We study the multi-phase medium in the vicinity of the active galactic nucleus Centaurus A (Cen A). Combined high-resolution observations with the ALMA and Chandra observatories indicate that the hot X-ray emitting plasma coexists with the warm and cold media in Cen A. This complex environment is a source of CO lines with great impact for its diagnostics. We present the images from the two above-mentioned instruments covering the nuclear region (diameter of 10'' i.e., ~180 pc), and we study the conditions for plasma thermal equilibrium and possible coexistence of cool clouds embedded within the hot X-ray emitting gas. Further, we demonstrate that the multi-phase medium originates naturally by the thermal instability (TI) arising due to the interaction of the high-energy radiation field from the nucleus with the ambient gas and dust. We demonstrate that cold gas clouds can coexist in the mutual contact with hot plasma, but even colder dusty molecular clouds have to be distanced by several hundred pc from the hot region. Finally, we propose a 3-D model of the appearance of the hot plasma and the CO line-emitting regions consistent with the Chandra image and we derive the integrated emissivity in specific molecular lines observed by ALMA from this model. To reproduce the observed images and the CO line luminosity the dusty shell has to be ~420 pc thick and located at ~1000 pc from the centre.
Narrow-band HST imaging has resolved the detailed internal structure of the 10 kpc diameter H alpha+[NII] emission line nebulosity in NGC4696, the central galaxy in the nearby Centaurus cluster, showing that the dusty, molecular, filaments have a width of about 60pc. Optical morphology and velocity measurements indicate that the filaments are dragged out by the bubbling action of the radio source as part of the AGN feedback cycle. Using the drag force we find that the magnetic field in the filaments is in approximate pressure equipartition with the hot gas. The filamentary nature of the cold g…
We report a detection (3.5x10^37 \pm 5.6x10^36 ergps) of the optical coronal emission line [Fe X]6374 and upper limits of four other coronal lines using high resolution VIMOS spectra centred on NGC 4696, the brightest cluster galaxy in the Centaurus cluster. Emission from these lines is indicative of gas at temperatures between 1 and 5 million K so traces the interstellar gas in NGC 4696. The rate of cooling derived from the upper limits is consistent with the cooling rate from X-ray observations (~10 solar masses per year) however we detect twice the luminosity expected for [Fe X]6374 emissio…
We present photometry of the nearby galaxy NGC 5128 (Centaurus A) observed with the PACS and SPIRE instruments on board the Herschel Space Observatory, at 70, 160, 250, 350 and 500 μm, as well as new CO J = 3-2 observations taken with the HARP-B instrument on the JCMT. Using a single component modified blackbody, we model the dust spectral energy distribution within the disk of the galaxy using all five Herschel wavebands, and find dust temperatures of ~30 K towards the centre of the disk and a smoothly decreasing trend to ~20 K with increasing radius. We find a total dust mass of (1.59 \pm 0.…
Supermassive black holes and supernovae explosions at the centres of active galaxies power cycles of outflowing and inflowing gas that affect galactic evolution and the overall structure of the Universe. While simulations and observations show that this must be the case, the range of physical scales (over ten orders of magnitude) and paucity of available tracers, make both the simulation and observation of these effects difficult. By serendipity, there lies an active galaxy, Centaurus A (NGC 5128), at such a close proximity as to allow its observation over this entire range of scales and acros…
$\require{mediawiki-texvc}$ Supermassive black holes at the centre of active galactic nuclei (AGN) produce relativistic jets that can affect the star formation characteristics of the AGN hosts. Observations in the ultraviolet (UV) band can provide an excellent view of the effect of AGN jets on star formation. Here, we present a census of star formation properties in the Northern Star-forming Region (NSR) that spans about 20 kpc of the large radio source Centaurus A hosted by the giant elliptical galaxy NGC 5128. In this region, we identified 352 UV sources associated with Cen A using new obser…
Wide-field $u'g'r'i'z'$ Dark Energy Camera observations centred on the giant elliptical galaxy NGC5128 covering $\sim21deg^2$ are used to compile a new catalogue of $\sim3200$ globular clusters (GCs). We report 2404 new candidates, including the vast majority within $\sim140$kpc of NGC5128. We find evidence for a transition at a galactocentric radius of $R_{\rm gc}\approx55$kpc from GCs intrinsic to NGC5128 to those likely to have been accreted from dwarf galaxies or that may transition to the intra-group medium of the Centaurus A galaxy group. We fit power-law surface number density profiles …
Centaurus A is the most nearby powerful AGN, widely studied at all wavelengths. Molecular gas has been found in the halo at a distance of ~20 kpc from the galaxy centre, associated with HI shells. The molecular gas lies inside some IR and UV bright star-forming filaments that have recently been observed in the direction of the radio jets. These archival data show that there is dust and very weak star formation on scales of hundreds of parsecs. On top of analysing combined archival data, we have performed searches of HCN(1-0) and HCO+(1-0) emission with ATCA at the interaction of the northern f…
XRISM Discovery of Multiple Ionized Fe-K Emission and Absorption Components in Centaurus A
Hiroshima University（广岛大学）；Tsinghua University（清华大学）；The University of Michigan（密歇根大学）；Saint Mary’s University（圣玛丽大学）；National Aeronautics and Space Administration, Goddard Space Flight Center（美国国家航空航天局戈达德太空飞行中心）；University of Maryland, Baltimore County (UMBC)（马里兰大学巴尔的摩县分校）；University of Maryland, College Park（马里兰大学帕克分校）；Université Paris-Saclay（巴黎萨克雷大学）；Université Paris Cité（巴黎西岱大学）
XRISM 发现 Centaurus A 中多个离子化 Fe-K 发射和吸收成分，揭示环核环境与 AGN 统一模型的关系。
We have explored the capabilities of dust extinction and $γ$ rays to probe the properties of the interstellar medium in the nearby anti-centre region. We have jointly modelled the $γ$-ray intensity and the stellar reddening, E(B-V) as a combination of H$_{\rm I}$-bright, CO-bright, and ionised gas components. The complementary information from dust reddening and $γ$ rays is used to reveal the dark gas not seen, or poorly traced, by H$_{\rm I}$, free-free, and $^{12}$CO emissions. We compare the total gas column densities, $N_{\rm{H}}$, derived from the $γ$ rays and stellar reddening with those…
The third data release (DR3) of the European Space Agency satellite Gaia provides coordinates, parallaxes, proper motions, and radial velocities for a sample of $\sim 34$ million stars. We use the combined 6-dimensional phase space information to search for hypervelocity stars (HVSs), unbound stars accelerated by dynamical processes happening in the Galactic Centre. By looking at the kinematics of Gaia DR3 stars in Galactocentric coordinates and by integrating their orbits in the Galactic potential, we do not identify any HVS candidates with a velocity higher than $700$ km s$^{-1}$ and robustl…
The high stellar density in the central parsecs around the Galactic Centre makes it a seemingly favourable environment for finding relativistic binary pulsars. These include pulsars orbiting other neutron stars, stellar-mass black holes or the central supermassive black hole, Sagittarius A*. Here we present multi-epoch pulsar searches of the Galactic Centre at four observing frequencies, (4.85, 8.35, 14.6 18.95) GHz, using the Effelsberg 100-m radio telescope. Observations were conducted one year prior to the discovery of, and during monitoring observations of, the Galactic Centre magnetar PSR…
A dynamical encounter between a stellar binary and Sgr A* in the Galactic Centre (GC) can tidally separate the binary and eject one member with a velocity beyond the escape speed of the Milky Way. These hypervelocity stars (HVSs) can offer insight into the stellar populations in the GC environment. In a previous work, our simulations showed that the lack of main sequence HVS candidates with precise astrometric uncertainties and radial velocities in current data releases from the \textit{Gaia} space mission places a robust upper limit on the ejection rate of HVSs from the GC of $3\times10^{-2} …
The centre of our Milky Way harbours the closest candidate for a supermassive black hole. The source is thought to be powered by radiatively inefficient accretion of gas from its environment. This form of accretion is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which it can be fed. However, the magnetization of the gas, a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of the accretion, remove angular momentum from the in…
Following a dynamical encounter with Sgr A*, binaries in the Galactic Centre (GC) can be tidally separated and one member star ejected as a hyper-velocity star (HVS) with a velocity beyond the escape speed of the Milky Way. As GC-born objects located in more observationally accessible regions of the sky, HVSs offer insight into the stellar population in the inner parsecs of the Milky Way. We perform a suite of simulations ejecting stars from the GC, exploring how detectable HVS populations depend on assumptions concerning the GC stellar population, focusing on HVSs which would appear in curren…
The Central Molecular Zone (CMZ) is the most extreme star-forming environment in the Milky Way in terms of gas pressures, densities, temperatures, and dynamics. It acts as a critical test bed for developing star formation theories applicable to the (high-redshift-like) conditions under which most stars in the Universe formed. We present a set of numerical simulations of molecular clouds orbiting on the 100-pc stream that dominates the molecular gas reservoir of the CMZ, with the goal of characterising their morphological and kinematic evolution in response to the external gravitational potenti…
The star formation rate in the Central Molecular Zone (CMZ) is an order of magnitude lower than predicted according to star formation relations that have been calibrated in the disc of our own and nearby galaxies. Understanding how and why star formation appears to be different in this region is crucial if we are to understand the environmental dependence of the star formation process. Here, we present the detection of a sample of high-mass cores in the CMZ's "dust ridge" that have been discovered with the Submillimeter Array as part of the CMZoom survey. These cores range in mass from ~ 50 - …
Ultra-compact dwarf galaxies (UCDs) were serendipitously discovered by spectroscopic surveys in the Fornax cluster twenty years ago. Nowadays, it is commonly accepted that many bright UCDs are the nuclei of galaxies that have been stripped. However, this conclusion might be driven by biased samples of UCDs in high-density environments, on which most searches are based. With the deep optical images of the Fornax Deep Survey, combined with public near-infrared data, we revisit the UCD population of the Fornax cluster and search for UCD candidates, for the first time, systematically out to the vi…
We model the inspiral of globular clusters (GCs) towards a galactic nucleus harboring a supermassive black hole (SMBH), a leading scenario for the formation of nuclear star clusters. We consider the case of GCs containing either an intermediate-mass black hole (IMBH) or a population of stellar mass black holes (BHs), and study the formation of gravitational wave (GW) sources. We perform direct summation $N$-body simulations of the infall of GCs with different orbital eccentricities in the live background of a galaxy with either a shallow or steep density profile. We find that the GC acts as an…
The environment within the inner few hundred parsecs of the Milky Way, known as the "Central Molecular Zone" (CMZ), harbours densities and pressures orders of magnitude higher than the Galactic Disc; akin to that at the peak of cosmic star formation (Kruijssen & Longmore 2013). Previous studies have shown that current theoretical star-formation models under-predict the observed level of star-formation (SF) in the CMZ by an order of magnitude given the large reservoir of dense gas it contains. Here we explore potential reasons for this apparent dearth of star formation activity.
The spectrum of any star viewed through a sufficient quantity of diffuse interstellar material reveals a number of absorption features collectively called diffuse interstellar bands (DIBs). The first DIBs were reported 90 years ago, and currently well over 500 are known. None of them has been convincingly identified with any specific element or molecule, although recent studies suggest that the DIB carriers are polyatomic molecules containing carbon. Most of the DIBs currently known are at visible and very near-infrared wavelengths, with only two previously known at wavelengths beyond one micr…
Gamma ray observations have found evidence of an extremely energetic outflow emanating from the Galactic Centre, and an `excess' of emission at GeV energies towards the Galactic Centre over that expected from current models. Determining whether the outflow is AGN- or star formation-driven, and whether the `excess' is astrophysical in origin or requires new physics (e.g. self-annihilation of dark matter), requires the accurate modelling of the expected energy injection from astrophysical sources and the subsequent interaction with the surrounding environment. We briefly summarise current constr…
The Engineering Development Array (EDA) is a single test station for Square Kilometre Array (SKA) precursor technology. We have used the EDA to detect low-frequency radio recombination lines (RRLs) from the Galactic Centre region. Low-frequency RRLs are an area of interest for future low-frequency SKA work as these lines provide important information on the physical properties of the cold neutral medium. In this project we investigate the EDA, its bandpass and the radio frequency interference environment for low-frequency spectroscopy. We present line spectra from 30 to 325 MHz for the Galacti…
Our aim is to understand the role of the environment in the quenching of star formation of galaxies located in the infall cluster region of Abell 85 (A85). This is achieved by studying the post-starburst galaxy population as tracer of recent quenching. By measuring the equivalent width (EW) of the [OII] and Hdelta spectral lines, we classify the galaxies in three groups: passive (PAS), emission line (EL), and post-starburst (PSB) galaxies. The PSB galaxy population represents about 4.5% of the full sample. Dwarf galaxies (Mr > -18.0) account for about 70 - 80% of PSBs, which indicates that mos…
The Galactic Centre is known to have undergone a recent star formation episode a few Myrs ago, which likely produced many T Tauri stars hosting circumstellar discs. It has been suggested that these discs may be the compact and dusty ionized sources identified as ``G-clouds''. Given the Galactic Centre's hostile environment, we study the possible evolutionary pathways these discs experience. We compute new external photoevaporation models applicable to discs in the Galactic Centre that account for the sub-sonic launching of the wind and absorption of UV photons by dust. Using evolutionary disc …
