Publication Search Results

ID: 158858
Type: article
Authors: Lower, Sidney; Narayanan, Desika; Leja, Joel; Johnson, Benjamin D.; Conroy, Charlie; Davé, Romeel
Abstract: The primary method for inferring the stellar mass (M_*) of a galaxy is through spectral energy distribution (SED) modeling. However, the technique rests on assumptions such as the galaxy star formation history (SFH) and dust attenuation law that can severely impact the accuracy of derived physical properties from SED modeling. Here we examine the effect that the assumed SFH has on the stellar properties inferred from SED fitting by ground-truthing them against mock observations of high-resolution cosmological hydrodynamic galaxy formation simulations. Classically, SFHs are modeled with simplified parameterized functional forms, but these forms are unlikely to capture the true diversity of galaxy SFHs and may impose systematic biases with underreported uncertainties on results. We demonstrate that flexible nonparametric SFHs outperform traditional parametric forms in capturing variations in galaxy SFHs and, as a result, lead to significantly improved stellar masses in SED fitting. We find a decrease in the average bias of 0.4 dex with a delayed-t model to a bias under 0.1 dex for the nonparametric model, though this is heavily dependent on the choice of prior for the nonparametric model. Similarly, using nonparametric SFHs in SED fitting results in increased accuracy in recovered galaxy star formation rates and stellar ages.

ID: 157814
Type: article
Authors: Paterson, K.; Fong, W.; Nugent, A.; Escorial, A. Rouco; Leja, Joel; Laskar, T.; Chornock, R.; Miller, A. A.; Scharwächter, J.; Cenko, S. B.; Perley, D.; Tanvir, N. R.; Levan, A.; Cucchiara, A.; Cobb, B. E.; de, K.; Berger, Edo; Terreran, G.; Alexander, K. D.; Nicholl, M.; Blanchard, P. K.; Cornish, D.
Abstract: We present the discovery of the optical afterglow and host galaxy of the Swift short-duration gamma-ray burst (SGRB) GRB 181123B. Observations with Gemini-North starting ≍9.1 hr after the burst reveal a faint optical afterglow with i ≍ 25.1 mag at an angular offset of 0"59 ± 0"16 from its host galaxy. Using grizYJHK observations, we measure a photometric redshift of the host galaxy of $z={1.77}_{-0.17}^{+0.30}$ . From a combination of Gemini and Keck spectroscopy of the host galaxy spanning 4500-18000 Å, we detect a single emission line at 13390 Å, inferred as Hβ at z = 1.754 ± 0.001 and corroborating the photometric redshift. The host galaxy properties of GRB 181123B are typical of those of other SGRB hosts, with an inferred stellar mass of ≍9.1 × 10⁹ M_☉, a mass-weighted age of ≍0.9 Gyr, and an optical luminosity of ≍0.9L*. At z = 1.754, GRB 181123B is the most distant secure SGRB with an optical afterglow detection and one of only three at z > 1.5. Motivated by a growing number of high-z SGRBs, we explore the effects of a missing z > 1.5 SGRB population among the current Swift sample on delay time distribution (DTD) models. We find that lognormal models with mean delay times of ≍4-6 Gyr are consistent with the observed distribution but can be ruled out to 95% confidence, with an additional ≍one to five Swift SGRBs recovered at z > 1.5. In contrast, power-law models with ∝t^-1 are consistent with the redshift distribution and can accommodate up to ≍30 SGRBs at these redshifts. Under this model, we predict that ≍1/3 of the current Swift population of SGRBs is at z > 1. The future discovery or recovery of existing high-z SGRBs will provide significant discriminating power on their DTDs and thus their formation channels.

ID: 154164
Type: article
Authors: Leja, Joel; Tacchella, Sandro; Conroy, Charlie
Abstract: The UVJ color-color diagram is a popular and efficient method to distinguish between quiescent and star-forming galaxies through their rest-frame U-V versus V-J colors. Here we explore the information content of this color-color space using the Bayesian inference machine Prospector. We fit the same physical model to two data sets: (i) UVJ fluxes alone, and (ii) full UV-mid IR (MIR) broadband spectral energy distributions from the 3D-HST survey. Notably this model uses both nonparametric star formation histories and a flexible dust attenuation curve, both of which have the potential to "break" the typical correlations observed in UVJ color-color space. Instead, these fits confirm observed trends between UVJ colors and observed galaxy properties, including specific star formation rate (sSFR), dust attenuation, stellar age, and stellar metallicity. They also demonstrate that UVJ colors do not, on their own, constrain stellar age or metallicity; the observed trends in the UVJ diagram are instead driven by galaxy scaling relationships and thus will evolve with cosmological time. We also show that UVJ colors "saturate" below {log}({sSFR}/{yr}}^-1)≲ -10.5, i.e., changing sSFR no longer produces substantial changes in UVJ colors. We show that far-UV and/or MIR fluxes continue to correlate with sSFR down to low sSFRs and can be used in color-color diagrams to efficiently target galaxies with much lower levels of ongoing star formation. We provide selection criteria in these new color-color spaces as a function of desired sample sSFR.

ID: 154621
Type: article
Authors: Mason, Charlotte A.; Naidu, Rohan P.; Tacchella, Sandro; Leja, Joel
Abstract: Modelling reionization often requires significant assumptions about the properties of ionizing sources. Here, we infer the total output of hydrogen-ionizing photons (the ionizing emissivity, \dot{N}_\textrm {ion}) at z = 4-14 from current reionization constraints, being maximally agnostic to the properties of ionizing sources. We use a Bayesian analysis to fit for a non-parametric form of \dot{N}_\textrm {ion}, allowing us to flexibly explore the entire prior volume. We infer a declining \dot{N}_\textrm {ion} with redshift at z > 6, which can be used as a benchmark for reionization models. Model-independent reionization constraints from the cosmic microwave background (CMB) optical depth and Ly α and Ly β forest dark pixel fraction produce \dot{N}_\textrm {ion} evolution ( d\log _{10}\dot{\mathbf {N}}_{ion}/ dz|_{z=6\rArr 8} = -0.31± 0.35 dex) consistent with the declining UV luminosity density of galaxies, assuming constant ionizing photon escape fraction and efficiency. Including measurements from Ly α damping of galaxies and quasars produces a more rapid decline: d\log _{10}\dot{\mathbf {N}}_{ion}/ dz|_{z=6\rArr 8} =-0.44± 0.22 dex, steeper than the declining galaxy luminosity density (if extrapolated beyond M_UV≳ -13), and constrains the mid-point of reionization to z = 6.93 ± 0.14.

ID: 155457
Type: article
Authors: Nelson, Erica J.; Tadaki, Ken-ichi; Tacconi, Linda J.; Lutz, Dieter; Förster Schreiber, Natascha M.; Cibinel, Anna; Wuyts, Stijn; Lang, Philipp; Leja, Joel; Montes, Mireia; Oesch, Pascal A.; Belli, Sirio; Davies, Rebecca L.; Davies, Richard I.; Genzel, Reinhard; Lippa, Magdalena; Price, Sedona H.; Übler, Hannah; Wisnioski, Emily
Abstract: A randomly chosen star in today's universe is most likely to live in a galaxy with stellar mass between the Milky Way and Andromeda. It remains uncertain, however, how the structural evolution of these bulge-disk systems proceeded. Most of the unobscured star formation we observe by building Andromeda progenitor s at 0.7 _e (dust) = 3.4 kpc, r _e (Hα)/r _e (dust) = 1.4, and r _e (UV)/r _e (dust) = 1.8. Crucially, however, the bulge and disk of this galaxy are building simultaneously; although the dust emission is more compact than the rest-optical emission (r _e (optical)/r _e (dust) = 1.4), it is somewhat less compact than the stellar mass (r _e (M _*)/r _e (dust) = 0.9). Taking the rest-500 μm emission as a tracer, the expected structural evolution can be accounted for by star formation: it will grow in size by ∆r _e /∆M _* ̃ 0.3 and in central surface density by ∆Σ_cen/∆M _* ̃ 0.9. Finally, our observations are consistent with a picture in which merging and disk instabilities drive gas to the center of galaxies, boosting global star formation rates above the main sequence and building bulges.

ID: 154411
Type: article
Authors: Nicholl, M.; Blanchard, Peter K.; Berger, Edo; Gomez, S.; Margutti, R.; Alexander, K. D.; Guillochon, James; Leja, Joel; Chornock, R.; Snios, Bradford; Auchettl, K.; Bruce, A. G.; Challis, Peter; D'Orazio, Daniel J.; Drout, M. R.; Eftekhari, T.; Foley, R. J.; Graur, Or; Kilpatrick, C. D.; Lawrence, A.; Piro, A. L.; Rojas-Bravo, C.; Ross, N. P.; Short, P.; Smartt, S. J.; Smith, K. W.; Stalder, B.
Abstract: We present and analyse a new tidal disruption event (TDE), AT2017eqx at redshift z = 0.1089, discovered by Pan-STARRS and ATLAS. The position of the transient is consistent with the nucleus of its host galaxy; the spectrum shows a persistent blackbody temperature T ≳ 20 000 K with broad H I and He II emission; and it peaks at a blackbody luminosity of L ≈ 10⁴⁴ erg s^-1. The lines are initially centred at zero velocity, but by 100 d, the H I lines disappear while the He II develops a blueshift of ≳ 5000 km s^-1. Both the early- and late-time morphologies have been seen in other TDEs, but the complete transition between them is unprecedented. The evolution can be explained by combining an extended atmosphere, undergoing slow contraction, with a wind in the polar direction becoming visible at late times. Our observations confirm that a lack of hydrogen a TDE spectrum does not indicate a stripped star, while the proposed model implies that much of the diversity in TDEs may be due to the observer viewing angle. Modelling the light curve suggests AT2017eqx resulted from the complete disruption of a solar-mass star by a black hole of ̃10^6.3 M_☉. The host is another Balmer-strong absorption galaxy, though fainter and less centrally concentrated than most TDE hosts. Radio limits rule out a relativistic jet, while X-ray limits at 500 d are among the deepest for a TDE at this phase.