Hello!

I'm Danny Horta, a PhD student working at Liverpool John Moores University (UK). I am also currently a visiting research student at the University of Queensland in Brisbane (Australia), where I will be working/living until November 2021.
I am originally from the Canary islands (Lanzarote), and was raised in north-east of Spain (Catalunya). At the age of seventeen, I left my home and relocated to the UK (Newcastle or "the Toon") on a basketball scholarship, where I ended up playing professionally alongside completing my undergraduate degree at Northumbria University in Physics with Astrophysics (MPhys).
I am a very active person, and like to play a lot of sports (such as basketball, judo, football...). I am also a food enthusiast and adventure seeker, and like to take any opportunity I can to go and explore the world on my travels.

Image credit: © Danny Horta Darrington (BBL Trophy Final 2017, Glasgow, UK).

Research Interests

I am primarily interested in the formation and evolution of the Milky Way and its Globular Cluster system, and how we can apply this to understanding the formation of other disk galaxies. Specifically, I attempt to answer the following questions: How did the Galaxy assemble? What is the nature of Galactic populations? Is the Milky Way a typical galaxy?

To try to understand these problems, I combine observational data from large surveys such as APOGEE and Gaia with cosmological numerical simulations such as EAGLE/FIRE.

Main research topics I am interested in:

Galactic archaeology; galaxy formation and evolution; mass assembly of galaxies; stellar populations; chemical tagging; Galactic dynamics; Galactic chemical evolution; first stars; globular cluster formation and evolution

Discovery of the Heracles

"Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy"

I am interested in the mass assembly history of the Galaxy via untangling of the field populations of the Galactic stellar halo. In 2020, I lead a paper which focused on chemo-dynamically disentangling accreted and "in situ" field populations in the Milky Way. Via the combination of APOGEE chemistry and IoM values determined using Gaia data, we discovered a new metal-poor substructure in the inner ~4 kpc from the Galactic centre that presented chemo-dynamical signatures of accreted populations. We dubbed this substructure the "Heracles" after the famous greek hero which according to the myth was involved in the creation of the Milky Way, and suggested it is the remnant of a major accretion event that likely merged early in the life of the Galaxy. Moreover, using the EAGLE numerical simulations, we showed that such accretion scenario is not unusual for Milky Way mass galaxies, and estimated a stellar mass for the progenitor of this substructure given by its Mg and Fe chemical compositions of ~5x10^8 Msun (almost twice the size of the recently discovered Gaia-Enceladus/Sausage accretion event and a third of the total mass of the Galactic stellar halo). Furthermore, our results showed that the fraction of "in situ" to accreted stars below [Fe/H] < -0.8 in the inner Galaxy is between 1:2 to 1:3, confirming theoretical predictions from various numerical cosmological simulations.

To see this paper, click here.

Mass arising from Globular Cluster disruption

"The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants"

Entwined with the question of how the Milky Way stellar halo formed, is the question of how much dissolved GCs contribute to the total mass of the Galaxy. In order to tackle this question, I lead a paper in which we modelled the density of halo field stars and stars arising from GC dissolution and/or evaporation (i.e. N-rich stars), in order to assess the contribution of N-rich stars to the total stellar halo mass budget as a function of Galactocentric radius. Using chemical tagging and Gaussian mixture modelling tehcniques, we targetted N-rich stars in APOGEE and assessed their ratio with respect to the halo field. Our results showed that GC disruption is higher in the inner ~2 kpc of the Galaxy, contributing an average of ~28% of the total stellar halo mass. We also showed that GC disruption also occurs at large Galactocentric distances (~10-15 kpc), however contributing a much smaller average fraction of the order of ~4%. Finally, we used our density models to integrate the mass from both disrupted GC stars, and halo field stars within a hollow sphere of ~20 kpc in radius. Our estimates yielded a total mean halo stellar mass of M ~ 9 x 10^8 Msun, and a mean mass from dissolved and/or evaporated GC stars of M ~1 x 10^8 Msun.

To see this paper, click here.

Globular cluster formation via the AMR

"Linking globular cluster formation at low and high redshift through the age-metallicity relation in E-MOSAICS"

The question of how ancient (metal-poor) globular clusters (GC) form is not an easy one to answer. Suggested theories have either invoked conditions unique to the early Universe in order to form GCs, or adopted very old ages, leading to the suggestion that they may pre-date the formation of their (eventual) host galaxy. A contrasting view is that ancient GCs share the same formation mechanism as intermediate age clusters (IACs) and young massive clusters (YMCs), and are the result of the star formation processes and environment of their host galaxies at early times. In this work we set out to compare the age-metallicity relation (AMR) of massive clusters from Magellanic Cloud-mass galaxies in the E-MOSAICS suite of numerical cosmological simulations with an amalgamation of observational data of massive clusters in the Large and Small Magellanic Clouds (LMC/SMC, respectively). We aimed to test if: i) star cluster formation proceeds according to universal physical processes; ii) massive clusters of all ages trace a continuous AMR, suggestive of a common formation mechanism for YMCs, IACs, and ancient GCs; iii) the AMRs of smaller mass galaxies show a lower relation when compared to more massive galaxies. Our results showed that, within the uncertainties, the predicted AMRs of L/SMC-mass galaxies with similar star formation history to the L/SMC follow the same relation as observations. We also find that the metallicity at which the AMR saturates increases with galaxy mass, which is also found for the field star AMRs. This suggests that low-metallicity clusters can still form in dwarfs galaxies. Given our results, we suggest that ancient GCs share their formation mechanism with IACs and YMCs, in which GCs are the result of a universal process of star cluster formation during the early episodes of star formation in their host galaxies.

To see this paper, click here.

Origin of the Galaxy's Globular Clusters

"The Chemical Compositions of Accreted and in situ Galactic Globular Clusters According to SDSS/APOGEE"

I am interested in the origin of the Galactic Globular Cluster (GC) system, and what it can tell us about the mass assembly history of the Milky Way. In 2019, I lead a paper which compared the mean chemical abundances of different kinematically identified GC subgroups, in order to dissentangle if they formed in the Galaxy or were accreted through the process of mergers. We used the latest dataset (DR16) of the APOGEE survey in order to target over 3,000 stars associated with 46 GCs. Our results provided a different avenue to classify the origin of Galactic GCs, and showed that: (i) both the "in situ" and accreted GCs presented alpha-iron abundances which matched those of their field counterparts, with the accreted GCs occupying the same locus; (ii) the metal-rich low energy GCs are consistent with being from in situ origin; (iii) NGC 6388 and NGC5904 are likely from an accreted origin, whilst Liller 1 is likely to have formed in situ.

To see this paper, click here.

Teaching & Outreach

Aside from my research, another important aspect of my career that I am passionate about is teaching and outreach. I believe that every scientist has a debt to pay in sharing their knowledge with the next generation, both via an academic and/or public platform. I believe the experience of inspiring young minds to succeed and learn about the Universe is one of the most rewarding aspects of being an astronomer.

Teaching experience

Senior Demonstrator, Practical Astrophysics (2019-2020)/(2020-present)

Teaching assistant, Introduction to Astrophysics (2018-2019)

Outreach activities

Public talk: "The Milky Way: How it formed?", LJMU Wexweek (July,2020)

LJMU Outreach committee member (2018-present)

Media & Highlights

I have been fortunate enough to have the opportunity to disseminate my work with the general public via several media platforms thanks to the attention some of my results have received. In particular, the results from a paper I lead on the discovery of Heracles -the galaxy within the Galaxy- have been significantly impactful. As part of this discovery, several articles have been written in different media outlets ranging from research highlights in prestigious journals, to news articles and radio podcasts... The majority of the highlights/media articles surrounding this discovery are listed below:

Research Highlights

Nature Astronomy, "Excavating a Galactic tomb" Link.

Press Releases

Sloan Digital Sky Survey"Astronomers discover new “fossil Galaxy” buried deep within the Milky Way" Link.

Media Articles (Most relevant only)

BBC (Newsround) "Meet 'Heracles', a 10 billion year old fossil galaxy” Link.

Sky & Telescope "Astronomers discover Galactic “fossil” inside the Milky Way” Link.

Media INAF "Una galassia fossile nel cuore della Via Lattea” Link.

CNET "Scientists spot signs of 'fossil galaxy' lurking in the heart of the Milky Way" Link.

Forbes "We’ve found an ancient ‘fossil Galaxy’ inside our Milky Way, say scientists" Link.

The Independent "Fossil Galaxy" found hidden in the Milky Way Link.

News interviews/podcasts

Interview on "Principio de incertidumbre", Canal Extremadura, Spanish News (2020), "Heracles, un fósil galáctico en el interior de la Vía Láctea" Link.

Image © Danny Horta/NASA/JPL-Caltech/SDSS "artist impression of the Milky Way with Heracles"

Feel free to get in touch!

Email: D.HortaDarrington@2018.ljmu.ac.uk

GitHub: DHortaD

Twitter: @DarringtonHorta

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