The Largest Planetary Nebula on the Sky
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ING Newsletter No. 8, September 2004

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Reference: ING Newsl., No. 8, page 6 - 8.
Article mirrored at: La Palma server | Cambridge server
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The Largest Known Planetary Nebula on the Sky

Paul Hewett* and Mike Irwin (IoA, Cambridge)

The enormous Sloan Digital Sky Survey (SDSS) spectroscopic catalogue has many applications but the discovery of Planetary Nebulae (PN) had not been recognised as among the potential scientific returns. However, the INT recently played a key role in the identification of a record breaking PN discovered serendipitously from the SDSS.

The vast majority of PN in our own galaxy have been identified via wide-field narrow-band Hα surveys of the type currently ongoing using the INT ( North/) or through wide-field low-resolution slitless spectroscopic surveys, with both techniques attempting to isolate objects showing very high equivalent width emission lines that are characteristic of PN. The potential of the relatively high-resolution, pointed spectra that make-up the SDSS spectroscopic database involved a serendipitous observation during the course of a search for high-redshift gravitational lenses. The idea behind the gravitational lens search is to target luminous (massive) galaxies at intermediate redshifts, 0.2<z<0.6, which constitute the optimal line-of-sight for detecting gravitationally lensed background sources (Hewett et al., 2000). The population of high-redshift star-forming galaxies, many of which possess strong Lya emission, provide a high surface density of readily detectable background sources. The first such object was discovered by Warren et al. (1996) and the application of the SDSS survey for lenses at lower redshifts has been demonstrated by Bolton et al. (2004).

Examining the results of an automated search of the SDSS DR1 spectroscopic database for emission lines from putative high-redshift sources, one particular galaxy showed an unambiguous emission line detection with a somewhat weaker feature to the blue. The emission line pair was immediately identifiable as emission from [OIII] 4959, 5007. Not an entirely unexpected occurrence but the unusual feature of the detection was that the wavelength of the detection placed the emission at essentially zero radial velocity. Querying the output of the emission line search for similar detections produced more spectra showing a similar signature. All of the objects possessing [OIII] emission occurred in an approximately circular region with a diameter of ~1.5°, with not a single detection anywhere else on the sky. Investigation of SDSS spectra of stars, quasars and even sky fibres revealed further detections, all concentrated in the same region of sky.

A series of checks fairly rapidly eliminated the majority of instrumental artifacts or transient phenomena as the cause of the emission. Discrete enquires of the SDSS team produced the news that [OIII] emission had occasionally been detected but this was due to auroral activity. However, the detection of the [OIII] emission in two SDSS spectroscopic fields observed on different nights and confirmation of the continued presence of [OIII] emission from a spectrum obtained at the MMT Observatory ruled out an explanation due to a transient phenomenon. Combining spectra beyond the boundaries of the region where [OIII] emission was detected in individual produced clear detections of [OIII] emission extending over a region more than 2° in diameter.

A smaller number of individual spectra also showed the presence of emission from Hα and [NII] 6548, 6583. The spatial distribution of the individual emission line detections revealed clear trends and composite spectra, made up from objects contiguous on the sky, confirmed the trends and even allowed the detection of [SII] 6718, 6732. Figure 1 shows the spatial distribution of line emission as derived from the SDSS spectra.

Figure 1
Figure 1. Spatial distribution of spectra with detectable [OIII] 4959, 5007 (dots), Hα (circles), and [NII] 6583 (crosses). The hatched area indicates a region where composite spectra also show unambiguous evidence of [OIII] 4959, 5007 emission. Positions of objects with SDSS spectra for which no individual detections were obtained are also indicated. The dashed outline shows the area included in the narrow-band images of Figure 2. The location of the white dwarf PG 1034+001 is marked by a box. [ JPEG | TIFF ]

Narrowband imaging of the central part of the region was undertaken during a WFC survey run on the INT in 2003 May. The object is hard work, with integrations of 1200 and 2700s in Hα and [OIII] 4959, 5007 respectively, necessary to allow the detection of emission over the majority of the field. However, the results were unambiguous, with excellent agreement between the surface brightness distribution evident in the INT images (Figure 2) and the emission line detections from the SDSS spectra. A striking feature of the images was the presence of a well-defined arc-like feature, perhaps suggestive of some form of shock (Hewett et al., 2003).

Figure 2
Figure 2. The left hand panel shows a mosaic of 6 INT WFC continuum–subtracted pointings in Hα+[NII] while the right panel shows the equivalent for [OIII]. The images are approximately 0.8° on a side with North to the top and East to the left. The location of the white dwarf PG1034+001 is indicated by a circle in the [OIII] image. Emission with complex structure is evident in the central regions of the images in both passbands. A well–defined arc, or boundary, is visible at center–right in the [OIII] image. [ JPEG | TIFF ]

A wide range of possible explanations for the emission line region were considered without success. Then, following the INT observations, a search of the region using SIMBAD revealed the presence, close to the region with the strongest [OIII] emission, of a very nearby, extremely hot DO white dwarf (PG 1034+001). The location of the white dwarf clinched the identification of the emission region as a PN. The diameter of more than 2° makes the object the largest known PN on the sky and Rauch et al. (2004) have identified evidence for an ionised halo some 10° across.

PG 1034+001 does not yet possess a parallax distance but the spectroscopic distance estimate of 155+58 pc (Werner et al., 1995) means the PN is certainly the second closest known and a parallax distance could confirm the nebula as the nearest PN to the Solar System. The unambiguous detection of a PN associated with a non-DA white dwarf is also a first. Determination of a reliable age for the PN will help constrain timescales associated with the late stages of evolution of post-asymptotic giant branch stars and the origin of helium-rich white dwarfs. The PN is certainly old, an estimate of the expansion age and a kinematic age estimate, derived from extrapolating the observed proper motion of PG 1034+001 back to the origin of the radius of curvature of the arc feature, both suggest an age of ~100,000yr. The strongly enhanced [NII] emission evident along the south western boundary of the PN is also indicative of the interaction of an old PN with the surrounding interstellar medium.

The strength of the [OIII] emission suggests that imaging of other hot non-DA white dwarfs might be rewarding and we have begun such a programme with the INT in collaboration with Matt Burleigh (Leicester). The first run earlier this year suffered from poor conditions but preliminary results suggest the detection of at least one new PN. ¤


*Email contact: Paul Hewett (

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