ACCESS – Hα imaging

Hα imaging

To complement the IFU spectroscopy from WiFeS which provides detailed kinematical and spectroscopic information, but relatively poor spatial resolution (1″x1″ pixels) we also have obtained high-quality Hα imaging of the Shapley supercluster galaxies from our WiFeS survey. This imaging was obtained using the Maryland-Magellan Tunable filter (MMTF) attached to IMACS on the Baade 6.5m Magellan telescope at Las Campanas Observatory in Chile over three half-nights in May 2012.

The MMTF uses a Fabry-Perot etalon to produce a narrow-band filter with transmission of width ~10A which is tuned to cover the Hα emission at the redshift of each galaxy. The MMTF is attached to IMACS on the f/2 focal position, which provides imaging with 0.2″ pixels over a 27’x27′ field of view.
IMACS in conjunction with the active optics of the Magellan telescope allowed us to obtain high-quality fully-sampled imaging at a spatial resolution of ~0.5″ FHWM, capable of detecting the ionized gas stripped from the supercluster galaxies and quantifying its morphology, revealing several “jellyfish”-like galaxies with long trails of clumpy extra-galactic gas.

More information on the telescope/instrument here

Data are currently being reduced. Below we show two sample raw images.

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A disc galaxy with ionized gas flowing out from the top-right part of the disc.

A case study for WiFeS

One of the first galaxies observed with WiFeS was a spiral L* galaxy at 1 Mpc from the centre of the cluster Abell 3558.This galaxy is star-forming (~8M yr-1) according to Spitzer + Galex data. The nuclear region is heavily obscured (Fig. 1).

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Figure 1. Composte BRK image of a galaxy in the SSC with two pointing of WiFeS indicated by the rectangles. The dust-obscured nuclear region comes out in K band. The bars indicating the orientation correspond to 15 kpc length.

The star formation (Fig. 2) is mainly concentrated in the centre of the galaxy and in a region 13 kpc SW from the nucleus, which account for 30% and 20% of the total SF respectively. The SF derived from H&alpha after corerection for dust attenuation closely matches the distribution of Spitzer 24 &mu m emission.

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Figure 2. WiFeS map of the star formation rate across the galaxy. The red contours mark the Spitzer 24 &mu m emission. Notice the coincidence of the IR emission with the SFR derived from the spectra. The SFR is derived from the H&alpha flux after correction for dust attenuation through the Balmer decrement.

The distribution of H&alpha flux shows the presence of ionized gas out of the disc (up to ~13 kpc in projection, see Fig 3).

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Figure 3. H&alpha flux map overlaid on the R-band image, showing extraplanar gas extending in projection ~15 kpc NW from the stellar disc.

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Figure 4. Radial velocity (left) and velocity dispersion (right) maps of the ionized gas. White contorurs indicate the brightness of the red continuum. Dashed lines mark the major and minor axes.

The kinematics of the ionized gas (Fig. 4) is complex. Notice that the extraplanar gas (i) broadly follows the motion of the disc gas and (ii) has a high velocity dispersion. High velocity dispersion is also observed ~2-5 kpc SE from the centre.

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Figure 5. Line flux ratio diagnostic diagrams. Colours of different galaxy regions are coded as in Fig. 6.

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Figure 6. Colour codes for the line flux ratio diagnostic diagrams in Fig. 5.

Figure 5 shows the line-flux ratio diagnostic diagrams of the different regions of the galaxy (colour-coded as shown in Fig. 6). A significant part of the galaxy is characterized by flux ratios of composite (HII+AGN) spectra, lying between the theoretical (red curve) and empirical (blue dashed curve) upper limits for pure star-forming regions. For these regions the source of excitation may be a combination of photo-ionizazion related to star formation and shock excitation.

These data should enable us to understand the origin of the extraplanar gas and the star formation in this galaxy.

The paper on this case was submitted.

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The galaxy on the right is experiencing gas-stripping towards the centre of the picture, where many Hα knots are visible. The field comprising the two galaxies was also covered by a WiFeS mosaic.

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