arXiv subjects

Jesus Gallego

Publications and source records attributed to Jesus Gallego.

24 records · Page 2Linked to original sources

arXiv2007

A contribution to the selection of emission-line galaxies using narrow-band filters in the optical airglow windows

Emission line galaxies are an invaluable tool for our understanding of the evolution of galaxies in the Universe. Imaging of deep fields with narrow-band filters allows not only the selection of these objects, but also to infer the line flux and the equivalent width of the emission line with some assumptions. The narrow-band filter technique provides homogeneous samples of galaxies in small comoving volumes in the sky. We present an analysis of the selection of emission-line galaxies using narrow-band filters. Different methods of observation are considered: broad-band -- narrow-band filters and two broad-band and one narrow-band filters. We study also the effect of several lines entering simultaneously inside the filters (this is the case of Halpha). In each case the equations to obtain the equivalent width and line flux from the photometry are obtained. Candidates to emission-line objects are selected by their color excess in a magnitude-color diagram. For different narrow-band filters, we compute the mean colors of stars and galaxies, showing that, apart from galaxies, some types of stars could be selected with certain filter sets. We show how to compute the standard deviation of the colors of the objects even in the usual case when there are not enough objects to determine the standard deviation from the data. We present also helpful equations to compute the narrow-band and the broad-band exposure times in order to obtain minimum dispersion in the ratio of fluxes of both bands with minimum total exposure time.

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arXiv2005

Are There Local Analogs of Lyman Break Galaxies?

To make direct comparisons in the rest-far-ultraviolet between LBGs at z~3 and more local star-forming galaxies, we use HST/STIS to image a set of 12 nearby (z<0.05) HII galaxies in the FUV and a set of 14 luminous compact blue galaxies (LCBGs) at moderate redshift (z~0.5) in the NUV, corresponding to the rest-FUV. We then subject both sets of galaxy images and those of LBGs at z~3 to the same morphological and structural analysis. We find many qualitative and quantitative similarities between the rest-FUV characteristics of distant LBGs and of the more nearby starburst samples, including general morphologies, sizes, asymmetries, and concentrations. Along with some kinematic similarities, this implies that nearby HII galaxies and LCBGs may be reasonable local analogs of distant Lyman break galaxies.

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arXiv2001

The Gas Content and Kinematics of Nearby Blue Compact Galaxies: Implications for Studies at Intermediate and High Redshift

We present Arecibo 21 cm spectroscopy, Keck HIRES Hb spectroscopy, and WIYN R-band images of 11 nearby blue compact galaxies selected to be similar to blue compact star-forming galaxies at intermediate redshifts (0.1<z<1). We detect HI in 10 of 11 sample galaxies, yielding HI masses of 0.3-4x10^9 M_odot, HI linewidths, W_20, of 133-249 km/s, dynamical masses of 0.5-5x10^10 M_odot, gas depletion timescales, tau_gas, of 0.3-7 Gyr, HI mass fractions of 0.01-0.58, and mass-to-light ratios of 0.1-0.8. These values span the range typical of nearby HII galaxies, irregulars and spirals. Despite the restricted morphological selection, our sample is quite heterogeneous in HI content, dynamical mass, and gas depletion timescale. Therefore, these galaxies should look very different from each other in 5 Gyr. Fading of intermediate-z luminous blue compact galaxies into NGC-type spheroidals is a viable evolutionary scenario for the least massive, most gas-poor objects. The most consistent characteristic of our sample is that the ratio, R, of HII linewidth to HI 21 cm linewidth (W_20) are systematically less than unity. The simplest explanation is that the ionized gas is more centrally concentrated than the neutral gas within the gravitational potential. On average R=0.66 +/- 0.16, similar to findings for local HII galaxies. We find R is also a function of linewidth; smaller linewidth galaxies have smaller values of R. Correcting optical linewidths by this factor raises derived masses and places these galaxies on the local luminosity--linewidth (Tully-Fisher) relation. If this ratio applies to intermediate-redshift galaxies, then the proposed luminosity evolution of the Tully-Fisher relation is much smaller and more gradual than suggested by studies using optical emission-linewidth measurements.

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arXiv1997

Optical Rotation Curves of Distant Field Galaxies: Sub-L* Systems

Moderate-resolution spectroscopic observations from the Keck 10m telescope are used to derive internal kinematics for eight faint disk galaxies in the fields flanking the Hubble Deep Field. The spectroscopic data are combined with high-resolution F814W WFPC2 images from the Hubble Space Telescope which provide morphologies and scale-lengths, inclinations and orientations. The eight galaxies have redshifts 0.15 < z < 0.75, magnitudes 18.6 < I_814 < 22.1 and luminosities -21.8 < M_B < -19.0 (H_0 = 75 and q_0 = 0.05). Terminal disk velocities are derived from the spatially-resolved velocity profiles by modeling the effects of seeing, slit width, slit misalignment with galaxy major axis, and inclination for each source. These data are combined with the sample of Vogt et al. (1996) to provide a high-redshift Tully-Fisher relation that spans three magnitudes. This sample was selected primarily by morphology and magnitude, rather than color or spectral features. We find no obvious change in the shape or slope of the relation with respect to the local Tully-Fisher relation. The small offset of < 0.4 B mag with respect to the local relation is presumably caused by luminosity evolution in the field galaxy population, and does not correlate with galaxy mass. A comparison of disk surface brightness between local and high-redshift samples yields a similar offset, ~0.6 mag. These results provide further evidence for only a modest increase in luminosity with lookback time.

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arXiv1997

On the Nature of the Strong Emission-Line Galaxies in Cluster Cl 0024+1654: Are Some the Progenitors of Low Mass Spheroidals?

We present new size, line ratio, and velocity width measurements for six strong emission-line galaxies in the galaxy cluster, Cl 0024+1654, at redshift z~0.4. The velocity widths from Keck spectra are all narrow (30<sigma<120 km/s), with three profiles showing double peaks. Four galaxies have low masses (M<10^{10} Mo). Whereas three galaxies were previously reported to be possible AGNs, none exhibit AGN-like emission line ratios or velocity widths. Two or three appear as very blue spirals with the remainder more akin to luminous H-II galaxies undergoing a strong burst of star formation. We propose that after the burst subsides, these galaxies will transform into quiescent dwarfs, and are thus progenitors of some cluster spheroidals (We adopt the nomenclature suggested by Kormendy & Bender (1994), i.e., low-density, dwarf ellipsoidal galaxies like NGC 205 are called `spheroidals' instead of `dwarf ellipticals') seen today.

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arXiv1996

Keck Spectroscopy of Redshift z~3 Galaxies in the Hubble Deep Field

We have obtained spectra with the 10-m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 2. Combined with 5 previously confirmed high-redshift galaxies in the HDF, the 16 confirmed sources yield a comoving volume density of n>2.5^-4 h50^3 Mpc^-3 for q0=0.05, or n>1.2^-3 h50^3 Mpc^-3 for q0=0.5. These densities are comparable to estimates of the local volume density of galaxies brighter than L*, and could be almost three times higher still if all 29 of the unconfirmed candidates in our original sample are indeed also at high redshift. The galaxies are small but luminous, with half-light radii 1.8 < r(1/2) < 6.5 h50^-1 kpc and absolute magnitudes -21.5 > M_B > -23. The HST images show a wide range of morphologies, including several with very close, small knots of emission embedded in wispy extended structures. Using rest-frame UV continuum fluxes with no dust correction, we calculate star formation rates in the range 7 - 24 or 3 - 9 h50^-2 Msun/yr for q0=0.05 and q0=0.5, respectively. The variety of morphologies and the high number density of z=3 galaxies in the HDF suggest that they represent a range of physical processes and stages of galaxy formation and evolution, rather than any one class of object, such as massive ellipticals. A key issue remains the measurement of masses. These high-redshift objects are likely to be the low-mass, starbursting building blocks of more massive galaxies seen today.

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