0Probing the UV to MIR spectra of local Little Red DotsJWST has uncovered a mysterious population of compact, red sources at z2-10 known as "Little Red Dots" (LRDs). Their observed properties defy nearly all typical AGN properties, leading researchers to hypothesize that they could be primordial supermassive stars or enshrouded supermassive black holes ('quasi-stars'), offering a direct window into heavy black hole seed formation. A small population of local analogs at z<0.5 have been recently identified, ~5-10 magnitudes brighter than those from JWST, providing an unparalleled window into the phenomena. We propose JWST/NIRSpec+MIRI + HST UV spectroscopy of six local LRD analogs to decipher their nature. Rest-frame 0.1-0.2 and 1-28 um spectra will probe key diagnostics: UV emission, absorption and outflows; dust continuum shape; silicate features; PAH emission; and fine-structure lines, and numerous absorption lines and bands. This sample will push further into the MIR than is possible for their distant cousins, thereby enabling novel insights into the enigmatic LRD population and their central power sources.We propose to observe the sample of 3 local LRDs with both HST COS GL130M + GL160M and WFC3 UVIS F300X, F475W, and F850LP. The proposed UV spectral and spatially resolved studies of faint compact emission are well-suited to HST's capabilities in space. Based on the high Balmer decrements, low metallicities and existing rest-frame UV spectra of BL-LRDs with JWST, we may expect that the UV regime of BL-BCDs may lack most broad and/or bright lines, aside from Ly-alpha and perhaps HeI and HeII. Instead, if the UV is dominated stellar light from a nuclear star cluster, we expect to see FUV signatures of very young stellar populations, namely, stellar absorptions.
For COS, we will use the two medium-resolution gratings, G130M and G160M (lam_cen=1291 and 1589, respectively) to cover the almost continuous spectral coverage between Ly-alpha and HeII (1640A), i.e., 1130--1780\AA with a few (short) gaps. We adopt a combination of G130M+G160M rather than a single setup with G140L because (i) each of them provides almost a 2x better throughput, so after binning along the wavelength we will get similar quality of continuum measurements; (ii) they provide much higher spectral resolution, which is crucial for the detection of narrow blue-shifted absorption lines associated with outflows, as we expect for our targets.
For UVIS WFC3, we will use F300X+F475W+F850LP to probe the faint extended emission associated with the underlying older stellar population, enabling constraints on the stellar mass, age, and morphology of the hosts. These particular filters were chosen so as to mitigate contamination from high-EW lines in the nucleus, the PSF of which we will need to model and remove. The latter motivates us to adopt 3 UVIS bands, which offer a somewhat better sampled PSF compared to the IR detector. Notably, F300X will probe young stars in the NUV, and is 3 times more efficient than F275W for the same exposure time. F850LP will probe a redder starlight to offer a useful upper limit on stellar mass, while avoiding H-alpha and HeI 10830A. F850LP will also serve as the primary filter for the two-dimensional light profile decomposition to estimate structural properties (dust attenuation is not considered important here vs. the bluer bands, while the width of the instrumental PSF is still small). F475W (SDSS g) will be crucial for the analysis of stellar population properties of host galaxies in a combination with F850LP, while avoiding H-beta+[O III] and only suffering minor continuum contamination from weaker Balmer lines and [O II].n/aGalaxies6falsefalsefalsefalsefalsefalsefalsetrueSECONDARYJWST12316falseSUPPORTEDGALAXYDWARF COMPACTYESThis object was generated by the targetselector and retrieved from the SIMBAD database.J2000J2000GALAXYDWARF COMPACTYESJ2000J2000GALAXYDWARF COMPACTNOJ2000J2000WFC3/UVISImagingNoNoF300XWFC3/UVISImagingNoNoF475WWFC3/UVISImagingNoNoF850LPWFC3/UVISImagingNoNoF300XWFC3/UVISImagingNoNoF475WWFC3/UVISImagingNoNoF850LPWFC3/UVISImagingNoNoF300XWFC3/UVISImagingNoNoF475WWFC3/UVISImagingNoNoF850LPCOS/FUVSpectroscopicNoNoG130M1291COS/FUVSpectroscopicNoNoG160M1589COS/FUVSpectroscopicNoNoG130M1291COS/FUVSpectroscopicNoNoG160M1589COS/FUVSpectroscopicNoNoG130M1291COS/FUVSpectroscopicNoNoG160M1589POS-TARGDITHER0.173LINE0.112falsefalse81.785423.884WFC3-UVIS-DITHER-BOX1falsesource shows mild asymmetric extension ~2" to NE.(Tic-Visit
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