The nuclear spiral in M31, consisting of ionized and neutral dusty gas clouds of typically sub-parsec sizes, shows optical emission lines characteristic of LINERs. Yet the lack of UV radiation from either an active nucleus or massive young stars makes the ionizing source of this nearest LINER a longstanding puzzle. We propose a WFC/UVIS observation of the nuclear spiral, using appropriate narrow-band filters to map the H alpha, [N II] and [O III] lines. The resolving power of HST is essential to measuring the line intensities and their ratios from discrete gas clouds across the nuclear spiral, based on which we can derive the physical properties of gas such as density and ionization state. We will then assess the relative importance of ionization/excitation mechanisms, in particular UV radiation provided by evolved stars and heating by cosmic-rays, by confronting with theoretical models. This study advances our understanding of the regulation of galactic circumnuclear environments, which is in turn crucial to understanding the evolution of the central super-massive black hole and the host galaxy.

With the WFC3/UVIS and 8 allocated orbits, we will map four emission lines, [O II]3727, [O III]5007, H-alpha6563, and [N II]6583, from the central 150 arcsec X 150 arcsec region about the M31 nucleus. A total of seven filters will be used, including four on-line filters and three off-line filters for measuring the continuum. Based on the expected line intensities and desired S/N, each of the three filters, F373N, F656N and F658N, will be ultilized for one orbit, and the F502N filter will be ultilized for two oribits. Each of the three off-line continuum filters, F390M, F547M and F665N, will be ultilized for one oribit. Each orbit will have the same exposure design. We intend to use a 3-point pattern which is similar to WFC3-UVIS-MOSAIC-LINE pattern, but with the dither offsets of 36.33 and 71.33 pixels in the x and y directions. This 1/3 subpixel offset is chosen to optimize the PSF sampling. This pattern could also help to remove cosmic-rays and the CCD gap of WFC3. The exposure time for each pointing is 900 second for WFC3. Therefore, the total exposure time for each orbit is 2700 second. The overhead incurred during each orbit is as follows: 1) 333 s for the guide-star acquisition and instrument setup, b) 2700 s science exposures, c) 232 s for UVIS overhead for preparing the second and third exposures. The total time used is 3265 s, which can be fixed into one orbit, according to the APT of the latest version. With the ACS and 8 allocated paralell orbits, we will also sample the outer regions (about 6 arcminutes from the center) with narrow-band filters, F502N, .F658N, F660N (2 orbit for each) and the corresponding continuum F550M and F625W (1 orbit for each). The effective exposure time for each orbit is 2382 s ISM IN EXTERNAL GALAXIES 12 false 0.0 0.0 false false false false false false SUPPORTED EXT-MEDIUM EMISSION LINE NEBULA NUCLEUS SPIRAL ARM This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 00 42 44.3100 +41 16 9.40 J2000 WFC3/UVIS Imaging No No F373N WFC3/UVIS Imaging No No F390M WFC3/UVIS Imaging No No F502N WFC3/UVIS Imaging No No F547M WFC3/UVIS Imaging No No F656N WFC3/UVIS Imaging No No F658N WFC3/UVIS Imaging No No F665N ACS/WFC Imaging No No F502N ACS/WFC Imaging No No F550M ACS/WFC Imaging No No F658N ACS/WFC Imaging No No F660N ACS/WFC Imaging No No F625W Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. We prefer setting the orientation angle to be 120 +/- 10 degree, in which case the parallel ACS observations will have an optimal coverage of an outer spiral arm. If this range is inconvenient to schedule, a broader range of 80-160 degree will still be good for the ACS field of view, as we tentatively set here for the Visit Orientation Requirements. We emphasize that the exact orientation has little effect for the prime WFC3 field of view, but we prefer having the same (or nearly same) orientation for all visits. (Tic-Visit :proposal-id 12174 :visit-id "01" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:29:42.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) false 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "02" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:00.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "03" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:03.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "04" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:06.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "04" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "05" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:08.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "05" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "06" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:10.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "06" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "07" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:12.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "07" :obset-id "07" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "07" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0 Same orient constraint links visits (01 02 03 04 05 06 07 08) The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12174 :visit-id "08" :proposal-version -1 :trans-version "20100505" :date "08-Jul-2010 17:30:14.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1718912 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12174 :visit-id "08" :obset-id "08" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12174 :visit-id "08" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3008 :al-overhead-at-end 108 :internal-dur-at-end 1019 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3341 :pcs-gap 0 ) true 1 0