{"id":176,"date":"2025-07-22T16:04:58","date_gmt":"2025-07-22T23:04:58","guid":{"rendered":"https:\/\/dornsife.usc.edu\/james-bullock\/?page_id=176"},"modified":"2025-07-28T16:32:32","modified_gmt":"2025-07-28T23:32:32","slug":"research","status":"publish","type":"page","link":"https:\/\/dornsife.usc.edu\/james-bullock\/research\/","title":{"rendered":"Research"},"content":{"rendered":"\n\n\n\n  \n    \n\n\n\n\n\n\n<div\n  class=\"cc--component-container cc--rich-text \"\n\n  \n  \n  \n  \n  \n  \n  >\n  <div class=\"c--component c--rich-text\"\n    \n      >\n\n    \n      \n<div class=\"f--field f--wysiwyg\">\n\n    \n  <h2><strong>The Nature of Dark Matter<\/strong><\/h2>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-211 alignright\" src=\"https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Dark-Matter-300x300.png\" alt=\"A simulation by former PhD student Shea Garrison-Kimmel of how dark matter may be distributed in the Local Group.\" width=\"300\" height=\"300\" srcset=\"https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Dark-Matter-300x300.png 300w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Dark-Matter-150x150.png 150w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Dark-Matter-320x320.png 320w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Dark-Matter.png 768w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/>There is robust observational <a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2016A%26A...594A..13P\/abstract\">evidence<\/a>\u00a0that approximately 85% of the mass in the universe is in the form of non-baryonic material, different from anything known in the periodic table of elements. \u00a0The leading idea is that dark matter consists of elementary particles beyond the\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Standard_Model\">standard model<\/a>\u00a0of particle physics. \u00a0Understanding the nature of dark matter and characterizing the microphysics of the dark matter particles stand among the most important quests in modern cosmology\u00a0<em>and<\/em>\u00a0particle physics. \u00a0Much of my work focuses on the use of numerical simulations in comparison with astronomical observations to constrain the nature of dark matter, specifically on the \u201csmall\u201d \u00a0size-scales of individual galaxies. \u00a0Interestingly, there are some indications that the standard\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Cold_dark_matter\">cold dark matter<\/a>\u00a0model may not be able to reproduce some observations on small scales, which motivates the exploration of alternative models like self-interacting dark matter.\u00a0<strong>The image above shows a\u00a0<a href=\"https:\/\/arxiv.org\/abs\/1806.04143\">simulation<\/a>\u00a0by former PhD student Shea Garrison-Kimmel of how dark matter may be distributed in the\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Local_Group\">Local Group<\/a>.<\/strong><\/p>\n<p><em>Here is a\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2017ARA%26A..55..343B\/abstract\">review article<\/a>\u00a0on the small scale crisis in cold dark matter I wrote with Mike Boylan-Kolchin.<\/em><\/p>\n<p><em>Here is the original\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2011MNRAS.415L..40B\/abstract\">Too Big to Fail paper<\/a>, where we pointed out one of the potential problems with cold dark matter.<\/em><\/p>\n<p><em>Here is a\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2013MNRAS.430...81R\/abstract\">self-interacting dark matter paper<\/a>\u00a0led by former PhD student Miguel Rocha, which used simulations to show how SIDM may help with small-scale problems.<\/em><\/p>\n\n\n\n<\/div>\n\n\n  <\/div><\/div>\n\n\n\n\n  \n    \n\n\n\n\n\n\n<div\n  class=\"cc--component-container cc--rich-text \"\n\n  \n  \n  \n  \n  \n  \n  >\n  <div class=\"c--component c--rich-text\"\n    \n      >\n\n    \n      \n<div class=\"f--field f--wysiwyg\">\n\n    \n  <h2><strong>Satellite Galaxies and Stellar Halos<\/strong><\/h2>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-213 alignright\" src=\"https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Stars-300x298.png\" alt=\"Image demonstrating how tiny satellite galaxies and diffuse stellar halo light may be distributed throughout the Local Group.\" width=\"300\" height=\"298\" srcset=\"https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Stars-300x298.png 300w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Stars-150x150.png 150w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Stars-768x763.png 768w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Stars.png 948w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/>The smallest galaxies in the universe are laboratories for galaxy formation in the extreme. \u00a0Dwarf galaxies are the most dark-matter dominated, lowest metallicity, and most numerous galaxies in the universe and therefore provide important testing grounds for theories of galaxy formation and the nature of dark matter. One of my longstanding research themes has been to understand the character and overall count of small dwarf galaxies in the vicinity of the Milky Way. \u00a0Many of the small galaxies that got close to the Milky Way in the past were completely destroyed by tidal interactions and their remnants are seen to exist as stellar streams and diffuse light in the <a href=\"https:\/\/en.wikipedia.org\/wiki\/Stellar_halo\">stellar halo.<\/a>\u00a0 Galaxy stellar halos therefore provide complementary laboratories for constraining galaxy formation physics and the merger histories of galaxies.\u00a0<strong>The image here shows stars in the same\u00a0<a href=\"https:\/\/arxiv.org\/abs\/1806.04143\">simulation<\/a>\u00a0as above, now demonstrating how tiny\u00a0satellite galaxies and diffuse stellar halo light may be distributed throughout the\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Local_Group\">Local Group<\/a>.<\/strong><\/p>\n<p><em>Here is a\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2005ApJ...635..931B\/abstract\">paper<\/a>\u00a0on stellar halo formation that I wrote with Kathryn Johnston.<\/em><\/p>\n<p><em>Here is a\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2010MNRAS.406.1220W\/abstract\">paper<\/a>\u00a0led by former PhD student Joe Wolf that derived a new standard mass estimator for dwarf galaxies.<\/em><\/p>\n<p><em>Here is a\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2008ApJ...688..277T\/abstract\">paper<\/a>\u00a0led by former PhD student Erik Tollerud that predicts the existence of many more dwarf galaxies soon to be discovered.<\/em><\/p>\n\n\n\n<\/div>\n\n\n  <\/div><\/div>\n\n\n\n\n  \n    \n\n\n\n\n\n\n<div\n  class=\"cc--component-container cc--rich-text \"\n\n  \n  \n  \n  \n  \n  \n  >\n  <div class=\"c--component c--rich-text\"\n    \n      >\n\n    \n      \n<div class=\"f--field f--wysiwyg\">\n\n    \n  <h2><strong>Galaxy Formation<\/strong><\/h2>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-medium wp-image-212 alignright\" src=\"https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Gas-300x300.png\" alt=\"Illustrates how an extended Circum-Galactic Medium (CGM) of multiphase gas likely exists throughout the Local Group.\" width=\"300\" height=\"300\" srcset=\"https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Gas-300x300.png 300w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Gas-150x150.png 150w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Gas-768x768.png 768w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Gas-320x320.png 320w, https:\/\/dornsife.usc.edu\/james-bullock\/wp-content\/uploads\/sites\/484\/2025\/07\/Gas.png 948w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/>Within the standard paradigm, the locations of galaxies in space and their global potentials are governed by dark matter halos. \u00a0But the detailed properties of galaxies \u2014 their sizes, shapes, evolutionary histories, and chemical makeup \u2014 are driven by complex astrophysical properties. \u00a0In order to fully understand galaxy formation, we must not only account for dark matter and dark energy, but also the baryonic material and associated astrophysics that governs stellar evolution, feedback, and the multiphase structure of cosmic gas.\u00a0<strong>This image shows gas in the same\u00a0<a href=\"https:\/\/arxiv.org\/abs\/1806.04143\">simulation<\/a>\u00a0and illustrates how an extended Circum-Galactic Medium (CGM) of multiphase gas likely exists throughout the\u00a0<a href=\"https:\/\/en.wikipedia.org\/wiki\/Local_Group\">Local Group<\/a>.<\/strong><\/p>\n<p><em>Here is a\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2011ApJ...738...39S\/abstract\">paper<\/a>\u00a0led by former PhD student Kyle Steward that introduced the idea that the gas that builds galaxies should have much more angular momentum than the dark matter.<\/em><\/p>\n<p><em>Here are\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2015MNRAS.454.2092O\/abstract\">two<\/a>\u00a0<a href=\"https:\/\/ui.adsabs.harvard.edu\/abs\/2015MNRAS.453.1305W\/abstract\">papers<\/a>\u00a0led by former Postdoc Jose O\u00f1orbe and former PhD student Coral Wheeler that simulate the formation of tiny galaxies using\u00a0<a href=\"https:\/\/fire.northwestern.edu\/\">FIRE<\/a>\u00a0simulations.<\/em><\/p>\n<p>&nbsp;<\/p>\n\n\n\n<\/div>\n\n\n  <\/div><\/div>\n","protected":false},"excerpt":{"rendered":"","protected":false},"author":11,"featured_media":276,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"page-chaptered-page.php","meta":{"_acf_changed":false,"footnotes":""},"class_list":["post-176","page","type-page","status-publish","has-post-thumbnail","hentry"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.1.1 - 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