Deep PN surveys in galaxies beyond the Local Group

Roberto H. Mendez

Institute for Astronomy, Honolulu

Outline of this talk:

1. Motivations for deep surveys beyond the Local Group

2. How to find the PNs3. Contamination of the PN samples4. Searching for PNs in regions of recent

massive star formation5. Deep spectroscopy of PNs in the elliptical

galaxy NGC 46976. The shape of the PN luminosity function

1. Motivations for deep surveys beyond the Local Group

• Distance determination using the PN luminosity function (PNLF).

• PNs as kinematic probes: dark matter and angular momentum distribution. PN detections at large angular distances from the galaxy’s center are especially valuable; they provide unique information.

• Intracluster PNs; tracers of an intergalactic population of stars presumably lost to the cluster galaxies by close non-destructive interactions.

• Abundances and abundance gradients from PNs in elliptical galaxies; probing abundances of individual stars as a complement to interpretation of integrated light.

2. How to find the PNs

The previous slide showed the basic idea for PN searches: to take images through an on-band filter passing the redshifted [O III] 5007 and an off-band filter passing no nebular emission lines.

Next slide shows an example in NGC 4697. To the on-band and off-band images a third image is added, taken through the on-band filter plus a grism.

Searching

with grisms

helps the

identification

and gives

information

about radial

velocities

Next slide shows how to calibrate the displacement produced by the insertion of the grism into the light path as a function of wavelength and position on the CCD.

(Taken from Mendez et al. 2001, ApJ, 563, 135).

3. Contamination of the PN samples

The main contaminants are background sources with emission lines redshifted into the on-band filter: [O II] 3727 at z=0.35 or Ly alpha at z=3.1.

The contamination increases as the surface density of PNs decreases. Negligible near the galaxy’s center, the contamination may become a problem in the intracluster medium.

Contamination in NGC 4697 A survey for PNs in this elliptical galaxy produced 535 detections (Mendez

et al. 2001, ApJ, 563, 135). In addition, some 20 objects of different kinds were found, namely:• Many extended sources, presumably star formation regions at low redshift.

Those pose no confusion problem because of their extended nature.• Two point sources bright in the on-band and fainter but visible in the off-

band. Could be high-z quasars or star forming regions or even a PN within a globular cluster in NGC 4697, as reported recently in the case of NGC 5128 by Minniti and Rejkuba. Again no confusion is possible since the continuum is visible.

• Three point sources invisible in the off-band. They were recognized as non-PNs because their radial velocities were wrong for NGC 4697. Another example of the usefulness of the grism images.

Therefore we can expect a few more of those, but with the right velocities; if they exist, we cannot distinguish them from PNs without more data. This level of contamination (a few in more than 500) is not important for our purposes.

Radial velocities

of 535 PNs in the

elliptical galaxy

NGC 4697

(Mendez et al. 2001,

ApJ, 563, 135)

Contamination of the PN samples

The contamination increases with distance to the galaxy. It is negligible for distances < 10 Mpc, as we just saw.

At the distance of the Virgo cluster the luminosity function of the contaminants imitates the PNLF of a low-surface-density PN population at that distance. This produced some confusion in early studies of the Virgo intracluster PNs (ICPNs).

It turns out that the surface density of Virgo ICPNs is not constant; in some places there are no PNs and all you detect are the contaminants. This is frustrating but it teaches us that the distribution of the ICPNs is far from relaxed in Virgo.

Contamination of the PN samples

Conclusion: in deep searches for intracluster PNs we need spectroscopic confirmation: i.e. we need to detect at least 2 emission lines to confirm the redshift (e.g. 4959, 5007).

The worst news is that working with PNs beyond 25 or 30 Mpc is going to be hard; the contaminants are in that case brighter than the PNs, and just a few of them can deform the PNLF. Spectroscopic confirmation will be essential; that will cost in terms of telescope time.

4. Searching for PNs in galaxies with recent massive star formation

Normally we would rather avoid these galaxies because of the confusion with H II regions ionized by massive hot stars.

If we just want a PNLF distance (e.g. to compare with the cepheid distance) then it is easy to restrict the search to regions without recent star formation, as done by Feldmeier, Ciardullo and Jacoby 1997, ApJ, 479, 231.

However, if we are interested in finding e.g. how the PNLF shape depends on stellar population effects, then we must purposefully search for PNs in difficult environments. Two methods have been proposed to disentangle PNs from unresolved massive star H II regions:

1. Central star not visible, while massive star is visible (Soffner et al. 1996, A&A, 306, 9).

2. Massive star H II regions tend to be of low excitation, while PNs at the bright end of the PNLF are of higher excitation (e.g. Ciardullo et al. 2002, ApJ, 577, 31).

PNs and H II regions in NGC 300

In collaboration with Wolfgang Gieren (Concepcion, Chile) we have taken images of PNs in the late spiral NGC 300 with the ESO-MPI 2.2m telescope + WFI.

We used 3 filters: on-band, off-band, Halpha. The images I will show were produced assigning blue color to the on-band, red to the off-band, green to Halpha. Therefore the stellar population looks reddish, high excitation is blue and low excitation is green.

The PNs are easily recognized as “pale blue dots”. But we also find several cases of high-excitation massive star H II regions (blue, extended). Therefore in principle there is a risk of contamination by compact, high-excitation massive star H II regions. The next two images show additional examples.

Conclusion

It looks dangerous to depend solely on the excitation criterion. The survey should be deep enough to ensure detection of individual massive hot stars in the offband. If that is accomplished, then we do not need to rely on the nebular excitation level.

5. Deep spectroscopy of PNs in NGC 4697

I am collaborating with Daniel Thomas, Roberto Saglia, Claudia Maraston, Rolf Kudritzki and Ralf Bender in an attempt to obtain information about abundances and abundance gradients in elliptical galaxies from PNs.

The idea is to compare with the data inferred from long-slit spectroscopy of ellipticals, and eventually extend the gradient to at least 3 effective radii (the usual limit for integrated light spectra is one effective radius).

We chose NGC 4697 because we had already found PNs (Mendez et al. 2001, ApJ, 563, 135).

Long-slit spectrograms

The following slide shows information extracted from long-slit FORS spectra of NGC 4697 along the major axis. In particular the central panel shows the run of metallicity as a function of the projected distance to the center of light of the galaxy. Plus signs and diamonds show the run in opposite directions from the center. From this figure we would predict a [Z/H] of about -0.6 beyond log r = 2.

Abundances of PNs in NGC 4697

We selected 14 bright PNs along the major axis of this flattened elliptical galaxy.

The multi-object spectral exposures were taken with the VLT + FORS1 (3 nights in 2002) and with Keck I + LRIS-B (1 night in 2003). The distance to NGC 4697 is about 10 Mpc. Next slide shows the positions of the PNs.

Average of the FORS1 and LRIS-B spectra of the 14 objects

Average of the LRIS-B spectra of the 14 objects

Very strong 5007/Hbeta ratios

We find many cases of 5007/Hbeta ratios near 20. Although we could not detect the diagnostic line [O III] 4363 in individual spectra, we can argue anyway that those PNs with strong 5007 must have O and Ne abundances near solar. The argument can be illustrated using the following figure from Dopita et al. 1992, ApJ, 389, 27. We have confirmed our conclusion using dozens of CLOUDY runs for a variety of nebular and central star parameters.

Conclusions

• We find lower limits for O and Ne abundances of 4 PNs beyond 2 effective radii from the center of NGC 4697. The lower limits are a bit below, but near solar values. This is much higher than expected from the gradient as measured from the integrated light spectra.

• We cannot say much about the abundances closer to the center because weak 5007 is not incompatible with supersolar metallicities. The gradient can very well be real.

• We are still discussing these results, but they probably mean there must be a large spread in metallicity in the stellar population. The PNs provide direct evidence of the existence of a metal-rich population far from the center of NGC 4697.

6. The shape of the PNLF

This is a last-minute addition to the presentation, triggered by George Jacoby’s description of evidence that the PN luminosity function has a “camel shape” or rather a deficit of PNs beyond the famous bright end used for distance determinations. In fact this kind of PNLF shape is predicted naturally from Monte Carlo simulations if we assume that most central stars are H-burners.

Monte Carlo simulations of PNLFs

(Mendez and Soffner 1997, A&A, 321, 898)

• Generate a set of central stars with random post-AGB ages and masses.

• Using evolutionary tracks, derive the corresponding central star luminosities and surface temperatures.

• Using recombination theory and empirical information about leaking of H-ionizing photons, calculate the nebular Hbeta luminosities.

• Assuming a distribution of ratios 5007/Hbeta, obtain the luminosities in 5007 and compute the PNLF.

Next slide shows a simulation of a collection of 1500 central stars burning H. The evolutionary tracks show a quick drop in luminosity as the H-burning shell is extinguished and the star goes into the white dwarf cooling track. For that reason there is a lack of central stars at log L between 2.5 and 3. This lack of central stars offers a natural explanation for the lack of intermediate-brightness PNs.

PNLF shapes

The next two slides show the analytical function used by Ciardullo et al. to fit the PNLF, compared with the PNLF shape predicted by the Monte Carlo simulations. The simulations clearly show the PN deficit at intermediate luminosities. The final shape of the PNLF is determined mostly by the evolutionary tracks and by how many H-ionizing photons are leaking from the PNs. For more details, please refer to Mendez 1999, in “Post-Hipparcos Cosmic Candles”, eds. Heck and Caputo, Kluwer, p. 161

PNLF shapes

More PNLF shapes