Download PPT presentation

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 PNs
3. Contamination of the PN samples
4. Searching for PNs in regions of recent
massive star formation
5. Deep spectroscopy of PNs in the elliptical
galaxy NGC 4697
6. 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 offband. 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 nonPNs 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-surfacedensity 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 ESOMPI 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