Early formation and recent starburst activity in the nuclear disc of the Milky Way (1910.06968v2)

Abstract: The nuclear disc is a dense stellar structure at the centre of the Milky Way, with a radius of $\sim$150 pc. It has been a place of intense star formation in the past several tens of millions of years but its overall formation history has remained unknown up to now. Here we report the first detailed star formation history of this region. The bulk of its stars formed at least eight billion years ago. This initial activity was followed by a long period of quiescence that was ended by an outstanding event about 1 Gyr ago, during which roughly 5% of its mass formed in a time window $\sim$100 Myr, in what may arguably have been one of the most energetic events in the history of the Milky Way. Star formation continued subsequently on a lower level, creating a few percent of the stellar mass in the past $\sim$500 Myr, with an increased rate up to $\sim$30 Myr ago. Our findings contradict the previously accepted paradigm of quasi-continuous star formation at the centre of the Milky Way. The long quiescent phase agrees with the overall quiescent history of the Milky Way and suggests that our Galaxy's bar may not have existed until recently, or that gas transport through the bar was extremely inefficient during a long stretch of the Milky Way's life, and that the central black hole may have acquired most of its mass already in the early days of the Milky Way.

Star Formation History in the Nuclear Disc of the Milky Way

This paper presents a comprehensive analysis of the star formation history (SFH) within the Nuclear Disc of the Milky Way, uncovering critical insights into its early formation processes and recent starburst activities. The Nuclear Disc, a dense stellar structure with a radius of ~150 pc at the Milky Way's center, has long puzzled astrophysicists due to its complex star-forming activities and largely unknown formation history. This paper leverages data from the GALACTICNUCLEUS survey, which provides detailed near-infrared imaging to obtain high-resolution insights into this elusive region's SFH.

The researchers have determined that the majority of the stars in the Nuclear Disc were formed over eight billion years ago. This early star-forming activity was succeeded by a substantial quiescent phase lasting over six billion years. Notably, this period was disrupted by an extraordinary starburst event approximately one billion years ago, during which approximately 5% of the disc's stellar mass formed within a remarkably short timeframe, estimated to be less than 100 million years. Subsequent star formation continued at a lower level, with increased activity evident in the last 30 million years.

This starburst event stands in stark contrast to the previously dominant model suggesting continuous star formation within the Milky Way’s central regions. Such findings challenge assumptions about the Milky Way's evolutionary dynamics and prompt reevaluation of the mechanisms regulating gas inflow and star formation suppression. The prolonged quiescence implies an inefficiency or alteration in gas transport to the Galactic Center, potentially due to an absent or inefficient galactic bar during this phase.

The results are supported by synthetic photometry and color-magnitude diagrams (CMDs) that reveal a bifurcation in the red clump (RC) sequence, indicating distinct populations correlating to old and intermediate-age stellar groups. Further validation comes from probabilistic modeling using the BaSTI and MIST stellar evolution models, yielding consistent SFH interpretations despite variations in specific event ages.

These insights suggest practical and theoretical implications. Practically, it highlights the need for reevaluating the processes influencing the SFH in central galactic regions and challenges existing simulations of galactic evolution. Theoretically, it raises questions about the Milky Way's capacity for feeding its central black hole, Sagittarius A*, and elucidates potential parallels with external starburst galaxies known for their high star formation rates in nuclear regions.

The findings underscore the necessity for continued observation and modeling to refine the temporal dynamics of SFH in the Nuclear Disc and draw more accurate correlations with observed galactic phenomena, such as the formation of the Fermi Bubbles. Future spectroscopic and high-resolution imaging are critical for disentangling these complex narratives and advancing our understanding of galactic structure and evolution.