80 research outputs found
The CFHTLS Deep Catalog of Interacting Galaxies I. Merger Rate Evolution to z=1.2
We present the rest-frame optical galaxy merger fraction between 0.2<z<1.2,
as a function of stellar mass and optical luminosity, as observed by the
Canada-France-Hawaii Telescope Legacy Deep Survey (CFHTLS-Deep). We developed a
new classification scheme to identify major galaxy-galaxy mergers based on the
presence of tidal tails and bridges. These morphological features are signposts
of recent and ongoing merger activity. Through the visual classification of all
galaxies, down to i_vega<22.2 (~27,000 galaxies) over 2 square degrees, we have
compiled the CFHTLS Deep Catalog of Interacting Galaxies, with ~1600 merging
galaxies. We find the merger fraction to be 4.3% +/-0.3% at z~0.3 and 19.0%
+/-2.5% at z~1, implying evolution of the merger fraction going as (1+z)^m,
with m=2.25 +/-0.24. This result is inconsistent with a mild or non-evolving
(m4sigma level of confidence. A mild trend, where massive
galaxies with M>10^10.7 M_sun are undergoing fewer mergers than less massive
systems M~10^10 M_sun), consistent with the expectations of galaxy assembly
downsizing is observed. Our results also show that interacting galaxies have on
average SFRs double that found in non-interacting field galaxies. We conclude
that (1) the optical galaxy merger fraction does evolve with redshift, (2) the
merger fraction depends mildly on stellar mass, with lower mass galaxies having
higher merger fractions at z<1, and (3) star formation is triggered at all
phases of a merger, with larger enhancements at later stages, consistent with
N-body simulations.Comment: e.g.: 17 pages, 14 figures, accepted for publication in Ap
The Space Density Evolution of Wet and Dry Mergers in the Canada-France-Hawaii Telescope Legacy Survey
We analyze 1298 merging galaxies with redshifts up to z = 0.7 from the Canada-France-Hawaii Telescope Legacy Survey, taken from the catalog presented in Bridge et al. (2010). By analyzing the internal colors of these systems, we show that so-called wet and dry mergers evolve in different senses, and quantify the space densities of these systems. The local space density of wet mergers is essentially identical to the local space density of dry mergers. The evolution in the total merger rate is modest out to z ∼ 0.7, although the wet and dry populations have different evolutionary trends. At higher redshifts dry mergers make a smaller contribution to the total merging galaxy population, but this is offset by a roughly equivalent increase in the contribution from wet mergers. By comparing the mass density function of early-type galaxies to the corresponding mass density function for merging systems, we show that not all the major mergers with the highest masses (M_(stellar) > 10^(11)M☉) will end up with the most massive early-type galaxies, unless the merging timescale is dramatically longer than that usually assumed. On the other hand, the usually-assumed merging timescale of ∼ 0.5–1 Gyr is quite consistent with the data if we suppose that only less massive early-type galaxies form via mergers. Since low-intermediate mass ellipticals are 10–100 times more common than their most massive counterparts, the hierarchical explanation for the origin of early-type galaxies may be correct for the vast majority of early-types, even if incorrect for the most massive ones
A Close-pair Analysis of Damp Mergers at Intermediate Redshifts
We have studied the kinematics of ~2800 candidate close-pair galaxies at 0.1 < z < 1.2 identified from the Canada-France-Hawaii Telescope Legacy Survey fields. Spectra of these systems were obtained using spectrometers on the 6.5 m Magellan and 5 m Hale telescopes. These data allow us to constrain the rate of dry mergers at intermediate redshifts and to test the "hot halo" model for quenching of star formation. Using virial radii estimated from the correlation between dynamical and stellar masses published by Leauthaud et al., we find that around 1/5 of our candidate pairs are likely to share a common dark matter halo (our metric for close physical association). These pairs are divided into red-red, blue-red, and blue-blue systems using the rest-frame colors classification method introduced in Chou et al.. Galaxies classified as red in our sample have very low star formation rates, but they need not be totally quiescent, and hence we refer to them as "damp," rather than "dry," systems. After correcting for known selection effects, the fraction of blue-blue pairs is significantly greater than that of red-red and blue-red pairs. Red-red pairs are almost entirely absent from our sample, suggesting that damp mergers are rare at z ~ 0.5. Our data support models with a short merging timescale (<0.5 Gyr) in which star formation is enhanced in the early phase of mergers, but quenched in the late phase. Hot halo models may explain this behavior, but only if virial shocks that heat gas are inefficient until major mergers are nearly complete
The Space Density Evolution of Wet and Dry Mergers in the Canada-France-Hawaii Telescope Legacy Survey
We analyze 1298 merging galaxies with redshifts up to z=0.7 from the
Canada-France-Hawaii Telescope Legacy Survey, taken from the catalog presented
in Bridge et al. (2010). By analyzing the internal colors of these systems, we
show that so-called wet and dry mergers evolve in different senses, and
quantify the space densities of these systems. The local space density of wet
mergers is essentially dentical to the local space density of dry mergers. The
evolution in the total merger rate is modest out to z ~ 0.7, although the wet
and dry populations have different evolutionary trends. At higher redshifts dry
mergers make a smaller contribution to the total merging galaxy population, but
this is offset by a roughly equivalent increase in the contribution from wet
mergers. By comparing the mass density function of early-type galaxies to the
corresponding mass density function for merging systems, we show that not all
the major mergers with the highest masses (M_stellar > 10^11 M_solar) will end
up with the most massive early-type galaxies, unless the merging timescale is
dramatically longer than that usually assumed. On the other hand, the
usually-assumed merging timescale of ~ 0.5-1 Gyr is quite consistent with the
data if we suppose that only less massive early-type galaxies form via mergers.
Since low-intermediate mass ellipticals are 10 --100 times more common than
their most massive counterparts, the hierarchical explanation for the origin of
early-type galaxies may be correct for the vast majority of early-types, even
if incorrect for the most massive ones.Comment: 10 pages, 8 figures. Accepted by A
The FUV to Near-IR Morphologies of Luminous Infrared Galaxies in the GOALS Sample
We compare the morphologies of a sample of 20 LIRGs from the Great
Observatories All-sky LIRG Survey (GOALS) in the FUV, B, I and H bands, using
the Gini (G) and M20 parameters to quantitatively estimate the distribution and
concentration of flux as a function of wavelength. HST images provide an
average spatial resolution of ~80 pc. While our LIRGs can be reliably
classified as mergers across the entire range of wavelengths studied here,
there is a clear shift toward more negative M20 (more bulge-dominated) and a
less significant decrease in G values at longer wavelengths. We find no
correlation between the derived FUV G-M20 parameters and the global measures of
the IR to FUV flux ratio, IRX. Given the fine resolution in our HST data, this
suggests either that the UV morphology and IRX are correlated on very small
scales, or that the regions emitting the bulk of the IR emission emit almost no
FUV light. We use our multi-wavelength data to simulate how merging LIRGs would
appear from z~0.5-3 in deep optical and near-infrared images such as the HUDF,
and use these simulations to measure the G-M20 at these redshifts. Our
simulations indicate a noticeable decrease in G, which flattens at z >= 2 by as
much as 40%, resulting in mis-classifying our LIRGs as disk-like, even in the
rest-frame FUV. The higher redshift values of M20 for the GOALS sources do not
appear to change more than about 10% from the values at z~0. The change in
G-M20 is caused by the surface brightness dimming of extended tidal features
and asymmetries, and also the decreased spatial resolution which reduced the
number of individual clumps identified. This effect, seen as early as z~0.5,
could easily lead to an underestimate of the number of merging galaxies at
high-redshift in the rest-frame FUV.Comment: Accepted for publication in the Astronomical Journal. The total page
count is 15 pages with 13 figures and 1 Tabl
Submillimetre observations of WISE-selected high-redshift, luminous, dusty galaxies
We present SCUBA-2 850um submillimetre (submm) observations of the fields of
10 dusty, luminous galaxies at z ~ 1.7 - 4.6, detected at 12um and/or 22um by
the WISE all-sky survey, but faint or undetected at 3.4um and 4.6um; dubbed
hot, dust-obscured galaxies (Hot DOGs). The six detected targets all have total
infrared luminosities greater than 10^13 L_sun, with one greater than 10^14
L_sun. Their spectral energy distributions (SEDs) are very blue from
mid-infrared to submm wavelengths and not well fitted by standard AGN SED
templates, without adding extra dust extinction to fit the WISE 3.4um and 4.6um
data. The SCUBA-2 850um observations confirm that the Hot DOGs have less cold
and/or more warm dust emission than standard AGN templates, and limit an
underlying extended spiral or ULIRG-type galaxy to contribute less than about
2% or 55% of the typical total Hot DOG IR luminosity, respectively. The two
most distant and luminous targets have similar observed submm to mid-infrared
ratios to the rest, and thus appear to have even hotter SEDs. The number of
serendipitous submm galaxies (SMGs) detected in the 1.5-arcmin-radius SCUBA-2
850um maps indicates there is a significant over-density of serendipitous
sources around Hot DOGs. These submm observations confirm that the
WISE-selected ultra-luminous galaxies have very blue mid-infrared to submm
SEDs, suggesting that they contain very powerful AGN, and are apparently
located in unusual arcmin-scale overdensities of very luminous dusty galaxies.Comment: 12 pages, 8 figures, 3 tables, accepted for publication in MNRA
The Infrared Properties of Sources Matched in the WISE All-sky and Herschel ATLAS Surveys
We describe the infrared properties of sources detected over ~36 deg^2 of sky in the GAMA 15 hr equatorial field, using data from both the Herschel Astrophysical Terahertz Large-Area Survey (H-ATLAS) and Wide-field Infrared Survey (WISE). With 5σ point-source depths of 34 and 0.048 mJy at 250 μm and 3.4 μm, respectively, we are able to identify 50.6% of the H-ATLAS sources in the WISE survey, corresponding to a surface density of ~630 deg^(–2). Approximately two-thirds of these sources have measured spectroscopic or optical/near-IR photometric redshifts of z < 1. For sources with spectroscopic redshifts at z < 0.3, we find a linear correlation between the infrared luminosity at 3.4 μm and that at 250 μm, with ±50% scatter over ~1.5 orders of magnitude in luminosity, ~10^9-10^(10.5) L_☉. By contrast, the matched sources without previously measured redshifts (r ≳ 20.5) have 250-350 μm flux density ratios which suggest either high-redshift galaxies (z ≳ 1.5) or optically faint low-redshift galaxies with unusually low temperatures (T ≾ 20). Their small 3.4-250 μm flux ratios favor a high-redshift galaxy population, as only the most actively star-forming galaxies at low redshift (e.g., Arp 220) exhibit comparable flux density ratios. Furthermore, we find a relatively large active galactic nucleus fraction (~30%) in a 12 μm flux-limited subsample of H-ATLAS sources, also consistent with there being a significant population of high-redshift sources in the no-redshift sample
The role of the most luminous, obscured AGN in galaxy assembly at z~2
We present HST WFC3 F160W imaging and infrared spectral energy distributions
for twelve extremely luminous, obscured AGN at , selected via "Hot,
Dust Obscured" mid-infrared colors. Their infrared luminosities span
L, making them among the most luminous objects in
the Universe at . In all cases the infrared emission is consistent with
arising at least in most part from AGN activity. The AGN fractional
luminosities are higher than those in either sub-millimeter galaxies, or AGN
selected via other mid-infrared criteria. Adopting the , M and
morphological parameters, together with traditional classification boundaries,
infers that three quarters of the sample as mergers. Our sample do not,
however, show any correlation between the considered morphological parameters
and either infrared luminosity or AGN fractional luminosity. Moreover, their
asymmetries and effective radii are distributed identically to those of massive
galaxies at . We conclude that our sample is not preferentially
associated with mergers, though a significant merger fraction is still
plausible. Instead, we propose that our sample are examples of the massive
galaxy population at that harbor a briefly luminous, "flickering" AGN,
and in which the and M values have been perturbed, due to either the
AGN, and/or the earliest formation stages of a bulge in an inside-out manner.
Furthermore, we find that the mass assembly of the central black holes in our
sample leads the mass assembly of any bulge component. Finally, we speculate
that our sample represent a small fraction of the immediate antecedents of
compact star-forming galaxies at .Comment: ApJ, accepted. Updated to reflect the accepted versio
Interferometric Follow-Up of WISE Hyper-Luminous Hot, Dust-Obscured Galaxies
WISE has discovered an extraordinary population of hyper-luminous dusty
galaxies which are faint in the two bluer passbands (m and m) but are bright in the two redder passbands of WISE (m and
m). We report on initial follow-up observations of three of these
hot, dust-obscured galaxies, or Hot DOGs, using the CARMA and SMA
interferometer arrays at submm/mm wavelengths. We report continuum detections
at 1.3 mm of two sources (WISE J014946.17+235014.5 and WISE
J223810.20+265319.7, hereafter W0149+2350 and W2238+2653, respectively), and
upper limits to CO line emission at 3 mm in the observed frame for two sources
(W0149+2350 and WISE J181417.29+341224.8, hereafter W1814+3412). The 1.3 mm
continuum images have a resolution of 1-2 arcsec and are consistent with single
point sources. We estimate the masses of cold dust are 2.0 for W0149+2350 and 3.9 for W2238+2653,
comparable to cold dust masses of luminous quasars. We obtain 2 upper
limits to the molecular gas masses traced by CO, which are 3.3 and 2.3 for W0149+2350 and W1814+3412,
respectively. We also present high-resolution, near-IR imaging with WFC3 on the
Hubble Space Telescope for W0149+2653 and with NIRC2 on Keck for W2238+2653.
The near-IR images show morphological structure dominated by a single,
centrally condensed source with effective radius less than 4 kpc. No signs of
gravitational lensing are evident.Comment: 13 pages, 3 figures. ApJ in pres
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