650 research outputs found
Studying the evolution of galaxies in compact groups over the past 3 Gyr - II. The importance of environment in the suppression of star formation
We present an in depth study on the evolution of galaxy properties in compact
groups over the past 3 Gyr. We are using the largest multi-wavelength sample
to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift
range of 0.01<z<0.23. To derive the physical properties of the galaxies we rely
on ultraviolet (UV)-to-infrared spectral energy distribution modeling, using
CIGALE. Our results suggest that during the 3 Gyr period covered by our sample,
the star formation activity of galaxies in our groups has been substantially
reduced (3-10 times). Moreover, their star formation histories as well as their
UV-optical and mid-infrared colors are significantly different from those of
field and cluster galaxies, indicating that compact group galaxies spend more
time transitioning through the green valley. The morphological transformation
from late-type spirals into early-type galaxies occurs in the mid-infrared
transition zone rather than in the UV-optical green valley. We find evidence of
shocks in the emission line ratios and gas velocity dispersions of the
late-type galaxies located below the star forming main sequence. Our results
suggest that in addition to gas stripping, turbulence and shocks might play an
important role in suppressing the star formation in compact group galaxies.Comment: (Accepted for publication in MNRAS, date of submission November 18,
2015
Accretion-Inhibited Star Formation in the Warm Molecular Disk of the Green-valley Elliptical Galaxy NGC 3226
We present archival Spitzer photometry and spectroscopy, and Herschel
photometry, of the peculiar "Green Valley" elliptical galaxy NGC~3226. The
galaxy, which contains a low-luminosity AGN, forms a pair with NGC~3227, and is
shown to lie in a complex web of stellar and HI filaments. Imaging at 8 and
16m reveals a curved plume structure 3 kpc in extent, embedded within the
core of the galaxy, and coincident with the termination of a 30 kpc-long HI
tail. In-situ star formation associated with the IR plume is identified from
narrow-band HST imaging. The end of the IR-plume coincides with a warm
molecular hydrogen disk and dusty ring, containing 0.7-1.1 10
M detected within the central kpc. Sensitive upper limits to the
detection of cold molecular gas may indicate that a large fraction of the H
is in a warm state. Photometry, derived from the UV to the far-IR, shows
evidence for a low star formation rate of 0.04 M yr
averaged over the last 100 Myrs. A mid-IR component to the Spectral Energy
Distribution (SED) contributes 20 of the IR luminosity of the galaxy,
and is consistent with emission associated with the AGN. The current measured
star formation rate is insufficient to explain NGC3226's global UV-optical
"green" colors via the resurgence of star formation in a "red and dead" galaxy.
This form of "cold accretion" from a tidal stream would appear to be an
inefficient way to rejuvenate early-type galaxies, and may actually inhibit
star formation.Comment: Accepted for Publication ApJ Oct 201
Star Formation in Nearby Early-Type Galaxies: The Radio Continuum Perspective
We present a 1.4 GHz Karl G. Jansky Very Large Array (VLA) study of a sample
of early-type galaxies (ETGs) from the volume- and magnitude-limited ATLAS-3D
survey. The radio morphologies of these ETGs at a resolution of 5" are diverse
and include sources that are compact on sub-kpc scales, resolved structures
similar to those seen in star-forming spiral galaxies, and kpc-scale radio
jets/lobes associated with active nuclei. We compare the 1.4 GHz, molecular
gas, and infrared (IR) properties of these ETGs. The most CO-rich ATLAS-3D ETGs
have radio luminosities consistent with extrapolations from H_2-mass-derived
star formation rates from studies of late-type galaxies. These ETGs also follow
the radio-IR correlation. However, ETGs with lower molecular gas masses tend to
have less radio emission relative to their CO and IR emission compared to
spirals. The fraction of galaxies in our sample with high IR-radio ratios is
much higher than in previous studies, and cannot be explained by a systematic
underestimation of the radio luminosity due to the presence extended,
low-surface-brightness emission that was resolved-out in our VLA observations.
In addition, we find that the high IR-radio ratios tend to occur at low IR
luminosities, but are not associated with low dynamical mass or metallicity.
Thus, we have identified a population of ETGs that have a genuine shortfall of
radio emission relative to both their IR and molecular gas emission. A number
of mechanisms may conspire to cause this radio deficiency, including a
bottom-heavy stellar initial mass function, weak magnetic fields, a higher
prevalence of environmental effects compared to spirals and enhanced cosmic ray
losses.Comment: accepted for publication in MNRA
Strong Far-IR Cooling Lines, Peculiar CO Kinematics and Possible Star Formation Suppression in Hickson Compact Group 57
We present [C II] and [O I] observations from Herschel and CO(1-0) maps from
the Combined Array for{\dag} Research in Millimeter Astronomy (CARMA) of the
Hickson Compact Group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG
57a has been previously shown to contain enhanced quantities of warm molecular
hydrogen consistent with shock and/or turbulent heating. Our observations show
that HCG 57d has strong [C II] emission compared to L and weak
CO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a lies
at the upper end of the normal distribution of [C II]/CO and [C II]/FIR ratios,
and its far-IR cooling supports a low density warm diffuse gas that falls close
to the boundary of acceptable PDR models. However, the power radiated in the [C
II] and warm H emission have similar magnitudes, as seen in other
shock-dominated systems and predicted by recent models. We suggest that
shock-heating of the [C II] is a viable alternative to photoelectric heating in
violently disturbed diffuse gas. The existence of shocks is also consistent
with peculiar CO kinematics in the galaxy, indicating highly non-circular
motions are present. These kinematically disturbed CO regions also show
evidence of suppressed star formation, falling a factor of 10-30 below normal
galaxies on the Kennicutt-Schmidt relation. We suggest that the peculiar
properties of both galaxies are consistent with a highly dissipative off-center
collisional encounter between HCG 57d and 57a, creating ring-like morphologies
in both systems. Highly dissipative gas-on-gas collisions may be more common in
dense groups because of the likelihood of repeated multiple encounters. The
possibility of shock-induced SF suppression may explain why a subset of these
HCG galaxies have been found previously to fall in the mid-infrared green
valley.Comment: ApJ accepted, 16 pages, 12 figures, 3 table
Theoretical and experimental study of positron annihilation with core electrons in solids
A theory for calculating the momentum distribution of annihilating positron-electron pairs in solids is presented. To test the theory, momentum distributions are measured by the Doppler broadening of the annihilation radiation for several bulk metals and semiconductors, as well as for semiconductor alloys and for positrons trapped at vacancies in semiconductors. The theory is based on a two-particle description of the annihilating electron-positron pair. Then, the electron-positron correlation effects, i.e., the enhancement of the electron density at the positron, depend on the electronic state in question. The theory is suited for calculating the high-momentum part of the annihilation spectrum that arises from the core electrons and which can be measured by the Doppler broadening using coincidence techniques. The ideas of the theory are justified by a good agreement between theory and experiment in the case of positron annihilation in undefected bulk lattices. Moreover, the comparison of the theoretical and experimental spectra for alloys and vacancy defects tests the theoretical description for the positron distribution in delocalized and localized states, respectively.Peer reviewe
The ATLAS3D project - XXV: Two-dimensional kinematic analysis of simulated galaxies and the cosmological origin of fast and slow rotators
We present a detailed two-dimensional stellar dynamical analysis of as ample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 x 1010Msun âŒâ€ Mstar âŒâ€ 6x 1011Msun. Kinematic maps of the stellar line-of-sight velocity, velocity dispersion, and higher-order Gauss-Hermite moments h3 and h4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the λR-parameter. The velocity, velocity dispersion, h3, and h4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS3D survey. This includes fast (regular), slow, and misaligned rotation, hot spheroids with embedded cold disk components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant in-situ formation of stars, or with additional gas-rich major mergers - resulting in a spin-up - in their formation history, form elongated fast rotators with a clear anti-correlation of h3 and v/Ï. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants. This formation path does not result in anti-correlated h3 and v/Ï. The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in-situ star formation since z âŒÂ 2, rotate slower and have older stellar populations. (shortened)PostprintPeer reviewe
Herschel observations of Hickson compact groups of galaxies: Unveiling the properties of cold dust
We present a Herschel far-infrared and sub-millimetre (sub-mm) study of a sample of 120 galaxies in 28 Hickson compact groups (HCGs). Fitting their UV to sub-mm spectral energy distributions with the model of da Cunha et al. (2008), we accurately estimate the dust masses, luminosities, and temperatures of the individual galaxies. We find that nearly half of the late-type galaxies in dynamically âoldâ groups, those with more than 25% of early-type members and redder UV-optical colours, also have significantly lower dustto-stellar mass ratios compared to those of actively star-forming galaxies of the same mass found both in HCGs and in the field. Examining their dust-to-gas mass ratios, we conclude that dust was stripped out of these systems as a result of the gravitational and hydrodynamic interactions, experienced owing to previous encounters with other group members. About 40% of the early-type galaxies (mostly lenticulars), in dynamically âoldâ groups, display dust properties similar to those of the UV-optical red late-type galaxies. Given their stellar masses, star formation rates, and UV-optical colours, we suggest that red late-type and dusty lenticular galaxies represent transition populations between blue star-forming disk galaxies and quiescent early-type ellipticals. On the other hand, both the complete absence of any correlation between the dust and stellar masses of the dusty ellipticals and their enhanced star formation activity, suggest the increase in their gas and dust content due to accretion and merging. Our deep Herschel observations also allow us to detect the presence of diffuse cold intragroup dust in 4 HCGs. We also find that the fraction of 250 ÎŒm emission that is located outside of the main bodies of both the red late-type galaxies and the dusty lenticulars is 15â20% of their integrated emission at this band. All these findings are consistent with an evolutionary scenario in which gas dissipation, shocks, and turbulence, in addition to tidal interactions, shape the evolution of galaxies in compact groups
First-principles calculations of the self-trapped exciton in crystalline NaCl
The atomic and electronic structure of the lowest triplet state of the
off-center (C2v symmetry) self-trapped exciton (STE) in crystalline NaCl is
calculated using the local-spin-density (LSDA) approximation. In addition, the
Franck-Condon broadening of the luminescence peak and the a1g -> b3u absorption
peak are calculated and compared to experiment. LSDA accurately predicts
transition energies if the initial and final states are both localized or
delocalized, but 1 eV discrepancies with experiment occur if one state is
localized and the other is delocalized.Comment: 4 pages with 4 embeddded figure
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