19 research outputs found
Barred Galaxies at z > 0.7: NICMOS HDFN Observations
Previous optical studies found an unexpected deficit of bars at z > 0.7. To
investigate the effects of bandshifting, we have studied the fraction of barred
spirals in the NICMOS Deep Field North. At z > 0.7 we find at least four barred
spirals, doubling the number previously detected. The number of barred galaxies
is small because these (and previous) data lack adequate spatial resolution. A
typical 5 kpc bar at z > 0.7 is only marginally detectable for WFPC2 at
0.8microns; the NICMOS data have even lower resolution and can only find the
largest bars. The average size of the four bars seen at z > 0.7 is 12 kpc. The
fraction of such large bars (4/95) is higher than that seen in nearby spirals
(1/44); all known selection effects suggest that the observed fraction is a
lower limit. However, important caveats such as small numbers and difficulties
in defining comparable samples at high and low redshifts should be noted. We
conclude that there is no significant evidence for a decrease in the fraction
of barred spirals beyond z ~ 0.7.Comment: Accepted for publication in ApJ Letters, 4 pages in emulate-apj
style, includes 3 figure
Dust Dynamics, Surface Brightness Profiles, and Thermal Spectra of Debris Disks: The Case of AU Mic
AU Microscopii is a 12 Myr old M dwarf that harbors an optically thin,
edge-on disk of dust. The scattered light surface brightness falls with
projected distance b from the star as b^-a; within b = 43 AU, a = 1-2, while
outside 43 AU, a = 4-5. We devise a theory to explain this profile. At a
stellocentric distance r = r_BR = 43 AU, we posit a ring of parent bodies on
circular orbits: the "birth ring," wherein micron-sized grains are born from
the collisional attrition of parent bodies. The "inner disk" at r < r_BR
contains grains that migrate inward by corpuscular and Poynting-Robertson (CPR)
drag. The "outer disk" at r > r_BR comprises grains just large enough to remain
bound to the star, on orbits rendered highly eccentric by stellar wind and
radiation pressure. How the vertical optical depth tau scales with r depends on
the fraction of grains that migrate inward by CPR drag without suffering a
collision. If this fraction is large, the inner disk and birth ring share the
same optical depth, and tau scales as r^-5/2 in the outer disk. By contrast,
under collision-dominated conditions, the inner disk is empty, and tau scales
as r^-3/2 outside. These scaling relations, which we derive analytically and
confirm numerically, are robust against uncertainties in the grain size
distribution. By simultaneously modeling the surface brightness and thermal
spectrum, we break model degeneracies to establish that the AU Mic system is
collision-dominated, and that its narrow birth ring contains a lunar mass of
decimeter-sized bodies. The inner disk is devoid of micron-sized grains; the
surface brightness at b < 43 AU arises from light forward scattered by the
birth ring. Inside b = 43 AU, the disk's V-H color should not vary with b;
outside, the disk must become bluer as ever smaller grains are probed.Comment: Final proofed version to be published in ApJ; no significant changes
from version
Evolution of the bar fraction in COSMOS: quantifying the assembly of the Hubble sequence
We have analyzed the redshift-dependent fraction of galactic bars over 0.2 < z < 0.84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg^2 field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z ~ 0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong bars (SBs) drops from about 30% to under 10%. It is clear that when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z ~ 0.84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z = 0.3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate
PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy
We present the discovery and characterisation of PTF10iya, a short-lived (dt
~ 10 d, with an optical decay rate of ~ 0.3 mag per d), luminous (M_g ~ -21
mag) transient source found by the Palomar Transient Factory. The
ultraviolet/optical spectral energy distribution is reasonably well fit by a
blackbody with T ~ 1-2 x 10^4 K and peak bolometric luminosity L_BB ~ 1-5 x
10^44 erg per s (depending on the details of the extinction correction). A
comparable amount of energy is radiated in the X-ray band that appears to
result from a distinct physical process. The location of PTF10iya is consistent
with the nucleus of a star-forming galaxy (z = 0.22405 +/- 0.00006) to within
350 mas (99.7 per cent confidence radius), or a projected distance of less than
1.2 kpc. At first glance, these properties appear reminiscent of the
characteristic "big blue bump" seen in the near-ultraviolet spectra of many
active galactic nuclei (AGNs). However, emission-line diagnostics of the host
galaxy, along with a historical light curve extending back to 2007, show no
evidence for AGN-like activity. We therefore consider whether the tidal
disruption of a star by an otherwise quiescent supermassive black hole may
account for our observations. Though with limited temporal information,
PTF10iya appears broadly consistent with the predictions for the early
"super-Eddington" phase of a solar-type star disrupted by a ~ 10^7 M_sun black
hole. Regardless of the precise physical origin of the accreting material, the
large luminosity and short duration suggest that otherwise quiescent galaxies
can transition extremely rapidly to radiate near the Eddington limit; many such
outbursts may have been missed by previous surveys lacking sufficient cadence.Comment: 18 pages, 8 figures; revised following referee's comment
Optical Flares from the Tidal Disruption of Stars by Massive Black Holes
A star that wanders too close to a massive black hole (BH) is shredded by the
BH's tidal gravity. Stellar gas falls back to the BH, releasing a flare of
energy. In anticipation of upcoming transient surveys, we predict the light
curves and spectra of tidal flares as a function of time, highlighting the
unique signatures of tidal flares in the optical and near-IR. Some of the gas
initially bound to the BH is likely blown away when the fallback rate is
super-Eddington at early times. This outflow produces an optical luminosity
comparable to that of a supernova; such events have durations of ~10 days and
may have been missed in supernova searches that exclude the nuclear regions of
galaxies. When the fallback rate subsides below Eddington, the gas accretes
onto the BH via a thin disk whose emission peaks in the UV to soft X-rays. Some
of this emission is reprocessed by the unbound stellar debris, producing a
spectrum of very broad emission lines (with no corresponding narrow forbidden
lines). These lines are strongest for BHs with MBH ~ 10^5 - 10^6 Msun and thus
optical surveys are particularly sensitive to the lowest mass BHs in galactic
nuclei. Calibrating our models to ROSAT and GALEX observations, we predict
detection rates for Pan-STARRS, PTF, and LSST and highlight observational
challenges in the optical. Pan-STARRS should detect at least several events per
year--many more if current theoretical models of super-Eddington outflows are
correct. These surveys will significantly improve our knowledge of stellar
dynamics in galactic nuclei, the physics of super-Eddington accretion, the
demography of intermediate mass BHs, and the role of tidal disruption in the
growth of massive BHs.Comment: Accepted for publication in MNRA