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Astro Seminar

The Cosmic Ultraviolet Baryon Survey (CUBS)

The Cosmic Ultraviolet Baryon Survey (CUBS) is a large HST Cycle 25 GO

program designed to map 'dark' baryonic structures in the crucial but

unconstrained epoch between redshift z = 0.4 and z ~ 2.  The CUBS

program aims to combine absorption-line spectroscopy of 15 UV-bright

QSOs at z > 0.8 with matching deep galaxy survey data.  This legacy

sample enables systematic studies of the co-evolution of galaxies and

diffuse circumgalactic and intergalactic gas at a time when the star

formation rate density undergoes its most dramatic changes, and

provide key insights into how galaxy growth is regulated by accretion

and outflows.  In this talk, I will show some of the first results

from the CUBS program.

Date:
-
Location:
BL 339

The Cosmic Ultraviolet Baryon Survey (CUBS)

The Cosmic Ultraviolet Baryon Survey (CUBS) is a large HST Cycle 25 GO

program designed to map 'dark' baryonic structures in the crucial but

unconstrained epoch between redshift z = 0.4 and z ~ 2.  The CUBS

program aims to combine absorption-line spectroscopy of 15 UV-bright

QSOs at z > 0.8 with matching deep galaxy survey data.  This legacy

sample enables systematic studies of the co-evolution of galaxies and

diffuse circumgalactic and intergalactic gas at a time when the star

formation rate density undergoes its most dramatic changes, and

provide key insights into how galaxy growth is regulated by accretion

and outflows.  In this talk, I will show some of the first results

from the CUBS program.

Date:
-
Location:
BL 339

AGN STORM discovers a mirror in an AGN

The AGN STORM (space telescope optical reverberation mapping) was an intense multi-wavelength campaign that observed NGC 5548, a type I Seyfert galaxy (z = 0.01717), for 180 months during 2014. The main goal was to determine the geometry and mass of the object through the reverberation mapping method, using 6 space-based and 21 ground-based telescopes.

NGC 5548 exhibited unusual behavior during the campaign: First, a strong soft X-ray absorption was present due to the appearance of a line of sight (LOS) obscurer between the central source and the observer. Then, it was discovered that broad emission lines and the UV continuum (emission-line holiday) decorrelated for 2 months during the observations. Finally, it was discovered that the same decorrelation happened between the narrow absorption lines and the continuum (absorption-line holiday).

We joined the campaign in 2017 to understand the behavior of NGC 5548 during the time of obscuration and holiday. We showed that LOS obscurer is the upper part of a symmetric continues disk-wind launched from the accretion disk. The base of this wind is called the equatorial obscurer and it persistently shields the BLR.  Based on Cloudy’s prediction, the variations of the covering factor of the LOS obscurer explains the absorption-line holiday, while changes in the column density/density of the equatorial obscurer explain the emission-line holiday.

Recently, we found out that the base of the wind has significant optical depth to electron scattering and must be a contributor to the Compton reflector. This reflector acts as a mirror and would reflect BLR emission. The wind’s optical depth explains why the far sides of the BLR are unexpectedly faint. During this talk, I will explain how our modeling resulted in this discovery, and also how the disk-wind model scenario fits into observations. 

Date:
-
Location:
BL 339

AGN STORM discovers a mirror in an AGN

The AGN STORM (space telescope optical reverberation mapping) was an intense multi-wavelength campaign that observed NGC 5548, a type I Seyfert galaxy (z = 0.01717), for 180 months during 2014. The main goal was to determine the geometry and mass of the object through the reverberation mapping method, using 6 space-based and 21 ground-based telescopes.

NGC 5548 exhibited unusual behavior during the campaign: First, a strong soft X-ray absorption was present due to the appearance of a line of sight (LOS) obscurer between the central source and the observer. Then, it was discovered that broad emission lines and the UV continuum (emission-line holiday) decorrelated for 2 months during the observations. Finally, it was discovered that the same decorrelation happened between the narrow absorption lines and the continuum (absorption-line holiday).

We joined the campaign in 2017 to understand the behavior of NGC 5548 during the time of obscuration and holiday. We showed that LOS obscurer is the upper part of a symmetric continues disk-wind launched from the accretion disk. The base of this wind is called the equatorial obscurer and it persistently shields the BLR.  Based on Cloudy’s prediction, the variations of the covering factor of the LOS obscurer explains the absorption-line holiday, while changes in the column density/density of the equatorial obscurer explain the emission-line holiday.

Recently, we found out that the base of the wind has significant optical depth to electron scattering and must be a contributor to the Compton reflector. This reflector acts as a mirror and would reflect BLR emission. The wind’s optical depth explains why the far sides of the BLR are unexpectedly faint. During this talk, I will explain how our modeling resulted in this discovery, and also how the disk-wind model scenario fits into observations. 

Date:
-
Location:
BL 339

Rapid High Amplitude Variability in Active Galactic Nuclei

I will report on the most recent discoveries by the NASA Neil Gehrels Swift mission of Active Galactic Nuclei (AGN) with high amplitude variabilities. While AGN typically vary with factors of 3 on times scales of days to years, some AGN exhibit outbursts or dramatic drops in the X-ray fluxes by factors of even more than 100. Among the most extreme cases are the Narrow-Line Seyfert 1 galaxy WPVS 007 which appears to be extremely X-ray faint, and the Seyfert 1.9 galaxy IC 3599 which has shown repeated X-ray flaring, most likely due to accretion disk instabilities. Swift capability of following objects for more than a decade with simultaneous X-ray and UV observations has allowed us to discover several of these extreme AGN.  Recent examples are IRAS 23226-3843, RX J2317-4422 and repeatedly Mkn 335 on which we triggered XMM/NuSTAR and HST observations several times in 2018 and 2019. In particular IRAS 23226-3843 is a so called changing look AGN that has changed its optical spectroscopic type several times. In most recent Swift observations IRAS 23226-3843 was found to be flaring again.

Date:
-
Location:
BL 339

Rapid High Amplitude Variability in Active Galactic Nuclei

I will report on the most recent discoveries by the NASA Neil Gehrels Swift mission of Active Galactic Nuclei (AGN) with high amplitude variabilities. While AGN typically vary with factors of 3 on times scales of days to years, some AGN exhibit outbursts or dramatic drops in the X-ray fluxes by factors of even more than 100. Among the most extreme cases are the Narrow-Line Seyfert 1 galaxy WPVS 007 which appears to be extremely X-ray faint, and the Seyfert 1.9 galaxy IC 3599 which has shown repeated X-ray flaring, most likely due to accretion disk instabilities. Swift capability of following objects for more than a decade with simultaneous X-ray and UV observations has allowed us to discover several of these extreme AGN.  Recent examples are IRAS 23226-3843, RX J2317-4422 and repeatedly Mkn 335 on which we triggered XMM/NuSTAR and HST observations several times in 2018 and 2019. In particular IRAS 23226-3843 is a so called changing look AGN that has changed its optical spectroscopic type several times. In most recent Swift observations IRAS 23226-3843 was found to be flaring again.

Date:
-
Location:
BL 339

The mystery of AGN NGC 5548

AGNs are the brightest persistent source of the electromagnetic radiation in the universe, enabling us to discover and study them across the cosmos. These are in the central regions of galaxies, and observing and tracing them informs us of the role of supermassive black holes (SBH) in the formation and evolution of galaxies. Fundamental properties such as the mass of the SBH and correlations with luminosity are based upon line-continuum reverberation mapping. Time delays between changes in the continuum luminosity and the response of the emission lines measure the physical size. The method is the basis for understanding phenomena near the SBH, which are far too small to be resolved even with the best telescopes.

During this talk, I will discuss how this correlation was broken in one of the well-studied AGNs, namely AGN NGC 5548.  In this object, the soft X-ray part of the SED was dramatically extinguished by an obscurer. During part of the time that this obscurer was present, the absorption and emission lines did not respond to variations of the continuum. We modeled the decorrelation of the absorption lines from the continuum in terms of a varying obscurer covering factor, and identify the physics which makes this possible. We identify a cycle in which the soft X-ray portion of the SED varies, causing changes in the ionization of helium.  The ionizing radiation produced in its recombination governs the ionization of the species observed with HST.  Photoionization models reproduce the sense of HST observations.  The obscurer is likely to be part of the broad-line region which happens to cover our sight line to the central object.  This shows the importance of cloud shadowing in understanding the physics of the emission-line clouds. 

Date:
Location:
Blazer 339
Event Series:

Astro Seminar: A Post-Mortem on Post-Starburst Galaxies

While much effort has been expended to detect the earliest galaxies and to follow their evolution to z~0, astronomers remain baffled by the present-day dichotomy between disky, star forming (aka late-type) galaxies and quiescent, spheroidal (aka early-type) galaxies. Finding galaxies in transition from one class to the other, whose spectra indicate intense recent star formation that has now ended, is key.  We have identified thousands of such "post-starburst galaxies" and discovered that they are often the products of late-type galaxy-galaxy mergers. Their current kinematics, stellar populations, and morphologies are consistent with late- to early-type galaxy evolution. I will discuss recent work that further demonstrates the importance of these galaxies in the study of galaxy evolution. In particular, we have shown that their molecular gas fractions decline rapidly in time after the starburst ends and in a manner consistent with feedback processes. Furthermore, we have determined that tidal disruption events, in which a star is disrupted by the supermassive black hole in a galaxy's center, favor post-starburst galaxies by factors of tens to hundreds. Like the well-known black hole-bulge mass correlation, this surprising connection between the properties of a galaxy on kpc scales and its supermassive black hole on pc scales requires explanation. 
 
Date:
-
Location:
CP 179
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