arXiv subjects

Adrienne Stilp

Publications and source records attributed to Adrienne Stilp.

10 recordsLinked to original sources

arXiv2017

How to Reconcile the Observed Velocity Function of Galaxies with Theory

Within a Lambda Cold Dark Matter (LCDM) scenario, we use high resolution cosmological simulations spanning over four orders of magnitude in galaxy mass to understand the deficit of dwarf galaxies in observed velocity functions. We measure velocities in as similar a way as possible to observations, including generating mock HI data cubes for our simulated galaxies. We demonstrate that this apples-to-apples comparison yields an "observed" velocity function in agreement with observations, reconciling the large number of low-mass halos expected in a LCDM cosmological model with the low number of observed dwarfs at a given velocity. We then explore the source of the discrepancy between observations and theory, and conclude that the dearth of observed dwarf galaxies is primarily explained by two effects. The first effect is that galactic rotational velocities derived from the HI linewidth severely underestimate the maximum halo velocity. The second effect is that a large fraction of halos at the lowest masses are too faint to be detected by current galaxy surveys. We find that cored dark matter density profiles can contribute to the lower observed velocity of galaxies, but only for galaxies in which the velocity is measured interior to the size of the core (~3 kpc).

astro-ph.GA↗

arXiv2013

The ACS Nearby Galaxy Survey Treasury XI. The Remarkably Undisturbed NGC 2403 Disk

We present detailed analysis of color-magnitude diagrams of NGC2403, obtained from a deep (m<28) Hubble Space Telescope (HST) Wide Field Planetary Camera 2 observation of the outer disk of NGC2403, supplemented by several shallow (m<26) HST Advanced Camera for Surveys fields. We derive the spatially resolved star formation history of NGC2403 out to 11 disk scale lengths. In the inner portions of the galaxy, we compare the recent star formation rates (SFRs) we derive from the resolved stars with those measured using GALEX FUV + Spitzer 24-micron fluxes, finding excellent agreement between the methods. Our measurements also show that the radial gradient in recent SFR mirror s the disk exponential profile to 11 scale lengths with no break, extending to SFR densities a factor of 100 lower than those that can be measured with GALEX and Spitzer (2x10^{-6} M_{\sun} yr^{-1} kpc^{-2}). Furthermore, we find that the cumulative stellar mass of the disk was formed at similar times at all radii. We compare these characteristics of NGC2403 to those of its "morphological twins," NGC300 and M33, showing that the structure and age distributions of the NGC2403 disk are more similar to those of the relatively isolated system NGC300 than to those of the Local Group analog M33. We also discuss the environments and HI morphologies of these three nearby galaxies, comparing them to integrated light studies of larger samples of more distant galaxy disks. Taken together, the physical properties and evolutionary history of NGC2403 suggest that the galaxy has had no close encounters with other M81 group members and may be falling into the group for the first time.

astro-ph.CO↗

arXiv2012

Implementing Molecular Hydrogen in Hydrodynamic Simulations of Galaxy Formation

Motivated by the observed connection between molecular hydrogen (H2) and star formation, we present a method for tracking the non-equilibrium abundance and cooling processes of H2 and H2-based star formation in Smoothed Particle Hydrodynamic simulations. The local abundances of H2 are calculated by integrating over the hydrogen chemical network. This calculation includes the gas-phase and dust grain formation of H2, shielding of HI and H2, and photodissociation of H2 by Lyman-Werner radiation from nearby stellar populations. Because this model does not assume equilibrium abundances, it is particularly well suited for simulations that model low-metallicity environments, such as dwarf galaxies and the early Universe. We further introduce an explicit link between star formation and local H2 abundance. This link limits star formation to "star-forming regions," represented by areas with abundant H2. With this implementation, we determine the effect of H2 on star formation in a cosmological simulation of a dwarf galaxy. This simulation is the first cosmological simulation with non-equilibrium H2 abundances to be integrated to a redshift of zero or to include efficient SN feedback. We find that our simulations are consistent with the observed Tully-Fisher, global Kennicutt-Schmidt, and resolved Kennicutt-Schmidt relations. We find that the inclusion of shielding of both the atomic and molecular hydrogen and, to a lesser extent, the additional cooling from H2 at temperatures between 200 and 5000 K increases the amount of cold gas in the galaxies. The changes to the ISM result in an increased amount of cold, dense gas in the disk of the galaxy and the formation of a clumpier interstellar media (ISM). The explicit link between star formation and H2 and the clumpier ISM results in a bluer galaxy with a greater spatial distribution of star formation at a redshift of zero. (abridged)

astro-ph.CO↗

arXiv2010

The ACS Nearby Galaxy Survey Treasury VI. The Ancient Star Forming disk of NGC 404

We present HST/WFPC2 observations across the disk of the nearby isolated dwarf S0 galaxy NGC 404, which hosts an extended gas disk. Our deepest field reaches the red clump and main-sequence stars with ages <500 Myr. Although we detect trace amounts of star formation at times more recent than 10 Gyr for all fields, the proportion of red giant stars to asymptotic giants and main sequence stars suggests that the disk is dominated by an ancient (>10 Gyr) population. Detailed modeling of the color-magnitude diagram suggests that ~70% of the stellar mass in the NGC 404 disk formed by z~2 (10 Gyr ago) and at least ~90% formed prior to z~1 (8 Gyr ago). These results indicate that the stellar populations of the NGC 404 disk are on average significantly older than those of other nearby disk galaxies, suggesting that early and late type disks may have different long-term evolutionary histories, not simply differences in their recent star formation rates. Comparisons of the spatial distribution of the young stellar mass and FUV emission in GALEX images show that the brightest FUV regions contain the youngest stars, but that some young stars (<160 Myr) lie outside of these regions. FUV luminosity appears to be strongly affected by both age and stellar mass within individual regions. Finally, we use our measurements to infer the relationship between the star formation rate and the gas density of the disk at previous epochs. We find that most of the history of the NGC 404 disk is consistent with star formation that has decreased with the gas density according to the Schmidt law. However, 0.5-1 Gyr ago, the star formation rate was unusually low for the inferred gas density, consistent with the possibility that there was a gas accretion event that reignited star formation ~0.5 Gyr ago. Such an event could explain why this S0 galaxy hosts an extended gas disk.

astro-ph.CO↗

arXiv2009

The ACS Nearby Galaxy Survey Treasury IV. The Star Formation History of NGC 2976

We present resolved stellar photometry of NGC 2976 obtained with the Advanced Camera for Surveys (ACS) as part of the ACS Nearby Galaxy Survey Treasury (ANGST) program. The data cover the radial extent of the major axis of the disk out to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth of M_F606W ~ 1, and an inner field was imaged to the crowding limit at a depth of M_F606W ~ -1. Through detailed analysis and modeling of these CMDs we have reconstructed the star formation history of the stellar populations currently residing in these portions of the galaxy, finding similar ancient populations at all radii but significantly different young populations at increasing radii. In particular, outside of the well-measured break in the disk surface brightness profile, the age of the youngest population increases with distance from the galaxy center, suggesting that star formation is shutting down from the outside-in. We use our measured star formation history, along with H I surface density measurements, to reconstruct the surface density profile of the disk during previous epochs. Comparisons between the recovered star formation rates and reconstructed gas densities at previous epochs are consistent with star formation following the Schmidt law during the past 0.5 Gyrs, but with a drop in star formation efficiency at low gas densities, as seen in local galaxies at the present day. The current rate and gas density suggest that rapid star formation in NGC 2976 is currently in the process of ceasing from the outside-in due to gas depletion. This process of outer disk gas depletion and inner disk star formation was likely triggered by an interaction with the core of the M81 group >~1 Gyr ago that stripped the gas from the galaxy halo and/or triggered gas inflow from the outer disk toward the galaxy center.

astro-ph.GA↗

arXiv2009

Increasing the Number of Underrepresented Minorities in Astronomy: Executive Summary

Promoting racial and ethnic diversity is critically important to the future success and growth of the field of astronomy. The raw ability, drive and interest required to excel in the field is distributed without regard to race, gender, or socioeconomic background. By not actively promoting diversity in our field we risk losing talented people to other professions (or losing them entirely), which means that there will be astronomical discoveries that simply won't get made. There is demonstrated evidence that STEM fields benefit from diverse perspectives on problems that require more complex thought processes. This is especially relevant to a field like astronomy where more and more work is being done collaboratively. The lack of notable growth in African American, Hispanic, and Native American representation in astronomy indicates that the 'pipeline' for these individuals is systemically leaky at critical junctures. Substantially more effort must be directed toward improving the educational and career development of minorities to insure that these potential colleagues are supported through the process. However, simply recognizing that the pipeline is faulty is woefully inadequate. There must be very specific, targeted solutions to help improve the situation. With this in mind, we offer two position papers addressing specific areas of improvement that we identify as (a) essential for any foreseeable progress in the field, and (b) attainable in the 2010-2020 decade. These position papers focus primarily on African Americans, Hispanics, and Native Americans. Although we do not directly address issues of Asian Americans, Pacific Islanders, and other groups, many of the recommendations made here can be adapted to address issues faced by these groups as well.

astro-ph.IM↗

arXiv2009

Increasing the Number of Underrepresented Minorities in Astronomy at the Undergraduate, Graduate, and Postdoctoral Levels (Paper I)

If the ethnic makeup of the astronomy profession is to achieve parity with the general population within one generation (~30 years), the number of underrepresented minorities earning graduate degrees in astronomy and astrophysics must increase in the coming decade by a factor of 5 to 10. To accomplish this, the profession must develop and invest in mechanisms to more effectively move individuals across critical educational junctures to the PhD and beyond. Early and continuous research engagement starting in the undergraduate years is critical to this vision, in which the federally funded research internship programs (e.g. NSF REU, NASA GSRP) and national centers/observatories play a vital role. Regionally based partnerships with minority-serving institutions (MSIs) are crucial for tapping extant pools of minority talent, as are post-baccalaurate and/or masters degree "bridging" programs that provide critical stepping stones to the PhD. Because of the strong undergraduate physics, engineering, and computer science backgrounds of many students from MSIs, we suggest that instrument development and large scale computing/data-mining are particularly promising avenues for engagement in the coming decade.

astro-ph.IM↗

arXiv2009

Increasing the Number of Underrepresented Minorities in Astronomy Through K-12 Education and Public Outreach (Paper II)

In order to attract, recruit and retain underrepresented minority students to pursue Astronomy and related fields, we must ensure that there continues to be a well qualified pool of graduate and undergraduate students from which to recruit. This required pool of people are today's elementary, middle and high school students. The Astronomy community must be proactive in demonstrating the importance of pursing scientific study and careers to these students and their parents. Only by actively engaging these communities can U.S Astronomy hope to increase the numbers of minority PhDs and continue to be a leader in Astronomical discovery and knowledge.

astro-ph.IM↗

arXiv2008

Connecting Gas Dynamics and Star Formation Histories in Nearby Galaxies: The VLA-ANGST Survey

In recent years, HST revolutionized the field of star formation in nearby galaxies. Due to its high angular resolution it has now become possible to construct star formation histories of individual stellar populations on scales of a few arcseconds spanning a range of up to ~600 Myr. This method will be applied to the ANGST galaxies, a large HST volume limited survey to map galaxies up to distances of 3.5-4.0 Mpc (excluding the Local Group). The ANGST sample is currently followed--up by high, ~6'' resolution VLA observations of neutral, atomic hydrogen (HI) in the context of VLA-ANGST, an approved Large VLA Project. The VLA resolution is well matched to that of the spatially resolved star formation history maps. The combination of ANGST and VLA-ANGST data will provide a new, promising approach to study essential fields of galaxy evolution such as the triggering of star formation, the feedback of massive stars into the interstellar medium, and the structure and dynamics of the interstellar medium.

astro-ph↗

arXiv2008

The Formation of Polar Disk Galaxies

Polar Ring Galaxies, such as NGC4650A, are a class of galaxy which have two kinematically distinct components that are inclined by almost 90 degrees to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain, and the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that Polar Ring Galaxies are simply an extreme example of the angular moment misalignment that occurs during the hierarchical structure formation characteristic of Cold Dark Matter cosmology. In our model, Polar Ring Galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy.

astro-ph↗