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Register a new userThe frequency of very young galaxies in the local Universe: II. The view from SDSS spectra
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Only a handful of galaxies in the local Universe appear to be very young. We estimate the fraction of very young galaxies (VYGs), defined as those with more than half their stellar masses formed within the last Gyr. We fit non-parametric star formation histories (SFHs) to ~280 000 galaxy spectra from a flux- and volume-limited subsample of the Main Galaxy Sample (MGS) of the SDSS, which is also complete in mass-to-light ratio, thus properly accounting for passive galaxies of a given mass. The VYG fractions decrease with increasing galaxy stellar mass, from ~50% at $m = 10^8,rm M_odot$ to ~0.1% at $m = 10^{11.5},rm M_odot$, with differences of up to 1 dex between the different spectral models used to estimate the SFH and on how we treat aperture effects. But old stellar populations may hide in our VYGs despite our conservative VYG sample built with galaxies that are globally bluer than within the region viewed by the SDSS fibre. The VYG fractions versus mass decrease more gradually compared to the Tweed et al. predictions using analytical and semi-analytical models of galaxy formation, but agree better with the SIMBA hydrodynamical simulation. These discrepancies highlight the usefulness of VYGs in constraining the strong uncertainties in both galaxy formation models and spectral modelling of galaxy SFHs. Given the lognormal cosmic SFH, these mean VYG fractions suggest that galaxies with $m > 10^8,rm M_odot$ undergo at most 4 major starbursts on average.
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In the local Universe, there is a handful of dwarf compact star-forming galaxies with extremely low oxygen abundances. It has been proposed that they are young, having formed a large fraction of their stellar mass during their last few hundred Myr. However, little is known about the fraction of young stellar populations in more massive galaxies. In a previous article, we analyzed 280 000 SDSS spectra to identify a surprisingly large sample of more massive Very Young Galaxies (VYGs), defined to have formed at least $50%$ of their stellar mass within the last 1 Gyr. Here, we investigate in detail the properties of a subsample of 207 galaxies that are VYGs according to all three of our spectral models. We compare their properties with those of control sample galaxies (CSGs). We find that VYGs tend to have higher surface brightness and to be more compact, dusty, asymmetric, and clumpy than CSGs. Analysis of a subsample with HI detections reveals that VYGs are more gas-rich than CSGs. VYGs tend to reside more in the inner parts of low-mass groups and are twice as likely to be interacting with a neighbour galaxy than CSGs. On the other hand, VYGs and CSGs have similar gas metallicities and large scale environments (relative to filaments and voids). These results suggest that gas-rich interactions and mergers are the main mechanisms responsible for the recent triggering of star formation in low-redshift VYGs, except for the lowest mass VYGs, where the starbursts may arise from a mixture of mergers and gas infall.
We present AKARI 2.5-5um spectra of 145 local luminous infrared galaxies in the Great Observatories All-sky LIRG Survey. In all of the spectra, we measure the line fluxes and EQWs of the polycyclic aromatic hydrocarbon (PAH) at 3.3um and the hydrogen recombination line Br-alpha, with apertures matched to the slit sizes of the Spitzer spectrograph and with an aperture covering ~95% of the total flux in the AKARI 2D spectra. The star formation rates (SFRs) derived from Br-alpha measured in the latter aperture agree well with SFRs(LIR), when the dust extinction correction is adopted based on the 9.7um absorption feature. Together with the Spitzer spectra, we are able to compare the 3.3 and 6.2um PAH features, the two most commonly used near/mid-IR indicators of starburst (SB) or active galactic nucleus (AGN) dominated galaxies. We find that the 3.3 and 6.2um PAH EQWs do not follow a linear correlation and at least 1/3 of galaxies classified as AGN-dominated using 3.3um PAH are classified as starbursts based on 6.2um PAH. These galaxies have a bluer continuum slope than galaxies that are indicated to be SB-dominated by both PAH features. The bluer continuum emission suggests that their continuum is dominated by stellar emission rather than hot dust. We also find that the median Spitzer spectra of these sources are remarkably similar to the pure SB-dominated sources indicated by high PAH EQWs in both 3.3 and 6.2um. We propose a revised SB/AGN diagnostic diagram using 2-5um data. We also use the AKARI and Spitzer spectra to examine the performance of our new diagnostics and to estimate 3.3um PAH fluxes using the JWST photometric bands in 0<z<5. Of the known PAH features and mid-IR high ionization emission lines used as SB/AGN indicators, only the 3.3um PAH feature is observable with JWST at z>3.5, because the rest of the features at longer wavelengths fall outside the JWST wavelength coverage.
We present the first study of bars in the local Universe, based on the Sloan Digitized Sky Survey (SDSS). The large sample of ~5000 local galaxies provides the largest study to date of local bars and minimizes the effect of cosmic variance. The sample galaxies have M_g<=-18.5 mag and cover the redshift range 0.01<=z<0.04. We use a color cut in the color-magnitude diagram and the Sersic index n to identify disk galaxies. We characterize bars and disks using r-band images and the method of iterative ellipse fits and quantitative criteria developed in Jogee at al. (2004, ApJL, 615, L105). After excluding highly inclined (i>60 degrees) systems our results are: (1) the optical (r-band) fraction of barred galaxies among local disk galaxies is 43%, which confirms the ubiquity of local bars, in agreement with other optical studies based on smaller samples (e.g.Eskridge et al. 2000, AJ, 119, 536, Marinova & Jogee 2006, astro-ph/0608039); (2) the optical bar fraction rises for bluer galaxies, suggesting a relation between bars and star formation; (3) preliminary analyzes suggest that the optical bar fraction increases steeply with the galaxy effective radius; (4) the optical bar fraction at z~0 is ~35% for bright disks (M_g<=-19.3 mag) and strong (bar ellipticity >0.4), large-scale (bar semi-major axis >1.5 kpc) bars, which is comparable to the value of 30+/-6% reported earlier (Jogee et al. 2004) for similar disks and bars at z~0.2-1.0.
Here I present results from individual galaxy studies and galaxy surveys in the Local Universe with particular emphasis on the spatially resolved properties of neutral hydrogen gas. The 3D nature of the data allows detailed studies of the galaxy morphology and kinematics, their relation to local and global star formation as well as galaxy environments. I use new 3D visualisation tools to present multi-wavelength data, aided by tilted-ring models of the warped galaxy disks. Many of the algorithms and tools currently under development are essential for the exploration of upcoming large survey data, but are also highly beneficial for the analysis of current galaxy surveys.
We probe the angular scale of homogeneity in the local Universe using blue galaxies from the SDSS survey as a cosmological tracer. Through the scaled counts in spherical caps, $ mathcal{N}(<theta) $, and the fractal correlation dimension, $mathcal{D}_{2}(theta)$, we find an angular scale of transition to homogeneity for this sample of $theta_{text{H}} = 22.19^{circ} pm 1.02^{circ}$. A comparison of this measurement with another obtained using a different cosmic tracer at a similar redshift range ($z < 0.06$), namely, the HI extragalactic sources from the ALFALFA catalogue, confirms that both results are in excellent agreement (taking into account the corresponding bias correction). We also perform tests to asses the robustness of our results. For instance, we test if the size of the surveyed area is large enough to identify the transition scale we search for, and also we investigate a reduced sample of blue galaxies, obtaining in both cases a similar angular scale for the transition to homogeneity. Our results, besides confirming the existence of an angular scale of transition to homogeneity in different cosmic tracers present in the local Universe, show that the observed angular scale $theta_{text{H}}$ agrees well with what is expected in the $Lambda$CDM scenario. Although we can not prove spatial homogeneity within the approach followed, our results provide one more evidence of it, strengthening the validity of the Cosmological Principle.
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