Universität zu Köln

Hosseini, Seyedeh Elaheh ORCID: 0000-0002-3004-6208 (2023). On the ambient medium and a stellar association around the supermassive black hole Sagittarius A*. PhD thesis, Universität zu Köln.

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Abstract

The center of the Milky Way is located at a distance of 8 kiloparsec from us and harbors a supermassive black hole (SMBH), known as Sagittarius A* (Sgr A*). The center of the Milky Way is the nearest galactic nucleus to us. In this thesis, I study the Galactic Center in the near and mid-infrared regime. I first focus on the ambient medium of the SMBH, Sgr A*. I analyze L′-band observations to investigate the probable bow shock features of the S2 star around the time of its periapse along its orbit around Sgr A* in May 2018. I place an upper limit on the density of the ambient medium of Sgr A*. However, the bow-shock synchrotron model provides a tighter upper limit at 1500 Schwarzschild radii. This upper limit allows for a wide variety of ambient media properties at the S2 star’s pricenter distance, but excludes the standard thin cold disk model. The improvements in the photometric sensitivity of the Extremely Large Telescope can provide tighter constraints on the ambient medium density around Sgr A*. Investigating the ambient medium around Sgr A*, I discovered an apparent concentration of stars at a distance of about 0.25 pc to the north-east of Sg A*. I investigated this sample of 42 stars which are located close to the bow shock source IRS 1W within a radius of 0.05 pc. The velocity dispersion of this subsample motivates me to consider them as an association of stars. I name them N-sources due to their northward moving direction. This association of stars can be a bound system due to a putative intermediate-mass black hole (IMBH) with a mass of about 1000-10000 M⊙. Another plausible scenario could be a projection of a disk-like distribution of young He-stars and/or dust-enshrouded stars. The non-detection of any bright source in X-ray at the position of N-sources weakens the IMBH scenario, whereas the location of IRS 13E and N-sources on the west and east side of Sgr A*, respectively, strengthens the disk-like distribution scenario. The most important component of the Galactic Center research is the SMBH, Sgr A*, which is a variable radio, near infrared, and X-ray source. For this reason, in the last part of my thesis, I conducted a research on the SMBH in near- and mid-infrared. In near-infrared, I investigate the simultaneous Ks- and L′-band observations in 2018. These observations were conducted in collaboration with the Event Horizon Telescope campaign to study the flare activities of Sgr A*. I detected a flare activity in Ks-band in the observations. However, after switching the filter to observe in L′-band, there the flare is not detectable anymore. This could be due to the time gap between switching the filters or it could be one of the Ks-band flares of Sgr A* which does not have a L′-band counterpart. The origins of these flares are believed to lie in the accretion flow onto Sgr A*. Most models include some form of synchrotron emission to explain the occurring flares from the radio to the NIR regime. However, the correlation pattern between the wavelengths is not yet completely understood. Related to this, in mid-infrared, I analyze observations of the Galactic Center in the N-band with the Very Large Telescope/VISIR. The previous attempts to detect the counterpart of Sgr A* in N-band led to placing an upper limit on the flux density of Sgr A*. I was able to obtain a matching upper limit of 10 mJy on the brightness of the mid-infrared counterpart of Sgr A*. This result is in agreement with the theoretically expected spectral energy density of the mean emission from Sgr A*. Finally, the launch of the most powerful infrared space telescope, the JamesWebb Space Telescope (JWST), opened a new era in astrophysics. Due to the delay in its launch, I hereby present preliminary results of the first JWST observations of the Galactic Center with NIRSpec. I compile a high-resolution, low-noise spectrum towards the Galactic Center, which demonstrates the great potential of JWST. Several lines can be identified in the spectrum, e.g. the [FeIII] line which might originate from the mini-spiral lying in the field of view. In general, the spectrum is in good agreement with the previous ground-based spectral studies of the Galactic Center. Furthermore, I present MIRI observations of three quasars, with redshifts of 0.4, 1.5, and 2.9, to establish the foundations for the upcoming MIRI Galactic Center observations. The quasars are observed in all four channels of MIRI. Heavy elements such as Cr, Fe, Mg, K, Si, Ar, and others are detected in their spectra once again demonstrating the great capabilities of JWST.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Hosseini, Seyedeh Elaheh hosseini@ph1.uni-koeln.de orcid.org/0000-0002-3004-6208 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-718389
Date:	22 December 2023
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language Galactic Center, Sagittarius A*, Ambient medium, Stellar association, Stars, Black hole, Intermediate mass black hole, IRS 13, IRS 1W English
Date of oral exam:	20 July 2023
Referee:	Name Academic Title Eckart, Andreas Prof. Dr. Zensus, J. Anton Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71838