6,347 research outputs found
The Merger-Triggered Active Galactic Nuclei Contribution to the Ultraluminous Infrared Galaxy Population
It has long been thought that there is a connection between ultraluminous
infrared galaxies (ULIRGs), quasars, and major mergers. Indeed, simulations
show that major mergers are capable of triggering massive starbursts and
quasars. However, observations by the Herschel Space Observatory suggest that,
at least at high redshift, there may not always be a simple causal connection
between ULIRGs and mergers. Here, we combine an evolving merger-triggered AGN
luminosity function with a merger-triggered starburst model to calculate the
maximum contribution of major mergers to the ULIRG population. We find that
major mergers can account for the entire local population of ULIRGs hosting AGN
and ~25% of the total local ULIRG luminosity density. By z ~ 1, major mergers
can no longer account for the luminosity density of ULIRGs hosting AGN and
contribute \lesssim 12% of the total ULIRG luminosity density. This drop is
likely due to high redshift galaxies being more gas rich and therefore able to
achieve high star formation rates through secular evolution. Additionally, we
find that major mergers can account for the local population of warm ULIRGs.
This suggests that selecting high redshift warm ULIRGs will allow for the
identification of high redshift merger-triggered ULIRGs. As major mergers are
likely to trigger very highly obscured AGN, a significant fraction of the high
redshift warm ULIRG population may host Compton thick AGN.Comment: Accepted ApJL, 3 figure
Relation Between Stellar Mass and Star Formation Activity in Galaxies
For a mass-selected sample of 66544 galaxies with photometric redshifts from
the Cosmic Evolution Survey (COSMOS), we examine the evolution of star
formation activity as a function of stellar mass in galaxies. We estimate the
cosmic star formation rates (SFR) over the range 0.2 < z < 1.2, using the
rest-frame 2800 A flux (corrected for extinction). We find the mean SFR to be a
strong function of the galactic stellar mass at any given redshift, with
massive systems (log (M/M(Sun)) > 10.5) contributing less (by a factor of ~ 5)
to the total star formation rate density (SFRD).
Combining data from the COSMOS and Gemini Deep Deep Survey (GDDS), we extend
the SFRD-z relation as a function of stellar mass to z~2. For massive galaxies,
we find a steep increase in the SFRD-z relation to z~2; for the less massive
systems, the SFRD which also increases from z=0 to 1, levels off at z~1. This
implies that the massive systems have had their major star formation activity
at earlier epochs (z > 2) than the lower mass galaxies.
We study changes in the SFRDs as a function of both redshift and stellar mass
for galaxies of different spectral types. We find that the slope of the SFRD-z
relation for different spectral type of galaxies is a strong function of their
stellar mass. For low and intermediate mass systems, the main contribution to
the cosmic SFRD comes from the star-forming galaxies while, for more massive
systems, the evolved galaxies are the most dominant population.Comment: 34 pages; 8 figures; Accepted for publication in Ap
Optical colours of AGN in the Extended Chandra Deep Field South: Obscured black holes in early type galaxies
We investigate the optical colours of X-ray sources from the Extended Chandra
Deep Field South (ECDFS) using photometry from the COMBO-17 survey, aiming to
explore AGN - galaxy feedback models. The X-ray sources populate both the
``blue'' and the ``red sequence'' on the colour-magnitude diagram. However,
sources in the ``red sequence'' appear systematically more obscured. HST
imaging from the GEMS survey demonstrates that the nucleus does not affect
significantly the observed colours, and therefore red sources are early-type
systems. In the context of AGN feedback models, this means that there is still
remaining material after the initial ``blowout''. We argue that this material
could not be only left-over from the original merger, but a secondary cold gas
supplier (such as minor interactions or self-gravitational instabilities) must
also assist.Comment: Submitted to A&
Mergers, AGN, and 'Normal' Galaxies: Contributions to the Distribution of Star Formation Rates and Infrared Luminosity Functions
We use a novel method to predict the contribution of normal star-forming
galaxies, merger-induced bursts, and obscured AGN, to IR luminosity functions
(LFs) and global SFR densities. We use empirical halo occupation constraints to
populate halos with galaxies and determine the distribution of normal and
merging galaxies. Each system can then be associated with high-resolution
hydrodynamic simulations. We predict the distribution of observed luminosities
and SFRs, from different galaxy classes, as a function of redshift from z=0-6.
We provide fitting functions for the predicted LFs, quantify the uncertainties,
and compare with observations. At all redshifts, 'normal' galaxies dominate the
LF at moderate luminosities ~L* (the 'knee'). Merger-induced bursts
increasingly dominate at L>>L*; at the most extreme luminosities, AGN are
important. However, all populations increase in luminosity at higher redshifts,
owing to increasing gas fractions. Thus the 'transition' between normal and
merger-dominated sources increases from the LIRG-ULIRG threshold at z~0 to
bright Hyper-LIRG thresholds at z~2. The transition to dominance by obscured
AGN evolves similarly, at factor of several higher L_IR. At all redshifts,
non-merging systems dominate the total luminosity/SFR density, with
merger-induced bursts constituting ~5-10% and AGN ~1-5%. Bursts contribute
little to scatter in the SFR-stellar mass relation. In fact, many systems
identified as 'ongoing' mergers will be forming stars in their 'normal'
(non-burst) mode. Counting this as 'merger-induced' star formation leads to a
stronger apparent redshift evolution in the contribution of mergers to the SFR
density.Comment: 16 pages, 9 figures (+appendices), accepted to MNRAS. A routine to
return the galaxy merger rates discussed here is available at
http://www.cfa.harvard.edu/~phopkins/Site/mergercalc.htm
The Wyoming Survey for H-alpha. III. H-alpha Luminosity Functions at z ~ 0.16, 0.24, 0.32, and 0.40
The Wyoming Survey for H-alpha, or WySH, is a large-area, ground-based
imaging survey for H-alpha-emitting galaxies at redshifts of z ~ 0.16, 0.24,
0.32, and 0.40. The survey spans up to four square degrees in a set of fields
of low Galactic cirrus emission, using twin narrowband filters at each epoch
for improved stellar continuum subtraction. H-alpha luminosity functions are
presented for each Delta(z) ~ 0.02 epoch based on a total of nearly 1200
galaxies. These data clearly show an evolution with lookback time in the
volume-averaged cosmic star formation rate. Integrals of Schechter fits to the
incompleteness- and extinction-corrected H-alpha luminosity functions indicate
star formation rates per co-moving volume of 0.010, 0.013, 0.020, 0.022 h_70
M_sun yr^{-1} Mpc^{-3} at z ~ 0.16, 0.24, 0.32, and 0.40, respectively.
Statistical and systematic measurement uncertainties combined are on the order
of 25% while the effects of cosmic variance are at the 20% level. The bulk of
this evolution is driven by changes in the characteristic luminosity L_* of the
H-alpha luminosity functions, with L_* for the earlier two epochs being a
factor of two larger than L_* at the latter two epochs; it is more difficult
with this data set to decipher systematic evolutionary differences in the
luminosity function amplitude and faint-end slope. Coupling these results with
a comprehensive compilation of results from the literature on emission line
surveys, the evolution in the cosmic star formation rate density over 0 < z <
1.5 is measured to be rho_dot_SFR(z) = rho_dot_SFR(0) (1+z)^{3.4+/-0.4}.Comment: Accepted for publication in ApJ Letter
Revealing the High-Redshift Star Formation Rate with Gamma-Ray Bursts
While the high-z frontier of star formation rate (SFR) studies has advanced
rapidly, direct measurements beyond z ~ 4 remain difficult, as shown by
significant disagreements among different results. Gamma-ray bursts, owing to
their brightness and association with massive stars, offer hope of clarifying
this situation, provided that the GRB rate can be properly related to the SFR.
The Swift GRB data reveal an increasing evolution in the GRB rate relative to
the SFR at intermediate z; taking this into account, we use the highest-z GRB
data to make a new determination of the SFR at z = 4-7. Our results exceed the
lowest direct SFR measurements, and imply that no steep drop exists in the SFR
up to at least z ~ 6.5. We discuss the implications of our result for cosmic
reionization, the efficiency of the universe in producing stellar-mass black
holes, and ``GRB feedback'' in star-forming hosts.Comment: 4 pages, 2 figures; ApJ Letters, in pres
Revisiting the Cosmic Star Formation History: Caution on the Uncertainties in Dust Correction and Star Formation Rate Conversion
The cosmic star formation rate density (CSFRD) has been observationally
investigated out to redshift z~10. However, most of theoretical models for
galaxy formation underpredict the CSFRD at z>1. Since the theoretical models
reproduce the observed luminosity functions (LFs), luminosity densities (LDs),
and stellar mass density at each redshift, this inconsistency does not simply
imply that theoretical models should incorporate some missing unknown physical
processes in galaxy formation. Here, we examine the cause of this inconsistency
in UV wavelengths by using a mock catalog of galaxies generated by a
semi-analytic model of galaxy formation. We find that this inconsistency is due
to two observational uncertainties: dust obscuration correction and conversion
from UV luminosity to star formation rate (SFR). The methods for correction of
obscuration and SFR conversion used in observational studies result in the
overestimation of CSFRD by ~ 0.1-0.3 dex and ~ 0.1-0.2 dex, respectively,
compared to the results obtained directly from our mock catalog. We present new
empirical calibrations for dust attenuation and conversion from observed UV LFs
and LDs into CSFRD.Comment: 12 pages including 11 figures. matches the published version (ApJ
2013 Jan. 20 issue
The Ha luminosity function and star formation rate up to z~1
We describe ISAAC/ESO-VLT observations of the Ha(6563) Balmer line of 33
field galaxies from the Canada-France Redshift Survey (CFRS) with redshifts
selected between 0.5 and 1.1. We detect Ha in emission in 30 galaxies and
compare the properties of this sample with the low-redshift sample of CFRS
galaxies at z~0.2 (Tresse & Maddox 1998). We find that the Ha luminosity,
L(Ha), is tightly correlated to M(B(AB)) in the same way for both the low- and
high-redshift samples. L(Ha) is also correlated to L([OII]3727), and again the
relation appears to be similar at low and high redshifts. The ratio
L([OII])/L(Ha) decreases for brighter galaxies by as much as a factor 2 on
average. Derived from the Ha luminosity function, the comoving Ha luminosity
density increases by a factor 12 from =0.2 to =1.3. Our results confirm a
strong rise of the star formation rate (SFR) at z<1.3, proportional to
(1+z)^{4.1+/-0.3} (with H_0=50 km/s/Mpc, q_0=0.5). We find an average SFR(2800
Ang)/SFR(Ha) ratio of 3.2 using the Kennicutt (1998) SFR transformations. This
corresponds to the dust correction that is required to make the near UV data
consistent with the reddening-corrected Ha data within the self-contained,
I-selected CFRS sample.Comment: 16 pages, 16 figures and 3 tables included, figures and text updated,
same results as in the 1st version, accepted in MNRA
A physical model for the origin of the diffuse cosmic infrared background
We present a physical model for origin of the cosmic diffuse infrared
background (CDIRB). By utilizing the observed stellar mass function and its
evolution as input to a semi-empirical model of galaxy formation, we isolate
the physics driving diffuse IR emission. The model includes contributions from
three primary sources of IR emission: steady-state star formation owing to
isolated disk galaxies, interaction-driven bursts of star formation owing to
close encounters and mergers, and obscured active galactic nuclei (AGN). We
find that most of the CDIRB is produced by equal contributions from objects at
z=0.5-1 and z>1, as suggested by recent observations. Of those sources, the
vast majority of the emission originates in systems with low to moderate IR
luminosities (L_{IR}<10^{12} $L_sun); the most luminous objects contribute
significant flux only at high-redshifts (z>2). All star formation in ongoing
mergers accounts for <10% of the total at all wavelengths and redshifts, while
emission directly attributable to the interaction-driven burst itself accounts
for <5%. We furthermore find that obscured AGN contribute <1-2% of the CDIRB at
all wavelengths and redshifts, with a strong upper limit of less than 4% of the
total emission. Finally, since electron-positron pair production interactions
with the CDIRB represent the primary source of opacity to very high energy
(VHE: E_\gamma > 1 TeV) \gamma-rays, the model provides predictions for the
optical depth of the Universe to the most energetic photons. We find that these
predictions agree with observations of high-energy cutoffs at TeV energies in
nearby blazars, and suggest that while the Universe is extremely optically
thick at >10 TeV, the next generation of VHE \gamma-ray telescopes can
reasonably expect detections from out to 50-150 Mpc.Comment: 14 pages, 13 figures, submitted to MNRA
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