arXiv subjects

Juna A. Kollmeier

Publications and source records attributed to Juna A. Kollmeier.

At least 73 records · Page 4Linked to original sources

arXiv2016

TGAS Error Renormalization from the RR Lyrae Period-Luminosity Relation

The Gaia team has applied a renormalization to their internally-derived parallax errors $σ_{\rm int}(π)$ $$ σ_{tgas}(π) = \sqrt{[Aσ_{int}(π)]^2 + σ_0^2}; \ \ \ \ (A,σ_0) = (1.4,0.20\ \rm mas) $$ based on comparison to Hipparcos astrometry. We use a completely independent method based on the RR Lyrae $K$-band period-luminosity relation to derive a substantially different result, with smaller ultimate errors $$ (A,σ_0) = (1.1,0.12\ \rm mas) \ \ \ \ (this\ paper). $$ We argue that our estimate is likely to be more accurate and therefore that the reported TGAS parallax errors should be reduced according to the prescription: $$ σ_{true}(π) = \sqrt{(0.79σ_{tgas}(π))^2 - (0.10\ \rm mas)^2}. $$

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arXiv2016

Predicted Information Content of Gaia-Hipparcos

The Gaia-Tycho release, scheduled for 14 September, is forecast to yield parallax errors of $σ(π)\sim 300\,μ$as for about 2 million Tycho stars. We show analytically that the actual performance should be $$ σ(π) = {\rm max}(σ_{1991}/96,20\,μ{\rm as}) $$ where $σ_{1991}$ is the positional error from the Hipparcos mission. For typical Tycho stars, $σ_{1991}\sim 30\,$mas, so this reproduces the usual claims. However, for the 100,000 star Hipparcos subset of this sample, $σ_{1991}$ is a factor 15 or more smaller. These much lower Hipparcos positional errors apply even to stars at the Hipparcos-Tycho limit, $V\sim 12$. This is especially important for RR Lyrae stars, as well as other special classes, that were systematically included in the Hipparcos catalog down to this limit because of their exceptional scientific importance. This predicted performance will provide an early test of the Gaia algorithms.

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arXiv2016

Gaia Parallax Zero Point From RR Lyrae Stars

Like Hipparcos, Gaia is designed to give absolute parallaxes, independent of any astrophysical reference system. And indeed, Gaia's internal zero-point error for parallaxes is likely to be smaller than any individual parallax error. Nevertheless, due in part to mechanical issues of unknown origin, there are many astrophysical questions for which the parallax zero-point error $σ(π_0)$ will be the fundamentally limiting constraint. These include the distance to the Large Magellanic Cloud and the Galactic Center. We show that by using the photometric parallax estimates for RR Lyrae stars (RRL) within 8kpc, via the ultra-precise infrared period-luminosity relation, one can independently determine a hyper-precise value for $π_{0}$. Despite their paucity relative to bright quasars, we show that RRL are competitive due to their order-of-magnitude improved parallax precision for each individual object relative to bright quasars. We show that this method is mathematically robust and well-approximated by analytic formulae over a wide range of relevant distances.

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arXiv2016

GW150914: Spin based constraints on the merger time of the progenitor system

We explore the implications of the observed low spin of GW150914 within the context of stellar astrophysics and progenitor models. We conclude that many of the recently proposed scenarios are in marked tension with this observation. We derive a simple model for the observed spin in the case that the progenitor system was a field binary composed of a black hole (BH) and a Wolf-Rayet star and explore the implications of the observed spin for this model. The spin observation allows us to place a lower limit for the delay time between the formation of the BH+BH binary and the actual merger, $t_{\textrm{merge}}$. We use typical values for these systems to derive $t_{\textrm{merge}}\gtrsim10^{8}\,\textrm{yr}$, which proves to be an important diagnostic for different progenitor models. We anticipate the next series of events, and the associated spin parameters, will ultimately yield critical constraints on formation scenarios and on stellar parameters describing the late-stage evolution of massive stars.

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arXiv2016

Ultrahigh-Energy Cosmic Rays from the "En Caul" Birth of Magnetars

Rapidly-spinning magnetars can potentially form by the accretion induced collapse of a white dwarf or by neutron star mergers if the equation of state of nuclear density matter is such that two low mass neutron stars can sometimes form a massive neutron star rather than a black hole. In either case, the newly born magnetar is an attractive site for producing ultrahigh-energy cosmic rays (particles with individual energies exceeding $10^{18}\,{\rm eV}$; UHECRs). The short-period spin and strong magnetic field are able to accelerate particles up to the appropriate energies, and the composition of material on and around the magnetar may naturally explain recent inferences of heavy elements in UHECRs. We explore whether the small amount of natal debris surrounding these magnetars allows the UHECRs to easily escape. We also investigate the impact on the UHECRs of the unique environment around the magnetar, which consists of a bubble of relativistic particles and magnetic field within the debris. Rates and energetics of UHECRs are consistent with such an origin even though the rates of events that produce rapidly-spinning magnetars remain very uncertain. The low ejecta mass also helps limit the high-energy neutrino background associated with this scenario to be below current IceCube constraints over most of the magnetar parameter space. A unique prediction is that UHECRs may be generated in old stellar environments without strong star formation in contrast to what would be expected for other UHECR scenarios, such as active galactic nuclei or long gamma-ray bursts.

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arXiv2015

Rest-frame UV single-epoch black hole mass estimates of low-luminosity AGN at intermediate redshifts

The ability to accurately derive black hole (BH) masses at progressively higher redshifts and over a wide range of continuum luminosities has become indispensable in the era of large-area extragalactic spectroscopic surveys. In this paper we present an extension of existing comparisons between rest-frame UV and optical virial BH mass estimators to intermediate redshifts and luminosities comparable to the local H$β$ reverberation mapped active galactic nuclei (AGN). We focus on the MgII, CIV, and CIII] broad emission lines and compare them to both H$α$ and H$β$. We use newly acquired near-infrared spectra from the FMOS instrument on the Subaru telescope for 89 broad-lined AGN at redshifts between 0.3 and 3.5, complemented by data from the AGES survey. We employ two different prescriptions for measuring the emission line widths and compare the results. We confirm that MgII shows a tight correlation with H$α$ and H$β$, with a scatter of ~0.25 dex. The CIV and CIII] estimators, while showing larger scatter, are viable virial mass estimators after accounting for a trend with the UV-to-optical luminosity ratio. We find an intrinsic scatter of ~0.37 dex between Balmer and carbon virial estimators by combining our dataset with previous high redshift measurements. This updated comparison spans a total of 3 decades in BH mass. We calculate a virial factor for CIV/CIII] logf(CIV/CIII])=0.87 with an estimated systematic uncertainty of ~0.4 dex and find excellent agreement between the local reverberation mapped AGN sample and our high-z sample.

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arXiv2015

The impact of environment and mergers on the HI content of galaxies in hydrodynamic simulations

We quantitatively examine the effects of accretion and environment on the HI content of galaxies within a cosmological hydrodynamic simulation that reproduces basic observed trends of HI in galaxies. We show that our model broadly reproduces the observed scatter in HI at a given stellar mass as quantified by the HI mass function in bins of stellar mass, as well as the HI richness versus local galaxy density. This shows that the predicted HI fluctuations and environmental effects are roughly consistent with data with few minor discrepancies. For satellite galaxies in >= 10^12M_* halos, the HI richness distribution is bimodal and drops towards the largest halo masses. The depletion rate of HI once a galaxy enters a more massive halo is more rapid at higher halo mass, in contrast to the specific star formation rate which shows much less variation in the attenuation rate versus halo mass. This suggests that, up to halo mass scales probed here (<= 10^14M_*), star formation is mainly attenuated by starvation, but HI is additionally removed by stripping once a hot gaseous halo is present. In low mass halos, the HI richness of satellites is independent of radius, while in high mass halos they become gas-poor towards the center. We show that the gas fraction of satellite and central galaxies decreases from z=5 to z=0,tracking each other until z~1 after which the satellites' HI content drops much more quickly, particularly for the highest halo masses. Mergers somewhat increase the HI richness and its scatter about the mean relation, tracking the metallicity in a way consistent with arising from inflow fluctuations, while star formation is significantly boosted relative to HI.

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arXiv2015

Baryon Cycling in the Low-Redshift Circumgalactic Medium: A Comparison of Simulations to the COS-Halos Survey

We analyze the low-redshift (z~0.2) circumgalactic medium by comparing absorption-line data from the COS-Halos Survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We extract for comparison direct observables including equivalent widths, covering factors, ion ratios, and kinematics. Both wind models are generally in good agreement with these observations for HI and certain low ionization metal lines, but show poorer agreement with higher ionization metal lines including SiIII and OVI that are well-observed by COS-Halos. These discrepancies suggest that both wind models predict too much cool, metal-enriched gas and not enough hot gas, and/or that the metals are not sufficiently well-mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by super-heating its ISM gas, which we show yields sufficient low ionisation absorption to be broadly consistent with observations, but also substantial OVI absorption that is inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of <50 kpc, recycling winds dominate the absorption of low ions and even HI, while OVI almost always arises from metals ejected longer than 1 Gyr ago. The similarity between the wind models is surprising, since we show that they differ substantially in their predicted amount and phase distribution of halo gas. We show that this similarity owes mainly to our comparison here at fixed stellar mass rather than at fixed halo mass in our previous works, which suggests that CGM properties are more closely tied to the stellar mass of galaxies rather than halo mass.

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arXiv2015

Torque-Limited Growth of Massive Black Holes in Galaxies Across Cosmic Time

We combine cosmological hydrodynamic simulations with analytic models to evaluate the role of galaxy-scale gravitational torques on the evolution of massive black holes at the centers of star-forming galaxies. We confirm and extend our earlier results to show that torque-limited growth yields black holes and host galaxies evolving on average along the Mbh-Mbulge relation from early times down to z = 0 and that convergence onto the scaling relation occurs independent of the initial conditions and with no need for mass averaging through mergers or additional self-regulation processes. Smooth accretion dominates the long-term evolution, with black hole mergers with mass ratios >1:5 representing typically a small fraction of the total growth. Winds from the accretion disk are required to eject significant mass to suppress black hole growth, but there is no need for coupling this wind to galactic-scale gas to regulate black holes in a non-linear feedback loop. Torque-limited growth yields a close-to-linear relation for the star formation rate and the black hole accretion rate averaged over galaxy evolution time scales. However, the SFR-AGN connection has significant scatter owing to strong variability of black hole accretion at all resolved time scales. Eddington ratios can be described by a broad lognormal distribution with median value evolving roughly as (1 + z)^1.9, suggesting a main sequence for black hole growth similar to the cosmic evolution of specific SFRs. Our results offer an attractive scenario consistent with available observations in which cosmological gas infall and transport of angular momentum in the galaxy by gravitational instabilities regulate the long-term co-evolution of black holes and star-forming galaxies.

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arXiv2014

The COS-Dwarfs Survey: The Carbon Reservoir Around sub-L* Galaxies

We report new observations of circumgalactic gas from the COS-Dwarfs survey, a systematic investigation of the gaseous halos around 43 low-mass z $\leq$ 0.1 galaxies using background QSOs observed with the Cosmic Origins Spectrograph. From the projected 1D and 2D distribution of C IV absorption, we find that C IV absorption is detected out to ~ 0.5 R$_{vir}$ of the host galaxies. The C IV absorption strength falls off radially as a power law and beyond 0.5 R$_{vir}$, no C IV absorption is detected above our sensitivity limit of ~ 50-100 m$Å$. We find a tentative correlation between detected C IV absorption strength and star formation, paralleling the strong correlation seen in highly ionized oxygen for L~L* galaxies by the COS-Halos survey. The data imply a large carbon reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass of $\gtrsim$ 1.2$\times 10^6$ $M_\odot$ out to ~ 110 kpc. This mass is comparable to the carbon mass in the ISM and more than the carbon mass currently in stars of these galaxies. The C IV absorption seen around these sub-L* galaxies can account for almost two-thirds of all $W_r$> 100 m$Å$ C IV absorption detected at low z. Comparing the C IV covering fraction with hydrodynamical simulations, we find that an energy-driven wind model is consistent with the observations whereas a wind model of constant velocity fails to reproduce the CGM or the galaxy properties.

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arXiv2014

The Circular Velocity Function of Group Galaxies

A robust prediction of $Λ{\rm CDM}$ cosmology is the halo circular velocity function (CVF), a dynamical cousin of the halo mass function. The correspondence between theoretical and observed CVFs is uncertain, however: cluster galaxies are reported to exhibit a power-law CVF consistent with $N$-body simulations, but that of the field is distinctly Schechter-like, flattened relative to $Λ{\rm CDM}$ expectations at circular velocities $v_c \lesssim 200\, {\rm km\, s^{-1}}$. Groups offer a powerful probe of the role environment plays in this discrepancy as they bridge the field and clusters. Here, we construct the CVF for a large, mass- and multiplicity-complete sample of group galaxies from the Sloan Digital Sky Survey. Using independent photometric $v_c$ estimators, we find no transition from a field- to $Λ{\rm CDM}$-shaped CVF above $v_c = 50\, {\rm km\, s^{-1}}$ as a function of group halo mass. All groups with $12.4 \lesssim \log M_{\rm halo} / M_{\odot} \lesssim 15.1$ (Local Group analogs to rich clusters) display similar Schechter-like CVFs marginally suppressed at low-$v_c$ compared to that of the field. Conversely, some agreement with $N$-body results emerges for samples saturated with late-type galaxies, with isolated late-types displaying a CVF similar in shape to $Λ{\rm CDM}$ predictions. We conclude that the flattening of the low-$v_c$ slope in groups is due to their depressed late-type fractions -- environment affecting the CVF only to the extent that it correlates with this quantity -- and that previous cluster analyses may suffer from interloper contamination. These results serve as useful benchmarks for cosmological simulations of galaxy formation.

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arXiv2014

The Photon Underproduction Crisis

We examine the statistics of the low-redshift Lyman-alpha forest from smoothed particle hydrodynamic simulations in light of recent improvements in the estimated evolution of the cosmic ultraviolet background (UVB) and recent observations from the Cosmic Origins Spectrograph (COS). We find that the value of the metagalactic photoionization rate required by our simulations to match the observed properties of the low-redshift Lyman-alpha forest is a factor of 5 larger than the value predicted by state-of-the art models for the evolution of this quantity. This mismatch results in the mean flux decrement of the Lyman-alpha forest being underpredicted by at least a factor of 2 (a 10-sigma discrepancy with observations) and a column density distribution of Lyman-alpha forest absorbers systematically and significantly elevated compared to observations over nearly two decades in column density. We examine potential resolutions to this mismatch and find that either conventional sources of ionizing photons (galaxies and quasars) must be significantly elevated relative to current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.

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arXiv2014

Calcium-rich Gap Transients: Solving the Calcium Conundrum in the Intracluster Medium

X-ray measurements suggest the abundance of Calcium in the intracluster medium is higher than can be explained using favored models for core-collapse and Type Ia supernovae alone. We investigate whether the Calcium conundrum in the intracluster medium can be alleviated by including a contribution from the recently discovered subclass of supernovae known as Calcium-rich gap transients. Although the Calcium-rich gap transients make up only a small fraction of all supernovae events, we find that their high Calcium yields are sufficient to reproduce the X-ray measurements found for nearby rich clusters. We find the $χ^{2}$ goodness-of-fit metric improves from 84 to 2 by including this new class. Moreover, Calcium-rich supernovae preferentially occur in the outskirts of galaxies making it easier for the nucleosynthesis products of these events to be incorporated in the intracluster medium via ram-pressure stripping. The discovery of a Calcium-rich gap transients in clusters and groups far from any individual galaxy suggests supernovae associated with intracluster stars may play an important role in enriching the intracluster medium. Calcium-rich gap transients may also help explain anomalous Calcium abundances in many other astrophysical systems including individual stars in the Milky Way, the halos of nearby galaxies and the circumgalactic medium. Our work highlights the importance of considering the diversity of supernovae types and corresponding yields when modeling the abundance of the intracluster medium and other gas reservoirs.

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arXiv2013

Hydrogen and Metal Line Absorption Around Low-Redshift Galaxies in Cosmological Hydrodynamic Simulations

We study the physical conditions of the circum-galactic medium (CGM) around z=0.25 galaxies as traced by HI and metal line absorption, using cosmological hydrodynamic simulations that include galactic outflows. Using lines of sight targeted at impact parameters from 10 kpc to 1 Mpc around galaxies with halo masses from 10^11-10^13 M_solar, we study the physical conditions and their variation with impact parameter b and line-of-sight velocity delta v in the CGM as traced by HI, MgII, SiIV, CIV, OVI, and NeVIII absorbers. All ions show a strong excess of absorption near galaxies compared to random lines of sight. The excess continues beyond 1 Mpc, reflecting the correlation of metal absorption with large-scale structure. Absorption is particularly enhanced within about v<300 km/sec and roughly 300 kpc of galaxies (with distances somewhat larger for the highest ion), approximately delineating the CGM; this range contains the majority of global metal absorption. Low ions like MgII and SiIV predominantly arise in denser gas closer to galaxies and drop more rapidly with b, while high ions OVI and NeVIII trace more diffusely distributed gas with a comparatively flat radial profile; CIV is intermediate. All ions predominantly trace T~10^4-4.5 K photo-ionised gas at all b, but when hot CGM gas is present (mostly in larger halos), we see strong collisionally-ionised OVI and NeVIII at b <= 100 kpc. Larger halo masses generally produce more absorption, though overall the trends are not as strong as that with impact parameter. These findings arise using our favoured outflow scalings as expected for momentum-driven winds; with no winds, the CGM gas remains mostly unenriched, while our outflow model with a constant velocity and mass loading factor produce hotter, more widely dispersed metals.

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arXiv2013

Tracing Inflows and Outflows with Absorption Lines in Circumgalactic Gas

We examine how HI and metal absorption lines within low-redshift galaxy halos trace the dynamical state of circumgalactic gas, using cosmological hydrodynamic simulations that include a well-vetted heuristic model for galactic outflows. We categorize inflowing, outflowing, and ambient gas based on its history and fate as tracked in our simulation. Following our earlier work showing that the ionisation level of absorbers was a primary factor in determining the physical conditions of absorbing gas, we show here that it is also a governing factor for its dynamical state. Low-ionisation metal absorbers (e.g. MgII) tend to arise in gas that will fall onto galaxies within several Gyr, while high-ionisation metal absorbers (e.g. OVI) generally trace material that was deposited by outflows many Gyr ago. Inflowing gas is dominated by enriched material that was previously ejected in an outflow, hence accretion at low redshifts is typically substantially enriched. Recycling wind material is preferentially found closer to galaxies, and is more dominant in lower-mass halos since high-mass halos have more hot gas that is able to support itself against infall. Low-mass halos also tend to re-eject more of their accreted material, owing to our outflow prescription that employs higher mass loading factors for lower-mass galaxies. Typical HI absorbers trace unenriched ambient material that is not participating in the baryon cycle, but stronger HI absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity measures of absorbers are generally poor at distinguishing between inflowing and outflowing gas, except in the case of very recent outflows. These results suggest that probing halo gas using a range of absorbers can provide detailed information about the amount and physical conditions of material that is participating in the baryon cycle.

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arXiv2013

A Preliminary Calibration of the RR Lyrae Period-Luminosity Relation at Mid-Infrared Wavelengths: WISE Data

Using time-resolved, mid-infrared data from WISE and geometric parallaxes from HST for four Galactic RR Lyrae variables, we derive the following Population II Period-Luminosity (PL) relations for the WISE [W1], [W2] and [W3] bands at 3.4, 4.6 & 12 um, respectively: M[W1] = -2.44 (+/- 0.95) x logP - 1.26 (+/- 0.25) sigma = 0.10 M[W2] = -2.55 (+/- 0.89) x logP - 1.29 (+/- 0.23) sigma = 0.10 M[W3] = -2.58 (+/- 0.97) x logP - 1.32 (+/- 0.25) sigma = 0.10 The slopes and the scatter around the fits are consistent with a smooth extrapolation of those same quantities from previously-published K-band observations at 2.2 um, where the asymptotic (long-wavelength) behavior is consistent with a Period-Radius relation having a slope of 0.5. No obvious correlation with metallicity (spanning 0.4 dex in [Fe/H]) is found in the residuals of the four calibrating RR Lyrae stars about the mean PL regression line.

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arXiv2013

The Spectrally Resolved Lyman-alpha Emission of Three Lyman-alpha Selected Field Galaxies at z~2.4 from the HETDEX Pilot Survey

We present new results on the spectrally resolved Lyman-alpha (LyA) emission of three LyA emitting field galaxies at z~2.4 with high LyA equivalent width (>100 Angstroms) and LyA luminosity (~10^43 erg/s). At 120 km/s (FWHM) spectral resolution, the prominent double-peaked LyA profile straddles the systemic velocity, where the velocity zero-point is determined from spectroscopy of the galaxies' rest-frame optical nebular emission lines. The average velocity offset from systemic of the stronger redshifted emission component for our sample is 176 km/s while the average total separation between the redshifted and main blueshifted emission components is 380 km/s. These measurements are a factor of ~2 smaller than for UV continuum-selected galaxies that show LyA in emission with lower LyA equivalent width. We compare our LyA spectra to the predicted line profiles of a spherical "expanding shell" LyA radiative transfer grid that models large-scale galaxy outflows. Specifically blueward of the systemic velocity where two galaxies show a weak, highly blueshifted (by ~1000 km/s) tertiary emission peak, the model line profiles are a relatively poor representation of the observed spectra. Since the neutral gas column density has a dominant influence over the shape of the LyA line profile, we caution against equating the observed LyA velocity offset with a physical outflow velocity, especially at lower spectral resolution where the unresolved LyA velocity offset is a convoluted function of several degenerate parameters. Referring to rest-frame ultraviolet and optical Hubble Space Telescope imaging, we find that galaxy-galaxy interactions may play an important role in inducing a starburst that results in copious LyA emission, as well as perturbing the gas distribution and velocity field which have strong influence over the LyA emission line profile.

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arXiv2013

The Neutral Hydrogen Content of Galaxies in Cosmological Hydrodynamic Simulations

We examine the global HI properties of galaxies in quarter-billion particle cosmological simulations using Gadget-2, focusing on how galactic outflows impact HI content. We consider four outflow models, including a new one (ezw) motivated by recent interstellar medium simulations in which the wind speed and mass loading factor scale as expected for momentum-driven outflows for larger galaxies and energy-driven outflows for dwarfs (sigma<75 km/s). To obtain predicted HI masses, we employ a simple but effective local correction for particle self-shielding, and an observationally-constrained transition from neutral to molecular hydrogen. Our ezw simulation produces an HI mass function whose faint-end slope of -1.3 agrees well with observations from the ALFALFA survey; other models agree less well. Satellite galaxies have a bimodal distribution in HI fraction versus halo mass, with smaller satellites and/or those in larger halos more often being HI-deficient. At a given stellar mass, HI content correlates with star formation rate and inversely correlates with metallicity, as expected if driven by stochasticity in the accretion rate. To higher redshifts, massive HI galaxies disappear and the mass function steepens. The global cosmic HI density conspires to remain fairly constant from z~5-0, but the relative contribution from smaller galaxies increases with redshift.

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