Planetary Nebulae and the Extragalactic Distance Scale

Planetary Nebulae Planetary Nebulae

and the and the

Extragalactic Distance Extragalactic Distance

ScaleScaleRobin Ciardullo

Why Measure Distances?The HST Key Project and WMAP say that H0 = 72 8 km/s/Mpc. But …

We still need distances to individual objects (AGN, MOND galaxies, etc.)

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Hf

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For precision cosmology, we need to know H0 to better than 10%. Note also that WMAP only constrains H0 if the universe w = P / = 1.

The current distance scale may contain systematic errors.

Why Observe Planetary Nebulae?

They’re very bright !!

The brightest PN in a galaxy have luminosities greater than 6000 L!

They can routinely be identified out to ~ 20 Mpc

From Dopita et al. (1994)


They’re easy to detect!



10% of the energy comes out at 5007 Å



[O III] 5007 Offband 5300[O III] Difference




They’re a high precision distance indicator.

M87

M86 M84

There is at least one [O III]-bright PN for every ~ 108 L. For large galaxies, the internal errors are a few percent!


They’re present in all galaxies!

E0 E3 S0

Sa

SbSc

SBa

SBbSBc

SBm Irr

The History of PN as Distance Indicators

• 1966: First Suggestion PN as Distance Indicators [Hodge]

In “Galaxies and Cosmology” by Hodge (McGraw-Hill 1966)

• 1966: First Suggestion PN as Distance Indicators [Hodge]• 1978: First PN-based distance estimate [Ford & Jenner]

The History of PN as Distance Indicators

• 1981: First use of PN for Local Group [Jacoby & Lesser]

• 1989: First use of Planetary Nebula Luminosity Function [Ciardullo et al.]

• 1990: First PNLF-based Hubble Constant [Jacoby et al.]• 1990: First Use of the PNLF in the Galaxy [Pottasch]

Reason for slow acceptance of method:individual PN are definitely not standard candles.

(Distances to Galactic PN are uncertain to factors of ~ 5)

The Method• Image the galaxy

through a redshifted [O III] 5007 filter



• Identify point sources that are invisibleinvisible in the continuum




• If the galaxy has star-formation, compare [O III] 5007 to H





• Perform photometry on a complete sample of objects

M5007 = 2.5 log F5007 – 13.74





• Perform photometry on a complete sample of objects

• Fit to an empirical function

M5007 = 2.5 log F5007 – 13.74

N (M) e 0.307 M {1 – e 3 ( M* – M ) }

The Calibration of M*

* Denotes metal-poor, low-luminosity galaxy

SMC* M33 M101 NGC 3351 NGC 3627

LMC M81 NGC 300* NGC 3368 NGC 4258

M31 NGC 2403 NGC 5253*

The PNLF cutoff magnitude, M*, is calibrated via measurements in galaxies with known Cepheid distances.

• M* is a constant for large, metal-rich galaxies

• Young and old populations have the same value of M*

• M* is fainter in small, low-metallicity galaxies.

This dependence was predicted by Dopita et al. (1992)!

M* = – 4.47

This consistency across > 1 dex in O/H strongly suggests that neither the Cepheids nor the PNLF need additional metallicity corrections.

M* = – 4.47

Are there systematic errors associated with the PNLF method? To answer this question, we can …

Testing the Method

• Perform Consistency Tests within Galaxies

• Perform Consistency Tests inside Galaxy Groups and Clusters

Can we find a case where the PNLF fails???

Tests within a Galaxy

• Five galaxies have large enough samples of PN to test for systematic shifts in the PNLF: M31 M81 M33 NGC 5128 NGC 4494

In these galaxies, the PNLF samples within galaxies always produce consistent distances

Samples of PN in M31

The bulge, disk, and halo PNLFs are indistinguishable

(M* < 0.05 mag)

Tests within a Cluster

• Six Galaxy Groups have PNLF distances to multiple galaxies: M81 Group: M81, NGC 2403 NGC 1023 Group: NGC 891, 1023 NGC 5128 Group: NGC 5102, 5128, 5253, (M83) Fornax Cluster: NGC 1316, 1380, 1399, 1404 Leo I Group: NGC 3351, 3368, 3377, 3379, 3384 Virgo Cluster: NGC 4382, 4472, 4486, 4649

(4374, 4406)

PNLF distances within clusters are always consistent to within ~ 1 Mpc, with no systematic behavior

All 5 galaxies within 1 Mpc Group Diameter

The Leo I GroupNGC 3379

NGC 3384

NGC 3377

NGC 3351

NGC 3368

Background NGC 4406 Group Resolved

The Virgo ClusterNGC 4649

NGC 4406

NGC 4374

NGC 4486

NGC 4382

NGC 4472

Does the PNLF ever Fail?

Actually yes – in the Virgo Cluster, there are intracluster stars!!!

Intracluster PN can be foreground to the target galaxy, and therefore appear “overluminous”.

Are Overluminous PN really Intracluster?

The density of PN within a galaxy should follow that of the galaxy’s light. Intracluster PN should not; they should scale with the area surveyed. “Overluminous” (foreground) objects should therefore be (primarily) found in the outskirts of galaxies.

0.25 mag

The Identification of Intracluster StarsIntracluster stars are not confined to the center of the Virgo Cluster. They’ve recently been found foreground to NGC 4526.

Are there Systematic Errors in the Distance Scale?

The PNLF and Surface Brightness Fluctuations

Cepheids provide the calibration for both the PNLF and the SBF method. Presumably, the distances of the two methods agree.

PNLF vs. SBF Distances

The PNLF zero point is calibrated via measurements in 13 galaxies with distances determined from Cepheids

The SBF zero point is calibrated via measurements in 6 galaxies with distances determined from Cepheids

How well do the two distance scales agree?

mean ~ 0.05 mag mean ~ 0.05 mag

PNLF vs. SBF Distances (29 Galaxies)

• The curve is the expected scatter in the measurements

• There are 3 outliers. The two worst are:

The zero points differ by ~ 0.15 mag! We have found a systematic error!

Most Likely Explanation – Internal Extinction

To derive the PNLF cutoff magnitude, M*, using Cepheid distances, one needs to know the reddening

If the reddening is underestimated, then M* is underestimated, and the inferred distance scale is too small.

M* = m* - Cep – R5007 E(B-V)

For the SBF method, however, the absolute fluctuation magnitude, MI, depends on color, i.e.,

So the zero point of the system is defined through

If the reddening is underestimated, then the color of the galaxy is overestimated, MI is overestimated, and therefore C is overestimated. The result is a distance scale that is too large.

MI = C + 4.5 (V-I)0

C = mI - Cep- RI E(B-V) - 4.5(V-I)obs + 4.5 E(V-I)

Because the SBF and PNLF methods react to reddening in opposite directions, a small amount of internal extinction can lead to a large discrepancy between the two distance scales:

= 4.2 E(B-V) = 7.7 E(B-V)

Only E(B-V) ~ 0.02 is needed to reconcile the PNLF and SBF distance scales

If only the SBF calibration is affected

If both the PNLF and SBF calibrations are affected

This error propagates up the entire distance ladder!

The PNLF and SN Ia

Until recently, the sample of galaxies with well-observed Type Ia supernovae and PNLF measurements was too small to be useful.

This is now changing.

PNLF vs. SN Ia Distances

• The PNLF calibration of the SN Ia distance scale agrees well with that of the Cepheids.

• (Any systematic error between the scales is less than ~ 4%)

(from Feldmeier, Philips, & Jacoby 2005)

PNLF, Cepheids, and GeometryThe PNLF distance scale is calibrated by Cepheids, and the

Cepheid scale assumes an LMC distance of (m-M)0 = 18.50. This can be checked via 2 geometric distance measurements.

The LMC (SN 1987A Light Echo)

A key benchmark of the extragalactic distance scale is the light echo measurement of SN 1987A. The classical analysis by Panagia et al. (1991) gives a distance of 51.2 3.1 kpc.

A more complex model by Gould & Uza (1998) gives a lower distance of D < 47.2 0.9 kpc.

Herrnstein et al. (1999) have analyzed the proper motions and radial velocities of NGC 4258’s nuclear masers. The orbits are Keplerian and yield a distance of 7.2 0.3 Mpc.

NGC 4258 (Nuclear Maser)

LMC NGC 4258

Method4258 - LMC

Geometry 18.50 0.05 29.29 0.09 10.79 0.10

Cepheids 18.50* 29.44 0.12 10.94 0.12

PNLF 18.50 0.11 29.43 0.09 10.93 0.14*Cepheid and PNLF values are based on (m-M)LMC = 18.50.

Ratio

Geometry vs. Cepheids vs. PNLF

LMC NGC 4258

Method4258 - LMC

Geometry < 18.37 0.04 29.29 0.09 10.92 0.10

Cepheids 18.50* 29.44 0.12 10.94 0.12

PNLF 18.50 0.11 29.43 0.09 10.93 0.14*Cepheid and PNLF values are based on (m-M)LMC = 18.50.

Ratio

Geometry vs. Cepheids vs. PNLF

The perfect agreement between the relative distances argues for a short distance to the LMC and a Hubble Constant that is 7% larger than the Key Project value (77 km s-1 Mpc-1).

Why Does the PNLF Work???The physics behind the PNLF is still controversial. However, there are clues …

• A PN’s [O III] 5007 luminosity depends on the luminosity of its central star. But there are mechanisms that can place a limit on the maximum [O III] flux a PN can emit.

• Collisional de-excitation of forbidden emission in young, dense nebulae




• Circumstellar extinction around massive (high luminosity) central stars









Why Does the PNLF Work???The real problem comes from the absolute luminosity of the PNLF cutoff …

M* = 4.47 corresponds to a luminosity of 600 L

To produce 600 L of [O III] emission, a central star must have a luminosity of L > 6,000 L.

A central star with L > 6,000 L must be more massive than M > 0.6 M. Such stars come from M > 2 M progenitors.

(Weidemann 2000)

Why Does the PNLF Work???

Elliptical galaxies do not have many (any?) 2 M main sequence stars. But they do have large numbers of 1 M stars. If some are in close binary systems which coalesce on the main sequence, the product may evolve into an [O III]-bright planetary.

The ratio of bright planetaries to blue stragglers is about equal to the ratio of the objects’ lifetimes.

Carrera et al. 2002

Summary

The Planetary Nebula Luminosity Function continues to be a useful tool for extragalactic astronomy and cosmology.

• The PNLF is the only standard candle capable of measuring distances to all the large galaxies of the local supercluster.

• The PNLF cutoff, M*, is the same for old and young populations.

• PNLF comparisons with Surface Brightness Fluctuations and Cepheid measurements suggest that small systematic errors in the distance scale still exist.

• The PNLF and Cepheid calibrations of SN Ia are in good agreement.

• The brightest PN in E/S0 galaxies may be the product of binary star evolution. But they are NOT binaries.

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Planetary Nebulae and the Extragalactic Distance Scale