8186( 3) - 02/17/99 18:01 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8186 Version: 3 Check-in Date: 17-Feb-1999 18:00:42 1.Proposal Title: The Properties of Gaseous Halos Around Disk Galaxies ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Joel Bregman University of Michigan CoI: Dr. Patricia Knezek The Johns Hopkins University N CoI: Mr. Eric Miller University of Michigan N CoI: Dr. Patrick Seitzer University of Michigan N ------------------------------------------------------------------------------------ 5. Abstract Gaseous halos are key components in the structure and evolution of spiral galaxies and here we propose to advance our understanding of these phenomena through absorption line studies of the halo gas around the LMC and the edge-on galaxy NGC 891. In the LMC we will determine the spatial extent of the halo and the mass of highly ionized gas by measuring the CIV line strength against three bright background AGNs at different disk locations. For the edge-on system NGC 891, we will observe an AGN that we discovered whose sightline passes through the X-ray, radio, and HAlpha halos, only 5 kpc from the disk. By observing the Mg II and Fe II resonance absorption lines, we will measure the filling factor, density, and pressure of this halo material and determine the mass of gas that cools at this height above the disk. ------------------------------------------------------------------------------------ 8186( 3) - 02/17/99 18:01 - [ 2] Observations Description ------------------------ To find a suitable background AGN projected behind the halo of an edge-on spiral, we assembled a list of edge-on galaxies that show line emission and X- ray emission but we found that there were no previously known AGNs projected suitable close to the disk. Consequently, we began a program to identify AGNs projected through the halos of these systems, using our combined X-ray and optical identification technique (Knezek and Bregman 1998). We found a single suitable target, an AGN that is projected upon the halo of NGC 891 only 106"\ (5.0 kpc) from the disk and in the inner part of the galaxy, 11"\ along the disk direction (r = 0.5 kpc); it has V = 18.7 and z = 1.18 (Fig. 1). This was extremely fortunate since the chance of finding such an object is not particularly high. In NGC 891, the hot X-ray halo has a density scale height of 3.5 kpc (Bregman and Houck 1997), the HAlpha emission has a component with a scale height of about 5 kpc (Rand 1997), and the radio continuum em ission is nearly cospatial with th e halo HAlpha and X-ray emission (Allen, Baldwin, and Sancisi 1978; Sukumar and Allen 1991). At the location of this AGN, emission from all three ISM tracers is visible, although it is the optical emission lines that will be especially valuable in our analysis. For the proposed absorption line study, we wish to use the strongest possible resonance lines to detect the cooled material, and based upon experience of absorption in our own and other galaxies, the Mg II doublet (at 2800 Angstrom) is the best candidate, followed by the Fe II lines from 2300--2600 Angstrom. The main reason for using this part of the spectrum is that a higher S/N can be achieved than in the 1200 --2000 Angstrom\ region. To estimate the expected Mg II and Fe II strengths we note that along this sightline, the column of HAlpha emitting gas is about 6times10^18 cm^-2 (Pildis, Bregman, and Schombert 1994; Rand 1997), and it is about 10\ and it will be spread out in velocity space due to the rotation and random motion, with an expected li n e width of about 200--300 km s^-1. For solar abundance and the conservative assumption that Mg II bearing gas is present in only 10\ equivalent width of about 1--2 Angstrom ; because of the velocity width, it will not be saturated (Tau = 0.2--0.4 at line center). The Fe II lines have a wide range of f values, but the Fe II 2600 Angstrom\ and 2382 Angstrom\ lines should have equivalent widths of about 0.8 and 0.7 Angstrom\ respectively. We obtain similar equivalent widths when we use the X- ray emission column in front of the AGN, 8times10^19 cm^-2, and then assume a steady-state halo where the cooling rate equals the replenishment rate of gas. We propose to use the G230L grating with the 0.5"*52"\ slit on STIS to detect these Mg II and Fe II lines. It order to detect the weaker Mg II doublet line at the 4Sigma level (assuming an equivalent width of 0.5 Angstrom ) and the stronger line at 8Sigma, we need a S/N = 24 per resolution element. In order to achieve a S/N = 24 at 2800 Angstrom , and for the redde ning typical of this region (E(B-V)= 0.13), an exposure time of 13 ksec is required (5 orbits). Luckily, one of our spectra (spectrum C; Pildis, Bregman, and Schombert 1994) happens to pass within 11"\ of the AGN, so we have modest spectral information for this region, in addition to the HAlpha images. However, we intend to obtain a higher quality optical spectrum with broader wavelength coverage of the halo emission at this location, which will help to better define the ionization state of the gas. Emission from Mg II and Fe II in our slit is very small and will not fill in an absorption feature. A STIS target acquisition exposure will be made (MODE=ACQ), which is adequate since we will be using the 0.5"*52"\ slit. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8186( 3) - 02/17/99 18:01 - [ 3] Data Distribution Paper Products: YES Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8186( 3) - 02/17/99 18:01 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC891-HALO ISM, ABSORPTION LINE RA=2H 22M 24.46S +/- 0.9", J2000 V = 18.7+/-0.8 SYSTEM - EXTRAGALACTIC, DEC=42D 21' 38.2" +/- .9", PLATE CORONA, HIGH VELOCITY -ID=01WN CLOUD, INTERMEDIATE 8186( 3) - 02/17/99 18:01 - [ 5] Visit: 01 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC891-HALO STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 10 S ONBOARD ACQ FOR 11, 21 CD P ------------------------------------------------------------------------------------------------------------------------------------ 11 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1100 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 12 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1162 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 21 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1400 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 22 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1335 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 31 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1400 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 32 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1335 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 41 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1335 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 42 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1400 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 51 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1335 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 52 NGC891-HALO STIS/N ACCUM 52X0.5 G230L 2376 1 1400 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 8186( 3) - 02/17/99 18:01 - [ 6] Summary Form for Proposal 8186 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.5 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 11, 21 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC891-HALO ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 ------------------------------------------------------------------------------------------------------------------------------------