Resumen de Neutral gas content of galaxies in different density environments: observations from vla and the alfalfa survey
M.Carmen Toribio Pérez
This dissertation deals with extragalactic detections of the 21-cm line of the neutral hydrogen atom (H1) obtained from both array and single dish radio telescopes. Among the wide range of topics that can be explored from extragalactic l-ll observations, which are one of the mainstays of our understanding of galaxy evolution and large-scale structure of the Universe, the work presented here is mostly framed within the scope of environmental studies. In particular, we present the results of two projects focused on galaxies lying in opposite environments.
The first project consists on the study of the H1 distribution and distance estimates of a handful of gas-poor spirals in the Virgo Cluster region from observations carried out with the Very Large Array (VLA). On the other hand, the second part of this thesis is devoted to analyze the optical properties and H1 content of a sample of field galaxies detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. The aims of our research regarding these two projects are complementary; while the first part is focused on gaining insight on the effects that high density environments have on galaxies, the second part seeks to set up new standards of normalcy for the H1 and optical properties of galaxies for future environmental studies.
The aperture synthesis observations in the 21 cm line of pointings centered on the Virgo Cluster region spirals NGC 4307, NGC 4356, NGC 441113, and NGC 4492 using the VLA in its CS configuration are presented. These galaxies were identified in a previous study of the three dimensional distribution of H1 emission in the Virgo region as objects with a substantial dearth of atomic gas and Tully--Fisher (TF) distance estimates that located them well outside the main body of the cluster. We have detected two other galaxies located in two of our fields and observed bands, the spiral NGC 4411A and the dwarf spiral VCC 740. For all these galaxies, we provide detailed information of the gas morphology and kinematics. Our new data confirm the strong HI deficiency of all the main targets but NGC 441 1B, which is found to have a fairly normal neutral gas content. Besides, the VLA observations have also been used to discuss the applicability of TF techniques to the five largest spirals we have observed. This analysis leads us to conclude that none of them is actually suitable for a TF distance evaluation, whether due to the radical trimming of their neutral hydrogen disks, which is the case of the targets NGC 4307, NGC 4356, and NGC 4492, or to their nearly face-on orientation (NGC 4411A and B).
The statistical analysis of H1 and optical properties of field galaxies constitutes the second project of this thesis. First, we report the results from the study of the HI content and stellar properties of nearby galaxies based on the Arecibo Legacy Fast ALFA (ALFALFA) blind 21-cm line survey observations on two strips covering a total area of 9 hr $\times$ 16 deg in the Virgo cluster direction, From these data, we have assembled a large homogeneous sample of 5496 galaxies with healthy amounts of neutral gas located in environments of low local galactic density. These are all galaxies with a counterpart in the Sloan Digital Sky Survey (SDSS), which has also granted us access to a vast number of their optical attributes. In this chapter, we focus exclusively on the distributions of morphology indicators such as structural parameters and colors, as well as their relationship with HI emission in an effort to shed more light on the nature of extragalactic H1 sources in the local universe. Our analysis confirms the extended view that red, passive objects of early type in the field tend to have less neutral hydrogen than bluer, star-forming late-type alaxies, a trend that ets emphasized with increasin distance in flux-limited datasets due to the positive correlation between global color and intrinsic luminosity. We find, however, that this latter relationship involves exclusively galaxies undetected in HI, whereas the color distribution for gas-healthy galaxies is Luminosity-independent. Moreover, our study demonstrates that the best way to discriminate between early and late-type galaxies is through the presence of significant I-ll emission, since sources With a _ true early-type morphology and substantial reserves of cold gas are rare exceptions to the norm.
Two smaller homogeneous samples containing several hundreds of isolated \alf\ galaxies each have also been extracted by applying different systematic identification criteria. These two subsets, which show neutral gas content distributions indistinguishable from Those found in our larger dataset of galaxies in low density environments, are nonetheless characterized by a higher relative abundance of objects with optical early-type attributes. This last trait, also observed in other catalogs of isolated galaxies existing ln the literature, highlights the selection bias arising from the aforementioned connection between color and luminosity and the application of methods for identifying isolated galaxy candidates partial to the brightest and, hence, the reddest members of a dataset.
Nextj we deal with a homogeneous subset of about three thousand objects extracted from the sample of Hl- healthy galaxies in environments of low local galactic density obeying the condition $\Fhi\ge l$~Jy \kms, for which we know that \alf\ is complete. We apply strategies of multivariate data analysis to these gas-rich galaxies in order to i) investigate the correlation structure of their main extensive 21cm and optical properties and reveal the true dimension of the space generated by them, ii) identify the intrinsic parameters that best define their HI content, and iii) explore the constraints that the joint distributions of our observables put on the scaling relations among fundamental variables. Our analysis conhrms previous claims that gaseous galaxies form essentially a one-parameter set, as well as that the most precise predictors of their HI mass are the stellar diameter, followed by the total luminosity (both in the $r$-band), and the maximum rotation speed. The most interesting findings regarding the associated scaling laws are that we get $90\%$ confidence intervals of $-7.54A{+l.l}_{-O.86}$ for the slope of the relation between magnitude and log rotation speed when the analysis is restricted to the galaxies with the highest inclinations ($i\geq 60\degr$), in good agreement with Tully-Fisher studies, and of $1.68{+0.09}_{-0.08}$ for the log slope ofthe HI mass-galaxy size relation. Given the completeness, homogeneity, and broad morphological coverage of our galaxy database, the latter result suggests that the constancy of the global (hybrid) HI surface density that some authors advocate for the entire spiral population is just a crude approximation of reality.
Finally, some ancillary results are presented. we investigate the main statistical properties of the observations and the completeness of the survey, display examples of rare, non-conventional, gas-rich early-type galaxies lying in low density regimes.
