The definition of a “galactic bulge” remains a matter of debate for the broad Astrophysics community. In BEARD, we refer to its classical definition: a bulge is a massive stellar structure supported by random motions located at the center of a spiral galaxy. Note that this is different to nuclear discs (kinematically-decoupled and small-scale stellar discs) and boxy/peanuts (X-shaped stellar structures form at the center of stellar bars).

Bulgeless galaxies are those either without a classical bulge or hosting a small bulge that, like in our Milky Way, accounts for less than 8% the mass of the main galaxy disc.

The hierarchical nature of galaxy growth in a ΛCDM paradigm predicts that nearby galaxies have undergone a number of mergers during their lifetime. This scenario works well for massive ellipticals. However, the stellar discs of spiral galaxies are fragile systems that may not survive: violent mergers destroy flat components making them much rounder, while minor mergers puff up structures. Cosmological hydrodynamical simulations have managed to form bulgeless dwarf disc galaxies with the aid of feedback and continuous cold gas accretion, but their recipes fail for Milky Way-mass galaxies (log(M_★/M_⦿)>10.5). How are bulgeless massive spirals built?

The Milky Way is the closest example of a bulgeless massive disc galaxy. With the advent of ground-based large spectroscopic surveys and the Gaia satellite, we are converging towards a view of our Galaxy composed of a thin disc with four spiral arms, a thick disc, and a central bar with a vertical boxy/peanut structure. The presence of a central bulge, if any, has been restricted to a very small structure accounting for no more than 8% of the disc mass.

Gaia-Enceladus was the last big merger event experienced by the Milky Way, around 10 Gyr ago. It did not destroy the disc, but rather triggered star formation. Other minor merger events have also been found, such as the Helmi, Sequoia, Thamnos, and Sagittarius streams. There therefore is a lack of major mergers during the Milky Way evolution, contrary to predictions from ΛCDM models. Whether this is a common feature of massive bulgeless spirals can only be understood using statistical samples of Milky Way-like galaxies.

BEARD was born to provide multi-mode and multi-frequency observational constraints, as well as a theoretical framework, to the challenge that Milky Way-like galaxies represent for a ΛCDM Universe.

The seed of BEARD was the multi-mode multi-wavelength observations awarded by the International Scientific Committee (CCI) during two consecutive years with an International Time Programme (ITP).

BEARD used several facilities at the Roque de los Muchachos Observatory:

• MEGARA@GTC for obtaining high spectral resolution integral-field spectroscopy
• WFC@INT for deep broad-band photometry
• IO:O@LT for narrow-band Hα photometry
• DOLORES@TNG and ISIS@WHT for long-slit low-resolution spectroscopy

Altogether, BEARD ITP observations were carried out during 78 nights. In parallel, the BEARD team have obtained further observations with other facilities, such as integral-field spectroscopy with WEAVE@WHT.

Much information is encoded at the galactic hearts, where different structures such as bulges, nuclear stellar clusters, and black holes coexist. What secrets do hide at the centres of Milky Way-like galaxies, where no prominent classical bulge can be found? Are the little spheroids actual classical bulges in miniature, are they rotating discs, or do we find compact, small stellar systems called nuclear star clusters? When and why did these structures form? And, the biggest secret of all: in a Universe where the coevolution between galaxies and supermassive black holes comes defined through correlations between black hole mass and bulge mass, which is the place of bulgeless galaxies? Galaxies like our own exist, and we are determined to understand how they assemble in the big puzzle of the Universe.

In BEARD, we are using supreme MEGARA@GTC (Gran Telescopio Canarias) integral-field spectra for unveiling all abovementioned secrets about the centres of Milky Way analogues.

The faint outskirts of galaxies, with their plethora of different components, contain key information about the merger formation of galaxies. For instance, tidal tails (elongated structures shaped by any tidal interaction) are globally associated to major mergers; shells have been demonstrated to raise from intermediate-mass mergers; and tidal streams are mostly found in less massive systems, with the material in the streams being different from that of the main galaxy.

We are exploring deep imaging of the BEARD Milky Way-like galaxies, taken with the Wide Field Camera (WFC) mounted on the 2.5 m Isaac Newton Telescope (INT), not only to unveil their merger history but also to characterise their properties, such as mass ratio, amount of gas involved, and fraction of accreted mass in the galaxy outskirts.

We are also using these images to explore the satellite population around Milky Way-like galaxies, to gain more clues about the merging processes they have gone through and to characterise whether they have an anisotropic distribution of satellites, like the Milky Way, or an isotropic distribution, as it might be expected within CDM.

The properties of the stars in the galaxies hold invaluable information about their formation and evolutionary processes. For example, a monolithic collapse scenario produces strong negative metallicity gradients across the galaxy, which get diluted after an active merger history.

We are exploring the stellar population properties, as well as the star formation histories and current star formation rate of BEARD galaxies with the DOLORES long-slit spectrograph mounted on the Telescopio Nazionales Galileo (TNG) telescope, the ISIS long-slit spectrograph on the William Herschel Telescope (WHT), and narrow-band H𝛂 images obtained with IO:O mounted on the Liverpool Telescope (LT).

Multiple structures shape late-type disc galaxies like our own Milky Way, which at least hosts a main galaxy disc, a bar, a nuclear stellar disc, a nuclear stellar cluster, and a bunch of spiral arms. This structural complexity hampers a good characterisation and understanding of the various phases that gave rise to our Galaxy, as well as other galaxies alike.

C2D is a novel two-dimensional (2D) multi-component spectrophotometric decomposition code. Its purpose is to untangle the galaxy light coming from every stellar structure (e.g. bulges, bars, discs) by working over integral-field spectra. It therefore provides not only an image (as regular 2D photometric decompositions do), but a whole integral-field datacube of each isolated structure (“spectrophotometric”), thus allowing a star formation history analysis of such component without contamination from the others.

We are applying C2D to ancillary MUSE@VLT, WEAVE@WHT, and MEGARA@GTC integral-field spectra of the BEARD Milky Way analogues to dissect these galaxies into their multiple components (bulges, if present, bars, and discs) and study in detail how they were assembled, from the innermost regions to the outskirts.

Numerical simulations are a fundamental tool to understand the physics of galaxies, now allowing a quantitative analysis of the merger history and distribution of satellites around Milky Way analogues. Our experts in numerical simulations are exploring unbiased samples of BEARD-like galaxies in the cosmological magnetohydrodynamical simulation Illustris TNG50 to unveil their properties and provide a theoretical framework to the observational results we are obtaining, while zoom-in simulations (Auriga, HESTIA, CLUES) offer a unique view of Milky Way-like galaxies.

BEARD galaxies were primarily selected from the SDSS-DR13 spectroscopic catalogue in a volume-limited fashion. We imposed the following conditions, resulting in a first draft of 61 galaxies:

• distance D<40 Mpc, using the SDSS spectroscopic redshift and a flat cosmology with  𝛀_m=0.3,  𝛀_𝛌=0.7, and a Hubble constant H₀=70 km s^-1 Mpc^-1;
• inclination i<60º, to avoid edge-on systems where projection effects hamper the morphological characterisation;
• concentration C<2.5, to guarantee that the BEARD parent sample is dominated by late-type disc galaxies;
• Petrosian radius R_petro>10 arcsec, avoiding extremely small galaxies with potential spatial resolution problems;
• total stellar masses M_★ > 10¹⁰ M_☉

The SDSS spectroscopic sample can be biased by fiber collision, which prevents detecting close targets. We therefore searched for massive discs in the HyperLeda catalogue, with the following criteria:

• Hubble type T>4 to select only late-type spirals; 
• 𝛅> 0º so they can be observed from the Northern hemisphere; 
• recessional velocities with respect to the cosmic microwave background, v_3k < 2800 km s^-1, in order to get galaxies within a distance of 40 Mpc; 
• B-band absolute magnitude B < -15 to avoid dwarf galaxies.

Through a cross-match with the SDSS photometric sample, 37 additional galaxies were found this way.

Colour images of the 61+37=98 galaxies were then visually inspected, finding out that 23 galaxies had been misclassified as massive spirals. The final BEARD parent sample consists of the remaining 75 Milky Way-like galaxies.