485 research outputs found
Major Galaxy Mergers and the Growth of Supermassive Black Holes in Quasars
Despite observed strong correlations between central supermassive black holes
(SMBHs) and star-formation in galactic nuclei, uncertainties exist in our
understanding of their coupling. We present observations of the ratio of
heavily-obscured to unobscured quasars as a function of cosmic epoch up to z~3,
and show that a simple physical model describing mergers of massive, gas-rich
galaxies matches these observations. In the context of this model, every
obscured and unobscured quasar represent two distinct phases that result from a
massive galaxy merger event. Much of the mass growth of the SMBH occurs during
the heavily-obscured phase. These observations provide additional evidence for
a causal link between gas-rich galaxy mergers, accretion onto the nuclear SMBH
and coeval star formation.Comment: Accepted for publication in Science. Published by Science Express on
March 25th. 17 pages, 5 figures, including supplemental online materia
The Multiple Timescales of Optical Variability of the Blazar 3C 279 During the 2001-2002 Outburst
During 2001-2002 the optically violent variable (OVV) blazar 3C 279 un-
derwent the most intense outburst seen during the entire fourteen year history
that this quasar has been studied at Colgate University's Foggy Bottom Obser-
vatory (FBO). This study concentrates on ~1600 R-filter images taken during
this period of activity. This data set includes twenty-nine nights of
microvari- ability coverage. The outburst began in March 2001, after 3C 279 had
faded to its faintest level, R = 15.5, in four years. The source reached its
brightest level, R = 12.5, in the fourteen years of our study in August 2001,
at which time it became unobservable due to its proximity to the Sun. Upon
becoming observable again in mid-December 2001, 3C 279 fluctuated between R =
13.9 and R = 14.7, until a dramatic decrease in flux level in June-July 2002
brought the source back down to a level comparable to its pre-outburst state.
The source exhibited numerous week-long flares of approximately one magnitude
during the outburst period. Superposed on these flares were night-to-night
variations of up to one half magnitude and intra-night microvariability of up
to 0.13 magnitude in three hours. We use visual inspection of the light curve
as well as numerical timescale analysis tools (the autocorrelation function,
the structure function, and the power spectrum) to characterize the multiple
timescales of variability ranging from 1.5 years to several hours.Comment: 44 pages, 21 figures, 3 tables Accepted for publication in the
Astronomical Journa
A Multiwavelength Study of a Sample of 70 μm Selected Galaxies in the COSMOS Field. II. The Role of Mergers in Galaxy Evolution
We analyze the morphological properties of a large sample of 1503 70 μm selected galaxies in the COSMOS field spanning the redshift range 0.01 10^(12) L_☉) being up to ~50%. We also find that the fraction of spirals drops dramatically with L_(IR). Minor mergers likely play a role in boosting the infrared luminosity for sources with low luminosities (L_(IR) 1 being difficult to classify and subject to the effects of bandpass shifting; therefore, these numbers can only be considered lower limits. At z 1, the fraction of major mergers is lower, but is at least 30%-40% for ULIRGs. In a comparison of our visual classifications with several automated classification techniques we find general agreement; however, the fraction of identified mergers is underestimated due to automated classification methods being sensitive to only certain timescales of a major merger. Although the general morphological trends agree with what has been observed for local (U)LIRGs, the fraction of major mergers is slightly lower than seen locally. This is in part due to the difficulty of identifying merger signatures at high redshift. The distribution of the U – V color of the galaxies in our sample peaks in the green valley (= 1.1) with a large spread at bluer and redder colors and with the major mergers peaking more strongly in the green valley than the rest of the morphological classes. We argue that, given the number of major gas-rich mergers observed and the relatively short timescale that they would be observable in the (U)LIRG phase, it is plausible for the observed red sequence of massive ellipticals (<10^(12) M_☉) to have been formed entirely by gas-rich major mergers
Near-Infrared MOSFIRE Spectra of Dusty Star-Forming Galaxies at 0.2<z<4
We present near-infrared and optical spectroscopic observations of a sample
of 450m and 850m-selected dusty star-forming galaxies (DSFGs)
identified in a 400 arcmin area in the COSMOS field. Thirty-one sources of
the 102 targets were spectroscopically confirmed at , identified
primarily in the near-infrared with Keck MOSFIRE and some in the optical with
Keck LRIS and DEIMOS. The low rate of confirmation is attributable both to high
rest-frame optical obscuration in our targets and limited sensitivity to
certain redshift ranges. The high-quality photometric redshifts available in
the COSMOS field allow us to test the robustness of photometric redshifts for
DSFGs. We find a subset (11/31%) of DSFGs with inaccurate () or non-existent photometric redshifts; these have very distinct
spectral energy distributions from the remaining DSFGs, suggesting a decoupling
of highly obscured and unobscured components. We present a composite rest-frame
4300--7300\AA\ spectrum for DSFGs, and find evidence of 20030 km s
gas outflows. Nebular line emission for a sub-sample of our detections indicate
that hard ionizing radiation fields are ubiquitous in high-z DSFGs, even more
so than typical mass or UV-selected high-z galaxies. We also confirm the
extreme level of dust obscuration in DSFGs, measuring very high Balmer
decrements, and very high ratios of IR to UV and IR to H luminosities.
This work demonstrates the need to broaden the use of wide bandwidth technology
in the millimeter to the spectroscopic confirmations of large samples of high-z
DSFGs, as the difficulty in confirming such sources at optical/near-infrared
wavelengths is exceedingly challenging given their obscuration.Comment: 14 pages, 13 figures, ApJ accepted. Composite DSFG Halpha spectrum
available at www.as.utexas.edu/~cmcasey/downloads.htm
Major mergers are not significant drivers of star formation or morphological transformation around the epoch of peak cosmic star formation
We investigate the contribution of major mergers (mass ratios > 1:5) to stellar mass growth and morphological transformations around the epoch of peak cosmic star formation (z ~ 2). We visually classify a complete sample of massive (M > 1010M_) galaxies at this epoch, drawn from the CANDELS survey, into late-type galaxies, major mergers, spheroids and disturbed spheroids which show morphological disturbances. Given recent simulation work, which indicates that recent (<0.3-0.4 Gyr) major-merger remnants exhibit clear tidal features in such images, we use the fraction of disturbed spheroids to probe the role of major mergers in driving morphological transformations. The percentage of blue spheroids (i.e. with ongoing star formation) that show morphological disturbances is only 21 +- 4 per cent, indicating that major mergers are not the dominant mechanism for spheroid creation at z ~ 2 - other processes, such as minor mergers or cold accretion are likely to be the main drivers of this process. We also use the rest-frame U-band luminosity as a proxy for star formation to show that only a small fraction of the star formation budget (~3 per cent) is triggered by major mergers. Taken together, our results show that major mergers are not significant drivers of galaxy evolution at z ~ 2
The DEIMOS 10k spectroscopic survey catalog of the COSMOS field
We present a catalog of 10718 objects in the COSMOS field observed through
multi-slit spectroscopy with the Deep Imaging Multi-Object Spectrograph
(DEIMOS) on the Keck II telescope in the wavelength range ~5500-9800A. The
catalog contains 6617 objects with high-quality spectra (two or more spectral
features), and 1798 objects with a single spectroscopic feature confirmed by
the photometric redshift. For 2024 typically faint objects we could not obtain
reliable redshifts. The objects have been selected from a variety of input
catalogs based on multi-wavelength observations in the field, and thus have a
diverse selection function, which enables the study of the diversity in the
galaxy population. The magnitude distribution of our objects is peaked at
I_AB~23 and K_AB~21, with a secondary peak at K_AB~24. We sample a broad
redshift distribution in the range 0<z<6, with one peak at z~1, and another one
around z~4. We have identified 13 redshift spikes at z>0.65 with chance
probabilities <4xE-4$, some of which are clearly related to protocluster
structures of sizes >10 Mpc. An object-to-object comparison with a multitude of
other spectroscopic samples in the same field shows that our DEIMOS sample is
among the best in terms of fraction of spectroscopic failures and relative
redshift accuracy. We have determined the fraction of spectroscopic blends to
about 0.8% in our sample. This is likely a lower limit and at any rate well
below the most pessimistic expectations. Interestingly, we find evidence for
strong lensing of Ly-alpha background emitters within the slits of 12 of our
target galaxies, increasing their apparent density by about a factor of 4.Comment: 28 pages, 11 figures and 5 tables. The full catalogue table is
available on http://cosmos.astro.caltech.edu. Accepted for publication in the
Astrophysical Journa
AGN X-ray variability in the XMM-COSMOS survey
We took advantage of the observations carried out by XMM in the COSMOS field
during 3.5 years, to study the long term variability of a large sample of AGN
(638 sources), in a wide range of redshift (0.1<z<3.5) and X-ray luminosity
(L(2-10)). Both a simple statistical method to asses the
significance of variability, and the Normalized Excess Variance
() parameter, where used to obtain a quantitative measurement
of the variability. Variability is found to be prevalent in most AGN, whenever
we have good statistic to measure it, and no significant differences between
type-1 and type-2 AGN were found. A flat (slope -0.23+/-0.03) anti-correlation
between and X-ray luminosity is found, when significantly
variable sources are considered all together. When divided in three redshift
bins, the anti-correlation becomes stronger and evolving with z, with higher
redshift AGN being more variable. We prove however that this effect is due to
the pre-selection of variable sources: considering all the sources with
available measurement, the evolution in redshift disappears.
For the first time we were also able to study the long term X-ray variability
as a function of and Eddington ratio, for a large sample of AGN
spanning a wide range of redshift. An anti-correlation between
and is found, with the same slope of the
anti-correlation between and X-ray luminosity, suggesting
that the latter can be a byproduct of the former one. No clear correlation is
found between and the Eddington ratio in our sample.
Finally, no correlation is found between the X-ray and the
optical variability.Comment: 14 Pages, 13 figures. Accepted to the Astrophysical Journal on
December 6, 201
SHELS: Optical Spectral Properties of WISE 22 \mu m-selected Galaxies
We use a dense, complete redshift survey, the Smithsonian Hectospec Lensing
Survey (SHELS), covering a 4 square degree region of a deep imaging survey, the
Deep Lens Survey (DLS), to study the optical spectral properties of Wide-field
Infrared Survey Explorer (WISE) 22 \mu m-selected galaxies. Among 507 WISE 22
\mu m-selected sources with (S/N)_{22\mu m}>3 (\simS_{22\mu m}>2.5 mJy), we
identify the optical counterparts of 481 sources (\sim98%) at R<25.2 in the
very deep, DLS R-band source catalog. Among them, 337 galaxies at R<21 have
SHELS spectroscopic data. Most of these objects are at z<0.8. The infrared (IR)
luminosities are in the range 4.5x10^8 (L_sun) < L_{IR} < 5.4x10^{12} (L_sun).
Most 22 \mu m-selected galaxies are dusty star-forming galaxies with a small
(<1.5) 4000 \AA break. The stacked spectra of the 22 \mu m-selected galaxies
binned in IR luminosity show that the strength of the [O III] line relative to
H\beta grows with increasing IR luminosity. The optical spectra of the 22 \mu
m-selected galaxies also show that there are some (\sim2.8%) unusual galaxies
with very strong [Ne III] \lambda 3869, 3968 emission lines that require hard
ionizing radiation such as AGN or extremely young massive stars. The specific
star formation rates (sSFRs) derived from the 3.6 and 22 \mu m flux densities
are enhanced if the 22 \mu m-selected galaxies have close late-type neighbors.
The sSFR distribution of the 22 \mu m-selected galaxies containing active
galactic nuclei (AGNs) is similar to the distribution for star-forming galaxies
without AGNs. We identify 48 dust-obscured galaxy (DOG) candidates with large
(\gtrsim1000) mid-IR to optical flux density ratio. The combination of deep
photometric and spectroscopic data with WISE data suggests that WISE can probe
the universe to z\sim2.Comment: 18 pages, 17 figures. To appear in Ap
Rest-frame Optical Emission Lines in Far-Infrared Selected Galaxies at z<1.7 from the FMOS-COSMOS Survey
We have used FMOS on Subaru to obtain near-infrared spectroscopy of 123
far-infrared selected galaxies in COSMOS and obtain the key rest-frame optical
emission lines. This is the largest sample of infrared galaxies with
near-infrared spectroscopy at these redshifts. The far-infrared selection
results in a sample of galaxies that are massive systems that span a range of
metallicities in comparison with previous optically selected surveys, and thus
has a higher AGN fraction and better samples the AGN branch. We establish the
presence of AGN and starbursts in this sample of (U)LIRGs selected as
Herschel-PACS and Spitzer-MIPS detections in two redshift bins (z~0.7 and
z~1.5) and test the redshift dependence of diagnostics used to separate AGN
from star-formation dominated galaxies. In addition, we construct a low
redshift (z~0.1) comparison sample of infrared selected galaxies and find that
the evolution from z~1.5 to today is consistent with an evolving AGN selection
line and a range of ISM conditions and metallicities from the models of Kewley
et al. (2013b). We find that a large fraction of (U)LIRGs are BPT-selected AGN
using their new, redshift-dependent classification line. We compare the
position of known X-ray detected AGN (67 in total) with the BPT selection and
find that the new classification line accurately selects most of these objects
(> 70%). Furthermore, we identify 35 new (likely obscured) AGN not selected as
such by their X-ray emission. Our results have direct implications for AGN
selection at higher redshift with either current (MOSFIRE, KMOS) or future
(PFS, MOONS) spectroscopic efforts with near-infrared spectral coverage.Comment: 7 pages, 3 figures, 2 tables. Accepted for publication in The
Astrophysical Journal Letter
- …