Volume 545, September 2012
|Number of page(s)||16|
|Published online||21 September 2012|
On the left, the relationship between FUV surface brightness and H i surface density is shown for NGC 4294. The colors indicate which of the three defined regions the pixels belong to: inner galaxy in blue, outer galaxy in red and external gas in green (as shown in the right-hand panel). The 1(3)σ FUV sky surface brightness level is indicated by the horizontal dotted (solid) line. The points with the error-bars indicate, from top to bottom, the mean and its dispersion within the H i surface density bins for each of the three regions (inner galaxy, extended galaxy and external gas). We have not applied any inclination corrections to these data. However, the arrow indicates how the pixel values should be corrected to account for inclination (based on the major/minor axis ratio of the galaxy). The correction in the FUV is very uncertain however, because it assumes the galaxy is optically thin. The two numbers on the left of each error bar, separated by an underscore symbol, indicate (left) the number of pixels used for this point, and (right) the number of pixels with a flux below the mean sky level in the bin. As using the latter pixels would give an infinite surface brightness they are not included in the computation of mean surface brightness. A large value for this second number is a good indication that we are very close to the sky value. The error bar near the lower-right corner (labeled “no H i”) shows the result of the same analysis when performed on the FUV emission outside the detected H i only, and provides an indication of our FUV detection limit (the dark- and light- grey shaded areas show, respectively, the corresponding 1 and 2σ limits). The yellow-shaded area indicates the location of nearby field galaxies from the THINGS sample (Bigiel et al. 2008). The vertically and horizontally shaded areas correspond respectively to dwarf galaxies and the outer parts of spirals, i.e., the points with error bars in Fig. 8 of Bigiel et al. (2010). A similar diagram for our complete sample is shown in our Fig. 3.
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We comment on each galaxy on the basis of the GALEX FUV images (Fig. 1), the optical NGVS images (Fig. 2) and their individual H i-FUV relationship as shown in on-line Figs. A.1 to A.8. In these figures, individual pixels above the solid (dotted) lines are detected at the 3(1)σ level, respectively. Although most points in the H i tails are barely detected in the FUV, we keep all pixels in our analysis, in particular because we compute the average FUV surface brightness in H i surface density bins from many pixels, thus reducing the noise level. In our figures, the point near the lower-right corner indicates the FUV intensity obtained by stacking in the same manner all pixels without detected H i, which provides an indication of the FUV emission detection limit for the binned analysis.
Chung et al. (2009) found no optical counterpart in SDSS images of the H i tail down to r = 26 mag arcsec-2, which we confirm at the much lower NGVS detection limit (about 29 mag arcsec-2 in the g-band at the 2σ level, Ferrarese et al. 2012). They also mention that the Hα emission is enhanced in the south of NGC 4299. This enhancement is also seen in our FUV images. It is however found within the optical radius of the galaxy only, and no clear UV clusters are seen outside the galaxy in the extended H i tail shared by NGC 4294 and NGC 4299. In our on-line Figs. A.1 and A.2, we have treated all external gas together, rather than making a separate analysis for each galaxy – in any case, the gas tail is almost common to both galaxies. For both galaxies, the inner galaxy H i-SFR relationship is in agreement with the data on spirals and dwarfs from Bigiel et al. (2008) at surface densities around a few M⊙ yr-1; we remind the reader that a comparison at larger densities is difficult because of the increasing dust attenuation and molecular fraction). The pixels in the outer regions of the two galaxies fall in the Bigiel et al. (2010) area for dwarfs and outer regions of spirals, as expected. The external FUV emission level (in the tail) is well below the FUV emission within the galaxy (inner or outer) at any given H i surface density. The tail’s FUV emission is moreover very close to the detection limit (a few sigma).
Note that NGC 4298 and NGC 4302 form a pair (like NGC 4294 and NGC 4299). While the interaction may affect the level and distribution of gas and star formation within NGC 4298, it is not believed to be undergoing ram-pressure stripping, and is therefore not included in the study. NGC 4302 has no UV emission associated with the H i tail to the north. The rest of the H i disk is truncated within the optical radius of the galaxy. A faint level of diffuse emission is seen in NGVS images at the position of the H i tail, but it does not show clear signs of young clusters or filaments, and it has similar levels of emission and colors as were found on the opposite side of the galaxy, where no gas is present. No FUV emission is seen in the tail, suggesting a low star formation efficiency in the external gas. In our pixel-by-pixel analysis, however, too few pixels are located outside of the optical body of the galaxy to derive any useful constraints.
Chung et al. (2007) found a UV tail along the edge of the H i to the southwest of the galaxy, displaced from the optical. Our FUV image also shows this extension (Fig. 1). Chung et al. (2007) did not see any optical counterpart to the H i on SDSS images. However, we clearly see blue clusters in our deeper NGVS images (Fig. 2) corresponding to the southwest FUV emission, but slightly displaced from it. The recent detailed study by Abramson et al. (2011) also found Hα and FUV emission, suggesting a low level of star formation in the tail, and confirmed the offset between the newly formed stars and the gas, which can occur if the ISM is continuously accelerated by ram-pressure, while stars decouple from it (Vollmer et al. 2012b). They also discussed nine extra-planar star forming regions found in the UV southeast of the galaxy. They estimated their ages to be between 100 and 350 Myr, from their FUV-NUV colors. These regions are not associated with gas peaks.
The gas disk of the galaxy is severely truncated, and the optical disk is much larger. For this reason, the young stars emission discussed above is found entirely “within” the galaxy, according to our definition of regions (Sect. 2.3), especially at the H i resolution used for studying the H i-FUV relationship (our pixel analysis actually has no points “outside” the galaxy, see Fig. A.4).
Some of the extra-planar star-forming regions are also detected as blue clusters in our NGVS images (Fig. 2). They are clearly found close to the galaxy only and not over the full extent of the external H i, as are the UV clusters, suggesting again a relative low star formation efficiency in the H i. A detailed analysis of the stellar population based on the NGVS data is beyond the scope of this paper.
This galaxy is known for its very long H i tail, probably made of stripped gas (Vollmer & Huchtmeier 2003; Oosterloo & van Gorkom 2005; Chung et al. 2009). We see no obvious optical or FUV counterparts in the tail, and the GALEX data are consistent with the absence of UV emission over the tail. Note that the optical/UV analysis cannot be performed over the full area of the tail because of the proximity of bright M86, which is masked in our pixel-based analysis. Because of the active nucleus of the galaxy (very bright in the UV) and of the very elongated shape of the WSRT H i beam (and thus of the Gaussian kernel used to attain this resolution with the UV data), part of the flux from “inside” the galaxy is found “outside” it, in our definition of the various regions. To avoid counting this flux as “external”, we had to mask it as much as possible. Although this masking (visible in the right part of Fig. A.5) introduces a small error in the H i and UV content of the galaxy, it avoids over-estimating the surface brightness and density values outside the galaxy.
Finally, we wish to remind that this work is performed at the H i resolution, so we cannot distinguish fine details within the disk (nor is this our purpose, as we try to estimate the amount of star formation in the tail, outside the optical body). However, some optical emission coinciding with Hα regions very close to the disk of the galaxy was found in NGVS images by Ferrière et al. (in prep.). While Yoshida et al. (2004) attributed the Hα emission along the H i tail to ionization of the stripped gas by an active nucleus, Ferrière et al. discuss the possibility that some of these regions, very close to the disk, may be associated with star formation.
Chung et al. (2007) did not note any UV emission or optical emission in the SDSS image which may be linked to the extended H i. Neither do we see anything in our GALEX UV images, nor blue clusters in the NGVS optical data. In the pixel-by-pixel analysis, very few pixels are in fact outside the galaxy (Fig. A.6) and they show a very low level of FUV emission.
Inside the galaxy, the FUV emission is relatively faint: it is consistent with the outer regions of spirals and with dwarfs, rather than the inner parts of normal spirals. The FUV image shows no wide-spread star formation in the gas outside the galaxy (Fig. A.7) and no obvious filaments/clumps are seen in the NGVS images.
NGC 4522 is undergoing a strong ram-pressure event despite its large distance to the center of the cluster (Kenney et al. 2004; Vollmer et al. 2006; Chung et al. 2009). Kenney & Koopmann (1999) and Kenney et al. (2004) showed the presence of H ii regions in the extra-planar gas. We indeed see some FUV emission when going out towards the extra-planar H i gas peaks, but none at the radius of the outermost isophote where H i is detected. The NGVS optical images also reveal blue filamentary structures and clusters in the same direction, which are barely visible in the B-band image of Kenney & Koopmann (1999). Once smoothed to the H i resolution, almost no pixels with detected H i are found beyond R25. Figure A.8 shows that the only contribution to the extended galaxy region (at R25 < R < 1.5 R25) is by this extra-planar gas. The FUV emission level in the extended region is clearly lower than in the inner galaxy, which itself seems, on the contrary, higher than the typical value in the THINGS sample (Bigiel et al. 2008) at the same gas surface density.
Crowl & Kenney (2006) found that the outer disk of the galaxy is consistent with a 2% (in stellar mass) very short duration starburst which occurred at the epoch of the truncation (100 Myr ago). The observed UV emission and blue filaments in the immediate proximity of the galaxy could correspond to stars formed during this brief episode.
© ESO, 2012
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