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

Elliptical galaxies with rapidly decreasing velocity dispersion profiles: NMAGIC models and dark halo parameter estimates for NGC 4494

We present here dynamical models for NGC 4494, one of the intermediate-luminosity elliptical galaxies inferred to have an unusually diffuse dark matter halo. These axisymmetric models were obtained with the NMAGIC made-to-measure method, fitting photometric and kinematic data for different inclinations and gravitational potentials. The extended kinematic data include light spectra in multiple slitlets out to 3.5Re, and hundreds of planetary nebula velocities out to ≃7Re, thus allowing us to probe the dark matter content and orbital structure in the halo.

We carried out an extended study on how well the characteristic parameters of dark matter halos can be recovered via NMAGIC modelling of these data, using Monte-Carlo simulations on NGC 4494-like mock galaxies to estimate the appropriate confidence boundaries.

Our best-fitting models have a dark-matter fraction of 0.6 (+-0.1) at 5 Re with a 70% confidence level, and an approximately flat circular velocity curve (CVC) outside 0.5 Re. Comparing with two other quasi-Keplerian ellipticals, NGC 3379 and NGC 4697, whose velocity dispersion profiles also decrease rapidly from the centre outwards, the outer CVCs and dark matter haloes are quite similar. NGC 4494 shows a particularly high dark matter fraction inside 3Re, and a strong concentration of baryons in the centre.
Parameter range
Range of valid models for NGC 4697 (yellow), NGC 3379 (green) and NGC 4494 (magenta). From top to bottom as a function of radius: circular velocity, dark matter fraction, and anisotropy parameter. Complete paper on ADS


Regularizing made-to-measure particle models of galaxies

Made-to-measure methods such as the parallel code NMAGIC have to solve underdetermined problems, where the number of constraints (photometric of kinematic observables) are usually much lower than the number of particle weights to define. A regularization method is therefore needed to make the method converge. Here we introduce a Moving Prior entropy Regularization method (MPR). The basic idea is to update the prior distribution needed by standard entropy regularization in parallel of the weight adaptation. The prior distribution is determined from the distribution of particles in phase-space. This allows one to construct smooth models from noisy data without erasing global phase-space gradients.
Final weight distribution
Typical particle weight distribution after a classical entropy regularization fit (black) and after Moving Priors entropy Regularization (red). MPR method avoid trails of extremely increased or decreased weights.

Priors smoothing
Particle model fits to the SAURON integral field kinematic data for NGC 3379. Top rows are the symmetrized SAURON data, middle rows are the best NMAGIC fit with classical entropy regularization and bottom rows are the best NMAGIC fit using Moving Priors entropy Regularization. Complete paper on ADS


Dynamical models of  NGC 4697 and NGC 3379

Circular velocity curves of NGC 3379Intrinsic kinematics of NGC 3379
Circular velocity curves of NGC 4697Internal kinematics of NGC 4697
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.


Gravitational potentials of nearby, massive elliptical galaxies

Potential of M87Potential of NGC 1399

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.


Distinct core and halo stellar population in the bright coma cluster galaxy NGC 4889

Models
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.

Poster

pdf


Publications

Morganti, L., Gerhard, O., Coccato, L., Martinez-Valpuesta, I., Arnaboldi, M. 2013,  MNRAS,  431,  3570.   
Elliptical galaxies with rapidly decreasing velocity dispersion profiles: NMAGIC models and dark halo parameter estimates
for NGC 4494. 2013MNRAS.431.3570M

Lyskova, N., Churazov, E., Zhuravleva, I., Naab, T., Oser, L., Gerhard, O., et al. 2012,  MNRAS,  423,  1813.   
Testing a simple recipe for estimating galaxy masses from minimal observational data. 2012MNRAS.423.1813L 

McNeil-Moylan, E. K., Freeman, K. C., Arnaboldi, M., Gerhard, O. E. 2012,  A&A, 539,  A11.   
Planetary nebula kinematics in NGC 1316: a young Sombrero. 2012A&A...539A..11M

Das, P., Gerhard, O., Mendez, R. H., Teodorescu, A. M., de Lorenzi, F. 2011,  MNRAS, 415,  1244.   
Using NMAGIC to probe the dark matter halo and orbital structure of the X-ray bright, massive elliptical galaxy, NGC 4649.
2011MNRAS.415.1244D 

Napolitano, N. R., Romanowsky, A. J., Capaccioli, M., Douglas, N. G., Arnaboldi, M.,
Coccato, L., et al. 2011,  MNRAS,  411,  2035.   
The PN.S Elliptical Galaxy Survey: a standard ΛCDM halo around NGC 4374?. 
2011MNRAS.411.2035N 

Das, P., Gerhard, O., Churazov, E., Zhuravleva, I. 2010,  MNRAS,  409,  1362.   
Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies. 2010MNRAS.409.1362D 

Coccato, L., Gerhard, O., Arnaboldi, M. 2010,  MNRAS,  407,  L26.   
Distinct core and halo stellar populations and the formation history of the
bright Coma cluster early-type galaxy NGC 4889. 2010MNRAS.407L..26C 

Ventimiglia, G., Gerhard, O., Arnaboldi, M., Coccato, L. 2010,  A&A,  520,  L9.   
The dynamically hot stellar halo around NGC 3311: a small cluster-dominated central galaxy. 2010A&A...520L...9V 

Coccato, L., Arnaboldi, M., Gerhard, O., Freeman, K. C., Ventimiglia, G., Yasuda, N. 2010,  A&A,  519,  A95.   
Kinematics and line strength indices in the halos of the Coma brightest cluster
galaxies NGC 4874 and NGC 4889. 2010A&A...519A..95C 

McNeil, E. K., Arnaboldi, M., Freeman, K. C., Gerhard, O. E., Coccato, L., Das, P.
2010,  A&A,  518,  A44.   
Counter-dispersed slitless-spectroscopy technique: planetary nebula velocities in the halo of NGC 1399. 2010A&A...518A..44M 

Churazov, E., Tremaine, S., Forman, W., Gerhard, O., Das, P., Vikhlinin, A., et al.
2010,  MNRAS,  404,  1165.   
Comparison of approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data.
 2010MNRAS.404.1165C  

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