Age dating the Galactic bar with the nuclear stellar disc

Baba, Junichi; Kawata, Daisuke

doi:10.1093/mnras/staa140

Cited by 43 publications

(35 citation statements)

References 135 publications

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“…Fux 1999; Li et al 2015;Ridley et al 2017;Sormani et al 2018a,b;Sormani & Barnes 2019;Tress et al 2020). The age distribution of the stars in the NSD can give us therefore hints about the formation epoch of the bar (Baba & Kawata 2020;Gadotti et al 2019). These authors also show that 3D velocity information is an effective way to reduce the contamination of bulge stars in the NSD as stars in the NSD are kinematically cool in contrast to the bulge.…”

Section: Introductionmentioning

confidence: 88%

The nuclear stellar disc of the Milky Way: A dynamically cool and metal-rich component possibly formed from the central molecular zone

Schultheis

Fritz

Nandakumar

et al. 2021

A&A

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Context. The nuclear stellar disc (NSD) is, together with the nuclear star cluster (NSC) and the central massive black hole, one of the main components in the central parts of our Milky Way. However, until recently, only a few studies of the stellar content of the NSD have been obtained owing to extreme extinction and stellar crowding. Aims. We study the kinematics and global metallicities of the NSD based on the observations of K/M giant stars via a dedicated KMOS (VLT, ESO) spectroscopic survey. Methods. We traced radial velocities and metallicities, which were derived based on spectral indices (Na I and CO) along the NSD, and compared those with a Galactic bulge sample of APOGEE (DR16) and data from the NSC. Results. We find that the metallicity distribution function and the fraction of metal-rich and metal-poor stars in the NSD are different from the corresponding distributions and ratios of the NSC and the Galactic bulge. By tracing the velocity dispersion as a function of metallicity, we clearly see that the NSD is kinematically cool and that the velocity dispersion decreases with increasing metallicity contrary to the inner bulge sample of APOGEE (|b|< 4°). Using molecular gas tracers (H2CO, CO(4−3)) of the central molecular zone (CMZ), we find an astonishing agreement between the gas rotation and the rotation of the metal-rich population. This agreement indicates that the metal-rich stars could have formed from gas in the CMZ. On the other hand, the metal-poor stars show a much slower rotation profile with signs of counter-rotation, thereby indicating that these stars have a different origin. Conclusions. Coupling kinematics with global metallicities, our results demonstrate that the NSD is chemically and kinematically distinct with respect to the inner bulge, which indicates a different formation scenario.

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Section: Introductionmentioning

confidence: 88%

The nuclear stellar disc of the Milky Way: A dynamically cool and metal-rich component possibly formed from the central molecular zone

Schultheis

Fritz

Nandakumar

et al. 2021

A&A

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“…The radial velocities of GIRAFFE observations were measured by fitting the cross-correlation peak with the solar spectra available at the ESO portal 8 , and the heliocentric radial velocities were derived. The final heliocentric radial velocities of GI-RAFFE spectra were determined by taking the average of the mean heliocentric velocity of each setup.…”

Section: Appendix A: Candidate Members Observed With Giraffementioning

confidence: 99%

“…Appendix C: Equivalent widths and atomic data 8 http://www.eso.org/observing/dfo/quality/GIRAFFE/pipeline/solar.html…”

Section: Appendix A: Candidate Members Observed With Giraffementioning

confidence: 99%

“…The Galactic bulge formation was probably a result of early mergers and/or dissipative collapse combined with a buckling bar, as suggested by the excellent modern data now available Baba & Kawata 2020). Within this context, globular clusters are important tracers of the early formation of the Galactic bulge, assuming that most of them were formed in situ.…”

Section: Introductionmentioning

confidence: 96%

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UVES analysis of red giants in the bulge globular cluster NGC 6522

Barbuy

Cantelli

Muniz

et al. 2021

A&A

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Context. NGC 6522 is a moderately metal-poor bulge globular cluster ([Fe/H] ~ −1.0), and it is a well-studied representative among a number of moderately metal-poor blue horizontal branch clusters located in the bulge. The NGC 6522 abundance pattern can give hints on the earliest chemical enrichment in the central Galaxy. Aims. The aim of this study is to derive abundances of the light elements C and N; alpha elements O, Mg, Si, Ca, and Ti; odd-Z elements Na and Al; neutron-capture elements Y, Zr, Ba, La, and Nd; and the r-process element Eu. We verify if there are first- and second-generation stars: we find clear evidence of Na–Al, Na–N, and Mg–Al correlations, while we cannot identify the Na–O anti-correlation from our data. Methods. High-resolution spectra of six red giants in the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with both the UVES and GIRAFFE spectrographs in FLAMES+UVES configuration. In light of Gaia data, it turned out that two of them are non-members, but these were also analysed. Spectroscopic parameters were derived through the excitation and ionisation equilibrium of Fe I and Fe II lines from UVES spectra. The abundances were obtained with spectrum synthesis. Comparisons of abundances derived from UVES and GIRAFFE spectra were carried out. Results. The present analysis combined with previous UVES results gives a mean radial velocity of vrhel = −15.62±7.7 km s−1 and a metallicity of [Fe/H] = −1.05 ± 0.20 for NGC 6522. Mean abundances of alpha elements for the present four member stars are enhanced with [O/Fe] = +0.38, [Mg/Fe] = ≈+0.28, [Si/Fe] ≈ +0.19, and [Ca/Fe] ≈ +0.13, together with the iron-peak element [Ti/Fe] ≈ +0.13, and the r-process element [Eu/Fe] = +0.40. The neutron-capture elements Y, Zr, Ba, and La show enhancements in the +0.08 < [Y/Fe] < +0.90, 0.11 < [Zr/Fe] < +0.50, 0.00 < [Ba/Fe] < +0.63, 0.00 < [La/Fe] < +0.45, and −0.10 < [Nd/Fe] < +0.70 ranges. We also discuss the spread in heavy-element abundances.

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“…Discy pseudobulges (nuclear discs) are thought to form from bar-driven gas inflow and subsequent star formation, and are thus the result of internal ('secular') processes that are unrelated to mergers and require the prior formation of a bar (e.g. Kormendy & Kennicutt 2004;Wozniak & Michel-Dansac 2009;Cole et al 2014;Baba & Kawata 2020). B/P bulges also require the formation of a bar, since they form out of (and remain part of) the bar; they are also the result of internal processes, albeit ones rather different from gas inflow (e.g.…”

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confidence: 99%

Composite bulges – II. Classical bulges and nuclear discs in barred galaxies: the contrasting cases of NGC 4608 and NGC 4643

Erwin

Seth

Debattista

et al. 2021

Monthly Notices of the Royal Astronomical Society

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We present detailed morphological, photometric, and stellar-kinematic analyses of the central regions of two massive, early-type barred galaxies with nearly identical large-scale morphologies. Both have large, strong bars with prominent inner photometric excesses that we associate with boxy/peanut-shaped (B/P) bulges; the latter constitute ∼30% of the galaxy light. Inside its B/P bulge, NGC 4608 has a compact, almost circular structure (half-light radius Re ≈ 310 pc, Sérsic n = 2.2) we identify as a classical bulge, amounting to 12.1% of the total light, along with a nuclear star cluster (Re ∼ 4 pc). NGC 4643, in contrast, has a nuclear disc with an unusual broken-exponential surface-brightness profile (13.2% of the light), and a very small spheroidal component (Re ≈ 35 pc, n = 1.6; 0.5% of the light). IFU stellar kinematics support this picture, with NGC 4608’s classical bulge slowly rotating and dominated by high velocity dispersion, while NGC 4643’s nuclear disc shows a drop to lower dispersion, rapid rotation, V-h3 anticorrelation, and elevated h4. Both galaxies show at least some evidence for V-h3correlation in the bar (outside the respective classical bulge and nuclear disc), in agreement with model predictions. Standard 2-component (bulge/disc) decompositions yield B/T ∼ 0.5–0.7 (and bulge n > 2) for both galaxies. This overestimates the true “spheroid” components by factors of four (NGC 4608) and over 100 (NGC 4643), illustrating the perils of naive bulge-disc decompositions applied to massive barred galaxies.

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Age dating the Galactic bar with the nuclear stellar disc

Cited by 43 publications

References 135 publications

The nuclear stellar disc of the Milky Way: A dynamically cool and metal-rich component possibly formed from the central molecular zone

The nuclear stellar disc of the Milky Way: A dynamically cool and metal-rich component possibly formed from the central molecular zone

UVES analysis of red giants in the bulge globular cluster NGC 6522

Composite bulges – II. Classical bulges and nuclear discs in barred galaxies: the contrasting cases of NGC 4608 and NGC 4643

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