The Line-of-Sight kinematics of Low Redshift Cosmological Galaxies

XuFen Wu; Ortwin Gerhard (MPE, Garching); Michael Hilz, Thorsten Naab (MPA, Garching) Email: xwu@mpe.mpg.de

Abstract

We study the line-of-sight kinematics of 42 galaxies at z=0 from the re-simulation of cosmological mergers (Oser et al. 2010) out to large radii. The enclosed stellar mass within 10% rvir of these galaxies range from 2.0 x 1010 to 3.8 x 1011 Msun. There are a few thousands to one million particles for each galaxy. We compute time-averaged line-of-sight kinematics which avoids the fluctuations of particles in one snapshot. We predict line-of-sight velocity fields and velocity dispersion profiles of stellar particles for different inclination angles, and then obtain the angular momentum parameter λ profiles of the stars. We find that 75% of the galaxies in the simulation are intrinsically fast rotators (edge-on view) and the rest are slow rotators. The λ profiles for the galaxies are almost flat from 2 Re to 5 Re. The stellar mass (r < 10% rvir), ellipticity and λ are also correlated: the more massive systems have smaller values of λ, whereas the less massive systems have more angular momentum; rounder systems have smaller λ while most of the flatter systems have larger values of λ, also to large radii.

Introduction:

Mass distributions and Rotation Curves:

Line-of-sight Kinematics of Stars to large radii

Conclusions1. 75% of the sample of cosmological galaxies (Oser et al. 2010) are fast rotators.

2. Rotation curves are almost flat at large radii, especially for massive galaxies.

3. In the outer part of the cosmological galaxies (2 Re - 5 Re), the angular momentum profiles λ(R) are almost flat. Besides, the λ, ellipticity and stellar mass of the galaxies are correlated. The less massive galaxies have larger angular momentum and they are flatter, whereas the more massive galaxies have smaller angular momentum and they are rounder.

Reference : Bois et al, 2011, NMRAS in press; Coccato et al. 2009, NMRAS, 394,1249; de Lorenzi et al. 2007, NMRAS, 376,71; Emsellem et al 2007, MNRAS, 379, 401; Naab et al. 2007, ApJ, 658, 710; Oser et al. 2010, ApJ, 725, 2312

http://apod.nasa.gov/apod/ap090508.html

Angular Momentum profiles : Fast and Slow rotators

High resolution cosmology simulations have been available to study the formation of galaxies, the luminosity and mass-size evolution of galaxies. However, the internal structure and projected kinematics haven’t been well studied. The remnants of galaxies’ major mergers and multiple minor mergers resemble Early-Type (or Early-Type-like) Galaxies. The SAUSON survey introduced a way of classifying the ETGs - slow and fast rotators. The observations of Planetary Nebulae kinematics enables people to study the outer kinematics of the ETGs. It is very necessary to statistically analyze the kinematics of galaxies from cosmological simulations and compare with observations.

Oser et al. (2010) studied the high resolution mergers from cosmological re-simulations. The comoving gravitational softening length is 0.4kpc/h for the baryonic components, allowing us to study the kinematics of galaxies where the radius is larger than 0.5 Re, especially for the outer regions. The galaxies studied here are from remnants of cosmological mergers in Oser et al. (2010). Here we analyze a sample of galaxies from Oser et al. (2010), which do not have massive satellites at z=0. The time-averaged kinematics in the outer part of the low redshift galaxies (at z=0) from cosmological simulations are studied here, as well as the angular momentum profiles (λ profiles) of stellar components. The predictions can be compared to the observations of PNe kinematics in the outer stellar halos of galaxies (Coccato et al. 2009).

Circular velocity of all the galaxies vcirc in bins of vcirc(5 Re) are plotted in Figure 1. The rotation curves are flat at large radii (R> 2Re), especially for the massive galaxies. The slope of vcirc at 5 Re (Figure 2) confirms this.

Fig 1 (left): Circular velocity of all the galaxies in bins of circular velocity at large radii (5Re): [160,220), [220, 300), [300,400) km/s at 5Re. The massive galaxies (with vcirc(5Re) > 220 km/s) have flat rotation curves, while the small galaxies (upper panel) have mildly decreasing rotation curve at large radii.

We used NMAGIC code (de Lorenzi et al. 2007) as an N-body code to integrate the orbits of the particles for one circular orbit at 10 Re (~ 10% rvir). To reduce the fluctuation of the particles at each snapshot, the observables of stars Δj are time averaged by the following equation (de Lorenzi et al. 2007):

Fig 2 (right): The slope of circular velocity of all the galaxies versus vcirc in the outer part (5Re). The massive galaxies (with large values of vcirc) have zero slopes (i.e. flat rotation curves) while the small galaxies have slopes of vcirc between flat (0.0) and Keplerian. (-0.5).

We computed velocity fields vj , velocity dispersion σj , projected mass mPj and the root mean square velocity vrms,j in the jth grid cell in an angular grid with radius Rj for the 42 galaxies with different inclination angles.

Fig 4: Velocity (upper panels), dispersion (second line of panels) and v/σ (lower panels) maps for the stellar components of three typical galaxies: the left panels are for a slow rotator, the middle column of panels are for a fast rotator with declining λ-profile inside 2Re and the right panels are for a faster rotator with increasing λ in 2Re. The surface densities of the stars (contour levels are: 107.5, 108.0, 108.5, 109.0, 109,5 Msun/kpc2) are overplotted on the v/σ maps.

Fig 3: The root mean square velocities vrms o f t h e s t e l l a r components of 42 galaxies in vcirc(5Re) bins. The vrms(R) is normalized by the vrms at 1 Re. The solid lines are for the fast rotators while the dotted lines are for the slow rotators. There is only one slow rotator when t he v c i rc ( 5Re ) i s lower than 220 km/s.

We then computed the stellar projected angular momentum λ (Eesellem et al. 2007) from the temporally smoothed observables:

!(R) =Ngrids!

j=1

mPj Rj |vj |

mPj Rj

"v2

j + "2j

The early-type galaxies can be classified into two families with the λ values at Re (Eesellem et al. 2007) : λ(Re)>0.1 are fast rotators and λ(Re)<0.1 are slow rotators.We found that among 42 galaxies, 32 are intrinsically fast rotators (from the edge-on view) and 10 are slow rotators.

Fig 5 (above): The λ-profiles binned with vcirc(5Re) with inclination angles 90 deg (edge-on, left panels) and 0 deg (face-on, right panels). The solid lines are for fast rotators whereas the dotted lines are for slow rotators. The slow rotators in general have small rotational speed at large radii.

Fig 6 (right): Upper left panel: local v/σ (Re) versus local v/σ (5Re) on the edge-on view; Upper right panel: λ (Re) versus λ (5Re). The two panels show that there is a correlation between the central rotation and stellar halo rotation, especially for the flattest galaxies.The lower panels show the local v/σ versus ellipticity at Re (left) and 5Re (right) on the edge-on view. The colours show the enclosed stellar mass in 10% rvir. The more massive galaxies are rounder and have smaller angular momentum, while most of the less massive galaxies are flatter and have more angular momentum at both small and large radii.

The 42 galaxies are binned with their circular velocity vcirc at 5Re. It is clear that most of the λ-profiles are flat for R > 2Re (Fig. 5). There is a strong correlation (upper right panel of Fig. 6) of λ(Re) and λ(5Re), which confirms the flat λ-profiles. There are two types of fast rotators: one has increasing λ within 2Re and flat λ at large radii, the other has decreasing λ within 2Re. In general, massive galaxies have smaller λ-profiles and most of the slow rotators have large values of vcirc(5Re).

T h e l o c a l v /σ ( i . e . , n o t accumulated with radius) shows that at 1Re and 5Re, the e l l i p t i c i t y a n d s p i n a r e correlated: rounder galaxies have smaller v/σ values while flatter galaxies have larger v/σ values.There is also a correlation between ellipticity and stellar mass (r<10% rvir) : Smaller galaxies are flatter and more massive galaxies are rounder.