Tracing the Dusty S-cluster Object on its orbit around the supermassive black hole in our galaxy. Near infrared observations with sinfoni at the Very Large Telescope

A faint gas cloud called G2 is found in 2011 on its way towards Sgr A* (that is associated with a supermassive black hole) in the center of the Milky Way. Since the first identification of this source (that is called Dusty S-cluster Object in the following), it is traced until spring 2014 in the red-shifted Br-gamma wavelength domain with the SINFONI instrument mounted at the VLT/Chile. It emits not only in the red-shifted Br_gamma domain (with respect to the rest wavelength at 2.1661 µm, the Dusty S-cluster Object (DSO) is also traceable in the red-shifted Pa-alpha and He-I regime. It is claimed, that the object in the years before the periapse (i.e., the closest approach to Sgr A*) already starts to dissolve due to the gravitational force of the black hole. Because of that, a tail is detected that follows the object towards Sgr A*. It is expected, that the 'head' and the 'tail' cause a firework-like event that leads to a bright accretion behavior of Sgr A*. In the observations that take place after May 2014, the emitted and prominent Br-gamma line is observed in the blue-shifted wavelength domain. This indicates that the source passed the supermassive black hole Sgr A*. Furthermore, the object is still intact after the periapse. In this work, a detailed and extensive analysis of the SINFONI data between 2006 and 2015 with a spatial pixel scale of 0".025 and a corresponding field of view of 0".8x0".8 per single data cube with the focus on the DSO is presented. In detail, the position of the DSO in the Br-gamma linemaps is compared to the position of the DSO in the Lucy-Richardson algorithm de- and re-convolved images. The K-band magnitude, as well as the flux from these images, are extracted. The derived orbit is based on the analyzed 10-year survey. Using the position and the velocity of the DSO from the data-cubes, position-velocity diagrams for every year between 2006 and 2015 are presented. Many authors believe that the object is rather a gas cloud than a dust embedded young stellar object. The question, why this topic raises such a controversial point of view, is discussed in this work. Also, the final answer about the nature of the DSO is given. For the analysis, the standard reduction procedure with the SINFONI and EsoRex pipeline is used. Because of not satisfying results, post-data corrections are applied in order to establish various calibration procedures. For the low-pass filtered images, the Lucy-Richardson deconvolution algorithm is applied to K-band images of the Galactic center region around Sgr A*. The analysis shows that the DSO can, without any confusion, be identified in the Br-gamma linemaps between 2006 and 2015. It can also be traced as a compact source in the deconvolved K-band images in every epoch from 2006 to 2015. A comparison between stacked K-band and Lucy-Richardson images show the robustness of the procedure. The derived position-velocity diagrams underline the compact nature of the DSO. These extensive analysis methods reveal a new source that is called OS1. It follows the DSO on a similar orbit around Sgr A* and can explain the claimed tail emission of the DSO. The magnitude and flux extraction show that neither a brightening nor an increase in flux density of the infrared excess source takes place before or after the pericenter passage. The fact that the DSO can be observed in the near-infrared SINFONI data as a compact source between 2006 and 2015 indicates a stellar origin of the object. It is highly unlikely that a non-stellar object, for example, a gas cloud, could survive the pericenter passage of a supermassive black hole and stay compact on a Keplerian orbit especially after the flyby event in 2014. A stretching that leads to the creation of a tail in the epochs before the periapse cannot be confirmed by judging the results in the linemaps, the low-pass filtered continuum images of the GC, or in the position-velocity diagrams

The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a rich population of dusty objects in the IRS 13 cluster

A detailed analysis of the Nuclear Stellar Cluster (NSC) concedes not only the existence of the Scluster with its fast-moving stars and the supermassive black hole (SMBH) Sgr A*. It also reveals an embedded region of gas and dust with an exceptionally high stellar density called IRS 13. The IRS 13 cluster can be divided into the northern and the eastern counterparts, called IRS 13N and IRS 13E, respectively. This work will focus on both regions and study their most prominent members using rich infrared and radio/submm data baselines. Applying a multiwavelength analysis enables us to determine a comprehensive photometric footprint of the investigated cluster sample. Using the raytracing-based radiative transfer model HYPERION, the spectral energy distribution of the IRS 13 members suggests a stellar nature of the dusty sources. These putative Young Stellar Objects (YSOs) have a comparable spectroscopic identification to the D and G sources in or near the S cluster. Furthermore, we report the existence of a population of dusty sources in IRS 13 that can be mostly identified in the H-, K-, and Lband. Together with the objects reported in literature, we propose that this population is the outcome of a recent star formation process. Furthermore, we report that these presumably young objects are arranged in a disk structure. Although it cannot be excluded that the intrinsic arrangement of IRS 13 does show a disk structure, we find indications that the investigated cluster sample might be related to the counterclockwise disk.Comment: 59 pages, 44 figures, accepted and published by the Ap

Mid-Infrared studies of dusty sources in the Galactic Center

Mid-Infrared (MIR) images of the Galactic center show extended gas and dust features along with bright IRS sources. Some of these dust features are a part of ionized clumpy streamers orbiting Sgr~A*, known as the mini-spiral. We present their proper motions over 12 year time period and report their flux densities in $N$-band filters {and derive their spectral indices}. The observations were carried out by VISIR at ESO VLT. High-pass filtering led to the detection of several resolved filaments and clumps along the mini-spiral. Each source was fit by a 2-D Gaussian profile to determine the offsets and aperture sizes. We perform aperture photometry to extract fluxes in two different bands. We present the proper motions of the largest consistent set of resolved and reliably determined sources. In addition to stellar orbital motions, we identify a stream-like motion of extended clumps along the mini-spiral. We also detect MIR counterparts of the radio tail components of the IRS7 source. They show a clear kinematical deviation with respect to the star. They likely represent Kelvin-Helmholtz instabilities formed downstream in the shocked stellar wind. We also analyze the shape and the orientation of the extended late-type IRS3 star that is consistent with the ALMA sub-mm detection of the source. Its puffed-up envelope with the radius of $\sim 2\times 10^6\,R_{\odot}$ could be the result of the red-giant collision with a nuclear jet, which was followed by the tidal prolongation along the orbit.Comment: 25 pages, 13 figures, 6 tables, accepted for publication in Ap

The Galactic Center with Roman

We advocate for a Galactic center (GC) field to be added to the Galactic Bulge Time Domain Survey (GBTDS). The new field would yield high-cadence photometric and astrometric measurements of an unprecedented ${\sim}$3.3 million stars toward the GC. This would enable a wide range of science cases, such as finding star-compact object binaries that may ultimately merge as LISA-detectable gravitational wave sources, constraining the mass function of stars and compact objects in different environments, detecting populations of microlensing and transiting exoplanets, studying stellar flares and variability in young and old stars, and monitoring accretion onto the central supermassive black hole. In addition, high-precision proper motions and parallaxes would open a new window into the large-scale dynamics of stellar populations at the GC, yielding insights into the formation and evolution of galactic nuclei and their co-evolution with the growth of the supermassive black hole. We discuss the possible trade-offs between the notional GBTDS and the addition of a GC field with either an optimal or minimal cadence. Ultimately, the addition of a GC field to the GBTDS would dramatically increase the science return of Roman and provide a legacy dataset to study the mid-plane and innermost regions of our Galaxy.Comment: 19 pages, 3 figures. Submitted to the NASA Roman Core Community Surveys White Paper Cal

MIDIS: JWST/MIRI reveals the Stellar Structure of ALMA-selected Galaxies in the Hubble-UDF at Cosmic Noon

We present deep James Webb Space Telescope (JWST)/MIRI F560W observations of a flux-limited, ALMA-selected sample of 28 galaxies at z=0.5-3.6 in the Hubble Ultra Deep Field (HUDF). The data from the MIRI Deep Imaging Survey (MIDIS) reveal the stellar structure of the HUDF galaxies at rest-wavelengths of >1 micron for the first time. We revise the stellar mass estimates using new JWST photometry and find good agreement with pre-JWST analysis; the few discrepancies can be explained by blending issues in the earlier lower-resolution Spitzer data. At z~2.5, the resolved rest-frame near-infrared (1.6 micron) structure of the galaxies is significantly more smooth and centrally concentrated than seen by HST at rest-frame 450 nm (F160W), with effective radii of Re(F560W)=1-5 kpc and S\'ersic indices mostly close to an exponential (disk-like) profile (n~1), up to n~5 (excluding AGN). We find an average size ratio of Re(F560W)/Re(F160W)~0.7 that decreases with stellar mass. The stellar structure of the ALMA-selected galaxies is indistinguishable from a HUDF reference sample of galaxies with comparable MIRI flux density. We supplement our analysis with custom-made, position-dependent, empirical PSF models for the F560W observations. The results imply that an older and smoother stellar structure is in place in massive gas-rich, star-forming galaxies at Cosmic Noon, despite a more clumpy rest-frame optical appearance, placing additional constraints on galaxy formation simulations. As a next step, matched-resolution, resolved ALMA observations will be crucial to further link the mass- and light-weighted galaxy structures to the dusty interstellar medium.Comment: 19 pages, 10 figures, 1 table, submitted to Ap

Observation of S4716 -- a Star with a 4 yr Orbit around Sgr A*

The ongoing monitoring of the Galactic center and Sgr A*, the central supermassive black hole, produces surprising and unexpected findings. This goes hand in hand with the technical evolution of ground- and space-based telescopes and instruments, but also with the progression of image filter techniques such as the Lucy Richardson algorithm. As we continue to trace the members of the S cluster close to Sgr A* on their expected trajectory around the supermassive black hole, we present the finding of a new stellar source, which we call S4716. The newly found star orbits SgrA* in about 4.0 yr and can be detected with NIRC2 (Keck), OSIRIS (Keck), SINFONI (VLT), NACO (VLT), and GRAVITY (VLTI). With a periapse distance of about 100 au, S4716 shows an equivalent distance toward Sgr A* as S4711. These fast-moving stars undergo a similar dynamical evolution, since S4711-S4716 share comparable orbital properties. We will furthermore draw a connection between the recent finding of a new faint star called S300 and the data presented here. Additionally, we observed a blend-star event with S4716 and another newly identified S star S148 in 2017.Comment: 33 pages, 23 figures, published at Ap

The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a Rich Population of Dusty Objects in the IRS 13 Cluster

A detailed analysis of the nuclear star cluster not only concedes the existence of the S cluster, with its fast-moving stars and the supermassive black hole Sgr A*. It also reveals an embedded region of gas and dust with an exceptionally high stellar density called IRS 13. The IRS 13 cluster can be divided into the northern and eastern counterparts, called IRS 13N and IRS 13E, respectively. This work will focus on both regions and study their most prominent members using rich infrared and radio/submillimeter data baselines. Applying a multiwavelength analysis enables us to determine a comprehensive photometric footprint of the investigated cluster sample. Using the ray-tracing-based radiative transfer model HYPERION, the spectral energy distribution of the IRS 13 members suggests a stellar nature of the dusty sources. These putative young stellar objects (YSOs) have a comparable spectroscopic identification to the D and G sources in or near the S cluster. Furthermore, we report the existence of a population of dusty sources in IRS 13 that can be mostly identified in the H , K , and L band. We propose that, together with the objects reported in the literature, this population is the outcome of a recent star formation process. Furthermore, we report that these presumably young objects are arranged in a disk structure. Although it cannot be excluded that the intrinsic arrangement of IRS 13 does show a disk structure, we find indications that the investigated cluster sample might be related to the counterclockwise disk

First observed interaction of the circumstellar envelope of an S-star with the environment of Sgr A*

Several publications highlight the importance of the observations of bow shocks to learn more about the surrounding interstellar medium and radiation field. We revisit the most prominent dusty and gaseous bow shock source, X7, close to the supermassive black hole, Sgr~A*, using multiwavelength analysis. For the purpose of this study, we use SINFONI (H+K-band) and NACO ($L'$- and $M'$-band) data-sets between 2002 and 2018 with additional COMIC/ADONIS+RASOIR ($L'$-band) data of 1999. By analyzing the line maps of SINFONI, we identify a velocity of $\sim 200$ km/s from the tip to the tail. Furthermore, a combination of the multiwavelength data of NACO and SINFONI in the $H$-, $K$-, $L'$-, and $M'$-band results in a two-component black-body fit that implies that X7 is a dust-enshrouded stellar object. The observed ongoing elongation and orientation of X7 in the Br$\gamma$ line maps and the NACO $L'$-band continuum indicate a wind arising at the position of Sgr~A* or at the IRS16 complex. Observations after 2010 show that the dust and the gas shell seems to be decoupled in projection from its stellar source S50. The data also implies that the tail of X7 gets thermally heated up due to the presence of S50. The gas emission at the tip is excited because of the related forward scattering (Mie-scattering), which will continue to influence the shape of X7 in the near future. In addition, we find excited [FeIII] lines, which underline together with the recently analyzed dusty sources and the Br$\gamma$-bar the uniqueness of this source.Comment: 28 pages, 12 figure, 10 tables, accepted for publication by the Astrophysical Journa

X3: A High-mass Young Stellar Object Close to the Supermassive Black Hole Sgr A*

To date, the proposed observation of young stellar objects (YSOs) in the Galactic center still raises the question of where and how these objects could have formed due to the violent vicinity of Sgr A*. Here, we report the multiwavelength detection of a highly dynamic YSO close to Sgr A* that might be a member of the IRS 13 cluster. We observe the beforehand known coreless bow-shock source X3 in the near- and mid-infrared (NIR/MIR) with SINFONI (VLT), NACO (VLT), ISAAC (VLT), VISIR (VLT), SHARP (NTT), and NIRCAM2 (KECK). In the radio domain, we use CO continuum and H30 α ALMA observations to identify system components at different temperatures and locations concerning the central stellar source. It is suggested that these radio/submillimeter observations in combination with the NIR Br γ line can be associated with a protoplanetary disk of the YSO, which is consistent with manifold VISIR observations that reveal complex molecules and elements such as PAH, S iv , Ne ii , and Ar iii in a dense and compact region. Based on the photometric multiwavelength analysis, we infer the mass of ${15}_{-5}^{+10}{M}_{\odot }$ for the YSO with a related age of a few 10 ^4 yr. Due to this age estimate and the required relaxation timescales for high-mass stars, this finding is an indication of ongoing star formation in the inner parsec. The proper motion and 3D distance imply a relation between X3 and IRS 13. We argue that IRS 13 may serve as a birthplace for young stars that are ejected due to the evaporation of the cluster