FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browserResubmitted with new visit 03 (repeat of visit 02 through HOPR 92365)301229481----- Attempting Submission 2 (Tue Jun 01 18:43:34 GMT 2021) -----
----- Attempting Submission 3 (Fri Aug 27 00:57:23 GMT 2021) -----
----- Attempting Submission 4 (Wed Jan 12 21:30:02 GMT 2022) -----
----- Attempting Submission 5 (Fri Jan 14 17:15:35 GMT 2022) -----
----- Attempting Submission 6 (Tue Oct 25 16:43:37 GMT 2022) -----Same targets as before (just added repeat visit). Flash leveles worked out by ISThe resolved PAH properties at low metallicityThe NIR-MIR SED of galaxies is dominated by spectral features from polycyclic aromatic hydrocarbon (PAH) molecules fluorescing in regions illuminated by FUV photons. In low metallicity systems (Z < 0.2 Zo), previous studies with Spitzer have revealed a substantial deficiency in the emission and abundance of PAHs, the origin of which remains unexplained due to the lack of deep, resolved, multi-band photometry or spectroscopy. We will obtain NIRCam and MIRI imaging to observe the 3.3, 7.7, and 11.3 micron PAH features in dusty star-forming regions in two low metallicity galaxies, IC 1613 (15% solar) and Sextans A (8% solar). Additionally, we will obtain HST/WFC3 imaging with F275W, F336W, F475W, and F814W to complement the JWST observations. The HST observations will allow us to derive extinction maps, which will be used to normalize the PAH abundance to that of big grains. To obtain the extinction maps, we will fit the NUV-NIR SED of thousands of stars in the HST and JWST images with the BEAST code (Gordon+2016) in order to constrain the stellar mass, age and dust extinction (A_V) toward each sight-line. A_V measurements toward individual stars will be combined on the sky to produce the extinction maps.
The observations will allow us to 1) estimate the PAH properties, particularly their abundance, ionized fraction, and size distribution; and 2) correlate those properties with other tracers of the ISM, such as the star formation rate density, the distance and UV spectrum of massive stars, the
distance to PAH-producing stellar sources, the ISM column density of gas and dust, and gas-phase abundances. By observing how the PAH properties and abundance vary with environment, we will constrain the mechanisms responsible for the formation, destruction, excitation, and low abundance of PAHs in those poorly shielded low metallicity environments. Since JWST will measure star formation rates as low as 10 Mo/yr from PAH emission at the peak of cosmic star formation at redshift z~2 when the average metallicity of the universe was about 10% solar, it is crucial that we deepen our physical understanding of PAHs at low metallicity.This program will obtain WFC3/UVIS imaging of the dusty star-forming regions in IC 1613 (15% solar) in order to map extinction. The filters (F275W, F336W, F475W, F814W) were selected to
cover the NUV-optical stellar + dust SEDs so as to disentangle stellar mass from reddening. This program is designed to support JWST GO-2391 (a joint JWST-HST program). The HST NUV-optical imaging will complement the NIRCam and MIRI imaging.
We will obtain single fields centered on the dustiest star-forming regions where PAH emission was previously detected with Spitzer. We will use the WFC3-UVIS-GAP-LINE dither to cover the chip gap for the most sensitive filters for which we are getting 2 exposures (F475W, F814W). We do not expect saturation in those filters. We will use a custom box dither pattern (a hydrid pattern covering the gap and providing sub-pixel sampling) for the less sensitive filters that require 4 exposures (F275W, F336W). The parameters of the custom dither pattern are explained in the associated comments.
Exposure times were computed to reach a S/N of 10 for A5 stars with A_V = 1.Dr. Julia Roman-Duval is an expert on the observational characterization of the properties of interstellar gas and dust in nearby galaxies. She is the PI of one medium and two two large HST COS and STIS programs (GOs 12581, 14675 and 15880) to measure gas-phase abundances in low metallicity galaxies, and study the variations of the dust-to-metal ratio in the context of chemical evolution (Roman-Duval et al. 2019).
She has published several papers on the properties of dense gas in the Milky Way observed from the CO 1-0 line in the mm regime (Roman-Duval et al. 2009, 2010, 2011, 2016), as well as papers on the properties of dust in the Magellanic Clouds (Roman-Duval et al. 2010, 2014, 2017).
Dr. Martha Boyer is an expert on evolved stars and stellar dust sources in the context of chemical evolution. She is the PI of the DUSTiNGS large Spitzer program and several HST programs, to observe AGB stars in nearby dwarf galaxies and measure their dust contribution to the overall dust budget of low metallicity galaxies.
Dr. Jeremy Chastenet is an expert on dust IR SED modeling to derive dust properties in nearby galaxies (e.g., Chastenet et al. 2017), with a particular focus on PAH properties (Chastenet et al. 2020)
Dr. Chris Clark is an expert on the observational characterizaton of dust
properties from FIR emission (e.g., Clark et al. 2015, 2018, 2019). He is
the PI of HST program GO-16222 to measure the dust extinction properties in the very low metallicity galaxy Leo-P.
Dr. Bruce Draine is an internationally renowned expert on the observations (e.g., Draine et al. 2007) and modeling (e.g., Draine & Li 2001) of
dust grains and PAHs. He has published dozens of papers advancing our understanding of dust grains and PAHs, the most recent one being highly relevant to the proposed study (Draine et al. 2020).
Dr. Karl Gordon is an expert on the observational characterization of dust properties in the Milky Way and nearby galaxies using UV-FIR data. He has published dozens of papers on dust UV extinction curves (e.g., Gordon et al. 2003), PAHs (e.g., Gordon et al. 2008), dust properties obtained from FIR emission (e.g., Gordon et al. 2014) and extinction (e.g., Gordon et al. 2016). He is the PI of several HST programs to measure dust extinction properties. He is also an expert on JWST observations and calibration (e.g., Gordon et al. 2015).
Dr. Aleksandra Hamanowicz is an expert on UV-optical-mm spectroscopy of high-redshift galaxies and quasar absorbers (e.g., Hamanowicz et al. 2020), as well as intestellar abundances in low metallicity systems. She also has expertise in sum-mm studies of molecular gas content in galaxies and CO emission.
Dr. Karin Sandstrom is an expert on dust and gas observations in nearby galaxies, particularly on PAHs (e.g., Sandstrom et al. 2010, 2012). She is
the PI of the SMIDGE HST program to measure dust extinction in the SMC.6falsefalsefalsefalsefalsefalsefalseSUPPORTEDGALAXYMAGELLANIC IRREGULARSTAR FORMING REGIONYESJ2000POS-TARGMOSAICDITHER42.414WFC3-UVIS-DITHER-BOXfalse81.785true0.1120.173285.759POS-TARGWFC3-UVIS-GAP-LINE23.8841falsePOS-TARGDITHER2.414For the custom dither pattern, the coordinates of the 4 points are (-2.25, -30.25), (2.25, 30.25), (-0.25, 27.75), (-4.75, -32.75). The first line is the same as the WFC3-UVIS-GAP-LINE, and the second line is parallel to the first, with the points offset by -2.5 pix in x and -2.5 pix in y. Using the a11, b10, and b11 coefficients given in the ISR https://www.stsci.edu/files/live/sites/www/files/home/hst/instrumentation/wfc3/documentation/instrument-science-reports-isrs/_documents/2010/WFC3-2010-09.pdf, we get the parameters of the dither pattern:
Point spacing = sqrt( (4.5a11)**2 + (4.5b11 + 60.5b10)**2)) = 2.41 arcsec (same as gap line, that makes sense)
Line spacing = sqrt( (2.5a11)**2 + (2.5b10 + 2.5b11)**2) = 0.1448 arcsec (not the same as the uvis box line spacing)
Pattern orient alpha = arctan( (60.5b10 + 4.5b11)/(4.5a11)) = 85.758 deg (same as gap line, expected)
Gamma = arctan ( (2.5b10 + 2.5b11)/(-2.5a11) ) = -46.84 deg
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