Planetary Nebulae in the Elliptical Galaxy NGC 4649 (M 60): Kinematics and Distance Redetermination

Abstract

Using a slitless spectroscopy method with (a) the 8.2 m Subaru telescope and its FOCAS Cassegrain spectrograph, and (b) the ESO Very Large Telescope (VLT) unit 1 (Antu) and its FORS2 Cassegrain spectrograph, we have detected 326 planetary nebulae (PNs) in the giant Virgo elliptical galaxy NGC 4649 (M 60), and we have measured their radial velocities. After rejecting some PNs more likely to belong to the companion galaxy NGC 4647, we have built a catalog with kinematic information for 298 PNs in M 60. Using these radial velocities we have investigated if they support the presence of a dark matter halo around M 60. The preliminary conclusion is that they do; based on an isotropic, twocomponent Hernquist model, we estimate the dark matter halo mass within 3R e to be 4×10 11 M ⊙ , which is almost one half of the total mass of about 10 12 M ⊙ within 3R e . This total mass is similar to that estimated from globular cluster, XMM-Newton and Chandra observations. The dark matter becomes dominant outside. More detailed dynamical modeling of the PN data is being published in a companion paper. We have also measured the m(5007) magnitudes of many of these PNs, and built a statistically complete sample of 218 PNs. The resulting PN luminosity function (PNLF) was used to estimate a distance modulus of 30.7±0.2 mag, equivalent to 14±1 Mpc. This confirms an earlier PNLF distance measurement, based on a much smaller sample. The PNLF distance modulus remains smaller than the surface brightness fluctuation (SBF) distance modulus by 0.4 mag. The reason is still unknown.

Planetary nebulae as kinematic tracers of galaxy stellar halos

“…Initial works that included a significant amount of PNe in the analysis were carried on massive nearby ellipticals, such as NGC 1316 (Arnaboldi et al 1998), NGC 5128 (Hui et al 1995;Peng et al 2004), and more recently on NGC 4649 (Teodorescu et al 2011) -just to cite few examples. The signature of the presence of dark matter was clearly present in the kinematic profile at large radii.…”

Planetary nebulae populations in external galaxies

“…Since the work of Arnaboldi et al (1994) on NGC 1399 and of Hui et al (1995) on Centaurus A (NGC 5128), there has been a considerable success in mapping the kinematics of the stellar population in the outer regions of ETGs out to 5R e (Arnaboldi et al 1996;Arnaboldi et al 1998;Méndez et al 2001;Romanowsky et al 2003;Coccato et al 2009;Napolitano et al 2009;McNeil et al 2010;Napolitano et al 2011;Teodorescu et al 2011;Cortesi et al 2011), and intracluster light (ICL) in nearby clusters (Arnaboldi et al 2004;Gerhard et al 2005;Gerhard et al 2007;Doherty et al 2009;Ventimiglia et al 2011).…”

“…There are some more complicated cases, as kinematic misalignment is more frequent at large radii (Shih & Méndez 2010, Teodorescu et al 2011.…”

Planetary nebula spectra in M 60 and M 82

“…This is unexpectedly high. If we were to uniformly correct the observed PN fluxes for the effect of this internal extinction, then the distance estimated using the PN luminosity function (PNLF distance) would become shorter, increasing the already large unexplained discrepancy with the SBF distance, as discussed by Teodorescu et al (2011). This certainly creates a motivation to enlarge the sample and acquire better individual PN spectra in M 60 and other elliptical galaxies.…”

“…This means that, in principle, a useful test for high metallicity could consist of detecting Hβ, which is much easier than detecting [O III] 4363. We have tried to explore this idea by observing selected bright PNs in M 60, previously discovered by Teodorescu et al (2011). For comparison, we include a selection of bright PNs in M 82, previously discovered by Johnson et al (2009).…”