101 research outputs found
The black hole mass metallicity relation and insights into galaxy quenching
One of the most important questions in astrophysics is what causes galaxies
to stop forming stars. Previous studies have shown a tight link between
quiescence and black hole mass. Other studies have revealed that quiescence is
also associated with 'starvation', the halting of gas inflows, which results in
the remaining gas being used up rapidly by star formation and in rapid chemical
enrichment. In this work we find the final missing link between these two
findings. Using a large sample of galaxies, we uncover the intrinsic
dependencies of the stellar metallicity on galaxy properties. In the case of
the star-forming galaxies, the stellar metallicity is driven by stellar mass.
However, for passive galaxies the stellar metallicity is primarily driven by
the black hole mass, as traced by velocity dispersion. This result finally
reveals the connection between previous studies, where the integrated effect of
black hole feedback prevents gas inflows, starving the galaxy, which is seen by
the rapid increase in the stellar metallicity, leading to the galaxy becoming
passive.Comment: 20 pages, 6 figures, submitted to Nature Astronom
The Tumultuous Formation of the Hubble Sequence at z > 1 Examined with HST/WFC3 Observations of the Hubble Ultra Deep Field
We examine in this paper a stellar mass selected sample of galaxies at 1 < z
< 3 within the Hubble Ultra Deep Field, utilising WFC3 imaging to study the
rest-frame optical morphological distribution of galaxies at this epoch. We
measure how apparent morphologies (disk, elliptical, peculiar) correlate with
physical properties, such as quantitative structure and spectral-types. One
primary result is that apparent morphology does not correlate strongly with
stellar populations, nor with galaxy structure at this epoch, suggesting a
chaotic formation history for Hubble types at z > 1. By using a locally defined
definition of disk and elliptical galaxies based on structure and
spectral-type, we find no true ellipticals at z > 2, and a fraction of
3.2+/-2.3% at 1.5 < z < 2. Local counterparts of disk galaxies are at a similar
level of 7-10%, much lower than the 75% fraction at lower redshifts. We further
compare WFC3 images with the rest-frame UV view of galaxies from ACS imaging,
showing that galaxies imaged with ACS that appear peculiar often contain an
`elliptical' like morphology in WFC3. We show through several simulations that
this larger fraction of elliptical-like galaxies is partially due to the
courser PSF of WFC3, and that the `elliptical' class very likely includes
early-type disks. We also measure the merger history for our sample using CAS
parameters, finding a redshift evolution increasing with redshift, and a peak
merger fraction of ~30% at z~2 for the most massive galaxies with M_*> 10^{10}
M_sol, consistent with previous results from ACS and NICMOS. We compare our
results to semi-analytical model results and find a relatively good agreement
between our morphological break-down and the predictions. Finally, we argue
that the peculiars, ellipticals and peculiar ellipticals have similar
properties, suggesting similar formation modes, likely driven by major mergers.Comment: 21 pages, submitted to MNRA
Interacting galaxies on FIRE-2: The connection between enhanced star formation and interstellar gas content
We present a comprehensive suite of high-resolution (parsec-scale), idealised
(non-cosmological) galaxy merger simulations (24 runs, stellar mass ratio
~2.5:1) to investigate the connection between interaction-induced star
formation and the evolution of the interstellar medium (ISM) in various
temperature-density regimes. We use the GIZMO code and the second version of
the 'Feedback in Realistic Environments' model (FIRE-2), which captures the
multi-phase structure of the ISM. Our simulations are designed to represent
galaxy mergers in the local Universe. In this work, we focus on the
'galaxy-pair period' between first and second pericentric passage. We split the
ISM into four regimes: hot, warm, cool and cold-dense, motivated by the hot,
ionised, atomic and molecular gas phases observed in real galaxies. We find
that, on average, interactions enhance the star formation rate of the pair
(~30%, merger-suite sample average) and elevate their cold-dense gas content
(~18%). This is accompanied by a decrease in warm gas (~11%), a negligible
change in cool gas (~4% increase), and a substantial increase in hot gas
(~400%). The amount of cold-dense gas with densities above 1000 cm^3 (the cold
ultra-dense regime) is elevated significantly (~240%), but only accounts for
0.15% (on average) of the cold-dense gas budget.Comment: 21 pages, 17 figures, accepted by MNRA
Elliptical Galaxies and Bulges of Disk Galaxies: Summary of Progress and Outstanding Issues
This is the summary chapter of a review book on galaxy bulges. Bulge
properties and formation histories are more varied than those of ellipticals. I
emphasize two advances: 1 - "Classical bulges" are observationally
indistinguishable from ellipticals, and like them, are thought to form by major
galaxy mergers. "Disky pseudobulges" are diskier and more actively star-forming
(except in S0s) than are ellipticals. Theys are products of the slow
("secular") evolution of galaxy disks: bars and other nonaxisymmetries move
disk gas toward the center, where it starbursts and builds relatively flat,
rapidly rotating components. This secular evolution is a new area of galaxy
evolution work that complements hierarchical clustering. 2 - Disks of
high-redshift galaxies are unstable to the formation of mass clumps that sink
to the center and merge - an alternative channel for the formation of classical
bulges. I review successes and unsolved problems in the formation of
bulges+ellipticals and their coevolution (or not) with supermassive black
holes. I present an observer's perspective on simulations of dark matter galaxy
formation including baryons. I review how our picture of the quenching of star
formation is becoming general and secure at redshifts z < 1. The biggest
challenge is to produce realistic bulges+ellipticals and disks that overlap
over a factor of 10**3 in mass but that differ from each other as observed over
that whole range. Second, how does hierarchical clustering make so many giant,
bulgeless galaxies in field but not cluster environments? I argue that we rely
too much on AGN and star-formation feedback to solve these challenges.Comment: 46 pages, 10 postscript figures, accepted for publication in Galactic
Bulges, ed. E. Laurikainen, R. F. Peletier, & D. A. Gadotti (New York:
Springer), in press (2015
The relationship between star formation rates, local density and stellar mass up to z ~ 3 in the GOODS NICMOS Survey
We investigate the relation between star formation rates and local galaxy
environment for a stellar mass selected galaxy sample in the redshift range 1.5
< z < 3. We use near-infra-red imaging from an extremely deep Hubble Space
Telescope survey, the GOODS-NICMOS Survey (GNS) to measure local galaxy
densities based on the nearest neighbour approach, while star-formation rates
are estimated from rest-frame UV-fluxes. Due to our imaging depth we can
examine galaxies down to a colour-independent stellar mass completeness limit
of log M\ast = 9.5 M\odot at z ~ 3. We find a strong dependence of star
formation activity on galaxy stellar mass over the whole redshift range, which
does not depend on local environment. The average star formation rates are
largely independent of local environment apart from in the highest relative
over-densities. Galaxies in over-densities of a factor of > 5 have on average
lower star formation rates by a factor of 2 - 3, but only up to redshifts of z
~ 2. We do not see any evidence for AGN activity influencing these relations.
We also investigate the influence of the very local environment on
star-formation activity by counting neighbours within 30 kpc radius. This shows
that galaxies with two or more close neighbours have on average significantly
lower star formation rates as well as lower specific star formation rates up to
z ~ 2.5. We suggest that this might be due to star formation quenching induced
by galaxy merging processes.Comment: 12 pages, 6 figures, accepted for publication in MNRA
Portraits of life: Patterns of events over the lifespan
This explorative content-analytic study completes earlier studies on the lifespan distributions of number and affect of past and future life-events, collected by means of the Life-line Interview Method (LIM), for three age groups of men and women (young, middle and late adulthood). LIM events were classified into 40 subcategories divided over 9 categories: Relations, School, Work, Health, Growth, Home, Birth, Death and Other. Compression of the full data set by age group, gender, affect, decade, and time perspective, disclosed various patterns of events underlying the human life-course, e.g., the âbump,â ârosy viewâ and âgender phase contrastâ patterns. The compressed data set provided detailed material for the composition of three written group portraits of life, reflecting the modal life story of young, middle-aged and older men and women. Patterns and portraits show a content shift of past memories and future expectations over the lifespan, supporting a more dynamic view on the human life-course
The Redshift and Mass Dependence on the Formation of the Hubble Sequence at \u3cem\u3ez\u3c/em\u3e \u3e 1 from CANDELS/UDS
In this paper we present a detailed study of the structures and morphologies of a sample of 1188 massive galaxies with M* â„ 1010âMâbetween redshifts z = 1 and 3 within the Ultra Deep Survey (UDS) region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. Using this sample we determine how galaxy structure and morphology evolve with time, and investigate the nature of galaxy structure at high redshift. We visually classify our sample into discs, ellipticals and peculiar systems and correct for redshift effects on these classifications through simulations. We find significant evolution in the fractions of galaxies at a given visual classification as a function of redshift. The peculiar population is dominant at z \u3e 2 with a substantial spheroid population, and a negligible disc population. We compute the transition redshift, ztrans, where the combined fraction of spheroidal and disc galaxies is equal to that of the peculiar population, as ztrans = 1.86 ± 0.62 for galaxies in our stellar mass range. We find that this transition changes as a function of stellar mass, with Hubble-type galaxies becoming dominant at higher redshifts for higher mass galaxies (ztrans = 2.22 ± 0.82), than for the lower mass galaxies (ztrans = 1.73 ± 0.57). Higher mass galaxies become morphologically settled before their lower mass counterparts, a form of morphological downsizing. We furthermore compare our visual classifications with the SĂ©rsic index, the concentration, asymmetry and clumpiness (CAS) parameters, star formation rate and rest-frame U â B colour. We find links between the colour of a galaxy, its star formation rate and how extended or peculiar it appears. Finally, we discuss the negligible z \u3e 2 disc fraction based on visual morphologies and speculate that this is an effect of forming disc appearing peculiar through processes such as violent disc instabilities or mergers. We conclude that to properly define and measure high-redshift morphology and structure a new and more exact classification scheme is needed
Measuring star formation in high-z massive galaxies: A mid-infrared to submillimeter study of the GOODS NICMOS Survey sample
We present measurements of the mean mid-infrared-to-submillimeter flux
densities of massive (M\ast \approx 2 \times 10^11 Msun) galaxies at redshifts
1.7 < z < 2.9, obtained by stacking positions of known objects taken from the
GOODS NICMOS Survey (GNS) catalog on maps: at 24 {\mu}m (Spitzer/MIPS); 70,
100, and 160{\mu}m (Herschel/PACS); 250, 350, 500{\mu}m (BLAST); and 870{\mu}m
(LABOCA). A modified blackbody spectrum fit to the stacked flux densities
indicates a median [interquartile] star-formation rate of SFR = 63 [48, 81]
Msun yr^-1 . We note that not properly accounting for correlations between
bands when fitting stacked data can significantly bias the result. The galaxies
are divided into two groups, disk-like and spheroid-like, according to their
Sersic indices, n. We find evidence that most of the star formation is
occurring in n \leq 2 (disk-like) galaxies, with median [interquartile] SFR =
122 [100,150] Msun yr^-1, while there are indications that the n > 2
(spheroid-like) population may be forming stars at a median [interquartile] SFR
= 14 [9,20] Msun yr^-1, if at all. Finally, we show that star formation is a
plausible mechanism for size evolution in this population as a whole, but find
only marginal evidence that it is what drives the expansion of the
spheroid-like galaxies.Comment: Accepted by MNRAS. 10 pages, 3 figures, 3 table
The demographics of galactic bulges in the SDSS database
We present a new database of our two-dimensional bulgeâdisk decompositions for 14,233 galaxies drawn from Sloan Digital Sky Survey DR12 in order to examine the properties of bulges residing in the local universe (0.005 < z < 0.05). We performed decompositions in the g and r bands by utilizing the galfit software. The bulge colors and bulge-to-total ratios are found to be sensitive to the details in the decomposition technique, and hence we hereby provide full details of our method. The g â r colors of bulges derived are almost constantly red regardless of bulge size, except for the bulges in the low bulge-to-total ratio galaxies (B/T r lesssim 0.3). Bulges exhibit similar scaling relations to those followed by elliptical galaxies, but the bulges in galaxies with lower bulge-to-total ratios clearly show a gradually larger departure in slope from the elliptical galaxy sequence. The scatters around the scaling relations are also larger for the bulges in galaxies with lower bulge-to-total ratios. Both the departure in slopes and larger scatters likely originate from the presence of young stars. The bulges in galaxies with low bulge-to-total ratios show signs of a frosting of young stars so substantial that their luminosity-weighted Balmer-line ages are as small as 1 Gyr in some cases. While bulges seem largely similar in optical properties to elliptical galaxies, they do show clear and systematic departures as a function of bulge-to-total ratio. The stellar properties and perhaps associated formation processes of bulges seem much more diverse than those of elliptical galaxies
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