Near-infrared luminosity function and colours of dwarf galaxies in the Coma Cluster

We present K-band observations of the low-luminosity galaxies in the Coma cluster, which are responsible for the steep upturn in the optical luminosity function at M_R ~ -16, discovered recently. The main results of this study are (i) The optical$-$near-infrared colours of these galaxies imply that they are dwarf spheroidals. The median M-K colour for galaxies with -19.3 < M_K < -16.3 is 3.6 mag. (ii) The K-band luminosity function in the Coma cluster at the faint-end is not wee constrained, because of the uncertainties due to the field-to-field variance of the background. However, within the estimate large errors, it is consistent with the R-band luminosity function, shifted by $\sim3$ magnitudes. (iii) Many of the cluster dwarfs lie in a region of the B-K vs. B-R colour-colour diagram where background galaxies are rare Local dwarf spheroidal galaxies lie in this region too. This suggests that a better measurement of the K-band cluster luminosity function can be made if the field-to-field variance of the background can be measured as a function of colour. (iv) If we assume that none of the galaxies in the region of the B-K vs. B-R plane given in (iii) in our cluster fields are background, and that all the cluster galaxies with $15.5 < K < 18.5$ lie in this region of the plane, then we measure alpha = -1.41 +/- 0.35 for -19.3 < M_K < -16.3, where alpha is the logarithmic slope of the luminosity function.Comment: 6 pages, 8 figs, 2 tabs, MNRAS in press; email: [email protected], [email protected]

Stellar Population of Ellipticals in Different Environments: Near-infrared Spectroscopic Observations

Near-infrared spectra of 50 elliptical galaxies in the Pisces, A2199 and A2634 clusters, and in the general field, have been obtained. The strength of the CO (2.3 micron) absorption feature in these galaxies is used to explore the presence of an intermediate-age population (e.g. Asymptotic Giant Branch stars) in ellipticals in different environments. We find the strongest evidence for such a population comes from ellipticals in groups of a few members, which we interpret as the result of recent minor merging of these galaxies with later type galaxies. Field galaxies from very isolated environments, on the other hand, show no evidence for young or intermediate-age stars as revealed by H beta and CO absorptions, and appear to form a very uniform, old population with very little scatter in metallicity and star formation history.Comment: 13 pages, 6 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Societ

An infrared study of a sample of optically selected galaxies

Infrared and optical photometry of a complete sub-sample of 194 galaxies in the Anglo-Australian Redshift Survey is presented. The Colour-Redshift diagrams are constructed and used, in a self-consistent manner, to reduce the data to zero-redshift colours at a fixed aperture size. Infrared and optical-to-ir Colour-Luminosity relations are analysed both as a function of morphological type and, in conjunction with published data, as a function of environment. An infrared Colour-Luminosity relation is found for all galaxy types. The slope seen for E-SO galaxies strengthen the hypothesis that metallicity is the driving factor in the relation at infrared and optical wavelengths. The Colour-Luminosity relations for spirals have slopes that are substantially steeper. The infrared Colour-Luminosity relations of spirals do not vary significantly with Hubble type and the type-dependence of their optical-ir Colour-Luminosity relation disappears when corrections for bulge contamination are made. The results imply that both bulge and disk material have distinct enrichment histories that depend on luminosity in a simple way irrespective of the host galaxy type. Comparison with stellar synthesis models has revealed that the optical-to-ir colours of spirals are also sensitive to metallicity variations. Moreover, a change in the slope of the initial mass function of star formation (from X=1.35 to X=3) is needed to produce their optical-ir colours over the observed range. The infrared Colour-Luminosity relation is fairly tight; the intrinsic scatter is small and does not appear to be environmental dependent. The relation for the field E-SOs, analysed in this study, agrees closely with that seen in both the Virgo and Coma clusters and is consistent with recent estimates of the Local Group motion; there is no evidence for any anomalous population of cool stars in the Virgo cluster galaxies. Although promising as a distance indicator in cluster sample, infrared photometry of high precision is required. The optical-ir relation for spirals, however, has an intrinsic scatter which is most likely due to varying amounts of star formation at a fixed luminosity and bulge-to-disk ratio. An application of the bulge-corrected optical-ir Colour-Luminosity relation to Bothun et al.'s sample of cluster spirals, reduces the scatter in the mean optical-ir colours amongst the clusters. However, the scatter is still larger than that observed in the infrared Tully-Fisher relation. A detailed analysis of the infrared luminosity function of 'field' galaxies is carried out in conjunction with its optical counterpart. The infrared luminosity function is found to be type-dependent. This implies that the E-SOs and the bulges of spirals have different evolutionary or formation histories. Moreover, the infrared luminosity function for a given morphological type of galaxies is found to be the same as its optical counterpart shifted by the respective mean optical- ir colours. Regarding this result, it is proposed that galaxies with strong/weak near-ir emission are not very common. The sensitivity of the luminosity function to inhomogeneities in the galaxian distribution is investigated. The best estimates for the luminosity function parameters are then used to predict deep number-magnitude counts at 2.2 µm (K ~ 18 - 20 mag)

The determination of the extragalactic distance scale

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Comparison of Observed Galaxy Properties with Semianalytic Model Predictions using Machine Learning

With current and upcoming experiments such as WFIRST, Euclid and LSST, we can observe up to billions of galaxies. While such surveys cannot obtain spectra for all observed galaxies, they produce galaxy magnitudes in color filters. This data set behaves like a high-dimensional nonlinear surface, an excellent target for machine learning. In this work, we use a lightcone of semianalytic galaxies tuned to match CANDELS observations from Lu et al. (2014) to train a set of neural networks on a set of galaxy physical properties. We add realistic photometric noise and use trained neural networks to predict stellar masses and average star formation rates on real CANDELS galaxies, comparing our predictions to SED fitting results. On semianalytic galaxies, we are nearly competitive with template-fitting methods, with biases of $0.01$ dex for stellar mass, $0.09$ dex for star formation rate, and $0.04$ dex for metallicity. For the observed CANDELS data, our results are consistent with template fits on the same data at $0.15$ dex bias in $M_{\rm star}$ and $0.61$ dex bias in star formation rate. Some of the bias is driven by SED-fitting limitations, rather than limitations on the training set, and some is intrinsic to the neural network method. Further errors are likely caused by differences in noise properties between the semianalytic catalogs and data. Our results show that galaxy physical properties can in principle be measured with neural networks at a competitive degree of accuracy and precision to template-fitting methods.Comment: 19 pages, 10 figures, 6 tables. Accepted for publication in Ap

Evolution of the near-infrared luminosity function in rich galaxy clusters

We present the K-band (2.2 microns) luminosity functions of the X-ray luminous clusters MS1054-0321 (z=0.82), MS0451-0305 (z=0.55), Abell 963 (z=0.206), Abell 665 (z=0.182) and Abell 1795 (z=0.063) down to absolute magnitudes M_K = -20. Our measurements probe fainte absolute magnitudes than do any previous studies of the near-infrared luminosity function of clusters. All clusters are found to have similar luminosity functions within the errors, when the galaxy populations are evolved to redshift $z=0$. It iw known that the most massive bound systems in the Universe at all redshifts are X-ray luminous clusters. Therefore, assuming that the clusters in our sample correspond to a single population seen at different redshifts, the results here imply that not only had the stars in present-day ellipticals in rich clusters formed by z=0.8, but that they existed in as luminous galaxies then as they do today. Additionally, the clusters have K-band luminosity functions which appear to be consistent with the Kband field luminosity function in the range -24 < M_K < -22, although the uncertainties in both the field and cluster samples are large.Comment: 21 pages, 7 figs, TeX, MNRAS in pres