We demonstrate a novel technology that combines the power of the multi-object spectrograph with the spatial multiplex advantage of an integral field spectrograph (IFS). The Sydney-AAO (Australian ...Astronomical Observatory) Multi-object IFS (SAMI) is a prototype wide-field system at the Anglo-Australian Telescope (AAT) that allows 13 imaging fibre bundles ('hexabundles') to be deployed over a 1-degree diameter field of view. Each hexabundle comprises 61 lightly fused multi-mode fibres with reduced cladding and yields a 75 per cent filling factor. Each fibre core diameter subtends 1.6 arcsec on the sky and each hexabundle has a field of view of 15 arcsec diameter. The fibres are fed to the flexible AAOmega double-beam spectrograph, which can be used at a range of spectral resolutions (R=λ/δλ≈ 1700-13 000) over the optical spectrum (3700-9500 Å). We present the first spectroscopic results obtained with SAMI for a sample of galaxies at z≈ 0.05. We discuss the prospects of implementing hexabundles at a much higher multiplex over wider fields of view in order to carry out spatially resolved spectroscopic surveys of 104-105 galaxies.
We investigate the total major (>1:4 by stellar mass) and minor (>1:100 by stellar mass) merger history of a population of 80 massive (Mlow * > 10 super(11) M middot in circle) galaxies at high ...redshifts (z = 1.7-3). We utilize extremely deep and high-resolution Hubble Space Telescope H-band imaging from the GOODS NICMOS Survey, which corresponds to rest-frame optical wavelengths at the redshifts probed. We find that massive galaxies at high redshifts are often morphologically disturbed, with a CAS (concentration, C; asymmetry, A; clumpiness, S) deduced merger fraction functionofm = 0.23 + or - 0.05 at z = 1.7-3. We find close accord between close pair methods (within 30 kpc apertures) and CAS methods for deducing major merger fractions at all redshifts. We deduce the total (minor + major) merger history of massive galaxies with Mlow * > 10 super(9) M middot in circle galaxies, and find that this scales roughly linearly with log-stellar-mass and magnitude range. We test our close pair methods by utilizing mock galaxy catalogs from the Millennium Simulation. We compute the total number of mergers to be (4.5 + or - 2.9)/(tau) from z = 3 to the present, to a stellar mass sensitivity threshold of ~1:100 (where tau is the merger timescale in Gyr which varies as a function of mass). This corresponds to an average mass increase of (3.4 + or - 2.2) x 10 super(11) M middot in circle over the past 11.5 Gyr due to merging. We show that the size evolution observed for these galaxies may be mostly explained by this merging.
From a volume-limited sample of 45 542 galaxies and 6000 groups with z ≤ 0.213, we use an adapted minimal spanning tree algorithm to identify and classify large-scale structures within the Galaxy And ...Mass Assembly (GAMA) survey. Using galaxy groups, we identify 643 filaments across the three equatorial GAMA fields that span up to 200 h
−1 Mpc in length, each with an average of eight groups within them. By analysing galaxies not belonging to groups, we identify a secondary population of smaller coherent structures composed entirely of galaxies, dubbed 'tendrils' that appear to link filaments together, or penetrate into voids, generally measuring around 10 h
−1 Mpc in length and containing on average six galaxies. Finally, we are also able to identify a population of isolated void galaxies. By running this algorithm on GAMA mock galaxy catalogues, we compare the characteristics of large-scale structure between observed and mock data, finding that mock filaments reproduce observed ones extremely well. This provides a probe of higher order distribution statistics not captured by the popularly used two-point correlation function.
We present an estimate of the galaxy stellar mass function and its division by morphological type in the local (0.025 < z < 0.06) Universe. Adopting robust morphological classifications as previously ...presented (Kelvin et al.) for a sample of 3727 galaxies taken from the Galaxy And Mass Assembly survey, we define a local volume and stellar mass limited sub-sample of 2711 galaxies to a lower stellar mass limit of
$\mathcal {M}=10^{9.0}\,{\rm M}_{{\odot }}$
. We confirm that the galaxy stellar mass function is well described by a double-Schechter function given by
$\mathcal {M}^{*}=10^{10.64}\,{\rm M}_{{\odot }}$
, α1 = −0.43,
$\phi _{1}^{*}=4.18\;\mathrm{dex}^{-1}\,\mathrm{Mpc}^{-3}$
, α2 = −1.50 and
$\phi _{2}^{*}=0.74\;\mathrm{dex}^{-1}\,\mathrm{Mpc}^{-3}$
. The constituent morphological-type stellar mass functions are well sampled above our lower stellar mass limit, excepting the faint little blue spheroid population of galaxies. We find approximately
$71{}_{-4}^{+3}$
per cent of the stellar mass in the local Universe is found within spheroid-dominated galaxies; ellipticals and S0-Sas. The remaining
$29{}_{-3}^{+4}$
per cent falls predominantly within late-type disc-dominated systems, Sab-Scds and Sd-Irrs. Adopting reasonable bulge-to-total ratios implies that approximately half the stellar mass today resides in spheroidal structures, and half in disc structures. Within this local sample, we find approximate stellar mass proportions for E : S0-Sa : Sab-Scd : Sd-Irr of 34 : 37 : 24 : 5.
We investigate the relationship between colour and structure within galaxies using a large, volume-limited sample of bright, low-redshift galaxies with optical–near-infrared imaging from the Galaxy ...And Mass Assembly survey. We fit single-component, wavelength-dependent, elliptical Sérsic models to all passbands simultaneously, using software developed by the MegaMorph project. Dividing our sample by n and colour, the recovered wavelength variations in effective radius (R
e) and Sérsic index (n) reveal the internal structure, and hence formation history, of different types of galaxies. All these trends depend on n; some have an additional dependence on galaxy colour. Late-type galaxies (n
r
< 2.5) show a dramatic increase in Sérsic index with wavelength. This might be a result of their two-component (bulge–disc) nature, though stellar population gradients within each component and dust attenuation are likely to play a role. All galaxies show a substantial decrease in R
e with wavelength. This is strongest for early types (n
r
> 2.5), even though they maintain constant n with wavelength, revealing that ellipticals are a superimposition of different stellar populations associated with multiple collapse and merging events. Processes leading to structures with larger R
e must be associated with lower metallicity or younger stellar populations. This appears to rule out the formation of young cores through dissipative gas accretion as an important mechanism in the recent lives of luminous elliptical galaxies.
We use a sample of 8298 galaxies observed as part of the Hubble Space Telescope (HST) H
160-band GOODS NICMOS Survey (GNS) to construct the galaxy stellar mass function both as a function of redshift ...and as stellar mass up to z= 3.5. Our mass functions are constructed within the redshift range z= 1-3.5 and consist of galaxies with stellar masses of M
*= 1012 M⊙ down to nearly dwarf galaxy masses of M
*= 108.5 M⊙ in the lowest redshift bin. We discover that a significant fraction of all massive M
* > 1011 M⊙ galaxies are in place up to the highest redshifts we probe, with a decreasing fraction of lower mass galaxies present at all redshifts. This is an example of 'galaxy mass downsizing', and is the result of massive galaxies forming before lower mass ones, and not just simply ending their star formation earlier as in traditional downsizing scenarios, whose effect is seen at z < 1.5. By fitting Schechter functions to our mass functions we find that the faint-end slope ranges from α=−1.36 to −1.73, which is significantly steeper than what is found in previous investigations of the mass function at high redshift. We demonstrate that this steeper mass function better matches the stellar mass added due to star formation, thereby alleviating some of the mismatch between these two measures of the evolution of galaxy mass. We furthermore examine the stellar mass function divided into blue/red systems, as well as for star-forming and non-star-forming galaxies. We find a similar mass downsizing present for both blue/red and star-forming/non-star forming galaxies, and further find that red galaxies dominate at the high-mass end of the mass function, but that the low-mass galaxies are mostly all blue, and therefore blue galaxies are creating the steep mass functions observed at z > 2. We furthermore show that, although there is a downsizing such that high-mass galaxies are nearer their z= 0 values at high redshift, this turns over at masses M
*∼ 1010 M⊙, such that the lowest mass galaxies are more common than galaxies at slight higher masses, creating a 'dip' in the observed galaxy mass function. We argue that the galaxy assembly process may be driven by different mechanisms at low and high masses, and that the efficiency of the galaxy formation process is lowest at masses M
*∼ 1010 M⊙ at 1 < z < 3. Finally, we calculate the integrated stellar mass density for the total, blue and red populations. We find the integrated stellar mass density of the total and blue galaxy population is consistent with being constant over z= 1-2, while the red population shows an increase in integrated stellar mass density over the same redshift range.
We present evidence for stochastic star formation histories in low-mass (M
* < 1010 M) galaxies from observations within the Galaxy And Mass Assembly (GAMA) survey. For ∼73 000 galaxies between 0.05 ...< z < 0.32, we calculate star formation rates (SFR) and specific star formation rates (SSFR = SFR/M
*) from spectroscopic Hα measurements and apply dust corrections derived from Balmer decrements. We find a dependence of SSFR on stellar mass, such that SSFRs decrease with increasing stellar mass for star-forming galaxies, and for the full sample, SSFRs decrease as a stronger function of stellar mass. We use simple parametrizations of exponentially declining star formation histories to investigate the dependence on stellar mass of the star formation time-scale and the formation redshift. We find that parametrizations previously fit to samples of z ∼ 1 galaxies cannot recover the distributions of SSFRs and stellar masses observed in the GAMA sample between 0.05 < z < 0.32. In particular, a large number of low-mass (M
* < 1010 M) galaxies are observed to have much higher SSFRs than can be explained by these simple models over the redshift range of 0.05 < z < 0.32, even when invoking mass-dependent staged evolution. For such a large number of galaxies to maintain low stellar masses, yet harbour such high SSFRs, requires the late onset of a weak underlying exponentially declining star formation history with stochastic bursts of star formation superimposed.
We use a sample of 8298 galaxies observed as part of the Hubble Space Telescope (HST) H160-band GOODS NICMOS Survey (GNS) to construct the galaxy stellar mass function both as a function of redshift ...and as stellar mass up to z= 3.5. Our mass functions are constructed within the redshift range z= 1-3.5 and consist of galaxies with stellar masses of M*= 1012Modot down to nearly dwarf galaxy masses of M*= 108.5Modot in the lowest redshift bin. We discover that a significant fraction of all massive M* > 1011Modot galaxies are in place up to the highest redshifts we probe, with a decreasing fraction of lower mass galaxies present at all redshifts. This is an example of 'galaxy mass downsizing', and is the result of massive galaxies forming before lower mass ones, and not just simply ending their star formation earlier as in traditional downsizing scenarios, whose effect is seen at z < 1.5. By fitting Schechter functions to our mass functions we find that the faint-end slope ranges from alpha =-1.36 to -1.73, which is significantly steeper than what is found in previous investigations of the mass function at high redshift. We demonstrate that this steeper mass function better matches the stellar mass added due to star formation, thereby alleviating some of the mismatch between these two measures of the evolution of galaxy mass. We furthermore examine the stellar mass function divided into blue/red systems, as well as for star-forming and non-star-forming galaxies. We find a similar mass downsizing present for both blue/red and star-forming/non-star forming galaxies, and further find that red galaxies dominate at the high-mass end of the mass function, but that the low-mass galaxies are mostly all blue, and therefore blue galaxies are creating the steep mass functions observed at z > 2. We furthermore show that, although there is a downsizing such that high-mass galaxies are nearer their z= 0 values at high redshift, this turns over at masses M* similar to 1010Modot, such that the lowest mass galaxies are more common than galaxies at slight higher masses, creating a 'dip' in the observed galaxy mass function. We argue that the galaxy assembly process may be driven by different mechanisms at low and high masses, and that the efficiency of the galaxy formation process is lowest at masses M* similar to 1010Modot at 1 < z < 3. Finally, we calculate the integrated stellar mass density for the total, blue and red populations. We find the integrated stellar mass density of the total and blue galaxy population is consistent with being constant over z= 1-2, while the red population shows an increase in integrated stellar mass density over the same redshift range.
We present the results of an extended narrow-band Hα study of the massive galaxy cluster XMMU J2235.3−2557 at z= 1.39. This paper represents a follow-up study to our previous investigation of star ...formation in the cluster centre, extending our analysis out to a projected cluster radius of 1.5 Mpc. Using the Near InfraRed Imager and Spectrograph on Gemini North Telescope we obtained deep H narrow-band imaging corresponding to the rest-frame wavelength of Hα at the cluster's redshift. We identify a total of 163 potential cluster members in both pointings, excluding stars based on their near-infrared colours derived from VLT/HAWK-I imaging. Of these 163 objects 14 are spectroscopically confirmed cluster members, and 20 per cent are excess line emitters. We find no evidence of star formation activity within a radius of 200 kpc of the brightest cluster galaxy in the cluster core. Dust-corrected star formation rates (SFRs) of excess emitters outside this cluster quenching radius,
kpc, are on average 〈SFR〉= 2.7 ± 1.0 M⊙ yr−1, but do not show evidence of increasing SFRs towards the extreme 1.5 Mpc radius of the cluster. No individual cluster galaxy exceeds an SFR of 6 M⊙ yr−1. Massive galaxies (log M*/M⊙ > 10.75) all have low specific SFRs (SSFRs, i.e. SFR per unit stellar mass). At fixed stellar mass, galaxies in the cluster centre have lower SSFRs than the rest of the cluster galaxies, which in turn have lower SSFRs than field galaxies at the same redshift by a factor of a few to 10. For the first time we can demonstrate through measurements of individual SFRs that already at very early epochs (at an age of the Universe of ∼4.5 Gyr) the suppression of star formation is an effect of the cluster environment which persists at fixed galaxy stellar mass.
We present the results of an extended narrow-band H alpha study of the massive galaxy cluster XMMU J2235.3-2557 at z= 1.39. This paper represents a follow-up study to our previous investigation of ...star formation in the cluster centre, extending our analysis out to a projected cluster radius of 1.5Mpc. Using the Near InfraRed Imager and Spectrograph on Gemini North Telescope we obtained deep H narrow-band imaging corresponding to the rest-frame wavelength of H alpha at the cluster's redshift. We identify a total of 163 potential cluster members in both pointings, excluding stars based on their near-infrared colours derived from VLT/HAWK-I imaging. Of these 163 objects 14 are spectroscopically confirmed cluster members, and 20per cent are excess line emitters. We find no evidence of star formation activity within a radius of 200kpc of the brightest cluster galaxy in the cluster core. Dust-corrected star formation rates (SFRs) of excess emitters outside this cluster quenching radius, kpc, are on average = 2.7 plus or minus 1.0 Modot yr-1, but do not show evidence of increasing SFRs towards the extreme 1.5Mpc radius of the cluster. No individual cluster galaxy exceeds an SFR of 6Modotyr-1. Massive galaxies (logM*/Modot > 10.75) all have low specific SFRs (SSFRs, i.e. SFR per unit stellar mass). At fixed stellar mass, galaxies in the cluster centre have lower SSFRs than the rest of the cluster galaxies, which in turn have lower SSFRs than field galaxies at the same redshift by a factor of a few to 10. For the first time we can demonstrate through measurements of individual SFRs that already at very early epochs (at an age of the Universe of similar to 4.5Gyr) the suppression of star formation is an effect of the cluster environment which persists at fixed galaxy stellar mass.
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