talks
abstracts, slides, and recordings
abstracts, slides, and recordings
Magnetic Field Studies in the Central Molecular Zone
Detailed mapping of gas kinematics in various tracers combined with recent numerical simulations have begun to assemble a coherent picture for the gas flow in the central molecular zone (CMZ). While probes of magnetic field strengths in different ISM components of the CMZ have alluded to the existence of strong magnetic fields, the topology of the field within the CMZ and its influence on the gas dynamics and subsequent star-formation is far from clear. This talk does not seek to provide a consensus on this topic (there is none), rather it is a presentation of the current data, open questions and systematic polarimetric surveys that are ongoing at various facilities that we can look forward to towards answering some of those questions.
Conditions of Star Formation in Dense-Gas Clumps in the Central Molecular Zone
Star formation in the Galactic central molecular zone (CMZ) has several characteristics that are rare elsewhere in the Galaxy: formation of young massive clusters, mini-starburst in the Sgr B2 complex, and low star formation efficiency in the vast majority of the GMCs. We investigate physical and chemical conditions in the dense-gas clumps identified using the HCN 4-3 complete mapping data of the CMZ. We first constructed three-dimensional (position-position-velocity) maps of volume density (nH2), gas column density (NH2), kinetic temperature (Tkin), and fractional abundances of popular atomic and molecular species by excitation analysis. We then used logistic regression analysis to search for the parameters that best distinguish between star-forming (SF) and non-SF clumps. The main results are summarized below. (1) Typical nH and temperature Tkin are measured to be 104.1 cm-3 and 101.8 K, respectively, for the gas visible in high-density tracer lines. On the other hand, low-J CO and [CI] lines are likely to originate from gas with lower density gas with n~103 cm-3. (2) The physical and chemical conditions of the CMZ clouds are generally affected by shockwaves (3) High atomic carbon abundance indicates high cosmic-ray flux or faster cloud lifecycle in the CMZ than in the Galactic disk region. (4) The parameters that distinguish the SF and non-SF clouds are reduced into the form of virial parameter (αvir). The threshold αvir is 6, which translates into nH of 104.6 cm−3 with the nH –αvir correlation. The scarcity of the low-αvir clumps, whose fraction to all HCN clumps is 0.1, can be considered as one of the immediate causes of the suppressed SF in the CMZ.
The circumnuclear disk revealed by ALMA - environments of star formation in the inner 10 pc of the Galaxy
The interaction between a supermassive black hole (SMBH) and the surrounding material is of primary importance in astrophysics. The detection of the molecular 2-pc circumnuclear disk (CND) immediately around the Milky Way SMBH, SgrA*, resembles the "molecular torus" in AGNs, providing a unique opportunity to study SMBH accretion and nuclear star formation at sub-parsec scales. However, the CND is transient if its gas density is under the tidal threshold of SgrA*/nuclear star clusters, thus depleting the source of fuel. Utilizing the Atacama Large Millimeter/submillimeter Array (ALMA), we present CS line maps in five rotational lines (Jupper=7,5,4,3,2) toward the CND of the Galactic Center. Our primary goal is to resolve the compact structures within the CND and the streamers, in order to understand the stability conditions of molecular cores in the vicinity of the supermassive black hole (SMBH) Sgr A*. Our data provide the first homogeneous high-resolution (1.3" = 0.05 pc) observations aiming at resolving density and temperature structures. The CS clouds have sizes of 0.05-0.2 pc with a broad range of velocity dispersion (FWHM=5-40 km/s). The CS clouds are a mixture of warm (Tk ~ 50-500 K, nH2=10^3-5 cm^-3) and cold gas (Tk < 50 K, nH2=10^6-8 cm^-3). A stability analysis based on the unmagnetized virial theorem including tidal force shows that 84 (+16/-37) % of the total gas mass (2.5X10^4 Msun) is tidally stable, which accounts for the majority of gas mass. Turbulence dominates the internal energy and thereby sets the threshold densities 10-100 times higher than the tidal limit at distance >1.5 pc to Sgr A*, and therefore, inhibits the clouds from collapsing to form stars near the SMBH. However, within the central 1.5 pc, the tidal force overrides turbulence and the threshold densities for a gravitational collapse quickly grow to > 10^8 cm^-3.
Star Formation Near Sgr A* and Beyond
Broadly speaking, the Galactic center hosts two different environments for formation of young stars within the inner 10pc. One is within a pc of Sgr A* where the stellar young cluster lies. Second, the region within the inner 2-7 pc of Sgr A* where the molecular ring is concentrated. I will discuss different modes of star formation in these environments. In particular, I describe disk-based mode of star formation near Sgr A* and review multiple signatures of on-going star formation that are found within the molecular ring.
Beyond a Sample Size of One: Star formation in nearby extreme analogs of the Galactic center.
The Milky Way center has long been the only galaxy nucleus in which we can study the initial stages of star formation at the high spatial resolutions needed to resolve individual molecular clouds. However, the advent of ALMA is now allowing us to observe gas in the centers of the most nearby galaxies at parsec to sub-parsec scales. In this talk, I will discuss ALMA observations of a small sample of galaxies, which contain both active starbursts and AGN. I will give an overview of these galaxies and how they compare to the Milky Way center, and summarize some recent results. In particular, I will focus on NGC 253, a barred spiral with a starbursting nucleus that contains an order of magnitude more molecular gas and star formation than the Milky Way. With ALMA, we resolve the emission from the H40 alpha line in the central 200 pc of this galaxy on spatial scales of ~4 pc. The emission primarily originates from a population of roughly a dozen embedded super star clusters in the early stages of formation. We measure electron temperatures and helium abundances for these clusters which are consistent with values measured for H II regions in the center of the Milky Way. We also find broad-linewidth recombination line emission originating from seven of the embedded clusters which we speculate contributes to driving the large-scale hot wind observed to emanate from the central starburst. Finally, I will conclude by discussing prospects for detailed comparison of gas properties between these galaxies, which will allow us to isolate the physical, chemical, and kinematic conditions during different stages of activity in a galaxy nucleus.
Modeling turbulence in galactic centers
Turbulence is a prevalent phenomenon in the interstellar medium, and in particular, the environment at the centers of galaxies. For example, detailed observations of the Milky Way’s Central Molecular Zone (CMZ) revealed that it has a complex and turbulent structure. In this talk I will introduce a proof-of-concept method to drive turbulence in galactic centers. Instead of relying on a particular physical mechanism, I have adopted a Fourier forcing module and have applied it using a smoothed particle hydrodynamics code. To test this method, I performed simulations of a simplistic model of the CMZ. This method induces a flocculent spiral structure and enhances mass accretion. This method is flexible and applicable to different physical scenarios, which makes it a promising new tool to simulate turbulence in galactic centers.
Star formation, feedback, and the global mass budget of the CMZ
In this talk I discuss recent simulations and models for the cycle of mass inflow, outflow, star formation, and feedback in the CMZ. I focus in particular on the question of which feedback mechanisms are dominant under the conditions found in the CMZ, and on what drives variations in the gas mass, star formation, rate, and other properties of the CMZ and galaxy centres more broadly. I critically assess the question of whether there is good reason to believe, observationally or theoretically, that star formation in the CMZ is subject to rapid timescale variations, and discuss the implications of that question for our broader understanding of how star formation is regulated.
Is Sagittarius B2 a progenitor of a super star cluster?
The giant molecular cloud complex Sagittarius B2 (hereafter, SgrB2) is one of the most massive regions with ongoing star formation in the Galaxy. It is located at a projected distance of about 100 pc along the plane to the Galactic Center and at 8.5 kpc from the Sun. The whole complex contains a total gas mass of 10^7 Msun, with the main sites of active star formation corresponding to the hot molecular cores SgrB2(N) and SgrB2(M), located at the center of the complex. They contain more than 50 high-mass stars with spectral types ranging from O5 to B0.
Recent high-spatial resolution ALMA observations have resolved the cluster population of SgrB2(N) and SgrB2(M). The two clusters have top-heavy core mass functions, with a lack of low-mass cores, and H2 volume densities of about 10^5-10^7 Msun/pc^3, one to two orders of magnitude higher than stellar densities of super star clusters. A network of converging filaments associated with large velocity gradients suggest that mass accretion towards the center of the cluster is important and still ongoing. All this poses the question: Does SgrB2 constitute an early stage of a protocluster that will evolve into a super star cluster like Arches and Quintuplet?
The structure and stellar population of the nuclear stellar disc
The nuclear stellar disc (NSD) is a dense disc-like stellar structure at the centre of the Milky Way that partially overlaps with the central molecular zone. It is characterised by an extremely variable star formation history (SFH), and constitutes a distinct component from the much larger Galactic bulge. Given its proximity, it is possible to resolve individual stars down to milli-parsec scales, being a perfect laboratory to better understand galactic nuclei. However, the high extinction and the extreme source crowding significantly hamper the analysis of its stellar population and structure. To overcome these difficulties, the GALACTICNUCLEUS survey, a high angular resolution (~0.2’’) photometric survey in the near infrared bands JHKs, was specially designed to observed the Galactic centre superseding all previous catalogues. Using its accurate photometry, I will present our results on the SFH of the NSD, the analysis of the extinction curve in the near infrared towards the NSD, and the determination of the distance towards three molecular clouds in the central molecular zone.
X-ray view of the Central Molecular Zone
X-ray observations of the diffuse emission at the Galactic center performed over the last two decades have revealed an intense and highly-variable non-thermal component, spatially correlated with dense molecular clouds present in the central three hundred parsecs. This reflection signal has been identified as echoes created by the past activity of the central supermassive black hole, Sagittarius A*. However, using these reflection features to reconstruct its precise history over the last centuries has been challenging. Through dedicated X-ray variability and spectral analyses, we are now able to derive an increasing number of constraints on the illuminating events but also on the reflecting clouds. Indeed, the X-ray echoes are currently processing the tomography of the Galactic center. Therefore, they can also be used to better understand both the fine structure of the molecular clouds and their 3D distribution within the Central Molecular Zone. I will explain these techniques and review the constraints that we have been obtaining thanks to Sagittarius A*'s past outbursts.
Star Formation (or not) in the Central Parsec
A dramatic star formation event clearly occurred in the central half-parsec of the Galaxy 4 to 6 million years ago, leaving the central cluster of massive young stars in its wake. There is presently very little gas in the volume occupied by that cluster, compared to what must have been present when that cluster formed, presumably because a combination of stellar winds, supernovae, and energetic outflows from Sgr A* has been effective in voiding the region of gas. However, there have been several suggestions in the literature that star formation has taken place since that starburst event, and even that it is still ongoing, with evidence presented for YSOs, protostars, masers, and protoplanetary disks. These claims will be discussed, and questioned. Then I’ll revisit the speculation that star formation in the central parsec, linked to the evolution of the circumnuclear disk, is a recurrent phenomenon following a limit cycle of activity driven by the inexorable slow accretion of gas from the Central Molecular Zone.
The Arches and Quintuplet Clusters: Evidence of an Unusual Stellar Initial Mass Function Near the Galactic Center?
As young massive clusters in the Central Molecular Zone, the Arches and Quintuplet clusters are valuable probes into the Initial Mass Function (IMF) in the extreme Galactic Center (GC) environment. I present the results of a multi-epoch Hubble Space Telescope study of both clusters, using proper motions to obtain high-quality samples of cluster members down to ~2 Msun. We forward model these observations to simultaneously constrain the cluster IMFs and other properties (such as age and total mass) while accounting for observational uncertainties, completeness, mass segregation, and stellar multiplicity. We find that the IMF of the Arches cluster exhibits a significant overabundance of high-mass stars compared to nearby star forming regions. Preliminary analysis yields similar results for the Quintuplet cluster. By comparing the Arches and Quintuplet to other young massive clusters in the Milky Way, we investigate if these IMF variations can be attributed to the GC environment.
Investigating star formation in the Galactic center cloud Sagittarius B2(N) from a chemical point of view
Despite its large reservoir of dense molecular gas, the central molecular zone (CMZ) appears to be overall deficient in star formation. The active star-forming region Sagittarius B2 stands out from the other CMZ clouds, with a large gas mass reservoir, a high star-formation rate, and one of the richest molecular inventories observed to date. From a chemical point of view, the extreme environmental conditions in the CMZ (strong radiation field, high cosmic-ray fluxes, X-rays) provide an excellent laboratory to investigate the building up of interstellar chemical complexity under extreme conditions.
I will present results from the analysis of the EMoCA line survey, which made possible the detection of 3 new hot cores in Sgr B2(N). The chemical composition and physical conditions of the hot cores are derived from the high-resolution and high-sensitivity data provided by ALMA at 3 mm. Based on the observational constraints (densities, temperatures), we use an astrochemical code to model the physico-chemical evolution of the hot cores. I will show how comparing the observations with the models helps us to constrain the environmental conditions (UV field, cosmic-ray ionization rate) in Sgr B2(N).
Hosted by:
Adam Ginsburg, Ashley Barnes, Cara Battersby, Jonathan Henshaw, Mattia Sormani, and Daniel Walker.