Kinematics of planetary nebulae in the outskirts of galaxies, from slitless FOCAS radial velocities

Roberto H. MendezInstitute for Astronomy,

University of Hawaii, Honolulu

Collaborators:

Ana M. Teodorescu (IfA, Univ. of Hawaii)

Rolf-Peter Kudritzki (IfA, Univ. of Hawaii)

Lent C. Johnson (University of Wyoming)

List of topics

• Motivations: kinematics of elliptical galaxies• Basic PN discovery technique• Slitless velocities with Subaru + FOCAS• Calibration mask and the local PN NGC 7293• New PNs discovered in NGC 4697• On the Keplerian decline of line-of-sight velocity

dispersion (losvd)• Future activities

Motivation• Kinematics of elliptical galaxies are difficult to study; they lack cold

H gas, and absorption-line studies of the integrated stellar spectrum are restricted to the central regions, where the surface brightness is sufficiently high.

• At the same time, any information about the dynamical behavior of the outer regions of ellipticals is valuable to test and even to guide theoretical ideas about galaxy formation. For example, ellipticals are expected to be surrounded by dark matter halos, and to show high rotation in the outskirts.

• Planetary nebulae (PNs) are excellent kinematic probes for dark matter and angular momentum distribution, particularly at large angular distances from the galaxy’s center, where they are easier to discover.

The Subaru + FOCAS version

• Dispersing element: echelle grism, used in 4th order. We get 0.5 Angstroms/pixel.

• Calibration of displacement using a rigid mask with approximately 1000 holes.

• Width of images: if 5 pixels, 140 km/s.

• Error in velocities: if 0.4 pixel, 10 km/s.

NGC 4697

FORS

(ESO VLT, Chile)

and

FOCAS

fields

Calibration Fun

Thorium-Argon Lamp

through an

Engineering Mask

970 points

100 pixel spacing

5028.655 Å

5017.254 Å

5009.350 Å5002.097 Å

5039.230 Å

Radial velocity calculations◊ Identify four closest calibration points that form a square around PN

◊ Calculate the redshifted wavelength of the [O III] emission feature at each of the four points

◊ Obtain the final wavelength by using a bilinear interpolation calculation – combining the four values according to the spatial relation of each of the 4 calibration points to the PN

◊ Final wavelength and heliocentric correction are used to calculate the radial velocity

NGC 7293, a local PN

Quality control using NGC 7293

1.Illuminate the calibration mask using NGC 7293 and take an image;

2.Insert the grism and take another image;3.Illuminate the mask using the Th-Ar lamp and take

another image through the grism.

1 and 2 give displacements that can be transformed into radial velocities. We call these “slitless” velocities.

2 and 3 give a velocity for each hole in the mask,using the holes as slits. We call these “classical” velocities.

Slitless vs. classical velocities

FORS vs. FOCAS

• Earlier data obtained with FORS + VLT (ESO, Cerro Paranal, Chile) yielded velocities for 535 PNs in the central regions (Mendez et al. 2001 ApJ 563, 135)

• New FOCAS data (218 PNs) in two fields partly overlapping the FORS data but extending outwards along the major axis of NGC 4697

FORS vs FOCAS velocities

We compare 162 PNs in NGC 4697, measured with both FORS and FOCAS. The FORS errors are 35 km/s. Comparing with random number simulations, we confirm that the FOCAS errors are 10 km/s.

Radial vels of PNs in NGC 4697

The 218 slitless PN velocities measured with FOCAS, as a function of projected distance along the major axis. The effect of rotation is visible, and the line-of-sight velocity dispersion decreases markedly outside. The solid lines indicate the escape velocity due to visible matter.

Rotation in NGC 4697

• Plus signs: absorption-line data

• Diamonds: PN data

As expected, the PNs rotate in the same sense as the stellar population. We detect no significant rotation beyond 5 Re.

NGC 4697 is almost edge-on

Line-of-sight velocity dispersion in NGC 4697

• Plus signs: absorption-line data

• Squares: PN data. The outer 4 points are based on the FOCAS velocities.

The solid line is a Hernquist model with a constant M/L ratio and a total mass of 1.5x10^11 Msun.

We find no evidence of dark matter within 5 Re.

Other cases Romanowsky et al. (2003 Science, 301, 1696) using the PN.S at La

Palma (Herschel telescope) have reported more examples of elliptical galaxies showing no evidence of dark matter halos:

• NGC 3379 (very convincing)• NGC 4494 and NGC 821 (not so convincing, more data are needed)

They originally claimed definite proof of absence of dark matter, but all these cases, including NGC 4697, can be interpreted as a consequence of radial anisotropy (predominance of radial orbits implies that far from the center we see less dispersion than for an isotropic system). More advanced modeling is in progress: De Lorenzi et al. 2007, MNRAS 376, 71 and 2008, MNRAS in press.

Not all ellipticals misbehave

There are some intermediate-mass ellipticals that do show evidence of a dark matter halo:

NGC 5128 (Hui et al. 1995 ApJ 449, 592) confirmed by Peng, Ford and Freeman 2004, ApJ 602, 685. This one shows also significat halo rotation.

NGC 1344 (Teodorescu et al. 2005, ApJ 635, 290).

So now we have to answer another question: how frequent is the Keplerian behavior?

At the present time

• As we collect more information, a variety in the amount and/or distribution of dark matter seems to be emerging. So far, the FOCAS data on PNs in NGC 4697 provide the best evidence of a Keplerian decline in the kinematics of an elliptical galaxy. We also confirm the same behavior in NGC 821 (Teodorescu et al., in preparation).

• Massive giant ellipticals: Gerhard et al. (2001 AJ 121, 1936) have reported a spread in the luminous-to-dark matter ratio.

• Are we seeing the same kind of behavior in “ordinary” ellipticals?

Alternatively

If we insist that dark matter has to be present everywhere, then…

in some cases these galaxies have such peculiar kinematics that they manage to conceal their dark matter halos; we can try to build consistent dynamical models and learn about their properties.

Next FOCAS project

We would like next to study PNs in the Virgo cluster elliptical M 60. This galaxy is more massive, and it shows hot X-ray emitting gas, giving direct evidence of the existence of a dark matter halo. Will the PNs show a correspondingly high losvd in the outskirts? Our first observing run in 2007 was affected by bad seeing, so we detected only about 40 PNs. We will try again next May.