Gaia: Astrometric Survey of the Galaxy

15

Galaktik Astronomi Çalıştayı Bildiriler KitabıGalactic Astronomy Workshop Proceedings Book

Çağrılı Bildiriler / Invited Papers

Submitted/Başvuru: 27.09.2019 Accepted/Kabul: 31.01.2020Corresponding author/Sorumlu yazar: Gerry Gilmore (Prof. Dr.), Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, United Kingdom. E-mail: [email protected] Citation/Atıf: Gilmore, G. 2021, in: Gaia: Astrometric Survey of the Galaxy, eds. S. Ak & S. Bilir, Galactic Astronomy Workshop Proceedings Book, 15. https://doi.org/10.26650/PB/PS01.2021.001.002

DOI: 10.26650/PB/PS01.2021.001.002

Gaia: Astrometric Survey of the GalaxyGerry GILMORE1

1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, United Kingdom

ORCID: G.G. 0000-0003-4632-0213

ABSTRACTGaia provides 5-D phase space measurements, 3 spatial coordinates and two space motions in the plane of the sky, for a representative sample of the Milky Way’s stellar populations (over 2 billion stars, being ~1% of the stars over 50% of the radius). Full 6-D phase space data is delivered from line-of-sight (radial) velocities for the 300 million brightest stars. These data make substantial contributions to astrophysics and fundamental physics on scales from the Solar System to cosmology.

1. Introduction

The ESA Gaia astrometric space mission is revolutionising astrophysics. Originally proposed in the early 1990’s to build on the proof of concept for absolute space astrometry demonstrated by the ESA HIPPARCOS mission, Gaia is currently operating superbly. The first two data releases have provided support for over 1000 research articles already, even though only a small subset of some types of the data being obtained have yet been calibrated, reduced and released.

A convenient overview of the whole Gaia mission and its capabilities is available in Gilmore (2018a), while very substantially more detailed descriptions are available in the many Gaia Data Release papers, and on the ESA Gaia website.

Gaia has in essence three scientific instruments, all based on a very large high-quality imaging billion-pixel camera. These instruments provide direct broad-band imaging, in the Gaia native (G-band) passband, which is analysed for photometry and astrometry, low-dispersion prism spectrophotometry in both blue (BP) and red (RP) wavelengths, together covering 300 nm to 1000 nm, and R = 12000 spectroscopy near the CaII triplet for radial velocities and spectroscopic analyses for brighter stars. The instruments and their observations are summarised in Figure 1.

https://orcid.org/0000-0003-4632-0213


12-13 Eylül 2019September 12-13, 2019

16

2. Gaia: The Early Years

The Gaia mission was proposed in early 1993, building on the technical success of the ESA HIPPARCOS mission, then in operation, which proved that absolute space astrometry was viable. The original Gaia proposal was in response to a CaII for interfometry missions. During the detailed Study Phase, which ran until 2000, it was soon realised that an interferometer was not the optimal solution. Rather a design loosely based on the parallel ROEMER proposal was adopted. This study led to development of the Gaia “RED BOOK” proposal to ESA. This proposal is still available via the ESA Gaia web portal1. The study led to a presentation of the Gaia mission for ESA adoption at a (very large) meeting held at UNESCO HQ, Paris, on September 13, 2000, and a summary article outlining the mission (Perryman et al., 2001). The various stages of design optimisation, construction and test led eventually to successful launch of Gaia in December 2013, and the start of science operations in mid 2014. At present (September 2019) Gaia has completed its “nominal” 5-year mission, the spacecraft is operating very well, and the mission is anticipated to continue data taking until the precision-control fuel is exhausted at the end of 2024. Over 1.3 trillion observations have been recorded at the mission mid-life point. The various key documents which led to the Gaia mission are illustrated in Figure 2, while a more detailed description is available in Gilmore (2018b).

Figure 1. An illustration of Gaia’s sky-scanning law (top left) and three forms of data gathering. The top centre row shows an image of the very large (1m × 0.6m, billion-pixel) Gaia focal-plane camera, with its three types of detectors, while the top-right shows an image, from which photometry and astrometry are derived (inset psf). The middle row shows the radial velocity spectrometer and a bright-star spectrum. The lower row shows the two low-dispersion (BP/RP) prisms, and time-series spectrophotometry from the prisms for a carbon star. This figure is taken from the mission overview presented by the DPACE Chair, Anthony Brown, at the ESA ESLAB#53 meeting in April 2019

3. Gaia and the Structure of the Milky Way

Figure 3 illustrates the basic stellar populations, which were known prior to Gaia launch, and whose detailed mapping and origins were the goal of the Gaia mission. Enormous progress has been made already from Gaia’s early astrometric and photometric data in quantifying the

1 https://www.cosmos.esa.int/web/gaia/home

https://www.cosmos.esa.int/web/gaia/home


12-13 Eylül 2019September 12-13, 2019

17

Galaxy. Gaia’s photometry is calibrated at the milli-mag level, substantially superior to previous large-area (percent-accuracy) surveys.

An interesting early conclusion from Gaia is that the earlier photometric studies, to which the Basle-Istanbul teams were major contributors, were in fact even more accurate than could reasonably have been expected. The most recent ground-based star-count Galactic structure analysis based on wide-area high-quality CCD data is that from the Dark Energy Survey (Pieres et al., 2020). This derives large-scale population parameters in good agreement with those derived from early photographic surveys.

As a Gaia-test, we show in Figure 4 the results from Gaia photometry for the density profiles towards both the Galactic poles for G-stars. This is essentially reproducing the experiment of Gilmore & Reid (1983) which discovered the Galactic Thick Disk.

Figure 2. The key documents which trace the Gaia mission from genesis to approval. Further details are available in Perryman et al. (2001) and in Gilmore (2018b).


12-13 Eylül 2019September 12-13, 2019

18

Figure 3. The high-latitude stellar populations. The figure shows a high-latitude stellar field with both optical (r, g-r: LHS) and near-infrared (K, H-K, RHS) colour-magnitude diagrams for the same stars. The optical data show the clear distinction between the (red-sequence) foreground thin-disk, the thick disk turnoff sequence (near g-r = 0.5) and the halo turnoff sequence (near g-r = 0.4). These distinctions are less evident in the near-infrared.

Figure 4. The stellar density profile towards both Galactic poles for G-stars. The various curves are as described in the figure. The resulting Bayesian reconstruction of the density profile illustrates the impressive consistency between early photographic studies and more recent CCD-based surveys. A consequence is that the local density of Dark Matter remains fully consistent with the first precision determination, which was in 1989. Not everything from Gaia is different! This figure is from Sanders & Gilmore (in prep).


12-13 Eylül 2019September 12-13, 2019

19

4. Gaia and the Evolution of the Milky Way

A primary ambition for Gaia was to determine the assembly history of the Milky Way. Remarkable progress has been made in this endeavour, through a combination of kinematic studies of the halo, to decipher the early merging history, and spectroscopy and stellar age determination to add a clock.

Studies of halo kinematics have shown, in several parallel and independently authored studies which agree remarkably closely, that a major merger took place between the proto-Milky Way galaxy and an infalling satellite, of Large Magellanic Cloud scale, early in the Galaxy’s history. The stars from the infalling galaxy now make up much of the inner halo. The Milky Way parent, which at the time was already a disk-like galaxy, was puffed up in the merger, with the primordial disk being transformed into what we now see as the thick disk. Later gas accretion created a new thin disk, which continues to form stars today. This picture was not new, but is in excellent agreement with what had been deduced from earlier studies of small samples of stars with accurate abundances and approximate kinematics.

Thus we see directly, in phase-space structure, the history of the Milky Way. An early disk-like proto-galaxy was forming stars and enriching, creating material enriched preferentially in “alpha” elements, products of Type II supernovae. These are primarily Mg, Si, Ti, Ca, and O. Over time more Type Ia supernovae had time to explode, creating more iron-peak elements, and diluting the alpha-rich material.

The merger which re-structured the galaxy also bought in much new low-metallicity gas. Thus, the level of chemical enrichment of the disk gas was substantially reduced at the time of the merger. Can we see this event in the chemistry? To do so we need both chemical element abundances for large samples of stars, and stellar ages. Until recently providing the ages was the limiting factor. There has been much recent progress here.

With Gaia parallaxes reasonably accurate ages can be determined for stars at the main-sequence turnoff or lower Red Giant Branch (RGB) from isochrone fitting. More generally, and allowing age determinations to very much larger distances, a new stellar age clock was derived which is based purely on RGB chemistry. Thus, it can be applied at any distance, even far beyond where accurate parallaxes are available from Gaia.

Masseron & Gilmore (2015) developed the [C/N]-age clock for RGB stars. This utilises the dredge-up of products from the main-sequence burning phase of evolution as a star rises on the RGB. This dredge-up is very dependant on stellar mass, and hence age. It includes main-sequence burning products, hence especially affects the [C/N] ratio, which is modified during the CNO-cycle. This clock had a major effect on Galactic archaeology, and is now widely applied. The basic idea is shown in Figure 5.


12-13 Eylül 2019September 12-13, 2019

20

Figure 5. The standard stellar population classification diagram, with [α/Fe] plotted against [Fe/H]. The upper sequence is the thick disk, the lower sequence is the thin disk. The colour coding is however in [C/N], which was shown by Masseron & Gilmore (2015) to be an age proxy. The value of [C/N] clearly reduces systematically from Solar or above for the most metal-poor thick disk stars, steadily down the thick disk sequence, and then continues after an abrupt offset from [Fe/H] about Solar to [Fe/H]=-0.6, up to super-solar abundances. This systematic change corresponds to a systematic age gradient.

Masseron & Gilmore (2015) calibrated the [C/N] age indicator using theoretical dredge-up stellar evolutionary models. More recently, as part of the Gaia-ESO Spectroscopic Survey (Gilmore et al., 2012) Casali et al. (2019) have calibrated the [C/N]-age relation on a consistent isochrone scale using observations of stars in open clusters with well-determined ages (Figure 6). This provides a well-calibrated age relation, allowing determination of the Galactic assembly history by-eye.

Figure 6 shows that the time during which the proto-Milky Way galaxy was forming stars and self-enriching was 2-3 Gyr. This period created what is now the thick disk. Some 9-10 Gyr ago the major merger which terminated the (thick-)disk formation also essentially instantaneously reset the [Fe/H] abundance down from Solar to [Fe/H] = -0.6 dex. From this new beginning the current thin disk has evolved until today. Interestingly, there is evidence here that no other significant merger has taken place, either before or after the single major event. This makes the Milky Way somewhat unusual compared to galaxy formation simulations.


12-13 Eylül 2019September 12-13, 2019

21

Figure 6. The current age distribution for thick and thin disk stars. This is based on stars which are members of open clusters with reliable isochrone age determinations. This figure is a product of the Gaia-ESO Survey, and is from Casali et al. (2019).

5. Gaia and the Future of Small-telescope Time-domain Astronomy

The substantial progress from analyses of Gaia astrometry which is reported above depends on sophisticated statistical analysis of very large data sets, and follow-up spectroscopy on 8-m telescopes. While exciting, this is beyond the resources of many research groups. In this section I outline a cutting-edge application which requires large amounts of time on small telescopes worldwide. Involvement in these projects makes ideal student training/masters projects, and a set of them makes for a strong and topical PhD.

As part of the processing of the Gaia photometric data, which takes place at the Cambridge Gaia data processing centre DPCI, near-real-time searches are made for new transient and variable sources. These are published for open follow-up immediately on discovery (see https://gaia.ac.uk/alerts). Many exciting sources have been discovered and become the basis of research collaborations by a wide community. The transient discoveries are dominated by supernovae and cataclysmic variables, as anticipated, but also include rare events-among the first super-luminous supernovae, Tidal Disruption Events, when a star is tidally destroyed while orbiting (too) close to a supermassive black hole, and many more. Among the bright sources are microlensing events, where Gaia may well find free-floating stellar-mass black holes. These cannot be detected in any other way, and are just one more example of the science potential of being involved in Gaia Alerts Follow-up. All interested are welcome.

https://gaia.ac.uk/alerts


12-13 Eylül 2019September 12-13, 2019

22

Figure 7. An application of small-telescope astronomy to gravitational micro-lensing. The source Gaia16aye was discovered by the Gaia Science Alerts system. It brightened to 12 mag, making it readily suitable for wide follow-up. Follow-up involved over 6,000 photometric time-series measures by a global network of mostly small telescopes. The complex source is a photometric and astrometric microlensed system, with the lens being a binary star. These figures are courtesy of Lukasz Wyrzykowski.

Acknowledgement: This brief article is based on a talk given at the workshop in İstanbul to celebrate the contributions, and birthday, of Salih Karaali. It has been a pleasure to know, and work with Prof Karaali over many years, and now to wish him a long and healthy retirement. I also acknowledge the generous hospitality of Tansel (and Serap) Ak and all their colleagues. Interestingly, the Workshop also coincided with the 19th anniversary of the presentation of the Gaia mission for adoption by ESA.

References

Casali, G., Magrini, L., Tognelli, E. et. al., 2019, A&A, 629A, 62 Gilmore, G., Reid, I. N., 1983, MNRAS, 202, 1025 Gilmore, G., Randich, S., Asplund, M. et al., 2012, The Messenger, 147, 25Gilmore, G., 2018a, Contemporary Physics, 59, 155Gilmore, G., 2018b, Astrometry and Astrophysics in the Gaia sky, Proceedings of the International Astronomical

Union, IAU Symposium, 330, 23Masseron, T., Gilmore, G., 2015, MNRAS, 453, 1855Pieres, A., Girardi, L., Balbinot, E. et al., 2020, MNRAS, 497, 1547Perryman, M., de Boer, K., Gilmore, G. et al., 2001, A&A, 369, 339Sanders & Gilmore (in prep)

Documents

Gaia: Astrometric Survey of the Galaxy