Comparing the properties of the X-shaped bulges of NGC 4710 and the Milky Way with MUSE

Abstract

Context. Our view of the structure of the Milky Way and, in particular, its bulge is obscured by the intervening stars, dust, and gas in the disc. While great progress in understanding the bulge has been achieved with past and ongoing observations, the comparison of its global chemodynamical properties with respect to those of bulges seen in external galaxies has yet to be accomplished. Aims. We used the Multi Unit Spectroscopic Explorer (MUSE) instrument installed on the Very Large Telescope (VLT) to obtain spectral and imaging coverage of NGC 4710. The wide area and excellent sampling of the MUSE integral field spectrograph allows us to investigate the dynamical properties of the X-shaped bulge of NGC 4710 and compare it with the properties of the X-shaped bulge of the Milky Way. Methods. We measured the radial velocities, velocity dispersion, and stellar populations using a penalised pixel full spectral fitting technique adopting simple stellar populations models, on a 1 × 1 area centred on the bulge of NGC 4710. We constructed the velocity maps of the bulge of NGC 4710 and investigated the presence of vertical metallicity gradients. These properties were compared to those of the Milky Way bulge and to a simulated galaxy with a boxy-peanut bulge. Results. We find the line-of-sight velocity maps and 1D rotation curves of the bulge of NGC 4710 to be remarkably similar to those of the Milky Way bulge. Some specific differences that were identified are in good agreement with the expectations from variations in the bar orientation angle. The bulge of NGC 4710 has a boxy-peanut morphology with a pronounced X-shape, showing no indication of any additional spheroidally distributed bulge population, in which we measure a vertical metallicity gradient of 0.35 dex/kpc. Conclusions. The general properties of NGC 4710 are very similar to those observed in the Milky Way bulge. However, it has been suggested that the Milky Way bulge has an additional component that is comprised of the oldest, most metal-poor stars, which is not part of the boxy-peanut bulge structure. Such a population is not observed in NGC 4710, but could be hidden in the integrated light we observed.

Peanut-shaped metallicity distributions in bulges of edge-on galaxies: the case of NGC 4710

“…We obtained spectral and imaging coverage of NGC 4710 using the Multi Unit Spectroscopic Explorer (MUSE) instrument installed on the Very Large Telescope (VLT). The observations were taken as part of the MUSE Science Verification observing run in June 2014 and their details can be found in the study of the kinematics of NGC 4710 presented in Gonzalez et al (2016). MUSE (Bacon et al 2010) is an optical wide-field integral-field spectrograph that uses the image slicing technique to cover a field of view (FOV) of 1 × 1 in wide-field mode resulting in a spatial sampling of 0.2 × 0.2 spaxels and resolving power from R ∼ 2000 at 4600Å to R ∼ 4000 at 9300Å .…”

“…Thus the simulation was able to follow the chemical evolution of all stellar populations. Gonzalez et al (2016) used the MUSE observations of NGC 4710 to map the kinematics of its bulge. They used the Penalised Pixel Fitting routine pPXF 1 (Cappellari & Emsellem 2004) to evaluate the galaxy's stellar kinematics by fitting the Single Stellar Population (SSP) MILES library of Vazdekis et al (2010) to the observed spectra in pixel space using a maximum penalised likelihood method.…”

“…We select a grid with ages ranging from 1 to 14.1 Gyr and metallicities 3 ranging from -1.31 to +0.22 dex, computed using a Salpeter (1955) IMF (unimodal with slope 1.3) and based on the Padova isochrones from Girardi et al (2000). We follow the same methodology as in Gonzalez et al (2016) and use pPXF to find the best fitting template spectrum over each spatial bin over the wavelength range (4750Å to 5550Å) to avoid regions potentially contaminated by residuals of strong sky emission lines. This spectral region has been used in similar analysis by the SAURON (Bacon et al 2001) and ATLAS 3D (Cappellari et al 2011) surveys, as it includes strong stellar population indicators such as Hβ (sensitive to age) as well as Mgb and Fe5250 (sensitive to metallicity).…”

“…We note that there is a region near the mid-plane (but clearly not in the centre of the galaxy) dominated by younger stellar populations (2 Gyr) than the rest of the disc, which coincides with a flux excess seen in the reconstructed image. In Gonzalez et al (2016) this region was identified as a stellar population located in the foreground disc because of its distinct kinematics and strong gas emission lines. The fact that the stellar population in this region is also very young and metal-poor with respect to the rest of the galaxy supports this conclusion and, furthermore, suggests that this is a recently triggered star forming region.…”

“…In this context, NGC 4710 is one of the best laboratories to test this prediction. NGC 4710, located at 16.9 Mpc, has a stellar mass of M ∼ 7 × 10 10 M (Cappellari et al 2013), is classified as an exactly edge-on barred galaxy with a B/PS bulge (Buta et al 2015), and has no co-spatial large-scale classical bulge (Gadotti 2012;Gonzalez et al 2016). Furthermore, the arms of its X-shaped bulge structure are seen very prominently, as a consequence of a nearly side-on orientation of the bar (Buta et al 2015).…”