76 research outputs found
Major Merging: The Way to Make a Massive, Passive Galaxy
We analyze the projected axial ratio distribution, p(b/a), of galaxies that
were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have
low star-formation rates. For these quiescent galaxies we find a rather abrupt
change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there
are hardly any galaxies with b/a<0.6, implying that essentially none of them
have disk-like intrinsic shapes and must be spheroidal. This transition mass is
~3-4 times higher than the threshold mass above which quiescent galaxies
dominate in number over star-forming galaxies, which suggests these mass scales
are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a
large range in axial ratios, implying a mix of bulge- and disk-dominated
galaxies. Our result strongly suggests that major merging is the most
important, and perhaps only relevant, evolutionary channel to produce massive
(>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.Comment: Minor changes to match published version in ApJ Letter
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
Active Disk Building in a local HI-Massive LIRG: The Synergy between Gas, Dust, and Star Formation
HIZOA J0836-43 is the most HI-massive (M_HI = 7.5x10^10 Msun) galaxy detected
in the HIPASS volume and lies optically hidden behind the Milky Way. Markedly
different from other extreme HI disks in the local universe, it is a luminous
infrared galaxy (LIRG) with an actively star forming disk (>50 kpc), central to
its ~ 130 kpc gas disk, with a total star formation rate (SFR) of ~20.5 Msun
yr^{-1}. Spitzer spectroscopy reveals an unusual combination of powerful
polycyclic aromatic hydrocarbon (PAH) emission coupled to a relatively weak
warm dust continuum, suggesting photodissociation region (PDR)-dominated
emission. Compared to a typical LIRG with similar total infrared luminosity
(L_TIR=10^11 Lsun), the PAHs in HIZOA J0836-43 are more than twice as strong,
whereas the warm dust continuum (lambda > 20micron) is best fit by a star
forming galaxy with L_TIR=10^10 Lsun. Mopra CO observations suggest an extended
molecular gas component (H_2 + He > 3.7x10^9 Msun) and a lower limit of ~ 64%
for the gas mass fraction; this is above average compared to local disk
systems, but similar to that of z~1.5 BzK galaxies (~57%). However, the star
formation efficiency (SFE = L_IR/L'_CO) for HIZOA J0836-43 of 140 Lsun (K km
s^{-1} pc^2)^{-1} is similar to that of local spirals and other disk galaxies
at high redshift, in strong contrast to the increased SFE seen in merging and
strongly interacting systems. HIZOA J0836-43 is actively forming stars and
building a massive stellar disk. Its evolutionary phase of star formation
(M_stellar, SFR, gas fraction) compared to more distant systems suggests that
it would be considered typical at redshift z~1. This galaxy provides a rare
opportunity in the nearby universe for studying (at z~0.036) how disks were
building and galaxies evolving at z~1, when similarly large gas fractions were
likely more common.Comment: Accepted for publication in The Astrophysical Journal. 16 pages, 8
figure
MASSIV: Mass Assembly Survey with SINFONI in VVDS. V. The major merger rate of star-forming galaxies at 0.9 < z < 1.8 from IFS-based close pairs
We aim to measure the major merger rate of star-forming galaxies at 0.9 < z
<1.8, using close pairs identified from integral field spectroscopy (IFS). We
use the velocity field maps obtained with SINFONI/VLT on the MASSIV sample,
selected from the star-forming population in the VVDS. We identify physical
pairs of galaxies from the measurement of the relative velocity and the
projected separation (r_p) of the galaxies in the pair. Using the well
constrained selection function of the MASSIV sample we derive the gas-rich
major merger fraction (luminosity ratio mu = L_2/L_1 >= 1/4), and, using merger
time scales from cosmological simulations, the gas-rich major merger rate at a
mean redshift up to z = 1.54. We find a high gas-rich major merger fraction of
20.8+15.2-6.8 %, 20.1+8.0-5.1 % and 22.0+13.7-7.3 % for close pairs with r_p <=
20h^-1 kpc in redshift ranges z = [0.94, 1.06], [1.2, 1.5) and [1.5, 1.8),
respectively. This translates into a gas-rich major merger rate of
0.116+0.084-0.038 Gyr^-1, 0.147+0.058-0.037 Gyr^-1 and 0.127+0.079-0.042 Gyr^-1
at z = 1.03, 1.32 and 1.54, respectively. Combining our results with previous
studies at z < 1, the gas-rich major merger rate evolves as (1+z)^n, with n =
3.95 +- 0.12, up to z = 1.5. From these results we infer that ~35% of the
star-forming galaxies with stellar masses M = 10^10 - 10^10.5 M_Sun have
undergone a major merger since z ~ 1.5. We develop a simple model which shows
that, assuming that all gas-rich major mergers lead to early-type galaxies, the
combined effect of gas-rich and dry mergers is able to explain most of the
evolution in the number density of massive early-type galaxies since z ~ 1.5,
with our measured gas-rich merger rate accounting for about two-thirds of this
evolution.Comment: Published in Astronomy and Astrophysics, 24 pages, 30 figures, 2
tables. Appendix with the residual images from GALFIT added. Minor changes
with respect to the initial versio
Dust-Obscured Star-Formation in Intermediate Redshift Galaxy Clusters
We present Spitzer MIPS 24-micron observations of 16 0.4<z<0.8 galaxy
clusters drawn from the ESO Distant Cluster Survey (EDisCS). This is the first
large 24-micron survey of clusters at intermediate redshift. The depth of our
imaging corresponds to a total IR luminosity of 8x10^10 Lsun, just below the
luminosity of luminous infrared galaxies (LIRGs), and 6^{+1}_{-1}% of M_V < -19
cluster members show 24-micron emission at or above this level. We compare with
a large sample of coeval field galaxies and find that while the fraction of
cluster LIRGs lies significantly below that of the field, the IR luminosities
of the field and cluster galaxies are consistent. However, the stellar masses
of the EDisCS LIRGs are systematically higher than those of the field LIRGs. A
comparison with optical data reveals that ~80% of cluster LIRGs are blue and
the remaining 20% lie on the red sequence. Of LIRGs with optical spectra,
88^{+4}_{-5}% show [O II] emission with EW([O II])>5A, and ~75% exhibit optical
signatures of dusty starbursts. On average, the fraction of cluster LIRGs
increases with projected cluster-centric radius but remains systematically
lower than the field fraction over the area probed (< 1.5xR200). The amount of
obscured star formation declines significantly over the 2.4 Gyr interval
spanned by the EDisCS sample, and the rate of decline is the same for the
cluster and field populations. Our results are consistent with an exponentially
declining LIRG fraction, with the decline in the field delayed by ~1 Gyr
relative to the clusters.Comment: 15 pages, 9 figure
Spectro-photometric close pairs in GOODS-S: major and minor companions of intermediate-mass galaxies
(Abriged) Our goal here is to provide merger frequencies that encompass both
major and minor mergers, derived from close pair statistics. We use B-band
luminosity- and mass-limited samples from an Spitzer/IRAC-selected catalogue of
GOODS-S. We present a new methodology for computing the number of close
companions, Nc, when spectroscopic redshift information is partial. We select
as close companions those galaxies separated by 6h^-1 kpc < rp < 21h^-1 kpc in
the sky plane and with a difference Delta_v <= 500 km s^-1 in redshift space.
We provide Nc for four different B-band-selected samples. Nc increases with
luminosity, and its evolution with redshift is faster in more luminous samples.
We provide Nc of M_star >= 10^10 M_Sun galaxies, finding that the number
including minor companions (mass ratio >= 1/10) is roughly two times the number
of major companions alone (mass ratio >= 1/3) in the range 0.2 <= z < 1.1. We
compare the major merger rate derived by close pairs with the one computed by
morphological criteria, finding that both approaches provide similar merger
rates for field galaxies when the progenitor bias is taken into account.
Finally, we estimate that the total (major+minor) merger rate is ~1.7 times the
major merger rate. Only 30% to 50% of the M_star >= 10^10 M_Sun early-type
(E/S0/Sa) galaxies that appear z=1 and z=0 may have undergone a major or a
minor merger. Half of the red sequence growth since z=1 is therefore unrelated
to mergers.Comment: Accepted in A&A. 14 pages, 6 figures, 8 tables. We have tested the
method with a local, volume-limited spectroscopic sample
Less than 10 percent of star formation in z=0.6 massive galaxies is triggered by major interactions
Both observations and simulations show that major tidal interactions or
mergers between gas-rich galaxies can lead to intense bursts of starformation.
Yet, the average enhancement in star formation rate (SFR) in major mergers and
the contribution of such events to the cosmic SFR are not well estimated. Here
we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24
micron SFRs from Spitzer and morphologies from two deep HST cosmological survey
fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a
function of projected galaxy separation. We apply two-point projected
correlation function techniques, which we augment with morphologically-selected
very close pairs (separation <2 arcsec) and merger remnants from the HST
imaging. Our analysis confirms that the most intensely star-forming systems are
indeed interacting or merging. Yet, for massive (M* > 10^10 Msun) star-forming
galaxies at 0.4<z<0.8, we find that the SFRs of galaxies undergoing a major
interaction (mass ratios <1:4 and separations < 40 kpc) are only 1.80 +/- 0.30
times higher than the SFRs of non-interacting galaxies when averaged over all
interactions and all stages of the interaction, in good agreement with other
observational works.
We demonstrate that these results imply that <10% of star formation at 0.4 <
z < 0.8 is triggered directly by major mergers and interactions; these events
are not important factors in the build-up of stellar mass since z=1.Comment: Submitted to ApJ. 41 pages, 11 figure
A coincidence of disturbed morphology and blue UV colour: minor-merger driven star formation in early-type galaxies at z~0.6
We exploit multi-wavelength photometry of early-type galaxies (ETGs) in the
COSMOS survey to demonstrate that the low-level star formation activity in the
ETG population at intermediate redshift is likely to be driven by minor
mergers. Splitting the ETGs into galaxies that show disturbed morphologies
indicative of recent merging and those that appear relaxed, we find that ~32%
of the ETG population appears to be morphologically disturbed. While the
relaxed objects are almost entirely contained within the UV red sequence, their
morphologically disturbed counterparts dominate the scatter to blue UV colours,
regardless of luminosity. Empirically and theoretically determined major-merger
rates in the redshift range z<1 are several times too low to account for the
fraction of disturbed ETGs in our sample, suggesting that minor mergers
represent the principal mechanism driving the observed star formation activity
in our sample. The young stellar components forming in these events have ages
between 0.03 and 0.3 Myrs and typically contribute <10% of the stellar mass of
the remnant. Together with recent work which demonstrates that the structural
evolution of nearby ETGs is consistent with one or more minor mergers, our
results indicate that the overall evolution of massive ETGs may be heavily
influenced by minor merging at late epochs and highlights the need to
systematically study this process in future observational surveys.Comment: MNRAS in press (significant revisions to version 1
Observational constraints on the co-evolution of supermassive black holes and galaxies
The star formation rate (SFR) and black hole accretion rate (BHAR) functions
are measured to be proportional to each other at z < ~3. This close
correspondence between SF and BHA would naturally yield a BH mass-galaxy mass
correlation, whereas a BH mass-bulge mass correlation is observed. To explore
this apparent contradiction we study the SF in spheroid-dominated galaxies
between z=1 and the present day. We use 903 galaxies from the COMBO-17 survey
with M* >2x10^10M_sun, ultraviolet and infrared-derived SFRs from Spitzer and
GALEX, and morphologies from GEMS HST/ACS imaging. Using stacking techniques,
we find that <25% of all SF occurs in spheroid-dominated galaxies (Sersic index
n>2.5), while the BHAR that we would expect if the global scalings held is
three times higher. This rules out the simplest picture of co-evolution, in
which SF and BHA trace each other at all times. These results could be
explained if SF and BHA occur in the same events, but offset in time, for
example at different stages of a merger event. However, one would then expect
to see the corresponding star formation activity in early-stage mergers, in
conflict with observations. We conclude that the major episodes of SF and BHA
occur in different events, with the bulk of SF happening in isolated disks and
most BHA occurring in major mergers. The apparent global co-evolution results
from the regulation of the BH growth by the potential well of the galactic
spheroid, which includes a major contribution from disrupted disk stars.Comment: 14 pages, 5 figures, accepted for publication in Ap
The Morphology of Galaxies in the Baryon Oscillation Spectroscopic Survey
We study the morphology of luminous and massive galaxies at 0.3<z<0.7
targeted in the Baryon Oscillation Spectroscopic Survey (BOSS) using publicly
available Hubble Space Telescope imaging from COSMOS. Our sample (240 objects)
provides a unique opportunity to check the visual morphology of these galaxies
which were targeted based solely on stellar population modelling. We find that
the majority (74+/-6%) possess an early-type morphology (elliptical or S0),
while the remainder have a late-type morphology. This is as expected from the
goals of the BOSS target selection which aimed to predominantly select slowly
evolving galaxies, for use as cosmological probes, while still obtaining a fair
fraction of actively star forming galaxies for galaxy evolution studies. We
show that a colour cut of (g-i)>2.35 selects a sub-sample of BOSS galaxies with
90% early-type morphology - more comparable to the earlier Luminous Red Galaxy
(LRG) samples of SDSS-I/II. The remaining 10% of galaxies above this cut have a
late-type morphology and may be analogous to the "passive spirals" found at
lower redshift. We find that 23+/-4% of the early-type galaxies are unresolved
multiple systems in the SDSS imaging. We estimate that at least 50% of these
are real associations (not projection effects) and may represent a significant
"dry merger" fraction. We study the SDSS pipeline sizes of BOSS galaxies which
we find to be systematically larger (by 40%) than those measured from HST
images, and provide a statistical correction for the difference. These details
of the BOSS galaxies will help users of the data fine-tune their selection
criteria, dependent on their science applications. For example, the main goal
of BOSS is to measure the cosmic distance scale and expansion rate of the
Universe to percent-level precision - a point where systematic effects due to
the details of target selection may become important.Comment: 18 pages, 11 figures; v2 as accepted by MNRA
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