Observations of Intracluster Planetary Nebulae

Prepared by F. Governato and B. Poggianti

We propose to use the wide field imager at the prime focus of LBT to obtain photometric and spectroscopic observations of intergalactic planetary nebulae (PNe) within nearby groups and clusters to study their formation history and dynamics. With this instrument it will be possbile to identify PNe over a large fraction of a group's virial radius using a narrow-band filter centered on the [OIII]$\lambda$5007Å line. This will provide, for the first time, a reliable estimate of the number of PNe that lie within groups and clusters that are not bound to galaxies, thus yielding the mass and radial variation of the diffuse stellar component. This will be compared to the distribution of galaxies and X-ray luminous gas, as well as the predictions of numerical simulations of ``Galaxy Harassment'' in groups and clusters in order to determine the effectiveness of galaxy interactions. Spectroscopic follow-up of the brightest PNe will provide very accurate information on the group dynamics, since PNe are unbiased test particles moving in the group potential.

Motivation: The discovery of PNe within groups that are not attached to individual galaxies (Theuns & Warren 1997, Arnaboldi et al. 1997), provides strong evidence for dynamical evolution of the galaxies in dense environments. Rapid tidal encounters with massive galaxies and the global tidal field -- ``galaxy harassment'' (Moore et al. 1996), will disrupt a substantial fraction of low surface brightness (LSB) and Sd-Sc spiral galaxies in groups and clusters, giving rise to a stellar population bound to the global group potential, but not to any individual galaxy. The detection of this stellar population would provide an invaluable tool to study the dynamics and formation history of dense environments. However, simple (and uncertain) calculations based on the amount of stars stripped and their colours, predict an average surface brightness of the diffuse light that could be as faint as $\sim 30\,$mag$_{\rm B}/$arcsec², extremely hard to observe directly. Much easier, at least for nearby groups, is to identify individual PNe that will trace the diffuse light and are observable beyond the Virgo cluster as point sources (Arnaboldi et al. 1997).

The two main scientific goals are:

$\bullet$ To estimate the mass and distribution of diffuse intra-group stellar material by measuring the number density of PNe as a function of position: The number of PNe can be linked to the total luminous mass of the intra-group stellar material using empirical calibrations (e.g. Ciardullo et al. 1989 for M31) or population synthesis models (Barbaro & Poggianti 1997, Bruzual & Charlot 1997). In general, we expect $\sim 4\,$PNe for every $10^7\,$L $^{\rm B}_\odot$in the complete PNe luminosity function, or about $\sim 1\,$PNe per $10^8\,$L $^{\rm B}_\odot$ when reaching about 1 magnitude fainter than the PNe cutoff magnitude $M^\ast$. Numerical simulations of galaxy evolution within groups demonstrate that most of the diffuse light will originate from harassed LSB disk galaxies (Figure 1). Thus the mass in diffuse material will constrain the numbers of LSB spirals that have been disrupted within groups. Moreover, galaxy harassment becomes weaker as the encounter timescale increases, giving rise to a unique signal in the distribution of the intra-group light. Up to $\sim 1/2\,$the virial radius, we would expect the PNe to follow closely the gravitational potential, since the mixing timescale is expected to be much smaller than a Hubble time, thus the projected surface number density of the PNe should fall as $\propto r^{-1}$for an isothermal profile. Beyond $\sim 1/2\,$the virial radius, the mixing timescale becomes comparable to the Hubble time so the PNe would not have had time to mix, and their spatial distribution should show the trails originated by stars stripped from the ``harassed'' galaxies.

$\bullet$ To use PNe as an unbiased tracer of the group potential: Galaxy measurements of the group velocity dispersion are very likely adversely affected if the group is embedded in an elongated filamentary structure (Hernquist, Katz & Weinberg 1995). In contrast the PNe have been stripped from galaxies and therefore trace only the virialised regions of the group potential. Thus for the first time we shall be able to accurately compare an unbiased dynamical measure of the group potential against results obtained from galaxy redshifts and the X-ray emission of the Intra-group medium. Substructure and phase space information of the PNe will provide information on the orbits of the disrupted galaxies and the strength of the gravitational encounters. This second aim will require spectroscopic follow-up of the brightest PNe.

Strategy: Identifying PNe is relatively simple since they emit most of their light in a few emission lines ([OIII]$\lambda$5007Å is generally the strongest). We will restrict our analysis to PNe obviously detached from individual galaxies defining a ``zone of avoidance'' around each galaxy with an area based on a conservative estimate of its virial radius, excluding PNe that are bound to individual galaxies.

Arnaboldi M., Freeman K.C., Mendez R.H. et al., 1996, ApJ 472, 145
Barbaro G. & Poggianti B.M., 1997, A&A 324, 490
Bruzual G. & Charlot S., 1997, in preparation
Ciardullo R., Jacoby G.H., Ford H., C., Neill J.D., 1989, ApJ 339, 53
Hernquist L., Katz N., Weinberg D.H., 1995, ApJ 442, 57
Moore B., Katz N., Lake G., Dressler A., Oemler A.Jr, 1996, Nature, 379, 613
Theuns T. & Warren S.J., 1997, MNRAS 284, L11

Figure 1: The image shows a simulated galaxy group in a standard CDM cosmology. Each side is 0.5h^-1Mpc and brighter colors indicate higher densities. At z=0.5 four small halos (circular velocity $\sim 150\,$km/sec) just about to infall into the group were replaced with pre-fabricated models of low surface brightness disk galaxies. Big green dots show their star component. The image shows the final state of the group at z=0. All the four galaxies went through pericentre (one is just off the right side of the image), and their dark matter halos have been severely stripped and have become part of the group DM halo. The star component has been affected by harassment. One galaxy has been severely harassed and its disk has been destroyed. Its stars are now distributed along its orbit. The disk of another galaxy has gone bar unstable. The disk of the remaining two galaxies thickened considerably even if they lost only a minor fraction of their stellar component to the intragroup medium.