Max Mutchler
Space Telescope Science Institute,
Baltimore, Maryland
James Flood
Shimadzu Scientific Instruments, and
Sperry Observatory,
Union College, New Jersey
The preliminary results below were presented as an AAS poster paper in Washington DC on 9 January 1998. A press release on 23 March 1998 resulted in articles in USA Weekend, MSNBC, and Astronomy magazine. These images were also featured as the "Astronomy Picture of the Day" on March 23 and July 1, 1998.
Larson & Tinsley, 1978, and van den Berg, 1978, noted that NGC 1808's peculiar optical morphology is similar to M82, and starburst activity may have been induced by tidal interaction with nearby NGC 1792, which also appears to have an asymmetric morphology.
NGC 1808 is a prominent radio source, and has been detected at frequencies from 0.03 to 14.8 GHz (Haynes, 1975, Large, 1981, Ekers, 1989). The radio emission is produced by supernova remnants (Dahlem, 1990). Sakia (1990) mapped the inner kpc region using the VLA at 1.5, 5, and 15 GHz. These high-resolution (1" at 5GHz) measurements revealed an intense nuclear source surrounded by several compact radio sources within the central 1kpc. There is little correspondence between the compact radio sources and the optical hotspots. Sakia created the A,B,C,D,E naming scheme for the most prominent hotspots, with hotspot D being the nucleus.
Krabbe, Sternberg, and Genzel (1994) conducted near IR spectral imaging of NGC 1808 and found no evidence for an embedded AGN -- the starburst clusters are the dominant sources of the thermal far-IR dust emission. They produced an extinction map of the circumnuclear region, with visual extinctions range from ~3 to ~5, so optical images are likely severely distorted by patchy foreground extinction toward the circumnuclear region. IR images are probably only slightly affected by foreground extinction. Much of the foregound extinction may be caused by the remarkable dust filaments, such that the optical hotspots trace regions of low foreground extinction. The nuclear starburst must be at least 50 million years old, and can be no older than 100 million years old. Star formation has been rapid and continuous. Without an influx of fresh molecular gas into the circumnuclear region, the star forming activity can only be maintained at this rate for another 6 to 20 million years. They found little evidence for very hot and massive star formation.
Ground-based images show that the outer spiral arms of the galaxy are indeed warped with respect to the inner arms (which display a prominent dark dust lane). This is evidence that NGC 1808 may have had a tidal interaction with another nearby galaxy, NGC 1792. Such an interaction could have created the bar morphology, and hurled gas towards the nucleus of NGC 1808, igniting the exceptional burst of star formation seen there.
We took images of NGC 1808 on 14 August 1997 with the
Wide Field Planetary Camera 2
(WFPC2)
onboard NASA's
Hubble Space Telescope (HST).
A total of six images were obtained using 3 different filters.
Two exposures were taken with each filter, so that cosmic rays could be
easily removed from the images.
We exposed the galaxy for a total of 240 seconds with the
broad band R filter F675W, 280 seconds with the
broad band I filter F814W, and 1400 seconds with the
narrow band [N II] filter F658N, which isolates redshifted
hydrogen-alpha emission. These are the resulting images:
The telescope pointing was a particular concern. The nucleus of the galaxy (i.e. the starburst region) is on the higher-resolution PC1 chip. The U3 ORIENT angle is 245 degrees (V3 position angle is 65 degrees), which places the line of HII regions in the galactic bar on the WF2 and WF4 chips. The following is some basic information about NGC 1808:
Right ascension (RA): 05h 07m 42.0s Declination (DEC): -37d 30' 51.0" Inclination angle: 57 degrees Position angle of major axis: 135 degrees (bar is 155 degrees) Recessional velocity: 989 +/- 10 km/s Distance (v/Ho) 13.2 Mpc = 43 million lightyears *
The coordinates given above are the J2000 coordinates used to point the telescope such that the nucleus would be near the center of the high-resolution PC1 chip. However, the central star cluster seen in the PC1 image is clearly offset from the center of the PC1 chip. We measured J2000 coordinates from our F675W image (use F658N image for this!) for what appears to be the center of the galaxy:
Right ascension (RA): 05h 07m 42.195s Declination (DEC): -37d 30' 47.29"
See our complete table of star cluster or "hotspot" coordinates.
* The distance estimate above assumes a Hubble contant (Ho) of 75 km/sec/Mpc. This implies that the full mosaicked image is about 25,000 light years, and the PC1 images are about 5700 light years on each side. Each WF pixel is 100 milli-arcseconds or about 16 light years wide, and each PC pixel is 46 milli-arcseconds or about 7 light years wide.
Garth Illingworth took a snapshot image of NGC 1808 in 1994, and Luis Colina took another in 2000. This data is also available in the HST archive, but the pointing is poor in both cases. Dahlem & Baggett (1997) applied unsharp masking techniques to Illingworth's image to enhance and analyze the filamentary structures.
The removal of hot pixels is the trickiest part of the reduction. There are a few different procedures, each with shortcomings. In the end, the multi-phase approach was used:
There is a small sub-pixel shift between the images (most noticeable in the PC1 images, since the pixels are "smaller"). We verified that the pointings executed perfectly, so the shifts are due solely to the "filter wedge effect", i.e. because each filter has a slightly different optical thickness. Before we can subtract the continuum R image from the H-alpha image, the images must be perfectly registered.
To create a continuum-subtracted H-alpha emission image, the R (F675W) image was subtracted from the H-alpha (F658N) image in the following manner: A BB/stellar/flat spectrum was chosen, and synphot was used to estimate the counts-per-sec in both the R and H-alpha filters (857 and 24, respectively). The R/NII flux ratio for WFPC2 is then 857/24 = 35.7. Sky backgrounds and dark frames were already subtracted from the images. We divided the R image by it's exposure time (240s) to create an R image in counts-per-second (Rcps). We divided the Rcps image by 35.7, the flux ratio. Then we multiplied the image by the H-alpha exposure time (1400s) to estimate the counts one would expect in the H-alpha image (based on the R image). We subtract this image from the H-alpha image to get a continuum subtracted H-alpha emission image.
The Hubble Space Telescope provides very high resolution images, which allow the complex structures in the core of this galaxy to be revealed. Many young star clusters can be seen in the closeup of the nuclear starburst -- and many more are likely obscured. This image shows that stars are often born in compact clusters within starbursts, and that dense gas and dust heavily obscures the starburst region. We can estimate the sizes of objects in the starburst. Each PC pixel is about 46 milli-arcseconds wide, which corresponds to about 7 light years (assuming Ho = 75 km/sec/Mpc). The starburst region is about 2400 light years (0.73 kiloparsecs) in diameter. The bright object at the very center of the galaxy is about 100 light years in diameter. It could be a giant cluster of stars, or a weak AGN. The other star clusters are about 10-50 light years in diameter.
Contours of the mosaicked F658N hydrogen alpha image identify star formation regions in the bar. F675W contours for the nucleus of the galaxy show two maxima. Either the galaxy has two nuclei (from a previous merger), or one of the dusty filaments happens to bisect the nucleus along our line of sight. These are the contour plots:
We made a a smoothed copy of our broad band red image and subtracted it from the original (unsharp masking) to enhance the filamentary edges (see Dahlem & Baggett, 1997). The filaments can be traced down to their origin in the circumnuclear region.
We made 2-color maps of the nucleus to identify color gradients, and sort out features by color. The contour plots show that the nucleus is brighter in H-alpha.
