0Assigned ID: 750
----- Attempting Submission 1 (Fri Apr 05 20:53:52 GMT 2019) -----The BOT classified the galaxy center as exceeding the Health/Safety Limits, but my calculation performed with the ETC (attached for both exposures) considering Galex FUV flux in the central region of the galaxy is well below the ACS/SBC safety limits reported in the Instrument Handbook.-1622027532----- Attempting Submission 1 (Wed Jul 24 16:37:23 GMT 2019) -----First-time direct imaging of newborn stars within a galactic outflowRecent models predict that star formation can occur directly within galactic outflows, either in active or star-forming galaxies. Tentative observational evidence of such phenomenon has been recently found in an AGN and in a few MaNGA galaxies based on the observed excitation of the ionized gas in the outflow. However, we are still missing a direct detection of the continuum emission from the newborn stars. Thanks to the unprecedented capabilities of MUSE at VLT, we have found tentative spectroscopic evidence of star formation occurring within the outflow in the nearby star-forming galaxy NGC 6810. We propose to image NGC 6810 in the FUV band with HST ACS/SBC to detect, for the first time, the direct emission from young OB stars in the outflow providing direct and conclusive evidence of this new star formation mode.We want to observe both sides of the outflow in NGC 6810 (see Fig. 1) with HST ACS/SBC in imaging mode in the F140LP filter, by making two separate exposures, one on the western side of the outflow and the other on the eastern side (as shown in Fig. 1b). At the redshift of NGC 6810 (z = 0.00677) the Lyalpha emission line is observed at a wavelength of 1223.5 Angstrom, outside the spectral range of the F140LP filter.
From VLT/MUSE data we measure an Halpha extinction-corrected flux for the outflowing gas (i.e. only from the Gaussian component associated to the outflow in our fits; see e.g. Fig. 1f) varying from ~10^-15 erg s^-1 cm^-2 arcsec^-2 in the inner regions down to ~2.5 x 10^-17 erg s^-1 cm^-2 arcsec^-2 in the outer regions at ~20''. We can then estimate the expected FUV continuum emission from the young stars giving rise to such ionized gas line emission, by employing the SFR tracers from Kennicutt & Evans (2012), i.e. log SFR = log L(Halpha) - 43.55 and log SFR = log L(FUV) - 41.27, giving log L(FUV) = log L(Halpha) + 2.08. This gives a FUV flux in vu*F_vu, which corresponds to a monochromatic flux at 1500 Angstrom, after having applied dust extinction (derived from Halpha/Hbeta line ratio of the outflow component), of F_lambda ~ 10^-17 erg s^-1 cm^-2 Angstrom^-1 arcsec^-2 and F_lambda ~ 10^-18 erg s^-1 cm^-2 Angstrom^-1 arcsec^-2 in the inner and in the outer regions of the outflow, respectively. A map of the expected observed FUV flux from the outflow is displayed in Fig. 1i, obtained from our MUSE Halpha and extinction maps with the procedure just described (the map represents the expected observed FUV emission from the outflow only, not also from the disk, having considered only the outflow spectral component in the Halpha line profiles for our calculation).
We employed the ACS imaging Exposure Time Calculator (ETC) to calculate the needed exposure time to request for SBC F140LP filter. We set a Bruzual Synthetic Stellar Spectra B1 V spectral shape of the continuum. We adopted an extinction E(B-V) = 0.2, average extinction in the outflow from VLT/MUSE data. We set average Earth shine and air glow and the zodiacal light value corresponding to the coordinates of NGC 6810. The estimated orbital visibility of the target, as given by the Astronomer's Proposal Tools (APT), is about 56 minutes per orbit. With the FUV fluxes estimated above we reach, in about 5400 s = 90 min (i.e. within two orbits), a S/N going from ~21 in the inner regions down to ~5 in the outer ones at ~20'' from the center, having considered an aperture of 0.5 arcsec diameter (containing about 80% of the PSF encircled energy, and corresponding to a box size of 15x15 pixel). The addition of the overheads, i.e. guide star acquisition (6 min for first-orbit acquisition, 4 min for reacquisition in subsequent orbits), instrument overheads (1.7 min for first exposure, 0.9 min for subsequent identical exposures), does not exceed two orbits (112 min).
Since we aim to carry out two observations, covering each of the two sides of the bipolar outflow of NGC 6810, as shown in Fig. 1b, we ask for a total of 4 orbits.
The actual observable time emerging from the Orbit Planner run in Phase 2 results to be higher than the one conservatively considered above for Phase 1, i.e. 6022 s instead of 5400 s in 2 orbits. Anyway, the S/N does not increase much and is consistent with that estimated above.
We performed a check for bright sources in the FOV of ACS/SBC, by making use of the ACS imaging Exposure Time Calculator (ETC). The brightest source in the FOV is the galaxy itself. We thus considered the emission from its central region, having a FUV AB magnitude from Galex RCS of 25.1 per arcsec2 and an extinction E(B-V)~0.65 measured from VLT/MUSE optical data. This results in ~2.5x10^-4 counts/s/pixel, which is well below the screening limit of 50 counts/s/pixel reported in the Instrument Handbook for ACS/SBC. We exclude for our science goal the possibility of having the issue of red leak, which consists in photons from NUV and optical bands contaminating ACS/SBC observations thereby falsely boosting the FUV count rate. In fact the target of our observations is the emission from hot OB newborn stars in the outflow and observations of such stellar types are basically unaffected by red leak, which instead affects solar- and redder-type stars. Our VLT/MUSE observations show in fact no indication of optical continuum emission associated with the outflow, thus excluding the presence of sensible emission from solar- or redder-type stars, which might cause read leak. Red leak might affect the galaxy disk in principle, but this is not a problem for our goal since our interest resides in the outflow. We would simply need to subtract the underlying disk emission (modelled through an isophotal profile) to isolate the intrinsic FUV emission from young OB stars in the outflow.
If HST will end up going in Reduced Gyro Mode (RGM) our target will still be in HST field of regard, having coordinates RA=295.892703 and Dec=-58.655590 (and in the Solar Avoidance Zone as it already is in 3 gyro-mode). The increase of target acquisition time by 2 minutes (180 s) will reduce the Actual Duration of each of our subexposures within a single orbit from ~3000 s to ~2820 s, which is still a bit higher than the time estimated in phase 1 based on the S/N needed for our science goal. We then conclude that the RGM will not impact significantly on our science case.The team members are experts in the field of galaxy evolution, outflows and feedback from outflows on their host galaxies. Most of them were part of the team which observed for the first time the process of star formation occurring within outflowing gas (Maiolino et al. 2017), which we want to investigate with the present proposal. Many members of the team have extensive experience with HST data from various instruments. Dr. F. Mannucci was the PI of a succesful HST proposal (#11735) with ACS and WFC3. Prof. R. Maiolino was the PI of two succesful HST proposals (#7865 and #9726) with NICMOS.Galaxies and the IGM6falsefalsefalsefalsefalsefalsefalseSUPPORTEDEXT-MEDIUMBIPOLAR OUTFLOWWINDYESThe V-magnitude reported above is the magnitude of the galaxy NGC 6810 from SIMBAD.
The FUV AB magnitude reported above is the magnitude per arcsec^2 of the central, brightest region of the galaxy (being also the brightest source within at least 2x2 arcmin^2), measured from Galex RCS; the extinction of this central region is E(B-V)~0.65 measured from VLT/MUSE optical data. Employing the ACS imaging ETC, this results in ~2.5x10^-4 counts/s/pixel, which is well below the screening limit of 50 counts/s/pixel reported in the Instrument Handbook for ACS/SBC.J2000J2000FUV AB mag = 25.1 arcsec^-2; E(B-V)~0.65EXT-MEDIUMBIPOLAR OUTFLOWWINDYESThe V-magnitude reported above is the magnitude of the galaxy NGC 6810 from SIMBAD.
The FUV AB magnitude reported above is the magnitude per arcsec^2 of the central, brightest region of the galaxy (being also the brightest source within at least 2x2 arcmin^2), measured from Galex RCS; the extinction of this central region is E(B-V)~0.65 measured from VLT/MUSE optical data. Employing the ACS imaging ETC, this results in ~2.5x10^-4 counts/s/pixel, which is well below the screening limit of 50 counts/s/pixel reported in the Instrument Handbook for ACS/SBC.J2000FUV AB mag = 25.1 arcsec^-2; E(B-V)~0.65ACS/SBCImagingNoNoF140LPPOS-TARGDITHER0.472LINEfalsefalse244.4ACS-SBC-DITHER-LINE1false/Users/giacomo/Google Drive INAF/Work/My proposals/HST - NGC6810 Cycle27/phase2/OP-cache/n6810_ph2-v01.xmlThe scheduling data was generated by CASM 27.2.CASM was run in Phase II mode.(Tic-Visit
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