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Register a new userDisentangling AGN and Star Formation Activity at High Redshift Using Hubble Space Telescope Grism Spectroscopy
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Differentiating between active galactic nuclei (AGN) activity and star formation in z ~ 2 galaxies is difficult because traditional methods, such as line ratio diagnostics, change with redshift while multi-wavelength methods (X-ray, radio, IR) are sensitive to only the brightest AGN. We have developed a new method for spatially resolving emission lines in HST/WFC3 G141 grism spectra and quantifying AGN activity through the spatial gradient of the [O III]/H$beta$ line ratio. Through detailed simulations, we show that our novel line-ratio gradient approach identifies ~ sim 40% more low-mass and obscured AGN than obtained by classical methods. Based on our simulations, we developed a relationship that maps stellar mass, star formation rate, and measured [O III]/H$beta$ gradient to AGN Eddington ratio. We apply our technique to previously studied stacked samples of galaxies at z ~2 and find that our results are consistent with these studies. Using this gradient method will also be able to inform other galaxy evolution science, such as inside-out quenching and metallicity gradients, and will be widely applicable to future spatially resolved JWST data.
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We present Hubble WFC3/IR slitless grism spectra of a remarkably bright $zgtrsim10$ galaxy candidate, GN-z11, identified initially from CANDELS/GOODS-N imaging data. A significant spectroscopic continuum break is detected at $lambda=1.47pm0.01~mu$m. The new grism data, combined with the photometric data, rule out all plausible lower redshift solutions for this source. The only viable solution is that this continuum break is the Ly$alpha$ break redshifted to ${z_mathrm{grism}=11.09^{+0.08}_{-0.12}}$, just $sim$400 Myr after the Big Bang. This observation extends the current spectroscopic frontier by 150 Myr to well before the Planck (instantaneous) cosmic reionization peak at z~8.8, demonstrating that galaxy build-up was well underway early in the reionization epoch at z>10. GN-z11 is remarkably and unexpectedly luminous for a galaxy at such an early time: its UV luminosity is 3x larger than L* measured at z~6-8. The Spitzer IRAC detections up to 4.5 $mu$m of this galaxy are consistent with a stellar mass of ${sim10^{9}~M_odot}$. This spectroscopic redshift measurement suggests that the James Webb Space Telescope (JWST) will be able to similarly and easily confirm such sources at z>10 and characterize their physical properties through detailed spectroscopy. Furthermore, WFIRST, with its wide-field near-IR imaging, would find large numbers of similar galaxies and contribute greatly to JWSTs spectroscopy, if it is launched early enough to overlap with JWST.
We present here a three-dimesional hydrodynamical simulation for star formation. Our aim is to explore the effect of the metal-line cooling on the thermodynamics of the star-formation process. We explore the effect of changing the metallicty of the gas from $Z/Z_{odot}=10^{-4}$ to $Z/Z_{odot}=10^{-2}$. Furthermore, we explore the implications of using the observational abundance pattern of a CEMP-no star, which have been considered to be the missing second-generation stars, the so-called Pop. III.2 stars. In order to pursue our aim, we modelled the microphysics by employing the public astrochemistry package KROME, using a chemical network which includes sixteen chemical species (H, H$^{+}$, H$^{-}$, He, He$^{+}$, He$^{++}$, e$^{-}$, H$_{2}$, H$_{2}^{+}$, C, C$^{+}$, O, O$^{+}$, Si, Si$^{+}$, and Si$^{++}$). We couple KROME with the fully three-dimensional Smoothed-particle hydrodynamics (SPH) code GRADSPH. With this framework we investigate the collapse of a metal-enhanced cloud, exploring the fragmentation process and the formation of stars. We found that the metallicity has a clear impact on the thermodynamics of the collapse, allowing the cloud to reach the CMB temperature floor for a metallicity $Z/Z_{odot}=10^{-2}$, which is in agreement with previous work. Moreover, we found that adopting the abundance pattern given by the star SMSS J031300.36-670839.3 the thermodynamics behavior is very similar to simulations with a metallicity of $Z/Z_{odot}=10^{-2}$, due to the high carbon abundance. As long as only metal line cooling is considered, our results support the metallicity threshold proposed by previous works, which will very likely regulate the first episode of fragmentation and potentially determine the masses of the resulting star clusters.
Solar system objects with perihelia beyond the orbit of Jupiter ($q >$ 5 AU) are too cold for water ice to generate an appreciable coma via sublimation. Despite this, numerous high perihelion objects (HPOs) including many comets and recently escaped Kuiper belt objects (``Centaurs) are observed to be active out at least to the orbit of Saturn ($q sim$ 10 AU). Peak equilibrium temperatures at 10 AU ($sim$125 K), while far too low to sublimate water ice, are sufficient to sublimate super-volatiles such as CO and CO$_2$ ice. Temperatures at 10 AU are also high enough to trigger the rapid crystallization of exposed amorphous ice, thus constituting another possible driver of distant activity. While supervolatile ices can sublimate strongly (as $r_H^{-2}$) to at least Kuiper belt (30 AU) distances, crystallization is an exponential function of temperature that cannot be sustained much beyond $sim$10 AU. The heliocentric dependence of the activity thus suggests an observational test. If activity in high perihelion objects is triggered by crystallization, then no examples of activity should be found with perihelia $q >>$ 10 AU. If, on the other hand, activity is due to free sublimation of exposed supervolatile ices, or another cause, then distant activity might be detected. We obtained sensitive, high resolution Hubble Space Telescope observations of HPOs to search for activity beyond the crystallization zone. No examples of activity were detected in 53 objects with $q >$ 15 AU, consistent with the crystallization trigger hypothesis. However, sensitivity limits are such that we cannot reject the alternative hypothesis that mass loss is driven by the sublimation of supervolatile ices. We also searched for binary companions in our sample, finding none and setting an empirical 3$sigma$ limit to the binary fraction of $<8$%.
We investigate the relation between AGN and star formation (SF) activity at $0.5 < z < 3$ by analyzing 898 galaxies with X-ray luminous AGN ($L_X > 10^{44}$ erg s$^{-1}$) and a large comparison sample of $sim 320,000$ galaxies without X-ray luminous AGN. Our samples are selected from a large (11.8 deg$^2$) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume ($sim 0.3$ Gpc$^3$) at $0.5 < z < 3$ minimizes the effects of cosmic variance and captures a large number of massive galaxies ($sim 30,000$ galaxies with $M_* > 10^{11} M_{odot}$) and X-ray luminous AGN. While many galaxy studies discard AGN hosts, we fit the SED of galaxies with and without X-ray luminous AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. We find that without this inclusion, stellar masses and star formation rates (SFRs) in AGN host galaxies can be overestimated, on average, by factors of up to $sim 5$ and $sim 10$, respectively. The average SFR of galaxies with X-ray luminous AGN is higher by a factor of $sim 3$ to $10$ compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high SFRs and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority ($> 95 %$) of galaxies with X-ray luminous AGN at $z=0.5-3$ do not show quenched SF: this suggests that if AGN feedback quenches SF, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity.
We present low-resolution ultraviolet spectra of 14 low redshift (z<0.8) quasars observed with HST/STIS as part of a Snap project to understand the relationship between quasar outflows and luminosity. By design, all observations cover the CIV emission line. Nine of the quasars are from the Hamburg-ESO catalog, three are from the Palomar-Green catalog, and one is from the Parkes catalog. The sample contains a few interesting quasars including two broad absorption line (BAL) quasars (HE0143-3535, HE0436-2614), one quasar with a mini-BAL (HE1105-0746), and one quasar with associated narrow absorption (HE0409-5004). These BAL quasars are among the brightest known (though not the most luminous) since they lie at z<0.8. We compare the properties of these BAL quasars to the z<0.5 Palomar-Green and z>1.4 Large Bright Quasar samples. By design, our objects sample luminosities in between these two surveys, and our four absorbed objects are consistent with the v ~ L^0.62 relation derived by Laor & Brandt (2002). Another quasar, HE0441-2826, contains extremely weak emission lines and our spectrum is consistent with a simple power-law continuum. The quasar is radio-loud, but has a steep spectral index and a lobe-dominated morphology, which argues against it being a blazar. The unusual spectrum of this quasar resembles the spectra of the quasars PG1407+265, SDSSJ1136+0242, and PKS1004+13 for which several possible explanations have been entertained.
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