A Study of the Abundances and Lyα Emission of Ultraviolet

Selected Samples of Star Forming Galaxies in the

Local UniverseRyan Mallery (Ph.D. thesis), R. Michael Rich, Jean-Michel Deharveng, and the Galex team

UCLA

GALEX

Telescope– Imaging and slitless grism

spectroscopy– FUV (1300-1800 Å) 4.5” psf FWHM– NUV (1800-2800 Å) 6” psf FWHM– 50 cm diameter – 1.2 degree diameter FOV– 8 Å spectral resolution

Survey Exp time Area mlim

Deep Imaging Survey

30,000s 80 deg2 25 AB

Medium Imaging Survey

1500s 1000 deg2 23 AB

All Sky Imaging Survey

100s 26,000 deg2 20.5 AB

Medium Spectroscopic Survey

150,000s 5 deg2 22 AB

Lyα Emitters Motivation• Issue: Lyα emission from galaxies is likely affected by a combination of dust

extinction, gas kinematics, and spatial geometry but the order of importance of each of these is unknown, and may vary from galaxy to galaxy.

• Opportunity: Galex grism mode in Far-UV (FUV) used to discover the first known sample of low redshift LAE galaxies. Instead of studying LAEs at z~4, study them at z~0.3

• Goal: Analyze photometry and spectroscopy of a local z~0.3 sample of ~76 LAEs and 107 non-LAEs to :– 1) determine why Lyα escapes from some galaxies and not others.– 2) determine the relative effect of gas kinematics, ISM geometry, and extinction

by dust on the Lyα emission.

• Importance: Lyα is presently one of the most widely utilized spectral features for the discovery and identification of galaxies at the highest redshifts. – Due to the low redshifts, and hence relatively bright UV+optical fluxes, the

sample represents a chance to understand the astrophysics of these sources in much more detail than can be done with high redshift LAEs

GALEX LAE Sample• The First Systematic low redshift

Survey for LAEs (first discovered by Deharveng et al. 2008)

• Northern Hemisphere Deep GALEX Spectroscopic Fields

– Cosmological Evolution Survey (COSMOS)– All Wavelength Extended Groth Strip

International Survey (AEGIS)– NOAO Deep Wide Field Survey (NDWFS)– Spitzer First Look Survey (FLS)

• 76 LAEs • LAEs are matched to sources in

publicly available datasets (SDSS, ACS, IRAC, MIPS) within 4”

– 75/76 with SDSS photometry– 25/76 with IRAC photometry– 20/76 with ACS photometry– 32/76 with MIPS 24μm photometry

COSMOS• UV spectroscopy exposure

time: 35 hours• 15 LAEs • 14/15 with SDSS• 15/15 with ACS imaging• 14/15 with MIPS 24 detections

AEGIS• UV spectroscopy exposure

time: 80 hours• 37 LAEs • 37/37 with SDSS• 6/37 with ACS imaging• 13/37 with MIPS 24 detections

NDWFS• UV spectroscopy exposure time: 43 hours• 17 LAEs • 17/17 with SDSS• 0/17 with ACS imaging• 0/17 with MIPS 24 detections

FLS• UV spectroscopy exposure time: 22 hours• 7 LAEs • 7/7 with SDSS• 0/7 with ACS imaging• 5/7 with MIPS 24 detections

Non-LAEs• Sources with GALEX

spectroscopy, within the same redshift range as the LAEs, that lack detections of Lyα.

107 non-LAEs•COSMOS: 39 sources •AEGIS: 39 sources•FLS: 29 sources

Non-LAEs: black hashed histogramLAEs: black histogram

Non-LAEs• Sources with GALEX

spectroscopy, within the same redshift range as the LAEs, that lack detections of Lyα.

107 non-LAEs•COSMOS: 39 sources •AEGIS: 39 sources•FLS: 29 sources

Non-LAEs: black hashed histogramLAEs: black histogram

•non-LAEs serve as a control sample.•Do LAEs have other physical properties that differ from non-LAEs?•Are these properties related to Lyα emission/escape?

UV, Optical, IR distributions

•Black hashed histogram: non-LAEs•Black Histogram: all LAEs•Blue Histogram: non-AGN LAEs•Red Histogram: AGN LAEs

•FUV: LAEs are on average slightly more luminous•Optical: LAEs are bimodal. The more luminous peak being due in part to AGN contamination.•IR: LAEs and non-LAEs have statistically similar distributions.

Color Magnitude Diagram: LAEs not special

• LAEs and non-LAEs span 2 orders of magnitude in optical luminosity

• Have similar NUV-r distributions

• At Mr>-21 the LAEs tend to have bluer UV colors, related to star formation history.

LEGEND•Blue: non-AGN LAEs•Red: AGN LAEsSquares: non-LAEs(symbols sized according to EWLyα)

Dust and Extinction

• LIR is derived from MIPS 24um and Chary & Elbaz (2001) dust models.

• LAEs tend to have FUV luminosities comparable to the highest FUV luminosities of non-LAEs.

• LAEs and non-LAEs span a similar range of IRX.

• 1/3 of LAEs have bluer UV colors than non-LAEs with similar IRX.

• Dust extinction is not likely to be the reason why Lyα is not detected in the non-LAEs.

Dust, Extinction, UV color and Starbursts

• Create a grid of Bruzual & Charlot (2003) Stellar population synthesis galaxy evolution models, varying star formation history and extinction. Half with and half without a bursts of star formation.

• The blue UV colors of LAEs can be explained by starbursts which form at least 10% of the galaxy’s stellar mass.

Morphology of LAEs

• Since there appears to be little evidence to suggest that dust extinction is inhibiting Lyα emission from the non-LAE sample, perhaps the sources have different morphologies/geometries?

• 20/76 LAEs & 64/107 non-LAEs with ACS F814W imaging with 0”.03 pix -1 resolution.

LAE/non-LAE Morphologies• The size of the LAEs correlates with optical luminosity.

• IFUV is likely underestimated for the large sources as the UV flux is likely emitted from HII regions within the disks.

• There are no compact low luminosity non-LAEs, which is likely a selection effect of the GALEX spectroscopy.

• Overall there is no major difference between the F814W geometries of the LAEs and non-LAEs.

Outflows and Star formation

• Heckman (2002) found that for a sample of nearby sources, outflows were present in all galaxies with a star formation rate surface density, ΣSFR > 0.1 M yr-1 kpc-2

• Rupke et al. (2004) found a correlation between SFR and outflow velocity.

log

(Out

flow

Vel

ocity

/ k

m s

-1)

ΣSFR of LAEs and non-LAEs

• All compact LAEs (r< 7 kpc) are offset from non-LAEs in ΣSFR - UV color.

• As ΣSFR is an indicator of outflows, this implies that the presence of outflows is the discriminating factor between LAEs and non-LAEs.

• Higher resolution UV imaging is required to investigate ΣSFR for the larger disk sources where the UV sizes may be poorly estimated by the F814W imaging.

Lyα Luminosity and the UV continuum

• Lyα to first order is correlated with the FUV luminosity, and to second order with the FUV luminosity density (IFUV).

What influences the emergent Lyα flux?

• For the GALEX LAEs, no correlation is found between the equivalent widths and UV luminosity, IR luminosity, or UV extinction (as indicated by IRX).

• This in contrast to the results of Shapley et al. (2003), Kornei et al. (2009) for z~3 LAEs, and Atek et al. (2009) for z~0 LAEs

Lyα Escape Fraction• fesc = Lyα/Hα/8.7 Hα/Hβ probes extinction, E(B-V)

Scarlata et al. (2010) Atek et al. (2009)

fesc, decreases with increasing extinctionLarge scatter: due to kinematics and/or different dust properties/distributions?

The Lyα escape fraction

• For 21 LAEs, flux calibrated optical spectroscopy was obtained with measured Hα and Hα fluxes.

• 13/21 were obtained with long slit aperture matched spectroscopy at Lick and KPNO.

• 8/21 were observed at Keck with 1”.5 long slit spectroscopy.

LEGENDDotted lines: CASE B recombintion ratios.Dashed line: Calzetti extinction law.Dot-dashed line: Cardelli exinction law

The mean escape fraction for these sources decreases with increasing

αβratios, indicating that dust extinction does have an effect on the amount of emergent Lyα flux

Escape Fraction and Equivalent width (EW)

• The EW increases for increasing escape fractions. – The notable exceptions

are the sources without aperture matched spectroscopic observations.

IRX vs Hα/Hβ• Hα/Hβ is uncorrelated with both

IRX and FUV-NUV.

• The global extinctions (e.g. IRX) do not correctly represent the line of sight extinctions into the HII regions as probed by the Balmer emission lines.

Escape, Extinction and ΣSFR

The Facts:• Sources with large EW >50 Å have both low E(B-V)≲0.07 and

high ΣSFR≳0.1

• Sources with low E(B-V) ≲0.07 and low ΣSFR ≲0.1 have low EW.

• One source, that has the lowest EW of the sample, has both a low E(B-V) and high ΣSFR but is highly inclined (b/a = 0.36)

Escape, Extinction and ΣSFR

Conclusions:• Both low E(B-V)≲0.07 and high ΣSFR≳0.1may be required for

large escape fractions and EW.• Both high E(B-V) ≳ 0.07 and low ΣSFR ≲0.1can greatly reduce

the observed escape fraction and EW.

• For the one source with both a low E(B-V) and high ΣSFR, the highly inclined (b/a = 0.36) morphology of the source may lead to increased scattering opacity along the line of sight which gives the source such a low EW.

Results

• The GALEX LAEs are a heterogeneous assortment of star forming galaxies. The distributions of IR-luminosities and optical luminosities span 2 orders of magnitude. The optical distribution is bimodal in part due to AGN contamination as 11/22 LAEs with 0.0Mr< -22.5 are AGN.

• Dust extinction is likely not the dominant effect inhibiting Lyα from escaping the non-LAE sources. It may play a role at high LIR.

• LAEs have higher ΣSFR than non-LAEs: this implies that the LAEs have outflows, while the non-LAEs do not.

Results (cont)

• The Lyα EWs of these sources are not correlated with the global photometric properties for the sample, including LFUV, and IRX (extinction). – For IRX, this is likely because the IRX values

probe a large area than the lines of sight into the HII regions that give rise to Lyα and the Balmer lines.

• Both geometry, extinction, and ΣSFR play a role in the Lya escape fraction: all 3 factors can inhibit the observed escape along the line of sight.

Project 2: Future Work

• Obtain aperture matched optical spectroscopy of entire GALEX LAE sample.

• Obtain high resolution UV spectra of sources with Cosmic Origins Spectrograph on HST– Measure kinematics

• Obtain high resolution UV images of the LAEs. Lyα imaging? – Compare ΣSFR vs kinematics

• Compare/contrast the GALEX LAEs to high redshift samples (i.e. SFH, M*, size, EW)

BACKUP SLIDES

Measuring Oxygen Abundances

• O3N2 {Pettini & Pagel et al 2005}– Calibration to Te abundance data, O/H = f(O3N2)– O3N2 = [OIII]/β / [NII]α– Not corrected for ionization

• M91 {McGaugh et al (1991)}– Photoionization Model, O/H = f(R23)

– R23= ([OIII]5007+[OIII]4959 + [OII]3727)/ β– O/H has 2 values for each R23 value

• T04 {Tremonti et al 2004}– Bayesian approach using photoionization models of Charlot & Longhetti (2001)– O/H < 8.2 : N/H ∝ (O/H) – O/H > 8.2 : N/H ∝ (O/H)2

Measuring the Nitrogen Abundance• Log N+/O+ = Log [NII6584]/[OII3727] + 0.307 - 0.02 Log TNII - 0.726/TNII

– from Pagel et al. (1992)– Empirical calibration from Te abundances– Applicability to high metallicities is unknown.

•TNII = .6065+.1600 log R23 + .1878 (log R23 )2 + .2803 (log R23)3

– from Thurston, Edmonds & Henry (1996)

•TNII from Cloudy photoionization models

•TNII~ 500K uncertainty