I am an ESA/AURA Astronomer at Space Telescope Science Institute. I recently joined the Hubble Space Telescope STIS instrument team. I am a spectroscopist, experienced in wavelengths from the Far-UV all the way to NIR, with ample experience collecting, calibrating and analyzing both space and ground observations.
My primary research interests are focused on investigating how galaxies of different types evolve chemically. In addition to my primary work, I explored star-forming galaxies, dust contents and their relevance to the epoch of reionization. I am particularly interested in the evolution of galaxies through cosmic time, including their stellar, neutral- and ionized-gas components. I am one of the members of the STScI research group, MINGLE (Metals IN GaLaxiEs).
Being complex systems containing vast amounts of gas, dust, and stars, galaxies allow us to study the Universe in great detail.It is inside these systems that stars form, and transform the simplest of elements, hydrogen, into heavy elements essential for life as we know it. I spend most of my days thinking of ways to obtain clues to their chemical enrichment histories and their evolution in general. Here are some of the topics I have explored:
As an exploratory project we devoted some time to investigate if detailed abundance analysis is feasible on extragalactic populations at distances of 4-5 Mpc away. Furthermore, we take on a couple of additional challenges: 1) we test whether detailed abundance analysis is possible for observations with relatively lower resolutions (R~8,000) than those required so far, R~20,000, 2) we shift the focus of the analysis from GCs (old populations) to YMCs (younger, more complex populations). Both challenges impose higher degrees of blending of the spectral features. To minimize the effects of strong blending, we devise optimized wavelength windows tailored for each element. We successfully measured alpha and Fe-peak elements of two YMC with ages <50 Myr, NGC1313-379 and NGC1705-1, providing some insight into the star formation histories of these two galaxies.
Dissecting the integrated light of a total sample of 23 Young Massive Clusters observed in the UV (15) and in the optical (8) in our latest publications (Hernandez et al. 2018 & Hernandez et al. 2019), we performed a metallicity study on the face-on spiral starburt M83. We identify two possible breaks in the gradient of this spiral galaxy. If the abundance breaks are genuine, the metallicity gradient of this galaxy follows a steep-shallow-steep trend, a scenario predicted by three-dimensional (3D) numerical simulations of disc galaxies.
Looking for insight into the chemical evolution of this early type galaxy, we study 20 globular clusters (GCs) using the ground-based spectrograph X-Shooter. The GC sample spans a broad range of metallicities (-1.92 < [Fe/H] < -0.13 ) and ages (~5-12 Gyr). Quantitatively we find a slightly higher average [alpha/Fe] ratio for GCs in NGC 5128 with [Fe/H]< -0.75, compared to GCs in the Milky Way at these same metallicities. One explanation of the observed enhancement in Centaurus A could be a difference in IMF, where in this elliptical galaxy we could be observing the effects of an IMF slightly skewed to high-mass stars. To confirm if the star formation histories between the Milky Way and NGC 5128 differ from one another, we recommend extending this analysis to a larger sample.
Star-forming Galaxies as
main contributors to the Epoch of Reionization
I have spent some time investigating if star-forming galaxies were indeed the main sources reionizing the Universe. I have used the Hubble spectrogrpah COS as its sensitivity goes below 900Å, allowing for studies of the Lyman continuum escape fractions in nearby starburst galaxies. In our first study (Leitherer et al. 2016) we observed three galaxies previously studied with FUSE, confirmed to have large-scale outflows driven by SNe and stellar winds, as well as inhomogeneous ISM, both ideal for escaping Lyman continuum photons. We measure absolute escape fractions of Lyman continuum photons of f(LyC)<7 %. However, in the absence of dust absorption we instead measure relative escape fractions of f(LyC)<60 %. In a more recent publication, Hernandez et al. 2018, we continued expanding our galaxy sample. Due to the low flux levels in the individual COS observations we combined all seven galaxies to measure the Lyman continuum fraction. We infer 2 sigma upper limits for the absolute and relative Lyman continuum escape fractions of f(LyC)<20 % and f(LyC)<30%. We conclude that these galaxies are optically thick to Lyman continuum radiation.
SCIENTIFIC PUBLICATIONS: FIRST AUTHOR
- Newly Improved Ionization Corrections for the Neutral Interstellar Medium: Enabling Accurate Abundance Determinations in Star-forming Galaxies throughout the Universe
Hernandez, S., Aloisi, A., James, B. L., Fox, A. J., Ferland, G. J., Tosi, M., Tumlinson, J., 2020, ApJ, 892, 19H
- The First Metallicity Study of M83 Using the Integrated UV Light of Star Clusters
Hernandez, S., Larsen, S., Aloisi, A., Berg, D. A., Blair, W. P., Fox, A. J., Heckman, T. M., James, B. L., Long, K. S., Skillman, E.D., Whitmore, B.C., 2019, ApJ, 872, 116H
- Detailed Abundance Analysis of Stellar Populations in NGC 5128 (Centaurus A)
Hernandez, S., Larsen, S., Trager, S., Kaper, L., Groot, P., 2018c, MNRAS, 476, 5189
- Investigating the Lyman photon escape in Local Starburst Galaxies with the Cosmic Origins Spectrograph
Hernandez, S., Leitherer, C., Boquien, M., Buat, V., Burgarella, D., Calzetti, D., and Noll, S., 2018b, MNRAS, 478, 1292
- Metallicities of Young Massive Clusters in NGC 5236 (M83)
Hernandez, S., Larsen, S., Trager, S., Kaper, L., Groot, P., 2018a, MNRAS, 473, 826
- Chemical Abundances of Two Extragalactic Young Massive Clusters
Hernandez, S., Larsen, S., Groot, P., Kaper, L., 2017, A&A, 603, A119
SCIENTIFIC PUBLICATIONS: CO-AUTHOR
- Analysis of Red-Supergiants in VdBH 222
Randa Asa'd, M. Kovalev, B. Davies, V. D. Ivanov, M. Rejkuba, A. Gonneau, S. Hernandez, C. Lardo, M. Bergemann, 2020, ApJ, accepted
- Hypercompact stellar clusters: realistic models of their spectroscopic, photometric, and morphological properties
Lena, D., Rauer, J. P., Jonker, P. G., Hernandez, S., Kostrzewa-Rutkowska, Z., 2020, MNRAS, 495, 1771
- Hubble Space Telescope photometry of multiple stellar populations in the inner parts of NGC 2419
Larsen, S., Baumgardt, H., Bastian, N., Hernandez, S., Brodie, J., 2018, A&A, 624A, 25L
- Probing the Outflowing Multiphase Gas ~1 kpc Below the Galactic Center
Savage, B. D., Kim, T-S., Fox, A. J., Massa, D., Bordoloi, R., Jenkins, E. B., Lehner, N., Bland-Hawthorn, J., Lockman, F. J., Hernandez, S., Wakker, B. P., 2017, ApJS, 232, 25
- Mapping the Nuclear Outflow of the MilkyWay: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble
Bordoloi, R., Fox, A. J., Lockman, F. J., Wakker, B. P., Jenkins, E. B., Savage, B. D., Hernandez, S., et al. 2017, ApJ , 834, 191
- Searching for GC-like abundance patterns in young massive clusters
Cabrera-Ziri, I., Lardo, C., Davies, B., Bastian, N., Beccari, G., Larsen, S. S., Hernandez, S, 2016, MNRAS, 460, 1869
- Direct Detection of Lyman Continuum Escape from Local Starburst Galaxies with the Cosmic Origins Spectrograph
Leitherer, C., Hernandez, S., Lee, J. C., Oey, M. S., 2016, ApJ, 823, 64
- Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way's Biconical Nuclear Outflow
Fox, A. J., Bordoloi, R., Savage, B. D., Lockman, F. J., Jenkins, E. B., Wakker, B. P., Bland- Hawthorn, J., Hernandez, S., et al. 2015, ApJL, 799, L7
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My complete CV is linked here.
- Ph.D. from Radboud Universiteit, Nijmegen, The Netherlands (Jan 2015 - Jun 2018)
- 13 refereed publications, 6 as first author (total citations: 191)
- 14 oral contributions at international meetings (including 6 invited talks)
Education & Work
2018 - present
Space Telescope Science Institute, Baltimore, MD
Supervisor: Alessandra Aloisi
Ph.D. in Astrophysics
Radboud Universiteit, Nijmegen, The Netherlands
Title: "Tales told by Galaxies: Clues to chemical enrichment and the Epoch of Reionization"
Supervisor: Soeren Larsen
B.A. Degree in Astronomy
University of Texas, Austin, TX