146 research outputs found
Direct Measurement of the Photon Statistics of a Triggered Single Photon Source
We studied intensity fluctuations of a single photon source relying on the
pulsed excitation of the fluorescence of a single molecule at room temperature.
We directly measured the Mandel parameter Q(T) over 4 orders of magnitude of
observation timescale T, by recording every photocount. On timescale of a few
excitation periods, subpoissonian statistics is clearly observed and the
probablility of two-photons events is 10 times smaller than Poissonian pulses.
On longer times, blinking in the fluorescence, due to the molecular triplet
state, produces an excess of noise.Comment: 4 pages, 3 figures, 1 table submitted to Physical Review Letter
A method for quantifying the gamma-ray burst bias. Application in the redshift range of 0â1.1
International audienceContext. Long gamma-ray bursts (LGRBs) are related to the final stages of evolution of very massive stars. As such, they should follow the star formation rate (SFR) of galaxies. We can use them to probe for star-forming galaxies in the distant universe following this assumption. The relation between the rate of LGRBs in a given galaxy and its SFR (which we call the LGRB bias) may however be complex, as we have good indications that the LGRB hosts are not perfect analogues to the general population of star-forming galaxies. Aims. In this work, we try to quantify how the LGRB bias depends on physical parameters of their host galaxy, such as SFR or stellar mass. These trends may reveal more fundamental properties such as the role of the metallicity of LGRBs and of their progenitors .Methods. We propose an empirical method based on the comparison of stellar mass functions (and SFR distributions) of LGRB hosts and of star-forming galaxies to find how the bias depends on the stellar mass or the SFR.Results. By applying this method to a sample of LGRB hosts at redshifts lower than 1.1, where the properties of star-forming galaxies are fairly well established and where the properties of LGRB host galaxies can be deduced from observations (limiting ourselves to stellar masses higher than 109.25Mâ and SFR higher than ~1.8 Mââyr-1), we find that the LGRB bias depends on both the stellar mass and SFR. We find that the bias decreases with the SFR; that is, we see no preference for highly star-forming galaxies, once we account for the higher number of massive stars in galaxies with larger SFR. We do not find any trend with the specific star formation rate (SSFR), but the dynamical range in SSFR in our study is narrow. Through an indirect method, we relate these trends to a possible decrease in the LGRBs rate / SFR ratio with the metallicity.Conclusions. The method we propose suggests trends that may be useful to constrain models of LGRB progenitors, showing a clear decrease in the LGRB bias with the metallicity. This is promising for the future as the number of LGRB hosts studied will increase
Dust Properties and Star-Formation Rates in Star-Forming Dwarf Galaxies
We have used the Spitzer Space Telescope to study the dust properties of a
sample of star-forming dwarf galaxies. The differences in the mid-infrared
spectral energy distributions for these galaxies which, in general, are low
metallicity systems, indicate differences in the physical properties, heating,
and/or distribution of the dust. Specifically, these galaxies have more hot
dust and/or very small grains and less PAH emission than either spiral or
higher luminosity starburst galaxies. As has been shown in previous studies,
there is a gradual decrease in PAH emission as a function of metallicity.
Because much of the energy from star formation in galaxies is re-radiated in
the mid-infrared, star-formation rate indicators based on both line and
continuum measurements in this wavelength range are coming into more common
usage. We show that the variations in the interstellar medium properties of
galaxies in our sample, as measured in the mid-infrared, result in over an
order of magnitude spread in the computed star-formation rates.Comment: 25 pages, 11 figures, 4 tables, ApJ accepte
The AGN Contribution to the Mid-IR Emission of Luminous Infrared Galaxies
We determine the contribution of AGN to the mid-IR emission of luminous
infrared galaxies (LIRGs) at z>0.6 by measuring the mid-IR dust continuum slope
of 20,039 mid-IR sources. The 24 micron sources are selected from a
Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Bo\"otes field and have
corresponding 8 micron data from the IRAC Shallow Survey. There is a clear
bimodal distribution in the 24 micron to 8 micron flux ratio. The X-ray
detected sources fall within the peak corresponding to a flat spectrum in
nufnu, implying that it is populated by AGN-dominated LIRGs, whereas the peak
corresponding to a higher 24 micron to 8 micron flux ratio is likely due to
LIRGs whose infrared emission is powered by starbursts. The 24 micron emission
is increasingly dominated by AGN at higher 24 micron flux densities (f_24): the
AGN fraction of the z>0.6 sources increases from ~9% at f_24 ~ 0.35 mJy to
74+/-20% at f_24 ~ 3 mJy in good agreement with model predictions. Deep 24
micron, small area surveys, like GOODS, will be strongly dominated by starburst
galaxies. AGN are responsible for ~ 3-7% of the total 24 micron background.Comment: 6 pages, accepted for publication in Ap
Infrared Luminosities and Dust Properties of z ~ 2 Dust-Obscured Galaxies
We present SHARC-II 350um imaging of twelve 24um-bright (F_24um > 0.8 mJy)
Dust-Obscured Galaxies (DOGs) and CARMA 1mm imaging of a subset of 2 DOGs, all
selected from the Bootes field of the NOAO Deep Wide-Field Survey. Detections
of 4 DOGs at 350um imply IR luminosities which are consistent within a factor
of 2 of expectations based on a warm dust spectral energy distribution (SED)
scaled to the observed 24um flux density. The 350um upper limits for the 8
non-detected DOGs are consistent with both Mrk231 and M82 (warm dust SEDs), but
exclude cold dust (Arp220) SEDs. The two DOGs targeted at 1mm were not detected
in our CARMA observations, placing strong constraints on the dust temperature:
T_dust > 35-60 K. Assuming these dust properties apply to the entire sample, we
find dust masses of ~3x10^8 M_sun. In comparison to other dusty z ~ 2 galaxy
populations such as sub-millimeter galaxies (SMGs) and other Spitzer-selected
high-redshift sources, this sample of DOGs has higher IR luminosities (2x10^13
L_sun vs. 6x10^12 L_sun for the other galaxy populations), warmer dust
temperatures (>35-60 K vs. ~30 K), and lower inferred dust masses (3x10^8 M_sun
vs. 3x10^9 M_sun). Herschel and SCUBA-2 surveys should be able to detect
hundreds of these power-law dominated DOGs. We use HST and Spitzer/IRAC data to
estimate stellar masses of these sources and find that the stellar to gas mass
ratio may be higher in our 24um-bright sample of DOGs than in SMGs and other
Spitzer-selected sources. Although larger sample sizes are needed to provide a
definitive conclusion, the data are consistent with an evolutionary trend in
which the formation of massive galaxies at z~2 involves a sub-millimeter
bright, cold-dust and star-formation dominated phase followed by a 24um-bright,
warm-dust and AGN-dominated phase.Comment: 16 pages, 7 figures, 6 tables; accepted to the Ap
The Star Formation Rate Density and Dust Attenuation Evolution over 12 Gyr with the VVDS Surveys
[Abridged] We investigate the global galaxy evolution over 12 Gyr
(0.05<z<4.5), from the star formation rate density (SFRD), combining the VVDS
Deep (17.5<=I<=24.0) and Ultra-Deep (23.00<=i<=24.75) surveys. We obtain a
single homogeneous spectroscopic redshift sample, totalizing about 11000
galaxies. We estimate the rest-frame FUV luminosity function (LF) and
luminosity density (LD), extract the dust attenuation of the FUV radiation
using SED fitting, and derive the dust-corrected SFRD. We find a constant and
flat faint-end slope alpha in the FUV LF at z1.7, we set alpha
steepening with (1+z). The absolute magnitude M*_FUV brightens in the entire
range 02 it is on average brighter than in the literature,
while phi* is smaller. Our total LD shows a peak at z=2, present also when
considering all sources of uncertainty. The SFRD history peaks as well at z=2.
It rises by a factor of 6 during 2 Gyr (from z=4.5 to z=2), and then decreases
by a factor of 12 during 10 Gyr down to z=0.05. This peak is mainly produced by
a similar peak within the population of galaxies with -21.5<=M_FUV<=-19.5 mag.
As times goes by, the total SFRD is dominated by fainter and fainter galaxies.
The presence of a clear peak at z=2 and a fast rise at z>2 of the SFRD is
compelling for models of galaxy formation. The mean dust attenuation A_FUV of
the global galaxy population rises by 1 mag during 2 Gyr from z=4.5 to z=2,
reaches its maximum at z=1 (A_FUV=2.2 mag), and then decreases by 1.1 mag
during 7 Gyr down to z=0. The dust attenuation maximum is reached 2 Gyr after
the SFRD peak, implying a contribution from the intermediate-mass stars to the
dust production at z<2.Comment: 23 pages, 15 figures, accepted for publication in A&
CASSIS: The Cornell Atlas of Spitzer/Infrared Spectrograph Sources. II. High-resolution observations
The Infrared Spectrograph (IRS) on board the Spitzer Space Telescope observed about 15,000 objects during the cryogenic mission lifetime. Observations provided low-resolution (R~60-127) spectra over ~5-38um and high-resolution (R~600) spectra over ~10-37um. The Cornell Atlas of Spitzer/IRS Sources (CASSIS) was created to provide publishable quality spectra to the community. Low-resolution spectra have been available in CASSIS since 2011, and we present here the addition of the high-resolution spectra. The high-resolution observations represent approximately one third of all staring observations performed with the IRS instrument. While low-resolution observations are adapted to faint objects and/or broad spectral features (e.g., dust continuum, molecular bands), high-resolution observations allow more accurate measurements of narrow features (e.g., ionic emission lines) as well as a better sampling of the spectral profile of various features. Given the narrow aperture of the two high-resolution modules, cosmic ray hits and spurious features usually plague the spectra. Our pipeline is designed to minimize these effects through various improvements. A super sampled point-spread function was created in order to enable the optimal extraction in addition to the full aperture extraction. The pipeline selects the best extraction method based on the spatial extent of the object. For unresolved sources, the optimal extraction provides a significant improvement in signal-to-noise ratio over a full aperture extraction. We have developed several techniques for optimal extraction, including a differential method that eliminates low-level rogue pixels (even when no dedicated background observation was performed). The updated CASSIS repository now includes all the spectra ever taken by the IRS, with the exception of mapping observations
A contribution of star-forming clumps and accreting satellites to the mass assembly of z ⌠2 galaxies
We investigate the contribution of clumps and satellites to the galaxy mass assembly. We analysed spatially resolved HubbleSpace Telescope observations (imaging and slitless spectroscopy) of 53 star-forming galaxies at z ⌠1â3. We created continuum and emission line maps and pinpointed residual âblobsâ detected after subtracting the galaxy disc. Those were separated into compact (unresolved) and extended (resolved) components. Extended components have sizes âŒ2 kpc and comparable stellar mass and age as the galaxy discs, whereas the compact components are 1.5 dex less massive and 0.4 dex younger than the discs. Furthermore, the extended blobs are typically found at larger distances from the galaxy barycentre than the compact ones. Prompted by these observations and by the comparison with simulations, we suggest that compact blobs are in situ formed clumps, whereas the extended ones are accreting satellites. Clumps and satellites enclose, respectively, âŒ20 perâcent and âČ80 perâcent of the galaxy stellar mass, âŒ30 perâcent and âŒ20 perâcent of its star formation rate. Considering the compact blobs, we statistically estimated that massive clumps (Mâ âł 109 Mâ) have lifetimes of âŒ650 Myr, and the less massive ones (108 < Mâ < 109 Mâ) of âŒ145 Myr. This supports simulations predicting long-lived clumps (lifetime âł 100 Myr). Finally, âČ30 perâcent (13 perâcent) of our sample galaxies are undergoing single (multiple) merger(s), they have a projected separation âČ10 kpc, and the typical mass ratio of our satellites is 1:5 (but ranges between 1:10 and 1:1), in agreement with literature results for close pair galaxies
Stellar Mass and Velocity Functions of Galaxies: Backward evolution and the fate of Milky Way siblings
We attempt in this paper to check the consistency of the observed Stellar
Mass Function (SMF), SFR functions and the cosmic star formation rate density
with simple backward evolutionary models. Starting from observed SMF for
star-forming galaxies, we use backwards models to predict the evolution of a
number of quantities, such as the SFR function, the cosmic SFR density and the
Velocity Function. The velocity being a parameter attached to a galaxy during
its history (contrary to the stellar mass), this approach allows us to quantify
the number density evolution of galaxies of a given velocity, e.g. of the Milky
Way siblings. Observations suggest that the SMF of star forming galaxies is
constant between redshift 0 and 1. In order to reproduce this result, we must
quench star formation in a number of star forming galaxies. The SMF of these
quenched galaxies is consistent with available data concerning the increase in
the population of quiescent galaxies in the same redshift interval. The SMF of
quiescent galaxies is then mainly determined by the distribution of active
galaxies that must stop star formation, with a modest mass redistribution
during mergers. The cosmic SFR density, and the evolution of the SFR functions
are relatively well recovered, although they provide some clue for a small
evolution of the SMF of star forming galaxies at the lowest redshifts. We thus
consider that we have obtained in a simple way a relatively consistent picture
of the evolution of galaxies at intermediate redshifts. We note that if this
picture is correct, 50 percent of the Milky-Way sisters (galaxies with the same
velocity as our Galaxy, i.e. 220 km/s) have quenched their star formation since
redshift 1 (and an even larger fraction for larger velocities). We discuss the
processes that might be responsible for this transformation.Comment: 12 pages, 14 figures, accepted in Astronomy and Astrophysic
Satellite content and quenching of star formation in galaxy groups at z ~ 1.8
We study the properties of satellites in the environment of massive star-forming galaxies at z ~ 1.8 in the COSMOS field, using a sample of 215 galaxies on the main sequence of star formation with an average mass of ~1011Mâ. At z> 1.5, these galaxies typically trace halos of mass âł1013Mâ. We use optical-near-infrared photometry to estimate stellar masses and star formation rates (SFR) of centrals and satellites down to ~ 6 Ă 109Mâ. We stack data around 215 central galaxies to statistically detect their satellite halos, finding an average of ~3 galaxies in excess of the background density. We fit the radial profiles of satellites with simple ÎČ-models, and compare their integrated properties to model predictions. We find that the total stellar mass of satellites amounts to ~68% of the central galaxy, while spectral energy distribution modeling and far-infrared photometry consistently show their total SFR to be 25-35% of the central's rate. We also see significant variation in the specific SFR of satellites within the halo with, in particular, a sharp decrease at <100 kpc. After considering different potential explanations, we conclude that this is likely an environmental signature of the hot inner halo. This effect can be explained in the first order by a simple free-fall scenario, suggesting that these low-mass environments can shut down star formation in satellites on relatively short timescales of ~0.3 Gyr
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