Akaxia Cruz

Physics PhD candidate, NSF Graduate Research Fellow,
University of Washington

  • Research

    Astrophysical implications of non-standard dark matter

  • Curriculum Vitae

    updated November 2022

  • Contact

    admcruz [at] uw.edu

  • Akaxia Cruz

      1. "Astrophysical Plasma Instabilities induced by Long-Range Interacting Dark Matter", A. Cruz and M. McQuinn (Submitted). 2022 Available from arXiv.

      2. "Dark Matter, Black Holes, and Cold Clumps in the Circumgalactic Medium", A.Cruz, J. Werk, T. R. Quinn, B. Shih, I. Butsky, N. N. Sanchez, A. Pontzen (in prep) 2022.

      3. "The Impact of Cosmic Rays on the Kinematics of the Circumgalactic Medium", I. S. Butsky, J. K. Werk, K. Tchernyshyov, D. B. Fielding, J. Breneman, D. R. Piacitelli, T. R. Quinn, N. N. Sanchez, A.Cruz, C. B. Hummels ApJ 2022. Available from ApJ.

      4. "Self-Interacting Dark Matter and the Delay of Super-Massive Black Hole Growth", A. Cruz, A. Pontzen, M. Volonteri, T. R. Quinn, M. Tremmel, A. M. Brooks, N. N. Sanchez, F. Munshi, A. DiCintio, MNRAS 2021. Available from MNRAS.

      5. "One-Two Quench: A Double Minor Merger Scenario", N. N. Sanchez, M. Tremmel, J. Werk, A. Pontzen, C. Christensen, T. Quinn, S. Loebman, A. Cruz
        ApJ (Submitted) 2020 Available from arXiv.

      6. "Not So Heavy Metals: Black Hole Feedback Enriches the Circumgalactic Medium", N. N. Sanchez, J. Werk, M. Tremmel, A. Pontzen, C. Christensen, T. Quinn, A. Cruz, ApJ 2019, Available from arXiv.
        Simulation of Milky-Way mass galaxies in CDM (left) and SIDM (right). Blue is cold gas, red is hot gas, and pink is stars. SIDM alters SMBH growth, which can be seen in the suppression of hot gas SMBH outflows.
    • Akaxia Cruz

      • In Cruz et al. 2020 I used fully cosmological galaxy simulations in cold DM (CDM) and self-interacting DM (SIDM) with a constant cross-section of 1 cm^2/g to examine how the co-evolution of SMBHs and their MW-mass host galaxies (Mvir ~ 7 x 10^11 Msol at z = 0) is influenced by different DM models. To do this I used physically motivated models of SMBH formation and growth and simulated a MW-mass galaxy with quenched star formation in CDM and SIDM to maximize the effects of SMBH growth on galaxy evolution. I then ran a Genetically Modified version of these galaxies to check for result dependent sensitivity to small changes. I determined that:

        1. SMBH formation is consistently suppressed in SIDM relative to a LCDM cosmology. My CDM simulations produced ~ 2 or 3 times more SMBHs than my SIDM simulations.
        2. SIDM delays SMBH growth through mergers by billions of years compared to CDM, as can be seen in the left panel of Figure 1.
        3. SIDM SMBHs generate less SMBH feedback compared to CDM SMBHs during the first 5 Gyrs of their evolution as can be seen in the right panel of Figure 1. Nonetheless, by z = 0.8 their SMBH masses differ only by around 0.2 dex , so that both CDM and SIDM runs remain compatible with the MBH-M* relation.
        4. SIDM galaxies have a larger central reservoir of gas available for star formation.
        5. SIDM galaxies form about 3 or 4 times more stars than CDM galaxies over their lifetimes.

      • This work suggests that upcoming multi-messanger SMBH observations will allow us to probe the microscopic nature of DM in an emerging new way.
    • Akaxia Cruz

      • Every simulation study of the CGM to date has used a CDM backdrop. However, interestingly, I have found that O VI column densities are significantly altered by the backdrop DM model used in simulations. A set of GM MW-mass galaxies simulations with baryons and SMBHs in CDM and SIDM have found that the evolution of the galaxies' SMBHs is delayed by billions of years in SIDM models compared to their CDM counterparts, due to SIDM coring in the high-z universe. This is a natural prediction of a DM model which lowers DM densities at high-z, as it naturally lowers SMBH halo occupation fractions and increases dynamical friction time scales. The delayed SMBH growth in SIDM has a plethora of physical implications, including enhanced O VI column densities and distinct absorption spectra. Thus, DM indirectly imprints itself onto feedback processes which may be detected using the CGM.
    • Akaxia Cruz

      • The existence of dark matter (DM) has been made robust through multiple orthogonal astrophysical probes, yet the microscopic properties of DM remain unknown. In particular, it is unclear whether DM interacts with itself and/or with the standard model (SM) non-gravitationally. Cold DM, a single, collision-less particle species with negligible primordial thermal dispersion is largely considered the “concordance” DM model in the astrophysical community. Yet, observations on dwarf scales suggest that DM might be self-interacting. Milli-charged DM (mDM) can couple non-gravitationally with itself and with the SM. I do analytic calculations which show streaming mDM can cause transverse EM Weibel plasma instabilities in galactic systems, such as DM falling onto Milky-Way like galaxies and in merging galaxies clusters.
    • Publications
    • SIDM and SMBH Growth
    • SIDM and Kinematics of the CGM
    • Milli-Charged DM


    • Upcoming conferences
    • Past Talks
    • Save the date!

      I will discuss the impact of self-interacting dark matter on the formation and growth of supermassive black holes, and upcoming surveys and space probes that will allow us to examine DM in a new way.

    • Past Talks

      ■ indicates invited talk

      • SIDM and the Delay of Supermassive Black Hole GrowthN-Body Shop Excellence Conference, NBody Shop 2021
      • SIDM and the Delay of Supermassive Black Hole GrowthCCAPP Astroparticle Lunch Talk, Ohio State University 2020
      • SIDM and the Delay of Supermassive Black Hole GrowthAstro Lunch Talk, Yale University 2020
      • SIDM and the Delay of Supermassive Black Hole GrowthKIPAC Tea Talk, Stanford University 2020
      • Astrophysical Implications of Non-Standard Dark MatterAstro Seminar, Rutgers University 2020
      • Examining SIDM vs. CDM in L*-Sized GalaxiesInstitut D'Astrophysique de Paris 2019
      • How SIDM effects Star formation in Simulated Milky Way-like GalaxiesUC Santa Cruz Galaxy Workshop 2018
      • Neutrino Oscillation and 1 to 10 GeV Neutrion/Nucleon ScatteringCU Denver SPS Colloqium 2016
      • On Women of Color in STEMGirls Inc of Metro Denver Girls and Guys Night out 2015
      • Modeling Few-GeV Neutrion Interactions in WaterPINGU Collaboration 2015
      • Modeling Few-GeV Neutrion Interactions in WaterHarvard School of Engineering and Applied Sciences 2015
      • Simulation and Examination of Few-GeV Neutrinos in WaterDOE SULI Poster Session 2015
      • Neutrino Interactions in WaterDOE SULI Poster Session 2014



Beyond my research interests, I am passionate about making physics and astronomy more equitable in its practices. In 2016, I worked with Girls Inc of Metro Denver to help design and launch a 5-year program to mitigate attrition of historically excluded girls from the STEM fields. During this time, I helped write a STEM guide for girls and women in collaboration with Dr. Violeta Garcia. I continue to volunteer and work with Girls Inc of Metro Denver. During graduate school I have helped to start two registered student organizations, Physicists for Inclusion and Equity (PIE) and Graduate Students of Color in Astronomy and Physics (GCAP), in which I am currently an officer.

  • Akaxia Cruz

  • Akaxia Cruz
  • About me

    Bueno! I am a fifth year PhD candidate and NSF Graduate Research Fellow in the Department of Physics at the University of Washington. My research lies at the intersection of galaxy formation, dark matter physics, and cosmology. These days, I am particularly interested in examining the influence of self-interacting dark matter on the formation and growth of supermassive black holes using hydrodynamics simulations, but I have dabbled in some other areas of physics, including examining milli-charged dark matter using analytic and numerical methods. Other miscellaneous facts:

    • I love viewing and creating art in all forms and consider myself to be an amateur photographer (the background photo was taken on film by me).
    • Coffee tasting is one of my favorite pastimes. I am always on the hunt for the best java in any city I visit (pre-COVID-19).
    • I welcome questions from prospective physics/astronomy undergraduate/graduate students. Please feel free to send me an email.