MIT Astrophysics Brown Bag Lunch Series - Spring 2012
Mondays at 12:05 PM in 37-212
MIT Kavli Institute for Astrophysics and Space Research
70 Vassar Street, Cambridge, MA
The Brown Bag lunch is a forum for visiting astronomers/astrophysicists to speak about their research. Talks begin at 12:05 and speakers should plan 40 minutes of material, to leave room for questions during and after the presentation. If you are visiting MIT and would like to give a lunch talk, you may either contact the organizers directly or have your local colleagues arrange a time. The present organizers of the series are Rob Simcoe, Nevin Weinberg, and Laura Lopez.
Abstract:
Abstract: The development of virial mass estimates for the central
black hole using one quasar spectrum has allowed a dramatic
improvement in our understanding of supermassive black hole evolution.
I will describe several new puzzles arising from the combination of
virial masses with luminosity and redshift measurements, many of which
are inconsistent with our current understanding of quasar evolution.
I will also describe a new class of quasars that does not appear to
fit easily into current models for quasar accretion.
Monday, Feb 13:
The Reddest Quasars: A Transitional Phase in Quasar/Galaxy Co-Evolution Eliat Glikman NSF Postdoctoral Fellow/Yale
Abstract:
Quasars are extremely luminous sources, powered by accretion of gas
onto a supermassive black
hole in the nucleus of some galaxies. Most of the >100,000 quasars
identified in the literature have
been identified in optical surveys through the "ultraviolet excess"
(UVX) method. However, these
samples are known to be incomplete and biased because of obscuration
and anisotropic radiation.
To overcome some of these biases and search for candidate obscured
quasars, we matched radio
sources from the FIRST 1.4 GHz survey, sensitive to 1 mJy, with the
2MASS near-infrared survey
and selected objects with red optical-to-near-infrared colors. We
followed up our candidates with
optical and/or near-infrared spectroscopy and identified 119
dust-reddened quasars, defined as
having at least one broad line and a reddening of E(B-V)> 0.1. The
sample spans a wide redshift
range, 0.1 < z < 3 and reaches a reddening, E(B-V) < 1.5. When
corrected for extinction, red
quasars are the most luminous objects at every redshift and the
fraction of red quasars increases
with luminosity. The properties of red quasars suggest that they are
revealing an emergent phase
where the heavily obscured quasar is emerging from its dusty
environment prior to becoming a
"normal" blue quasar. We compute the fraction of quasars that are in
this red phase and determine
that its duration is 20% as long as the unobscured quasars phase: a
few million years.
Monday, March 5:
X-ray Emission from Star Forming Galaxies Stefano Mineo CfA
Abstract:
Star-forming galaxies provide us with a unique and complete insight on
several aspects of X-ray astrophysics, from the accretion processes into
compact objects to the emission mechanisms from the hot inter-stellar
medium (ISM).
Based on a homogeneous set of X-ray, infrared and ultraviolet observations
from Chandra, Spitzer, GALEX and 2MASS archives, we studied the
populations of high-mass X-ray binaries (HMXBs) and the X-ray emission
from the diffuse hot gas in a sample of 29 nearby star-forming galaxies.
In agreement with previous results, we find that HMXBs are a good tracer
of the recent star formation activity in the host galaxy and their
collective luminosity and number scale with the star formation rate (SFR).
The luminosity function of HMXBs was derived, modeled and interpreted with
a statistical accuracy that exceeded by far that achieved in any of the
previous studies. In fact it included over 700 X-ray binaries and a
special care was taken to minimize the contamination by low-mass X-ray
binaries (LMXBs), to subtract the background AGN contribution and to
control the incompleteness of the Chandra source lists. This allowed to
find evidence, for the first time, of a high luminosity break at the
Eddington luminosity of a ~100 M_sun black hole. We studied the
implications of these results on binary evolution theory. We derived the
fraction of compact objects that once upon their lifetime experienced an
X-ray active phase powered by accretion from a high mass companion. We
also constrained the mass distribution of the secondary star in HMXBs. We
studied the physical properties of the hot ISM and their relation with the
SFR. Finally, we investigated the total X-ray emission (i.e. due to both
X-ray binaries and gas) of a sample of 56 star-forming galaxies, 27 of
which are late-type galaxies from the Chandra Deep Fields with redshift up
to z~1.3.
Monday, March 26:
Abstract:
Type I supernovae (SNe Ia) are believed to be thermonuclear explosions of CO white dwarfs when they approach the Chandrasekhar mass. Empirically they have been found to be standardizable distance candles and have provided the first observational evidence for an accelerating Universe. Despite their cosmological importance, the nature of their progenitors has remained hotly debated. In this talk I review the various methods that have been used in recent years to identify possible progenitor systems. I then show some very recent results from the Palomar Transient Factory, including some unpublished ones, that provide strong constraints in two cases, one of which is the first unambiguous discovery of a system with a hydrogen-rich donor star in a normal SN Ia, probably similar to the Galactic recurrent nova binary RS Oph.
Monday, May 21:
On the role of radiative cooling in determining the observed emission
from Microquasar jets Asaf Pe'er CfA/ITC
Abstract:
I will present a new model of emission from jets in Microquasars, which
implements elements from the study of jets in gamma-ray bursts to
these objects. This model inherently considers radiative and adiabatic
cooling of electrons along the jet. I will show theoretical results
that can explain some of the key observations. In particular, I will
show that: (I) a flat radio spectrum, as is frequently seen, is a
natural outcome of the model; (II) Strong magnetic field results in a
flux decay in the optical -- X band as F_\nu~\nu^{-1/2}, irrespective
of many of the model uncertainties. This may provide a natural
explanation to the breaks frequently seen in the X-ray emission of
many objects in the low/hard state. (III) An increase of the magnetic
field above a critical value of ~10^5 G leads to a sharp decrease in
the flux at the radio band, while the flux at higher frequencies
saturates to a constant value. I conclude that scatter in the values
of the magnetic field may provide a natural explanation to the
observed scatter in the radio/X ray luminosity correlation seen in
these objects.
Monday, June 25: