Science Keywords: Select from 2 to 5 items. At least one target must be specified in Phase I 0 Assigned ID: 3802 ----- Attempting Submission 1 (Tue Aug 30 15:04:38 GMT 2022) ----- Joint Chandra/HST 0 ----- Attempting Submission 1 (Wed Sep 28 15:52:58 GMT 2022) ----- A population of intermediate black holes (IMBH) will allow us to solve one of the long-standing problems in modern astrophysics, the origin of supermassive black holes. We explore the published sample of 305 optically selected IMBH candidates (3e4<M_BH<2e5 M_Sun), which reside in galaxy centers and appear as weak type-I AGN. As of now, 24 of them were confirmed in X-ray using archival data. Here we propose to observe 23 objects from that list with Chandra and 5 most compact host galaxies with the HST to double the number of X-ray confirmed IMBHs and get the first insight on structural properties of compact IMBH host galaxies. This will enable detailed studies of the IMBH population and help to constrain the masses of seed black holes in the early Universe. This is a joint Chandra/HST program. We propose (i) to observe with Chandra ACIS-S 23 IMBH candidates from a sample of optically selected low-redshift (0.021<z<0.1) AGN from SDSS exhibiting optical variability and/or broad line component in H-alpha present in multiple epochs with the expected X-ray fluxes detectable in <20 ksec; and (ii) obtain high-resolution NUV-to-optical images for the host galaxies of the five most compact objects using HST WFC3/UVIS. The candidates have virial BH masses between 30,000 and 200,000 MSun. Our observations will nearly double the currently available sample of X-ray confirmed IMBHs and triple the sample of IMBH host galaxies with high-resolution images, which is a crucially important aspect of our understanding of the physics behind the massive black hole growth. The X-ray luminosity combined with the MBH estimate from optical spectra will yield Eddington rates. The light profile decomposition done on high-resolution imaging will allow us to place IMBHs on the MBH-Mbulge diagram to test the BH/host co-evolution. The use of two optical colors will allow us to detect stellar population gradients in bulges, which can clarify the bulge growth scenario: for in situ ``inside-out'' pseudo-bulge growth there should be a negative metallicity gradient, while if a bulge is formed by mergers, the color and metallicity profiles should be flat because of the mixing of stars that occurs during mergers. Finally, the observations will place limits on the UV accretion SEDs. We propose to use the F300X, F350LP and F600LP filters. (i) the near-UV F300X is the deepest UV option in WFC3/UVIS; it will to isolate a compact active nucleus from star formation in the bulge (ii) a difference of F350LP and F600LP is much deeper than any blue optical WFC3 filter; it will reveal dust morphology in the inner part and will be crucial for the stellar population analysis of host galaxies combined with pure F600LP; (ii) F600LP will be the primary filter for the 2D light profile decomposition; (iii) F350LP and F600LP will provide the best stellar mass estimates for structural components. We computed stellar population models using PEGASE.2 for the proposed filter setup and confirmed its suitability. We use the WFC3 ETC to estimate the signal-to-noise ratio using best-fitting models of the light profiles of our targets from the Simard11 catalog. We estimate that in 700sec per filter in the optical F350LP and F600LP bands we will reach S/N ratios between 40 and 100 per pixel in the central regions of the IMBH host galaxies where we need to perform a two-dimensional light profile decomposition. It is difficult to make predictions for the UV AGN continuum in the proposed targets, as this regime has yet to be probed systematically. However, based on the results of past observational campaigns of small SMBHs such as proposal 13943 by Reines et al. available in the HST archive, which have significant nucleus detections, we request similar F300X integration times of 700sec per target. In total, we request 5 orbits to complete our project. IC, IZ, and IK have been working together for over a decade; KG joined us in 2016 during his 1st undergraduate year and already co-authored several refereed papers (ApJ and ApJS; three of those papers were followed with press releases and attracted media attention), and gave numerous presentations at professional conferences including an invited talk (tutorial on the preparation of observations) at the 40th anniversary of the MMT observatory. VT, VG and Ivan Kuzmin (all supervised by IC) performed the analysis of XMM-Newton, Chandra, and Swift X-ray data that led to the selection of targets for this proposal and also did search/identification of new intermediate-mass black hole candidates (paper in preparation). MD is a MSc student supervised by IC, she is in charge of the analysis of optical light curves for IMBH candidates based on ZTF and ATLAS transient surveys. Franz Bauer is a professor at Pontifica Universidad Catolica (Chile). He is an expert in multi-wavelength studies of AGN and star-forming galaxies with Chandra, XMM-Newton, HST, ALMA, and a wide range of optical/near-IR ground-based facilities. FB is a professor at Pontifica Universidad Catolica (Chile). He is an expert in multi-wavelength studies of AGN and star-forming galaxies with Chandra, XMM-Newton, HST, ALMA, and a wide range of optical/near-IR ground-based facilities. EC is a graduate student supervised by FB. We have solid experience in the cutting-edge Virtual Observatory powered research in astrophysics which resulted in 2 publications in AAAS Science (Chilingarian et al. 2009 and Chilingarian and Zolotukhin 2015). With very limited manpower, we have successfully completed several projects, which included large-scale data analysis efforts, e.g. development of a dedicated data analysis technique and subsequent analysis of 1 million SDSS spectra. We compiled a multi-wavelength FUV-to-NIR Reference Catalog of Spectral Energy Distributions of galaxies (RCSED; Chilingarian et al. 2017 ApJS 228 14; http://rcsed.sai.msu.ru/) and identified the first bona fide population of intermediate-mass black holes using it (Chilingarian et al. 2018 ApJ 863 1). During the last 3 years we have been running a very successful multi-wavelength campaign on search and characterization of intermediate-mass black holes and their host galaxies using Chandra, XMM-Newton, Swift, intermediate-resolution ground-based optical spectroscopy using MagE at the 6.5-m Magellan Telescope, NIR ground-based imaging using Magellan and MMT. Our team has a 20-yr long expertise in reducing, analyzing, and publishing data from many IFU spectrographs operated on intermediate sized and large ground based telescopes (including but not limited to SAURON@WHT, PMAS@CalarAlto~3.5, GMOS-IFU@Gemini, VIMOS-IFU@VLT, FLAMES@VLT, NIFS@Gemini, SINFONI@VLT) including NIR spectrographs equipped with image slicers. We have performed analysis of multi-wavelength data for several projects in the past, including the IMBH search and identification project published in 2018 in ApJ which falls into the 2\% of most cited papers in astrophysics published in 2018. 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