Outer haloes of elliptical galaxies
Introduction Recent results Publications MPE OPINAS Dynamics Group
Introduction
Outer haloes of elliptical galaxies are fascinating regions to study because dark matter dominates there and dynamical timescales of the stars are longer and therefore the formation history is better preserved. The advent of high-resolution instruments such as XMM-Newton and Chandra have enabled us to trace the hot gas density and temperature distributions out to large radii and from hydrostatic equilibrium also the gravitational potential of nearby, massive elliptical galaxies. The availability of planetary nebulae (PNe) and globular clusters (GCs) as kinematic tracers of the projected stellar velocity distribution has allowed us to create dynamical models using NMAGIC, probing the dynamics of the stars out into the halo. Moreover, with the use of deep medium resolution spectra, we can also measure the stellar population content in these regions. The combination of kinematics and stellar population content can therefore constrain their evolution and formation process.
Recent results
1. Dynamical models of NGC 4697 and NGC 3379
De
Lorenzi et al. (2008a,b) combine long-slit kinematic data, SAURON data
in the central region and PNe velocities and velocity dispersions in
the halo to create dynamical models of the intermediate-luminosity
galaxies NGC 4697 and NGC 3379 using the N-body particle code, NMAGIC.
They find a range of potentials and anisotropy profiles that are
consistent with the photometric and kinematic data. Therefore the
inavailabilty of higher-order moments of the LOSVD in the haloes of
these galaxies means the models suffer at the hands of the degeneracy
existing between mass and anisotropy (and to a lesser extent, shape).
As
presently the discrete data provided by PNe and GCs are not of a high
enough signal-to-noise ratio to derive higher order moments of the
LOSVD, we need a different way to distinguish between the range of
potentials and anisotropy profiles that are consistent with the data.
2. Gravitational potentials of nearby, massive elliptical galaxies
Massive ellipticals harbour a hot (T ~ 1 keV) interstellar medium (ISM) in the form of a low density (n < 0.1 cm-3) plasma trapped in the galaxy's gravitational potential. In massive ellipticals, the observed X-ray spectrum is dominated by emission from the ISM primarily via thermal bremsstrahlung and X-ray lines of heavy elements (Forman et al., 1985). In quiescent galaxies, where the gas is not significantly distorted by interactions with companion galaxies, one can assume hydrostatic equilibrium and use the temperature and density profiles derived from X-ray spectra to obtain a mass profile (e.g., Fukazawa et al., 2006). Churazov et al. (2008) obtain the potential of NGC 1399 and M87 and compare their profiles with that found in stellar dynamical models.
3. Distinct core and halo stellar population in the bright coma cluster galaxy NGC 4889
We
construct radial profiles of line strength indices along the major axis
of NGC 4889 by combining literature data for the central regions and
new deep spectroscopic data for the halo regions. We then derive age,
metallicity and alpha-enhancement radial profiles and their gradients
using Single Stellar Population models by Thomas et al.
(2003). This represents the most spatially extended dataset
with both stellar kinematics and line strength indices for a brightest
cluster galaxy.
We observe a different population content and gradient between the central regions of the galaxy (R<18 kpc) and the outer halo (R>18 kpc). The inner ~18 kpc (~1.2 Re) of NGC 4889 are characterized by a strong [Z/H] gradient and a nearly constant values of [alpha/Fe]. The outer regions (18 kpc < R < 60 kpc) are characterized by a constant metallicity content strong negative
gradient in the abundance ratio and older ages.
These data indicate that the central parts of NGC 4889 and its halo have undergone different formation mechanisms. Data in the center indicate a short star formation timescale, where the stars formed outside-in, reminiscent of a quasi-monolithic dissipative collapse. On the contrary, the data in the halo suggest that it was accreted from shredded satellite galaxies, as suggested also by numerical
simulations, over the central galaxy that was already in place.
Our measurements are also consistent with recent results on the size evolution of bright ETGs with redshift, i.e. at high redshifts ETGs are smaller and more compact than ETGs of similar mass at z = 0. Their effective radius evolves as Re ~ (1+z)-1.3 (van Dokkum et al. 2010). Scaling the present Re of NGC 4889 with this relation would predict Re = 6.2 kpc, at z = 1, which is consistent with the half light radius measured if considering the central regions of the galaxy only, on the assumption that outer regions of NGC 4889 were accreted later, at z < 1. Our finding for NGC 4889 suggests that we may have found local stellar population signatures of the observed ETG size evolution.
Publications
Outer haloes of elliptical galaxies are fascinating regions to study because dark matter dominates there and dynamical timescales of the stars are longer and therefore the formation history is better preserved. The advent of high-resolution instruments such as XMM-Newton and Chandra have enabled us to trace the hot gas density and temperature distributions out to large radii and from hydrostatic equilibrium also the gravitational potential of nearby, massive elliptical galaxies. The availability of planetary nebulae (PNe) and globular clusters (GCs) as kinematic tracers of the projected stellar velocity distribution has allowed us to create dynamical models using NMAGIC, probing the dynamics of the stars out into the halo. Moreover, with the use of deep medium resolution spectra, we can also measure the stellar population content in these regions. The combination of kinematics and stellar population content can therefore constrain their evolution and formation process.
Recent results
1. Dynamical models of NGC 4697 and NGC 3379
2. Gravitational potentials of nearby, massive elliptical galaxies
Massive ellipticals harbour a hot (T ~ 1 keV) interstellar medium (ISM) in the form of a low density (n < 0.1 cm-3) plasma trapped in the galaxy's gravitational potential. In massive ellipticals, the observed X-ray spectrum is dominated by emission from the ISM primarily via thermal bremsstrahlung and X-ray lines of heavy elements (Forman et al., 1985). In quiescent galaxies, where the gas is not significantly distorted by interactions with companion galaxies, one can assume hydrostatic equilibrium and use the temperature and density profiles derived from X-ray spectra to obtain a mass profile (e.g., Fukazawa et al., 2006). Churazov et al. (2008) obtain the potential of NGC 1399 and M87 and compare their profiles with that found in stellar dynamical models.
3. Distinct core and halo stellar population in the bright coma cluster galaxy NGC 4889
We observe a different population content and gradient between the central regions of the galaxy (R<18 kpc) and the outer halo (R>18 kpc). The inner ~18 kpc (~1.2 Re) of NGC 4889 are characterized by a strong [Z/H] gradient and a nearly constant values of [alpha/Fe]. The outer regions (18 kpc < R < 60 kpc) are characterized by a constant metallicity content strong negative
gradient in the abundance ratio and older ages.
These data indicate that the central parts of NGC 4889 and its halo have undergone different formation mechanisms. Data in the center indicate a short star formation timescale, where the stars formed outside-in, reminiscent of a quasi-monolithic dissipative collapse. On the contrary, the data in the halo suggest that it was accreted from shredded satellite galaxies, as suggested also by numerical
simulations, over the central galaxy that was already in place.
Our measurements are also consistent with recent results on the size evolution of bright ETGs with redshift, i.e. at high redshifts ETGs are smaller and more compact than ETGs of similar mass at z = 0. Their effective radius evolves as Re ~ (1+z)-1.3 (van Dokkum et al. 2010). Scaling the present Re of NGC 4889 with this relation would predict Re = 6.2 kpc, at z = 1, which is consistent with the half light radius measured if considering the central regions of the galaxy only, on the assumption that outer regions of NGC 4889 were accreted later, at z < 1. Our finding for NGC 4889 suggests that we may have found local stellar population signatures of the observed ETG size evolution.
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Publications
Coccato, L., Garhard, O., Arnaboldi, M. 2010, MNRAS letter, in press (arXiv: 1006.2382).
Distinct core and halo stellar populations and the formationhistory of the bright Coma cluster early-type galaxy NGC 4889.
2010arXiv1006.2382C
Coccato, L., Arnaboldi, M., Gerhard, O., Freeman, K. C., Ventimiglia, G., Yasuda, N., 2010 A&A, in press (arXiv: 1006.1894).
Kinematics and line strength indices in the halos of the Coma Brightest Cluster Galaxies NGC 4874 and NGC 4889 .
2010arXiv1006.1894C
Churazov, E., Tremaine, S., Forman, W., Gerhard, O., Das, P., Vikhlinin, A., et al. 2010, MNRAS, 359.
Comparison of approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data.
2010MNRAS.tmp..359C
Doherty, M., Arnaboldi, M., Das, P., Gerhard, O., Aguerri, J. A. L., Ciardullo, R., et al. 2009, A&A, 502, 771.
The edge of the M 87 halo and the kinematics of the diffuse light in the Virgo cluster core. 2009A&A...502..771D
de Lorenzi, F., Gerhard, O., Coccato, L., Arnaboldi, M., Capaccioli, M., Douglas, N. G., et al. 2009, MNRAS, 395, 76.
Dearth of dark matter or massive dark halo? Mass-shape-anisotropy degeneracies revealed by NMAGIC dynamical
models of the elliptical galaxy NGC 3379. 2009MNRAS.395...76D
Coccato, L., Gerhard, O., Arnaboldi, M., Das, P., Douglas, N. G., Kuijken, K., et al. 2009, MNRAS, 394, 1249.
Kinematic properties of early-type galaxy haloes using planetary nebulae. 2009MNRAS.394.1249C
Napolitano, N. R., Romanowsky, A. J., Coccato, L., Capaccioli, M., Douglas, N. G., Noordermeer, E., et al. 2009, MNRAS, 393, 329.
The Planetary Nebula Spectrograph elliptical galaxy survey: the dark matter in NGC 4494. 2009MNRAS.393..329N
Churazov, E., Forman, W., Vikhlinin, A., Tremaine, S., Gerhard, O., Jones, C. 2008, MNRAS, 388, 1062.
Measuring the non-thermal pressure in early-type galaxy atmospheres: a comparison of X-ray and optical potential
profiles in M87 and NGC 1399. 2008MNRAS.388.1062C
de Lorenzi, F., Gerhard, O., Saglia, R. P., Sambhus, N., Debattista, V. P., Pannella, M., et al. 2008, MNRAS, 385, 1729.
Dark matter content and internal dynamics of NGC 4697: NMAGIC particle models from slit data and planetary nebula
velocities. 2008MNRAS.385.1729D
Das, P., Gerhard, O., Coccato, L., Churazov, E., Forman, W., Finoguenov, A., et al. 2008, AN, 329, 940.
The orbital structure of the massive elliptical galaxy NGC 5846. 2008AN....329..940D
Coccato, L., Gerhard, O., Arnaboldi, M., Das, P., Douglas, N. G., Kuijken, K., et al. 2008, AN, 329, 912.
Probing the kinematics of early-type galaxy halos using planetary nebulae. 2008AN....329..912C
Douglas, N. G., Napolitano, N. R., Romanowsky, A. J., Coccato, L., Kuijken, K., Merrifield, M. R., et al. 2007, ApJ, 664, 257.
The PN.S Elliptical Galaxy Survey: Data Reduction, Planetary Nebula Catalog, and Basic Dynamics for NGC 3379.
2007ApJ...664..257D
Sambhus, N., Gerhard, O., Méndez, R. H. 2006, AJ, 131, 837.
Kinematic Evidence for Different Planetary Nebula Populations in the Elliptical Galaxy NGC 4697. 2006AJ....131..837S
Napolitano, N. R., Capaccioli, M., Romanowsky, A. J., Douglas, N. G., Merrifield, M. R., Kuijken, K., et al. 2005, MNRAS, 357, 691.
Mass-to-light ratio gradients in early-type galaxy haloes. 2005MNRAS.357..691N