Compact Narrow Emission Line Galaxies Research Articles

A Search for Low Surface Brightness Structure around Compact Narrow Emission Line Galaxies

As the most extreme members of the rapidly evolving faint blue galaxy population at intermediate redshift, the compact narrow emission line galaxies (CNELGs) are intrinsically luminous (-22 < M_B < -18) with narrow emission linewidths (30 < \sigma < 125 km/s). Their nature is heavily debated: they may be low-mass starbursting galaxies that will fade to present-day dwarf galaxies or bursts of star formation temporarily dominating the flux of more massive galaxies, possibly related to in situ bulge formation or the formation of cores of galaxies. We present deep, high-quality (~0.6 - 0.8 arcsec) images with CFHT of 27 CNELGs. One galaxy shows clear evidence for a tidal tail; the others are not unambiguously embedded in galactic disks. Approximately 55% of the CNELGS have sizes consistent with local dwarfs of small-to-intermediate sizes, while 45% have sizes consistent with large dwarfs or disks galaxies. At least 4 CNELGs cannot harbor substantial underlying disk material; they are low-luminosity galaxies at the present epoch (M_B > -18). Conversely, 15 are not blue enough to fade to low-luminosity dwarfs (M_B > -15.2). The majority of the CNELGs are consistent with progenitors of intermediate-luminosity dwarfs and low-luminosity spiral galaxies with small disks. CNELGs are a heterogeneous progenitor population with significant fractions (up to 44%) capable of fading into today's faint dwarfs (M_B > -15.2), while 15 to 85% may only experience an apparently extremely compact CNELG phase at intermediate redshift but remain more luminous galaxies at the present epoch.

The Fornax Spectroscopic Survey: The Number of Unresolved Compact Galaxies

We describe a sample of thirteen bright (18.5<Bj<20.1) compact galaxies at low redshift (0.05<z<0.21) behind the Fornax Cluster. These galaxies are unresolved on UK Schmidt sky survey plates, so would be missing from most galaxy catalogs compiled from this material. The objects were found during initial observations of The Fornax Spectroscopic Survey. This project is using the Two-degree Field spectrograph on the Anglo-Australian Telescope to obtain spectra for a complete sample of all 14000 objects, stellar and non-stellar, with 16.5<Bj<19.7, in a 12 square degree area centered on the Fornax cluster of galaxies. The surface density of compact galaxies with magnitudes 16.5<Bj<19.7 is 7+/-3 /sq.deg., representing 2.8+/-1.6% of all local (z<0.2) galaxies to this limit. There are 12+/-3 /sq.deg. with 16.5<Bj<20.2. They are luminous (-21.5<Mb<-18.0, for H0=50 km/s/mpc) and most have strong emission lines (H alpha equivalent widths of 40-200 A) and small sizes typical of luminous HII galaxies and compact narrow emission line galaxies. Four out of thirteen have red colors and early-type spectra, so are unlikely to have been detected in any previous surveys.

Chemical Properties of Star‐forming Emission‐Line Galaxies atz=0.1–0.5

We measure the H II-region oxygen and nitrogen abundances for 14 star-forming emission-line galaxies (ELGs) at intermediate redshifts (0.11 -1.4, similar to local spiral galaxies. These chemical properties are inconsistent with unevolved objects undergoing a first burst of star formation. Comparison with local galaxies showing similar chemical properties suggests that these intermediate-z objects contain substantial old stellar populations that were responsible for the bulk of the heavy elements presently seen in the ionized gas. Four of the 14 galaxies exhibit small half-light radii and narrow emission-line profiles (compact narrow emission-line galaxies [CNELGs]; Koo et al. 1995) consistent with small dynamical masses despite their large optical luminosities and high levels of chemical enrichment. We find that the four CNELGs are indistinguishable from the 10 other emission-line galaxies (ELGs) in the sample on the basis of their metallicity and luminosity alone. Because of their morphological similarity to H II and spheroidal galaxies, CNELGs have been proposed as the starburst progenitors of today's spheroidal galaxies. Our assessment of the stellar chemical abundances in nearby spheroidal galaxies reveals that the majority of the CNELGs are presently ~4 mag brighter and ~0.5 dex more metal rich than the bulk of the stars in well-known metal-poor dwarf spheroidal galaxies such as NGC 205 and NGC 185. Two of the four CNELGs exhibit oxygen abundances higher than the planetary nebula oxygen abundances in NGC 205, making an evolution between these two CNELGs and metal-poor dwarf spheroidal galaxies highly improbable. However, the data are consistent with the hypothesis that more luminous and metal-rich spheroidal galaxies like NGC 3605 may become the evolutionary endpoints of some CNELGs after 1-3 mag of fading. We suggest that the z=0.1-0.4 ELGs, and perhaps some of the CNELGs, are the precursors to today's spiral galaxies during an episode of vigorous bulge star formation ~5 Gyr ago.

Optical Structure and Colors of Faint Compact Narrow Emission Line Galaxies

We present the results of Hubble Space Telescope Wide Field Planetary Camera imaging in V606 and I814 of five compact narrow emission line galaxies (CNELGs) and four slightly more extended faint blue galaxies (FBGs) with 20.3 ≤ B ≤ 22.4 and redshifts z ~ 0.22-0.66. Half-light radii are measured to span from 014 to 067, thus yielding small sizes of Re ~ 1-5 kpc (H0=50 km s-1 Mpc-1, q0=0.1). The new scale lengths, in combination with previous Keck measurements of velocity widths, continue to confirm that CNELGs, though quite luminous (MB~-21), have masses comparable to dwarf galaxies (i.e., M/M☉ 1010). Their low mass-to-light ratios are consistent with values corresponding to very early phases of a major burst of star formation. The spatial contour maps and surface brightness profiles of CNELGs reveal the presence of high surface brightness knots surrounded by a diffuse exponential-like component and sometimes accompanied by fans and tails. Since the knots tend to also have bluer colors, they are identified as the location of the current starburst. The diffuse component is interpreted to be an older underlying population. With the current burst estimated from the rest-frame (B - V) colors and mass-to-light ratios to involve typically ~10% of the galaxy mass, CNELGs are predicted to fade by 4 mag to reach the low luminosities and surface brightnesses of bright spheroidal galaxies, such as NGC 205. These new results provide further support for our previous suggestion that these distant, low-mass, starburst galaxies may be the progenitors of today's bright spheroidals. In contrast, although the four FBGs have similar colors and mass-to-light ratios to CNELGs, they will most likely not evolve into spheroidals but will resemble small, star-forming disk galaxies.

On the Nature of the Faint Compact Narrow Emission-Line Galaxies: The Half-Light Radius–Velocity Width Diagram

We present new measurements of emission-line velocity widths for five faint compact narrow emission-line galaxies (CNELGs) with 20.5 #B #21.5 and redshifts z10.19 to 0.35. The spectra were taken at the Keck telescope using the HIRES spectrograph with a resolution of 8 km s 21 (FWHM). Emission-line profiles are roughly Gaussian with velocity widths s130 to 50 km s 21 . The new sdata, in combination withHubble Space Telescope(HST) measurements of half-light radiiRe, indicate that these CNELGs are low-mass stellar systems (i.e.,M110 9 MJ),whiletheirunusuallylowmass-to-lightratios(typically ,0.2MJ/LJ)areconsistentwithbeing undergoing a major burst of star formation. SinceRe and sare roughly independent of the fading of the stellar population, theRe‐sdiagram is particularly useful for comparing the properties of these young galaxies to those of evolved stellar systems. Several physical processes that may modify the position of CNELGs in this diagram during their evolution, including dissipation, mergers, stripping, and winds, are discussed briefly. We conclude that the new data support a simple evolutionary scenario in which these low-mass, young galaxies will fade after the starburst, without major changes inRe or s, to become today’s spheroidal galaxies. Subject headings: cosmology: observations—galaxies: compact—galaxies: evolution—galaxies: formation— galaxies: fundamental parameters

Characterizing distant blue galaxies with HST images and Keck spectra

Keck spectra and HST images have been used to derive characteristic velocity and length scales for an enigmatic component of the faint, blue, field galaxy population: the compact, narrow emission-line galaxies (CNELGs). These galaxies are very luminous, but have been found to be quite low mass systems (with typical masses ∼ 109 M⊙). Their blue colors and strong emission lines indicate that they are undergoing a major burst of star formation. Following the completion of their current burst they will fade, becoming, in the absence of further major bursts, objects very similar to contemporary spheroidal galaxies. With mean sizes Re ∼ 1.4 kpc and Gaussian velocity profiles with mean σ = 45 km s–1, the length scales and velocity widths of CNELGs are also quite consistent with the measured length scales and velocity widths of current spheroidals.

High-resolution spectra of distant compact narrow emission line galaxies: Progrenitors of spheroidal galaxies

Emission-line velocity widths have been determined for 17 faint (B approximately 20-23) very blue, compact galaxies whose redshifts range from z = 0.095 to 0.66. The spectra have a resolution of 8 Km/s and were taken with the HIRES echelle spectrograph of the Keck 10 m telescope. The galaxies are luminous with all but two within 1 mag of M(sub B) approximately -21. Yet they exhibit narrow velocity widths between sigma = 28-157 km/s, more consistent with typical values of extreme star-forming galaxies than with those of nearby spiral galaxies of similar luminosity. In particular, objects with sigma is less than or equal to 65 km/s follow the same correlations between sigma and both blue and H beta luminosities as those of nearby H II galaxies. These results strengthen the identification of H II glaxies as thier local counterparts. The blue colors and strong emission lines suggest these compact galaxies are undergoing a recent, strong burst of star formation. Like those which characterize some H II galaxies, this burst could be a nuclear star-forming event within a much larger, older stellar population. If the burst is instead a major episode in the total star-forming history, these distant galaxies could fade enough to match the low luminosities and surface brightnesses typical of nearby spheroidals like NGC 185 or NGC 205. Together with evidence for recent star formation, exponential light profiles, and subsolar metallicities, the postfading correlations between luminosity and velocity width and bewtween luminosity and surface brightness suggest that among the low-sigma galaxies, we may be witnessing, in situ, the progenitors of today's spheroidal galaxies.