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|Research interests / Nadia L. Zakamska
I am interested in many topics in observational and theoretical astrophysics. Below is a brief description of some of the topics of my past and current research, from quasars to exo-planets to black hole jets to cosmology.
A wide range of projects is available for graduate and undergraduate students at JHU. Some examples are listed below, and there are many other options. Do not hesitate to contact me (email is best; make sure to include your CV).
Quasar feedback is one of the major puzzles in galaxy formation theory. The word "feedback" implies that the quasar must have a strong effect on its large-scale environment: its entire host galaxy or even the inter-galactic matter. Quasar feedback can provide a natural explanation for the upper limit on the mass of galaxies in the local universe; for the black-hole / bulge correlations; and for the similarity between the black hole accretion history and the star formation history of the Universe. The only problem is that the direct observations of this phenomen have been extremely hard to come by!
Our group obtained ground-breaking observations of powerful quasar winds, and we are now using Magellan, Gemini, Herschel, EVLA, Hubble, Chandra and ALMA data to further characterize their phenomenology and physical conditions. We are studying this phenomenon both at low redshifts, where high-quality observations can yield a detailed physical description of this process, and at high redshifts, when quasar winds made the greatest impact on the formation of massive galaxies. Several observational projects on quasar feedback are available for students and postdocs. Here are some of our recent papers on this topic:
Sun et al. 2017 led by my Princeton collaborators on the sizes of galactic winds driven by active black holes
Wylezalek et al. on the relationship between circumnuclear and galactic outflows in active galaxies
Wylezalek and Zakamska 2016 on a possible detection of suppression of specific star formation as a function of quasar wind kinematics -- but only in gas-rich galaxies, where outflow efficiently couples to the gas
Zakamska et al. 2016b on the most extreme [OIII]5007 outflows known (which defy common narrow line / broad line definitions!)
Stern et al. 2016
Much more about feedback can be found on this page for the workshop on feedback which took place at JHU in Fall 2013.
[Picture: example kinematic maps of quasar winds (intensity, radial velocity, FWHM); credit: Liu, Zakamska et al. MNRAS, 2013a, 2013b, 2014 based on Gemini GMOS data]
[Picture: Alexandroff and Zakamska, JVLA images of four radio-quiet quasars and their [OIII]4959,5007A emission line kinematics. From Alexandroff et al. 2016.]
The puzzle of radio emission from radio quiet quasars. The most extended, powerful and beautiful sources in the radio sky are due to relativistic jets launched by supermassive black holes in centres of galaxies, but only a minority of active black holes produce these structures. The majority of black holes in the universe are "radio-quiet" and do not have extended bright jets, but they are not necessarily radio silent. Many of them appear as weak point sources in deep radio surveys, such as FIRST, and likely dominate number counts of radio sources above 0.1 mJy.
Among actively accreting black holes -- quasars -- only about 10% show extended jets and are "radio-loud". The nature of the radio emission in the remaining 90% of objects is not known. In 2014, we discovered a relationship between the radio emission of quasars and their extended gas kinematics and hypothesized that radio emission can be a bi-product of winds launched by the quasars and now propagating through the galactic interstellar medium, driving shocks and accelerating relativistic particles, which would produce radio emission. Here are some of our recent papers on this topic:
Obscured (type 2) quasars are luminous accreting black holes in the centers of galaxies whose central regions are shielded from us by large amounts of gas and dust. Until our work, only a handful of such quasars had been known, and their very existence in large numbers was frequently questioned. Over the last few years, my collaborators and I have identified hundreds of obscured quasars in the Sloan Digital Sky Survey and extensively studied their structure, demographics and effects on their host galaxies using space-based and ground-based telescopes. One of the highlights of this work is the detection of the light from the buried nucleus scattered into our line of sight by the material in the host galaxy and visible in our Hubble Space Telescope images (above). These observations provided the first direct measurement of the obscuration covering factor at high quasar luminosities.
Our most recent sample of obscured quasars (the largest available by far) includes nearly 1000 objects, and it is now clear that they are at least as common as the `normal' unobscured quasars at low (z< 0.7) redsfhits. This work was subject of a press release, and Reinabelle Reyes, a Princeton student who worked with me on this project, received the Chambliss Award Honorable Mention for our AAS poster.
Despite extensive observational effort across many wavelengths of the electromagnetic spectrum, the obscured quasar population remains poorly characterized at high redshifts -- at the peak of quasar activity -- and only a few dozen objects are known, although it is suspected that such objects represent a major phase of black hole growth. I am involved in a series of observational projects to find these objects in large numbers, to characterize their host galaxies and determine their impact on galaxy evolution:
Yuan et al. 2016, the largest optical catalog of type 2 quasars at z < 1 with all kinematic measurements publicly available
Wylezalek et al. 2016 on the connection between type 2 quasar activity and mergers of ellipticals with gas-rich galaxies
Obied et al. 2016 on giant scattering cones
[Picture: three obsured quasars; credit HST/SDSS/Nadia Zakamska]
Ultraluminous infrared galaxies (ULIRGs) are among the most powerful objects in the local Universe, comparable in luminosity to the brightest known quasars, but powered mostly by star formation. They are rare now but were much more abundant in the past. These objects form stars at a rate which is hundreds of times higher than that of the Milky Way, and thus they represent a rather extreme environment, called "starburst".
My paper "H_2 emission arises outside photodissociation regions in ultraluminous infrared galaxies" came out in Nature and demonstrated that ULIRGs have more warm molecular gas emission than would be expected based on their star formation rate. With undergraduate student Matthew Hill, we demonstrated that the excess emission likely originates in neutral interstellar gas shock-heated by outflows, which are driven by the supernovae explosions or an active nucleus.
[Picture: ULIRG NGC 6240 optical+IR which shows strong anomalous molecular hydrogen emission; credit NASA/JPL/Caltech/STScI/ESA]
[Picture: outflows in ULIRGs are driven by supernovae associated with recent star-formation activity or an active nucleus, distinguishable by the outflow velocity. From Hill and Zakamska, MNRAS 2014.]
Relativistic jets with toroidal fields:
Accretion of matter onto compact objects (neutron stars or black holes) is frequently accompanied by collimated relativistic outflows. My collaborators and I found a new class of self-similar analytical solutions for the structure of pressurized relativistic jets with toroidal magnetic fields. As the jet material expands, it runs into the ambient medium, resulting in a pile-up of material along the jet boundary. The magnetic field acts as a collimating force and produces a magnetic pinch along the axis of the jet. We constructed models which take into account these forces, and then used these models to predict the intensity and polarization of the observed synchrotron emission based on the physical properties of the jet, such as its Lorenz-factor, energy flux and magnetization. This work demonstrated that projection effects and the emissivity pattern of the jet have a strong effect on the observed polarization signal and provided an explanation for puzzling polarization patterns seen in some BL Lac objects.
I continue working on stability of jets with toroidal fields. I am also interested in phenomena that occur when a jet expands laterally to the point when different regions within the jet become causally disconnected from one another.
[Picture: one of our models; credit: Zakamska, Begelman, Blandford, 2008]
Eccentricities of extrasolar planets. The high eccentricities of extrasolar planets came as a surprise since it was usually believed that planets formed from a disk and therefore were expected to have nearly circular orbits. In collaboration with S.Tremaine, I have explored the possibility that eccentricities are excited in the outer parts of an extended planetary disk by encounters with stars passing at a distance of a few hundred AU. The eccentricities then propagate toward inner planets, as we described using Laplace-Lagrange secular perturbation theory. High eccentricities ( > 0.1) may be excited in planetary systems around stars that are formed in relatively dense, long-lived open clusters.
More recently, with collaborators M.Pan and E.Ford, I investigated observational biases in the measurements of exoplanet orbital parameters -- especially eccentricity -- obtained from radial velocity observations. We created a mock catalog of radial velocity data, choosing input planet masses, orbital periods, and observing patterns from actual radial velocity surveys and varying input eccentricities. We then analyzed the simulated data sets using Markov chain Monte-Carlo simulations and compared calculated orbital parameters with the input values. We found a significant bias in the determination of small eccentricities in radial velocity surveys. Since eccentricity is positive definite, eccentricities of planets on nearly circular (e < 0.05) orbits are preferentially overestimated. Though the extrasolar planet catalogs report eccentricities below 0.05 for just 19% of single-planet systems, we estimate that the true fraction of e < 0.05 orbits is about 35%. These methods can be applied to multi-planet systems and to other types of planetary surveys.
[Picture: some results of our simulations; credit: Zakamska, Pan, Ford, 2011]
Solar system acceleration from pulsar timing.
Whether or not the solar system contains as yet undiscovered massive planets or possesses a distant stellar companion has been the subject of intensive research. One of the ways to constrain the mass and position of a putative companion is to constrain the acceleration of the known solar system barycenter. In my work with S.Tremaine, I used pulsar timing data to determine limits on such acceleration. The constraint can be obtained by comparing the observed orbital period decay of binary pulsars to the value expected from general relativity. An independent constraint can be obtained in a statistical sense from millisecond pulsars. While we did not find any massive companions, the sensitivity in accelerations achieved using pulsar timing (a/c=a few x 10^(-19) sec^(-1)) is comparable to the acceleration in the Galactic potential at the position of the Sun.
[Picture: one of our sky maps showing acceleration limits; credit: Zakamska, Tremaine, 2005]
Thermal conduction in clusters of galaxies:
The mechanism of powering the X-ray emission of hot gas in clusters of galaxies is largely unknown. One possibility is the transport of thermal energy from the outer regions of the cluster into the center via thermal conduction. With R.Narayan, I constructed models of intracluster gas incorporating thermal conduction and showed that these models provided a good description of observed temperature and electron density profiles. We further showed that thermal conduction was a viable energy source for about half of the clusters in our study, whereas another half required additional energy sources, probably AGNs, in their centers. We also showed that conduction may prevent the gas from becoming thermally unstable.
[Picture: our models for one of the clusters; credit: Zakamska, Narayan, 2003]