Star formation history of the Galactic bulge from deep HST imaging of low reddening windows

Bernard, Edouard J.; Schultheis, M.; Matteo, P. Di; Hill, V.; Haywood, M.; Calamida, A.

doi:10.1093/mnras/sty902

Cited by 72 publications

(94 citation statements)

References 51 publications

(58 reference statements)

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“…When taking into account distance, reddening, and metallicity effects, Haywood et al (2016) showed that the MS-TO color spread of a purely old stellar population would be wider than what is observed, and thus advocating for the presence in the bulge of a conspicuous population of young and intermediate-age stars. Very similar results are presented by Bernard et al (2018) who calculated the star formation history of four bulge fields, including that of Clarkson et al (2011). Their findings suggest that over 80% of the stars are older than 8 Gyr, but also suggest the presence of star formation as recent as ∼1 Gyr.…”

Section: Introductionsupporting

confidence: 77%

“…Therefore, prior to any attempt to age-dating the bulge population from its observed CMD, special care must be taken to remove the contribution of the intervening disk population along the line of sight. This can be done either by using proper motions (see Clarkson et al 2008Clarkson et al , 2011Kuijken & Rich 2002;Bernard et al 2018) or statistically with a disk controlfield (e.g., Zoccali et al 2003;Valenti et al 2013). Both methods require some assumptions, and therefore have their own pros and cons.…”

Section: Disk Decontamination Proceduresmentioning

confidence: 99%

“…As is evident in Fig. 4 in Bernard et al (2018), along different lines of sight the proper motions of disk and bulge largely overlap, which means that there is no simple clear cut in µ l and/or µ b that would help differentiate disk from bulge; although a filter in proper motions can still be applied, it would inevitably end with an equally problematic cross-contamination between the populations. In addition, from the observations point of view the determination of proper motions is very time consuming.…”

Section: Disk Decontamination Proceduresmentioning

confidence: 99%

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Mapping the stellar age of the Milky Way bulge with the VVV

Surot

Valenti

Hidalgo

et al. 2019

A&A

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Context. Recent observational programs are providing a global view of the Milky Way bulge that serves as a template for detailed comparison with models and extragalactic bulges. A number of surveys (VVV, GIBS, GES, ARGOS, BRAVA, APOGEE) are producing comprehensive and detailed extinction, metallicity, kinematics, and stellar density maps of the Galactic bulge with unprecedented accuracy. However, the still missing key ingredient is the distribution of stellar ages across the bulge. Aims. To overcome this limitation, we aim to age-date the stellar population in several bulge fields with the ultimate goal of deriving an age map of the bulge. This paper presents the methodology and the first results obtained for a field along the bulge minor axis, at b = −6°. Methods. We use a new PSF-fitting photometry of the VISTA Variables in the Vía Láctea (VVV) survey data to construct deep color–magnitude diagrams of the bulge stellar population down to ∼2 mag below the main sequence turnoff. To address the contamination by foreground disk stars we adopt a statistical approach by using control-disk fields located at different latitudes (spanning approximately the bulge’s range) and longitudes −30° and +20°. We generate synthetic photometric catalogs of complex stellar populations with different age and metallicity distributions, including the observational errors and completeness. The comparison between the color–magnitude diagrams of synthetic and observed disk-decontaminated bulge populations provides constraints on the stellar ages in the observed field. Results. We find the bulk of the bulge stellar population in the observed field along the minor axis to be older than ∼7.5 Gyr. In particular, when the metallicity distribution function spectroscopically derived by GIBS is used, the best fit to the data is obtained with a combination of synthetic populations with ages in between ∼7.5 Gyr and 11 Gyr. However, the fraction of stars younger than ∼10 Gyr strongly depends upon the number of blue straggler stars present in the bulge. Simulations show that the observed color–magnitude diagram of the bulge in the field along the minor axis is incompatible with the presence of a conspicuous population of intermediate-age and young (≲5 Gyr) stars.

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Section: Introductionsupporting

confidence: 77%

Section: Disk Decontamination Proceduresmentioning

confidence: 99%

Section: Disk Decontamination Proceduresmentioning

confidence: 99%

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Mapping the stellar age of the Milky Way bulge with the VVV

Surot

Valenti

Hidalgo

et al. 2019

A&A

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“…No a-priori constraint on the age-metallicity relation is adopted: TheStorm code solves simultaneously for age and metallicity within the range covered by the simple stellar populations. For more details on the fitting procedure and error calculation, the reader is referred to published references 52,63,68 .…”

Section: Methods 1 Data Selectionmentioning

confidence: 99%

Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

Gallart¹,

Bernard²,

Brook³

et al. 2019

Nat Astron

Self Cite

238

205

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Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, being comparison with stellar models 1 required. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting 2 , till now mainly employed to study nearby galaxies. The new distances to individual Milky Way stars from the Gaia mission 3 have allowed us to use this technique to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced colour-magnitude diagram of the kinematically hot local halo 4 , which blue-sequence has been linked to a major accretion event 5, 6 . Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation 7 , these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red-sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.The new accurate parallaxes and luminosities provided in the second data release (DR2) of the Gaia mission 3 have allowed us to construct, for the first time, colour-magnitude diagrams (CMD) in the absolute plane for stars located in a large volume of the Milky Way, encompasing different Galactic structural components. These CMDs, in units of absolute magnitudes and colours, are what is required to derive star formation histories 2 and stellar age distributions, by comparing them with theoretical CMDs derived from stellar evolution models 8 .The top panels of Figure 1 show the CMD of two sub-populations of Milky Way stars taken from a parent population that lies within a sphere of 2 Kpc around the Sun, as observed by Gaia. In this volume, accurate distances and absolute magnitudes can be derived directly from parallaxes. The CMD in the top left panel contains about sixty thousand stars from this spherical region with large tangential velocities relative to the Sun (greater than 200 km/s). Stars with such high velocities are classified in this study as belonging to a kinematically defined stellar halo 4 . The CMD in the top right panel is of some arXiv:1901.02900v2 [astro-ph.GA] 29 Jul 2019 half million stars from the same spherical region but selected to be at least 1.1 Kpc above or below the Galactic plane. At this distance from the plane, the majority of stars are expected to belong to the thick disc 9 , rather than to the young thin disc component. We have excluded from this sample the stars with high velocity (greater than 200 km/s) that have been included in the halo CMD. Note that o...

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“…On the other hand, although the majority of MWB stars are known to be old (∼10 Gyr), it remains to be understood if there is a significant fraction of young stars (<5 Gyr), specifically at the metal-rich end (see Renzini et al 2018, and references therein). However, the MWB age-metallicity relation recently found by Bernard et al (2018), that favours a wide range of ages for metalrich stars, can be used to show that in this case (for the well known spatial variations in mean metallicity) the mean age of the MWB could spatially vary from 10 Gyr in the outer regions to up to 6 Gyr closer to the plane. Even when these maximum population gradients are considered, the theoretical mean magnitude variation of the RC would be of < 0.10 magnitudes (see Fig.…”

Section: Introductionmentioning

confidence: 93%

The structure behind the Galactic bar traced by red clump stars in the VVV survey

Gonzalez

Minniti

Valenti

et al. 2018

Monthly Notices of the Royal Astronomical Society: Letters

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Red clump stars are commonly used to map the reddening and morphology of the inner regions of the Milky Way. We use the new photometric catalogues of the VISTA Variables in the Vía Láctea survey to achieve twice the spatial resolution of previous reddening maps for Galactic longitudes −10 • < l < 10 • and latitudes −1.5 • < b < 1.5 • . We use these de-reddened catalogues to construct the K s luminosity function around the red clump in the Galactic plane. We show that the secondary peak (fainter than the red clump) detected in these regions does not correspond to the bulge red-giant branch bump alone, as previously interpreted. Instead, this fainter clump corresponds largely to the over-density of red clump stars tracing the spiral arm structure behind the Galactic bar. This result suggests that studies aiming to characterise the bulge redgiant branch bump should avoid low galactic latitudes (|b| < 2 • ), where the background red clump population contributes significant contamination. It furthermore highlights the need to include this structural component in future modelling of the Galactic bar.

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Star formation history of the Galactic bulge from deep HST imaging of low reddening windows

Cited by 72 publications

References 51 publications

Mapping the stellar age of the Milky Way bulge with the VVV

Mapping the stellar age of the Milky Way bulge with the VVV

Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

The structure behind the Galactic bar traced by red clump stars in the VVV survey

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