas was primilarly ionized through pure photo-ionization or also by a central Active Galactic Nucleus (AGN). We also use these diagnostic diagrams on our system, this time on each resolution element. The resulting BPT diagrams are shown in the upper pannel of Fig.4. The blue points corresponds to spatial regions were the emission lines are consistent with originating from pure photo-ionization: their spatial location is shown on the lower panel, for each diagram. The red points are however not consistent with pure photo-ionization models. We see that a extended regions of the dwarf galaxy, namely around the main star forming region, are not on the photo-ionization region of the [OI]/ Hα BPT diagram, which was unexpected.

Fig.3: FORS and MUSE views of NGC 5191N. The left image uses the optical bands V, R and I. The MUSE images uses three emission lines: blue is [OIII] (5007 Å), green is Hα and red is [NII] (6583 Å). The entire image spans 18 kpc x 18 kpc.

Fig.4: BPT diagnostics for different emission line ratios. On the [OIII]/Hβ vs [NII]/Hα diagram, the dotted line delineates the pure starburst region, as defined by Kauffmann et al. (2003) using a set of Sloan Digital Sky Survey (SDSS) spectra. The spaxels belonging to this region are shown in blue, those outside it in orange; their spatial location is shown in the figure below. The field of view is the same as in Fig. 3. The solid line traces the upper theoretical limit to pure Hii regions measured by Kewley et al. (2001). For the other diagrams, the dashed lines defined by Kewley et al. (2006) separate the starburst (blue points) from the AGN/LINER (red points) regions. The solid lines further distinguish between AGN ionization (above the line) and LINER ionization (below). The corresponding spatial distribution of starburst/non-starburst 33 binned spaxels are shown below. The highest fraction of spaxels inconsistent with a photoionization by a starburst show up on the [OIII]/Hβ vs [OI]/Hα diagram. Their spectra have been stacked and the resulting extracted line ratios are shown with the black point, together with the error bar.

We noticed that the outskirts of the galaxy also showed a larger velocity dispersion than the center of the dwarf. A dispersion velocity map of the dwarf is shown in Fig.5. This suggest the possibility of shock-ionization in this region. We have tested the shock hypothesis with the fast radiative shock model from Allen et al. (2008), which is based on MAPPINGS III (Sutherland & Dopita 1993). The resulting grids superimposed on the BPT diagrams are shown in Fig. 6. The shaded grey regions shows the area covered by the shock model: we see that shocks can indeed be an explanation to the unexpected emission line ratios that we found on the [OI]/ Hα BPT diagram. A discussion on the plausible origin of these shocks is presented in Fensch et al., 2016 .

Fig.5: Velocity dispersion measured from the width of the Hα emission line. We subtracted a constant value of 2.34 Å to the width of the line, corresponding to the average Gaussian FWHM close to the Hα line.

Fig.6: BPT diagrams Shock models on the BPT diagrams. Are plotted only points with detected [OI] emission. The blue (resp. red) points lie in (resp. out of) the star formation locus on the [OI] / Hα BPT diagram. The purple dashed line is the ionization model prediction, using a metallicity Z = 0.5 Z and log(P/k) = 5.2. The ionization parameter goes from log U = -2 to log U = -3.5, as indicated on the third diagram. The two grids come from the MAPPINGS III model for fast-shock, as described in the text, and are drawn for shock only and shock + precursor. The locus of both grids are shaded.

References:

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