Investigate the stellar populations in well resolved galaxies. The main goal is to probe the evolved stellar population (AGBs) and the very young YSOs. For the AGBs, these observations provide a measure of their dust mass loss. For the YSOs, this provides a meausre of the circumstellar dust mass. In general, near-infrared photometry probes the stellar atmosphere and the mid-infrared photometry probes the circumstellar environment. This program builds on the successful Spitzer SAGE programs in the Magellanic Clouds by extending this type of work to more distant galaxies as well as better sampling the mid-infrared SEDs (e.g., the 10 micron silicate feature).
4 positions in M31 to sample up to 10 kpc (nucleus to 10 kpc star forming ring).
3 position each in the LMC, SMC, and NGC 6822 to sample a range of metallicities and environments.
2 positions in M33, M51, M81, & M82 to sample center and disk.
1 position in NGC 925, NGC 1097, NGC 3351, NGC 4125, NGC 4594, NGC 5866 & NGC 6946.
Imaging: Each region will be observed with 2 NIRCam filters and 5 MIRI filters to provide good photometric SEDs to measure the 5 MIRI filters to provide good photometric SEDs to measure the stellar and circumstellar properties of the stars.
Mosaics for MIRI (4x4) and NIRCam (4x1) are done to provide a large enough region to sample the evolved stellar population as well as different star forming environments.
Parallel Observations Possible (yes/no/pure paralell)?
Ideal for parallel observations. Larger regions can be mosaiced with NIRCam/MIRI by taking observations separated by 6 months in parallel. Data volume could be managed by only saving data from one side of NIRCam. Idea is to do something similar to what has been done for the PHAT M31 MCT.
NIRCam images in the F090W and F277W filters (obtained simultaneously in the short and long wavelength channels, respectively) will be used to survey the low-mass stellar content of nine Local Group galaxies closer than 100 kpc. Each of these galaxies will be mapped along two strips that are approximately orthogonal, as done in the incarnation of this program that appeared in the 2005 SODRM (Program #410). At each position in the map, a half hour of exposure time will provide stellar photometry on the lower main sequence. The faint limit will vary from galaxy to galaxy, but will roughly reach 5.5 bolometric magnitudes below the old main-sequence turn-off. The resulting color-magnitude diagrams will be used to study luminosity and mass functions for the low-mass main-sequence stars as a function of position in the galaxy, along the two axes mapped.
For reference, assume an isochrone of a 10 Gyr population with a metallicity of Z=0.008 ([Fe/H]=-0.4) from the Padova isochrone library, which shows that a 0.25 M☉ star has an effective temperature of 3750K (M0V) and the following absolute magnitudes (relative to Vega): MV=10.9, MI=9.0, MJ=8.0, MH=7.1 mag. This point is 5.5 bolometric magnitudes below the turnoff.
Parallel Observations Possible (yes/no/pure parallel)?
MIRI parallel imaging would provide interesting diagnostics on cool populations.
The overwhelming majority of observational studies of star formation have been conducted on Milky Way star formation regions. However, star formation in the Universe is dominated by star formation at low metallicity, i.e. sub solar. The metallicity of the interstellar medium (ISM) during the peak star formation rate in the Universe, which occurs at a redshift of z ~ 1.5, is comparable to that of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). In the Spitzer-SAGE programs, thousands of new candidates young stellar objects were discovered a marked increase over the 1 YSO known prior in the SMC and 20 YSOs known in the LMC. This program will both followup candidate YSOs from Spitzer using MIRI MRS and NIRSpec IFU spectroscopy. We will also conduct sensitive imaging projects with NIRCam and MIRI to detect and characterize YSOs with lower mass and dust content than reachable by Spitzer. The targeted flux limit should reach individual 2 solar mass T-Tauri type star at the distance of the Magellanic Clouds.
For MIRI & NIRSpec spectroscopy: 100 YSOs in the LMC, some in clusters and some isolated; 50 YSOs in the SMC, some in clusters and some isolated; see the APT file for the source list.
For MIRI & NIRCam imaging: 4 fields in the LMC and 2 fields in the SMC.
This set of observations is a follow-up on a large sample of individual YSOs in the Magellanic Clouds and on several of the large star formation regions in which they are organized.
We have selected 150 YSOs as targets for spectroscopic observation with JWST - 100 in the LMC and 50 in the SMC. Objects were chosen to be randomly distributed in both galaxies and sample a range in 8.0 m flux. The random selection is a proxy for source total luminosity, as a realistic proxy for a scientifically driven need to sample different types of YSOs and different environments. All chosen targets have an 8.0 m flux greater than 2 mJy.
The Large Magellanic Cloud in Spitzer/MIPS 24 micron band in grey scale with the location of the targeted YSOs, marked by red dots. Some are clustered and others are isolated.
To cover the full range of spectral wavelengths afforded by JWST, we will use both MIRI Medium Resolution Spectroscopy and NIRSpec IFU Spectroscopy. In the mid-IR, the selected YSOs have typical continuum fluxes of >2 mJy. In 10 minutes of on source integration, MIRI can attain a continuum SNR for the dimmest sources of ~50, and >1000 for the brightest. Emission lines, often seen in the mid-IR spectra of YSOs to be an order of magnitude above the continuum will be detected to a SNR of at least several hundred. Each source requires three integrations (one for each wavelength region - short, medium, and long), so each target requires 30 minutes of on source integration. With 150 targets at 30 minutes of integration each, MIRI's spectroscopy will require 75 hours of on-source integration. Overheads for each source include slewing to the source (1800 seconds), guide star acquisition (240 seconds), target acquisition (600 seconds), dither slews for the 4 dithers anticipated (30 seconds each, 12 dither slews total, 360 seconds total), three grating/dichroic changes (60 s each, 180 s total), and detector overheads (5 s configure + 42 seconds deadtime = 47 sec per dither slew, 564 s total). The total overheads time is 3744 seconds, 62.4 minutes or 1.04 hours per source.
The Small Magellanic Cloud and its wing, and tail in Spitzer/MIPS 24 micron band in grey scale with the location of the targeted YSOs, marked by red dots. Some are clustered and others are isolated.
We use all high resolution gratings (G140H, G235H, and G395H) to cover the complete NIRSpec wavelength range. This wavelength range is particularly important for detecting ices via absorption and gas-state molecules in emission. Even the dimmest sources can attain a high continuum SNR (>50) in 5 minutes of integration. For the three filters this will require 50 minutes of integration per source. For 150 sources, this amounts to 37.5 hours of on source integration time. By taking the spectrum using NIRSpec immediately following the same source's MIRI Spectral observation, we can significantly reduce the overheads by avoiding a full slew to the source. In this case the overheads include guide star acquisition (240 seconds), target acquisition (600 seconds), dither slews for the 4 dithers anticipated (30 seconds each, 12 dither slews total, 360 seconds total), three grating/filter changes (150 s each, 450 s total), and detector overheads (20 s configure + 15.9 seconds deadtime = 25.9 sec per dither slew, 430.8 s total). The total overheads time is 2080.8 seconds, 34.68 minutes or 0.578 hours per source.
We have selected six (four in the LMC and two in the SMC) star formation regions for imaging with both MIRI and NIRCam. The observations will require mosaicking multiple pointings to completely cover the regions. In the LMC, we have selected N44, N105, N113, and N159, while in the SMC we have selected NGC 602 and N66. Each of these regions requires 3-9 pointings each (see the figures below).
LMC star formation regions with the tiling shown for the MIRI and NIRCam mapping strategy. Top to Bottom: N159, N105, N113, and N113.
We have estimated the observing time required to detect a 2 solar mass T-Tauri type star at the distance of the Magellanic Clouds, and therefore should be sensitive to objects above this mass threshold. Depending on the exact nature of the source (presence of circumstellar envelope, etc.), we may be sensitive to even lower mass objects. With the MIRI imager, we will observe with two different filters in the mid-IR, F560W and F2100W. To reach the desired YSO mass detection threshold (~10 sigma detection) will require 10 minutes of integration at 5.6 m and 60 minutes of integration at 21 m, or 70 minutes per pointing. There are a total of 37 pointings to cover the regions, giving a total on source integration time of 43.16 hours. Overheads for each source include slewing to the source (1800 seconds), tile slew (100 seconds per mosaic pointing), guide star acquisition (240 seconds), dither slews for the 4 dithers anticipated (30 seconds each, 120 seconds per mosaic tile), filter changes (60 s each), and detector overheads (5 s configure + 42 seconds dead time = 47 sec per dither slew). The total overheads varies for each source.
SMC star formation regions with the tiling shown for the MIRI and NIRCam mapping strategy. NGC602 (top), and NGC 346 (a.k.a. N66; bottom).
Yes, one can imagine taking some spectra while imaging is prime and taking some imaging frames with MIRI or aging is prime and taking some imaging frames with MIRI or NIRCam while taking spectra.
The purpose of this program is to image the gas in a range of star forming regions using NIRCam imaging with a range of narrow filters. Medium filters are used to characterize the stellar populations and allow continuum subtraction to achieve pure emission line images.
We have selected some Magellanic Cloud H II regions, and other star forming regions in Local group galaxies (M31, M33) as well as mosaics of nearby galaxies M51, M83, M101, NGC 4449 and NGC 4214.
3-point Intramodule primary dithers are assumed to cover gaps, but no sub-pixel dithers are required. Note: Both modules are ps, but no sub-pixel dithers are required. Note: Both modules are used with paired filters, and FULL arrays.
No constraints are assumed, although one could imagine it might be desirable to constrain the position angles of some of it might be desirable to constrain the position angles of some of the mosaics.
Parellel Observations Possible (yes/no/pure parallel)?
A parallel program of similar observations could be crafted, but this specific proposal does not seem like a good parallel opportunity because it targets specific objects.
Extragalactic globular cluster (GC) systems provide an excellent tool to trace the star formation and chemical enrichment histories of their host galaxies. Optical imaging programs using HST have established that several nearby early-type galaxies are known to host rich GC systems. In several such galaxies, the imaging yielded hitherto unknown and unexpected evidence for the presence of significant subpopulations of intermediate-age GCs with ages of 1-5 Gyr, i.e., formation redshifts 0.1 < z < 1. However, optical colors are strongly degenerate in age and metallicity. Spectroscopy of such GCs are needed (i) to confirm or deny the intermediate ages and (ii) to provide information on the chemical composition of such GCs. Unfortunately, these subpopulations are mainly found in the inner regions of galaxies, and the strong and sometimes complex galaxy background in these regions has so far prevented one from obtaining sufficient S/N in spectra, even when using ground-based 10-m class telescopes. With NIRSPEC on JWST, we can finally obtain adequate spectra of these targets down to relevant levels of the GC mass function, using multi-object spectroscopy. This program will obtain such observations in nearby galaxies in which evidence for the presence of intermediate-age GCs has been found. In addition to spectroscopy of NIR features that are sensitive to age and metallicity, imaging with a variety of filters will be taken to compare optical/NIR colors with the relative line strengths of spectral features in the NIR.
Candidate young or intermediate-age globular clusters in the inner regions of nearby galaxies.
Spectroscopy: In each galaxy, multi-object spectroscopy will be used to produce spectra of several GCs per pointing. For will be used to produce spectra of several GCs per pointing. For NIRSpec, two gratings are needed (G140M/F070LP and G235M/F170LP) to cover the temperature (and hence age)-sensitive lines Paγ and Paδ as well as metallicity-diagnostic lines (Na, Fe, CO, N). We assume that 2 pointings (and target ACQs) will be used per galaxy target.
Imaging: Each galaxy will be observed with 4 NIRCam filters (i.e., 2 pairs: F090W/F277W and F200W/F356W) to provide good photometric coverage over the entire wavelength range of the spectral observations. Use 8 dithers per filter pair (INTRAMODULE dithers, 4 primary, 2 subpixel).
Taking parallel imaging with NIRCam or NIRISS while NIRSpec or NIRCam is prime would be great for stellar populations RSpec or NIRCam is prime would be great for stellar populations work.
This program could be expanded to include MIRI imaging and/or more targets.
Monitor a number of nearby SNe of various types to look for evidence of dust formation. For the purpose of this SODRM program, assume 5 SNe of two different core-collapse types (SN II-P and II-n) should be monitored for dust formation over a range of timescales relevant to the SODRM. Assume Texp + 20 days, 40 days, 80 days, and 120 days, and 160 days.
We have selected five recent SNe as representative of actual targets that might be observed. We have fictitiously named the SN as if they were objects discovered in 2018 and 2019.
MIRI LRS observations cover the 5-14 um range at R=100. Then MIRI imaging with a number of medium band filters is used to look for the 18 um silicate feature (plus sample continuum on either side) and to observe the longer wavelength continuum to provide a longer lever arm on the dust temperature.
For LRS, we assume ''PT SOURCE'' for dither type (e.g. two dither steps) and readout pattern SLOW, for no other reason than it eats up more time and makes the integrations longer. No great care has gone into selecting the exposure times.
For the imaging, we assume 3-pt dithers and sub-pixel, nominally to produce the best image quality for photometry.
The observations are ToO, in the sense that it is assumed five different SNe will be targeted at some point in the SODRM (1.5 year) period, with the first observation at 20 days post-explosion. Then a sequence of observations are requested for monitoring the targets for possible changes indicating the formation of dust in and around the SNe. For purposes of the SODRM, we assume the desire to monitor at post-explosion times of 20, 40, 80, 120, and 160 days for each SN.
We have thus added timing links (with a +/-5 day for scheduling flexibility), and we have used a ''group within 2 hours'' requirement to force the LRS and Imaging observations to be done at the same time.
This would not be a good parallel program opportunity.
For the SODRM exercise, we should decide on how to specify when to start the clock for each of the SNe (e.g. when does each SN pop off) in order to allow a staggered start to the monitoring sequence for each SN. At the moment, we have just suggested start times from the assumed beginning of the SODRM cycle.
Since the target brightness will vary over time, this may complicate the target acquisition procedure.
If any SNe to be observed are in spatially large galaxies, this might also affect the ability to find appropriate es, this might also affect the ability to find appropriate guide stars.
In reality, the timeframe for observations of dust formation are probably quite a bit longer than the time frame assumed here. Hence, this program is provided more as a way to enter some modest ToO and monitoring-type ovided more as a way to enter some modest ToO and monitoring-type observations into the SODRM than anything else.
The goal of this proposal is to investigate the centers of 75 nearby well studied galaxies (the SINGS sample) to study the ISM from 1 to 5 microns. This wavelength range gives us access to a wide variety of spectral features at sensitivities that are unprecedented. The project has two goals. The first is to learn about the ISM in these galaxies using the unique capabilities of the IFU on the JWST NIRSpec instrument. The spatial resolution and sensitivity will allow us to study star formation and topics such as black hole fueling without being affected by the dust extinction that can be significant in the optical bands. The second goal is to calibrate potentially new diagnostics against the well calibrated galaxy characteristics of the sample galaxies. The SINGS sample is an excellent sample for this given the large variety of observations of these galaxies at all wavelengths. This calibration will be extremely useful for other JWST NIRSpec observations.
Spectroscopy: Each HII region will be observed with the MIRI and NIRSpec IFUs to cover at least a 5''x5'' region (requires multiple mini-map/mosaic positions). For MIRI, all 3 grating settings will be taken. For NIRSpec, two grating settings are needed to cover the 3.3 micron aromatic feature as well as obtain good diagnostic atomic/molecular emission lines (G395H/F290LP & G235H/F170P). This will produce full spectra at each point from 1.7-28.3 microns.
Imaging: Each HII region will be observed with 4 NIRCam filters and 6 MIRI filters to provide good photometric coverage over the entire wavelength range of the spectral observations.
Parallels with either of the cameras and/or NIRISS would in principle be useful.
The mid-infrared aromatic (PAH) features have been seen to be nearly ubiquitous in dusty astrophysical objects and dust and environmental properties of star formation regions in nearby and distant galaxies. The aromatic features have been seen to vary strongly as a function of environment from Spitzer observations. These Spitzer observations have shown that the strength of the aromatic features is roughly constant up to some threshold in ionization (as probed be the [NeIII]/[NeII] line ratio) and then drops quickly with increasing ionization. While the strength of the aromatic features changes by over a factor of 10, the ratio of different features does not vary by more than a factor of two.
This result has been seen in M101 HII regions (Gordon et al. 2008, ApJ, 682, 336) and a large sample of nearby starburst galaxies (Engelbracht et al. 2008, ApJ, 678, 804). This lack of significant variation in the aromatic feature ratios is unexpected given that the most probably carrier of these features is polycyclic aromatic hydrocarbon molecules (PAHs). If the carriers are PAH molecules, the feature ratios are predicted to vary strongly due to ionization and size selective destruction mechanisms. Unfortunately, the wavelength resolution and sensitivity of the Spitzer/IRS instrument was not high enough to probe for feature ratio variations larger than about a factor of two or to measure the aromatic features for faint HII regions in nearby galaxies.
With JWST, we can obtain much higher spectral and spatial resolution observations for both bright and faint targets. This program will obtain such observations in M101 and other nearby galaxies. In addition to the spectroscopy, imaging with a variety of filters will be taken to calibrate a broad band based measurements of the aromatic features.
M101 HII regions from Gordon et al. (2008).
Imaging:Each HII region will be observed with 4 NIRCam filters and 6 MIRI filters to provide good photometric coverage over the entire wavelength range of the spectral observations.
Taking parallel NIRCam/MIRI Imaging while the other was prime would be good for stellar populations work. Parallel NIRCam/MIRI imaging while the MIRI/NIRSpec IFU observations would be great for the same reasons. A +/-180 degree flip still provides useful information.
This is a program that is similar to one that was discussed by the MIRI Science Team (discussion lead by Gordon) and by the NIRCam Science Team (lead by Engelbracht).
This program could be expanded to more targets. Galaxies that would be of interest would be M31, M33, NGC 6946, & M81 (to just name 4).
The goals are to answer fundamental questions about AGN: How are AGNs and host galaxies related/connected?
b. Some type of feedback mechanism must be at work.
This program uses the coronagraphic capabilities of NIRCam and MIRI to generate high contrast images of the vicinity of the active nucleus. Coronagraphy is needed even for those objects that are Type 2, because the dust extinction drops dramatically in the near- to mid-IR.
We will use the Sinc masks in NIRCam with the F210M, F335M, F430M filters, and the 4QPM and Lyot masks in MIRI.
The sample consists of 6 of the nearest AGNs: three of Type 1 and three of Type 2. Mrk 231, NGC 1068, NGC 1275, NGC 4151, NGC 7469, Circinus Plus six reference stars of similar brightness and rough color.
All NIRCam coronagraphy for a specific target and its associated reference star must be obtained within one day.
All MIRI coronagraphy for a specific target and its associated reference star must be obtained within one day.
In general parallels are not useful on the galaxies in question since they are typically small (¿ few arcmin). However, parallel imaging as a general rule (e.g. for nearby regions) could work here.
Determining the Hubble Constant H0 was one of the original key projects for the Hubble Space Telescope. The current era of precision cosmology demands a measurement of H0 precise to ~1%, because that, in concert with other cosmological measurements, can break degeneracies and constrain cosmological parameters including the equation of state of dark energy, the energy density of cold dark matter, and the mass of neutrinos (Hu 2005; Freedman & Madore 2010).
Two groups are currently building a path for a 1% measurement of H0 with JWST. One group is using HST and concentrating on the near-IR (PI Riess), and the other is using Spitzer and concentrating on 3.5 micron (PI Freedman). Both groups agree that Cepheid parallaxes from GAIA will improve the precision of the Cepheid distance scale. In addition, both groups envision a strategy in which JWST observes Cepheids in galaxies that have hosted Type Ia supernovae or have a Tully Fisher distance, to tie the Cepheid distance scale to either the Type Ia SNe or the TF distance scales, respectively.
Supernovae are rare; Hubble is only capable of observing Cepheids in less than 20 Ia SNe host galaxies, which will result in a 1.4% precision on H0. (Riess, talk at 2011 JWST Frontiers Workshop). JWST will be able to reach more like ~45 SNe that are currently known, plus an additional ~30 that will be discovered between now and 2020. This will give an ultimate total of ~75 galaxies with both a Ia and a Cepheid distance, producing a 0.7% precision on H0 (Riess, ibid).
This SODRM program is envisioned as the first phase in that larger effort. The goal of this first phase is to get Cepheid magnitudes at 1.15 and 3.6 um for the 20 nearest galaxies that have hosted modern (good photometry) Type Ia SNe. All already have Cepheid periods measured by Hubble, so only one JWST epoch is required. These observations will return SNR>25 (15) down to Cepheid periods of 20d (30d) in typical integrations of 0.5hr (1.5hr) at 1.15um (3.6um). This program would be tie together the Cepheid distance scales from HST, JWST, and Spitzer, as well as the JWST distance scales for Type Ia SNe and Cepheids.
As such, we have put together a plausible logical first H0 proposal for JWST. It would not solve the whole JWST H0 enchilada, but it is a plausible program for the first 1-2 years of JWST science.
A plausible second phase for the future, not part of this proposal, would be to observe Cepheids in the 25 additional Type Ia hosts that are accessible to JWST but not WFC3/HST, as well as host galaxies of new Type Ia SNe that explode between now and the end of the JWST mission. Phase 2 would require regularly-spaced, multi-epoch observations, since the Cepheids must be discovered and their periods measured.
20 nearby host galaxies of Type Ia SNe. The targets themselves have not been vetted, and are probably wrong in detail, but the RA/DEC range should be approximately right.
How far JWST can see Cepheids at 3.6um: JWST is more sensitive at 3.6um than 1.1um, but Cepheids are fainter. Here's the math: From the LMC PL relation from Scowcroft 11, we would have about a dozen LMC Cepheids (ignoring the extremely long-P ones) at a mag cutoff of m(3.6)=11.0, which corresponds to P>30d. The distance modulus to the LMC is 18.5. So, we want to reach an absolute magnitude of M=-7.5. The ETC says that, for a G0I star, we can get SNR=30 in 30 min at m(Vega,3.6um)=23.7. If we relax to SNR=15 and t=1.5hr, we can get to m(Vega,3.6um)=25.1. These correspond, respectively, to distance moduli of u=31.2 and 32.6 (or distances of 17 and 33 Mpc). So, at 3.6um, JWST can see Cepheids about as far away as HST+WFC3 can. Taking WFC3 numbers from Adam's Frontier's talk, this is Cepheids in ~20 Type Ia SNe hosts.
So, for this program, we're targeting 20 objects with NIRCAM, duplexing 90 min integrations at 1.15μm and 90 min integrations at 3.6um. So that's 40 hr of observations without overheads. The dither pattern can be quite simple, enough for good photometry. No mosaicking is required. We will need to avoid saturation on the brighter Cepheids; I haven't checked this yet**.
Target selection: Were this a real proposal, there would be a careful selection of targets, to make sure that these targets all have HST-measured Cepheids, and that the Type Ia SN in each galaxy was well-measured with modern photometry, and was a well-behaved supernova. We have not done any of this. We merely retrieved from NED the closest 20 galaxies that have hosted Type Ia SNe. So in detail the target list is unreliable. But the RA,DEC distribution should be roughly right, which is what matters here. A real proposal would also carefully choose roll angles to cover the most known Cepheids possible per galaxy, and might mosaic on the largest galaxies. We have not done so.
Timing: Cepheids in a galaxy may be observed long after the SN has faded, so the SN imposes no timing constraints. In the 20 closest galaxies, the Cepheids will have known periods from HST, and thus require fewer photometric epochs per galaxy. Let's say one epoch.
In the logical follow-up to this proposal (''Phase 2''), the Cepheids will not have been discovered or have known periods, so JWST must observe many (a dozen?) epochs in the span of something like 40d. So that phase will require timing constraints. But thait's not in this proposal. The position angle doesn't matter. Nor are there TOO needs.
Pure parallel is possible, I suppose, although it is hard to see a strong science case.
Obtain Near-IR and Mid-IR imaging of early-type galaxies at a 2-pix resolution of 10-30 pc in the near-IR (1-2.5 mm), similar to optical HST imaging of nearby galaxies out to slightly beyond the Virgo cluster. For the near-IR observations, reach a depth that allows one to perform accurate multi-component galaxy model fitting, measure surface brightness fluctuations and radial stellar population gradients, characterize dynamical and population properties of globular cluster systems and ultra-compact dwarf galaxies, measure the effect of galaxy environment upon morphological components, and scaling relations among galaxies in different environments. For the mid-IR observations, reach a depth that allows one to detect the presence of a significant intermediate-age (TP-AGB) component or warm ISM in otherwise ''old, quiescent'' galaxies.
Inner regions of galaxies at distances out to m-M ~ 35 mag.
Each galaxy will be observed with 4 NIRCam filters (i.e., 2 pairs: F090W/F277W and F200W/F356W) to provide wavelength coverage that yields adequate metallicity and mass determination. Use 8 dithers per filter pair (INTRAMODULE dithers, 4 primary, 2 subpixel).
Observe nearby galaxies with two filters (F770W, F1280W) to enable detection of intermediate-age component or warm ISM.
As to the targets: For this exercise, we randomly selected 10 early-type galaxies (T <= 0) at high Galactic latitude (|b| > 45 degrees) with 4000 < v [km/s] < 4500 (for NIRCam and MIRI observations) and 20 more galaxies with 4500 < v [km/s] < 7500 (for only NIRCam observations). We used the SQL query in HyperLEDA to define the sample.
Yes, taking parallel imaging with MIRI or NIRCam while NIRCam or MIRI is prime, respectively, would be great for stellar populations-related science.
This program could be expanded to include more targets.
The detailed star formation history in spiral galaxy halos has only been characterized in the Local Group. The analysis requires photometry reaching stars on the old main sequence, below the main sequence turnoff (MV of 4 to 4.5 mag). These low-mass stars (~0.8 M☉) are only accessible to Hubble within approximately 1 Mpc, and well beyond the reach of ground-based observatories. The increased sensitivity of Webb will allow us to probe such populations beyond the confines of the Local Group. Color-magnitude diagrams constructed from the F090W and F200W filters on NIRCam provide an accurate probe of the age and metallicity distribution in a population (i.e., the star formation history), rivaling the most commonly-used probes on Hubble (e.g., ACS F606W & F814W), given the long wavelength baseline between the NIRCam filters. The well-known age-metallicity degeneracy can be broken if the color-magnitude diagram includes the main sequence, subgiant branch, and red giant branch.
The spiral galaxy NGC 55 is a good example of a spiral galaxy well beyond the reach of Hubble but perfectly suitable to Webb. It is inclined nearly edge-on, lies at a distance of 1.9 Mpc, and has a low foreground reddening of E(B-V)=0.013 mag. The bare minimum depth needed to obtain an accurate star formation history is a color-magnitude diagram reaching 0.5 mag below the turnoff at a signal-to-noise ratio of 5. Assuming an old (12 Gyr) population of somewhat low metallicity ([Fe/H]=-1.1) in the halo, this point on the NGC 55 main sequence will have an effective temperature of 6200 K and an apparent ence will have an effective temperature of 6200 K and an apparent magnitude of V= 31 Vega magnitudes.
Minor axis field in NGC55.
To obtain a signal-to-noise ratio of 5 in each filter, the NIRCam ETC claims we need 200 ksec in F090W and 290 ksec in F200W for an F8V star at V=31 mag. This corresponds to 136 hours of exposure time, or 170 hours of actual time with overheads, assuming an efficiency of 80% (given the similarity to other deep NIRCam imaging programs in the draft overheads report). While exposing in the F090W and F200W filters in the short-wavelength detectors, we will employ the F277W in the long-wavelength detectors. The time in F277W will provide a lower signal-to- noise ratio (3.7) at a point 0.5 mag below the turnoff, but will provide additional metallicity and temperature leverage. The exposures can be obtained with a simple PSF-resampling dither pattern, and there is no need to tile a mosaic larger than the NIRCam field of view, nor fill in the various gaps between the detectors and modules. However, all of the images should be obtained at the same orientation to maximize area covered at the full depth.
Same orientation for all exposures.
Yes. The other instruments will have access to other parts of the NGC 55 environment, so one can imagine a variety of of possible parallel observations (e.g., deep NIRISS imaging in similar filters, deep spectroscopy of composite disk populations).
NGC 55 is a representative target in this program. There are other galaxies beyond the Local Group where similar science could be pursued using a program analogous to the one described here.
A promising new method to probe chemical abundances in external galaxies is with red supergiants supergiants (RSGs). With their peak flux at ~1μm and with luminosities in excess of 104 L☉, they are extremely bright in the near-IR (MJ = -7 to -11). Davies, Kudritzki & Figer (2010) developed a new technique to use RSGs as chemical 'beacons' to map the star-forming history of their host galaxies, via analysis of absorption-line spectroscopy in the J-band. Optical spectroscopy of blue supergiants has been used to estimate metallicities (via oxygen) in some external galaxies (e.g. Bresolin et al. 2002; Urbaneja et al. 2005; Evans et al. 2007; Kudritzki et al. 2008). However, the RSG technique will provide direct estimates of iron abundances, as well as for α-elements such as Si and Mg, allowing the ratio α/Fe to be studied, an important diagnostic of the star-formation history of the host system.
The new technique has been studied in the context of ELTs and JWST by Evans et al. (2011) It requires good signal-to-noise (S/N>50) at only moderate spectral resolving power (R ~3000), in a part part of the J-band where the diagnostic lines are well separated. With JWST-NIRSpec we will be able to map stellar abundances and approximate radial velocities across entire galaxies at distances well beyond the Local Group, providing a 3D picture of their star-formation histories and leading to new constraints on the mass-metallicity relation.
RSGs in two of the closest 'Grand Design' spirals: M83 (at 4.5 Mpc) and M51 (8.5 Mpc), and the face-on spiral NGC 3621 (6.7 Mpc).
Each galaxy requires multiple NIRSpec pointings to map the radial gradients along the primary axes. Only J-band spectroscopy is required. Using the NIRSpec MSA ETC, adopting: G140H grating/F100 LP filter (R~2700), M0I spectral template, λ(S/N)=1.25 &956;m, modest E(B-V)~0.15mag, for MJ -8 at S/N>50, requires total exposures of: M83 (J=20.3): 5hrs/pointing; NGC 3621 (J=21.1): 15 hrs/pointing; M51 (J=21.65): 35 hrs/pointing. Three pointings in each galaxy leads to a total of 165hrs.
Will require repeat observations with the MSA at the same position angle on the target galaxy.
Taking parallel NIRCam/MIRI Imaging would be useful for studies of stellar populations (in many instances be useful for studies of stellar populations (in many instances complementing WFPC2/ACS/WFC3 optical observations).